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| Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work. |
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OT - Rotations of a low tire?
The "NPR "Car Talk" show's "Puzzler" a couple of weeks ago gave an answer stating that some car's computer "knew" a front tire was low on air because the ABS system noted that wheel was rotating "a heck of a lot faster" than the other wheels when the car was driven. I didn't buy that one. Sure, the rolling radius of a low tire is less than that of a fully inflated one, but the overall circumference, particularly on a steel belted tire, remains the same. Barring slippage, that circumference must lay its whole length on the road once per revolution, just like the circumference of a full tire does. From my TSD rallying days I remember that low tire pressures made some slight differences in odometer measurements, but these were in the second decimal place, hardly "a heck of a lot". Am I missing something here? What do the great minds on rcm think about this one? Jeff -- Jeffry Wisnia (W1BSV + Brass Rat '57 EE) "Truth exists; only falsehood has to be invented." |
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The "NPR "Car Talk" show's "Puzzler" a couple of weeks ago gave an
answer stating that some car's computer "knew" a front tire was low on air because the ABS system noted that wheel was rotating "a heck of a lot faster" than the other wheels when the car was driven. I don't buy it either. I know a GM mechanic and he states there are sensors in the wheel that transmit tire pressure to the computer. His bitch is that it complicates simple things like rotating tires because now he has to connect the scan tool and tell the computer how he rotated the tires. cs |
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"a heck of a lot faster"
This is indeed the "indirect" tire pressure rotation system. You're right, it actually requires that the pressure be so low that the outermost belts are sort-of buckled in, but this is what happens if you are 5 or more PSI down on a big vehicle. I think SUV's are soon (already?) required to have a "direct" method (after the tire fiasco of a few years ago), an actual pressure sensor. Tim. |
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"Jeff Wisnia" wrote in message ... The "NPR "Car Talk" show's "Puzzler" a couple of weeks ago gave an answer stating that some car's computer "knew" a front tire was low on air because the ABS system noted that wheel was rotating "a heck of a lot faster" than the other wheels when the car was driven. I didn't buy that one. Sure, the rolling radius of a low tire is less than that of a fully inflated one, but the overall circumference, particularly on a steel belted tire, remains the same. Barring slippage, that circumference must lay its whole length on the road once per revolution, just like the circumference of a full tire does. From my TSD rallying days I remember that low tire pressures made some slight differences in odometer measurements, but these were in the second decimal place, hardly "a heck of a lot". Am I missing something here? What do the great minds on rcm think about this one? Jeff -- Jeffry Wisnia Jeff I wonder if it would be legit to consider the distance from the axel center to the pavement becomes smaller when the tire is underinflated. The tire is free to flex and scrub as it wishes. The important parameter would be the "rolling radius", wouldnt it?? The picture gets pretty clear if you'd allow the underinflated tire to get thrown off. Then it would really have to turn alot faster to keep up with the other wheel with the good tire. Jerry |
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On Thu, 18 Aug 2005 13:09:28 -0400, Ned Simmons
wrote: In article , says... The "NPR "Car Talk" show's "Puzzler" a couple of weeks ago gave an answer stating that some car's computer "knew" a front tire was low on air because the ABS system noted that wheel was rotating "a heck of a lot faster" than the other wheels when the car was driven. I didn't buy that one. Sure, the rolling radius of a low tire is less than that of a fully inflated one, but the overall circumference, particularly on a steel belted tire, remains the same. Barring slippage, that circumference must lay its whole length on the road once per revolution, just like the circumference of a full tire does. But if the circumference remains constant as the rolling radius decreases there has to be slippage. Underinflated tires run hot, and some of that heat surely comes from excess flexing of the tire, but I imagine a large proportion is a result of the rubber scrubbing against the pavement. "a heck of a lot faster" may be exaggeration, unless the tire is seriously under inflated, but I'm sure the effect is measurable under controlled conditions even with small changes in pressure. I guess the question is how sensitive can the system really be without causing nuisance alarms? Ned Simmons Picture a spoked wheel with string instead of spokes, and the strings 1/2" too long. Just because the axle is closer to the road doesn't mean the tire is slipping, or that the tire's radius has actually changed. The heat is probably almost exclusively from the flexing, primarily in the sidewall. Pete Keillor |
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Pete Keillor wrote:
On Thu, 18 Aug 2005 13:09:28 -0400, Ned Simmons wrote: In article , says... The "NPR "Car Talk" show's "Puzzler" a couple of weeks ago gave an answer stating that some car's computer "knew" a front tire was low on air because the ABS system noted that wheel was rotating "a heck of a lot faster" than the other wheels when the car was driven. I didn't buy that one. Sure, the rolling radius of a low tire is less than that of a fully inflated one, but the overall circumference, particularly on a steel belted tire, remains the same. Barring slippage, that circumference must lay its whole length on the road once per revolution, just like the circumference of a full tire does. But if the circumference remains constant as the rolling radius decreases there has to be slippage. Underinflated tires run hot, and some of that heat surely comes from excess flexing of the tire, but I imagine a large proportion is a result of the rubber scrubbing against the pavement. "a heck of a lot faster" may be exaggeration, unless the tire is seriously under inflated, but I'm sure the effect is measurable under controlled conditions even with small changes in pressure. I guess the question is how sensitive can the system really be without causing nuisance alarms? Ned Simmons Picture a spoked wheel with string instead of spokes, and the strings 1/2" too long. Just because the axle is closer to the road doesn't mean the tire is slipping, or that the tire's radius has actually changed. The heat is probably almost exclusively from the flexing, primarily in the sidewall. Pete Keillor I like the free wheel with rubber band spokes. When you shine a strong beam of light onto the spokes on just one side of the wheel it heats them up, they shrink, the wheel goes out of balance, and it rotates, continuing to turn as long as the light is on. "They Shrink when heated?", you ask. Yep. I thought I knew about lots of things but I lived over 60 years before I learned that about rubber. It is composed of funny molecules that do the opposite of what I'd come to think of as normal, like shrinking when heated. If you've never tried this one it might suprise you. Stretch a rubber band between your hands, hold it stretched for a few seconds to let it come to near room temperature and then touch your upper lip to the center of the band and bring your hands together quickly. Feel it get colder? Jeff -- Jeffry Wisnia (W1BSV + Brass Rat '57 EE) "Truth exists; only falsehood has to be invented." |
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On Thu, 18 Aug 2005 17:37:52 -0400, Jeff Wisnia
wrote: Pete Keillor wrote: On Thu, 18 Aug 2005 13:09:28 -0400, Ned Simmons wrote: In article , says... The "NPR "Car Talk" show's "Puzzler" a couple of weeks ago gave an answer stating that some car's computer "knew" a front tire was low on air because the ABS system noted that wheel was rotating "a heck of a lot faster" than the other wheels when the car was driven. I didn't buy that one. Sure, the rolling radius of a low tire is less than that of a fully inflated one, but the overall circumference, particularly on a steel belted tire, remains the same. Barring slippage, that circumference must lay its whole length on the road once per revolution, just like the circumference of a full tire does. But if the circumference remains constant as the rolling radius decreases there has to be slippage. Underinflated tires run hot, and some of that heat surely comes from excess flexing of the tire, but I imagine a large proportion is a result of the rubber scrubbing against the pavement. "a heck of a lot faster" may be exaggeration, unless the tire is seriously under inflated, but I'm sure the effect is measurable under controlled conditions even with small changes in pressure. I guess the question is how sensitive can the system really be without causing nuisance alarms? Ned Simmons Picture a spoked wheel with string instead of spokes, and the strings 1/2" too long. Just because the axle is closer to the road doesn't mean the tire is slipping, or that the tire's radius has actually changed. The heat is probably almost exclusively from the flexing, primarily in the sidewall. Pete Keillor I like the free wheel with rubber band spokes. When you shine a strong beam of light onto the spokes on just one side of the wheel it heats them up, they shrink, the wheel goes out of balance, and it rotates, continuing to turn as long as the light is on. "They Shrink when heated?", you ask. Yep. I thought I knew about lots of things but I lived over 60 years before I learned that about rubber. It is composed of funny molecules that do the opposite of what I'd come to think of as normal, like shrinking when heated. If you've never tried this one it might suprise you. Stretch a rubber band between your hands, hold it stretched for a few seconds to let it come to near room temperature and then touch your upper lip to the center of the band and bring your hands together quickly. Feel it get colder? Jeff Yup, learned that in P-chem about 33 years ago. Weird. Pete Keillor |
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On Thu, 18 Aug 2005 11:52:04 -0400, Jeff Wisnia
wrote: The "NPR "Car Talk" show's "Puzzler" a couple of weeks ago gave an answer stating that some car's computer "knew" a front tire was low on air because the ABS system noted that wheel was rotating "a heck of a lot faster" than the other wheels when the car was driven. I didn't buy that one. Sure, the rolling radius of a low tire is less than that of a fully inflated one, but the overall circumference, particularly on a steel belted tire, remains the same. Barring slippage, that circumference must lay its whole length on the road once per revolution, just like the circumference of a full tire does. From my TSD rallying days I remember that low tire pressures made some slight differences in odometer measurements, but these were in the second decimal place, hardly "a heck of a lot". Am I missing something here? What do the great minds on rcm think about this one? Jeff First, you have to realize that you can't pressurize a tire enough to not have some deflection when weight is placed on it. Even solid rubber forklift tires compress in the contact area. Here is the data from a popular sized truck tire; overall diameter 40.84", loaded radius 19.20"; revolutions per mile 509; max inflation 110 psi. Before belted tires, not just steel belted radials, but any belted tires, the tread of bias tires created a tread wave in front of the contact patch. And sometimes continued into the contact patch, depending on speed. This was caused by the arched tread having to assume the nearly flat profile of the road surface. The wave was simply tread rubber waiting to be compressed as it went through the weight bearing area. All this flexing heated up the sidewalls of the tire and the scuffing of the tread as the compressed rubber exited the compressed area caused the bias tires to not last long. ABS brakes and indirect pressure monitoring systems have made it hard for hotrodders to put different size tires on the front and rear. There is more to come as our government tries to protect us as we get dumber, read lazier. All 2007 model year vehicles under 10,000 GVW will have direct pressure monitoring. (The public will feel they are absolved of maintaining their tire pressure) Each tire will have a pressure sensor mounted in the tire. Most are attached via the valve stem. Price? Between $175 and $300 each. Bumping a curb and breaking that non-replacable valve is going to be very expensive. Some high-end vehicles have them now. Corrosion is already a problem because of the brass, stainless, aluminum and steel components of the sensor, stem, core, nut, washer and wheel. And what do we get for this expense? The law already passed states that the monitor must alert the driver when a tire is 25% below its recommended pressure. That in itself is absurd but the monitor has 20 minutes to determine if a tire is low and alert the driver. The law is pushed by auto makers and monitor peddlers. They want to be relieved of any responsibility such as the Ford Explorer rollovers. ( As an aside, the same Firestone tires were used on F150 pickups. Recall a rash of pickup rollovers? Me neither.) Let's assume you have just checked the air in your tires at the corner self-serve-pay-for-air station. You were careful not to damage that valve, right? As you pull onto the freeway, you hit a piece of glass that cuts your tire. A cut that will deflate your tire to zero, not 75%, in 5 minutes. But it is 20 minutes before your high priced monitor comes alive. Another shredded tire, another irate motorist cursing the #&%* no-account tire. Or maybe, another casualty. The Rubber Manufacturer's Association, several tire companies and many consumer groups have sued the feds to either require tighter monitoring (less pressure loss and quicker detection) or scrap the law until such is available. They feel a false sense of protection is more dangerous than no protection. Changes you can expect. Who will check the air in your tires for free when exposed to the risk of $1200 of damage? Roadside assistance will no longer repair tires. They will mount your spare. But then who is responsible for reprogramming the computer to tell it where that spare is now. And what will it cost to repair the flat tire when simply taking the core out of the valve can cost $300? I'm guessing $30 to $50. For this kind of money, I feel a system could be developed to inflate the tires while traveling. There could still be a caution light or whatever. Maybe with suspension height sensors to detect load and air regulators, proper pressure could be maintained constantly. |
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On Thu, 18 Aug 2005 22:51:44 -0400, Ned Simmons
wrote: In article , says... On Thu, 18 Aug 2005 13:09:28 -0400, Ned Simmons wrote: In article , says... The "NPR "Car Talk" show's "Puzzler" a couple of weeks ago gave an answer stating that some car's computer "knew" a front tire was low on air because the ABS system noted that wheel was rotating "a heck of a lot faster" than the other wheels when the car was driven. I didn't buy that one. Sure, the rolling radius of a low tire is less than that of a fully inflated one, but the overall circumference, particularly on a steel belted tire, remains the same. Barring slippage, that circumference must lay its whole length on the road once per revolution, just like the circumference of a full tire does. But if the circumference remains constant as the rolling radius decreases there has to be slippage. Underinflated tires run hot, and some of that heat surely comes from excess flexing of the tire, but I imagine a large proportion is a result of the rubber scrubbing against the pavement. "a heck of a lot faster" may be exaggeration, unless the tire is seriously under inflated, but I'm sure the effect is measurable under controlled conditions even with small changes in pressure. I guess the question is how sensitive can the system really be without causing nuisance alarms? Ned Simmons Picture a spoked wheel with string instead of spokes, and the strings 1/2" too long. Just because the axle is closer to the road doesn't mean the tire is slipping, I don't think it's the fact that the axle is closer to the road that's causing the tire to slip relative to the pavement. When the tire deforms the radial distance from the axle to the ground across the length of the contact patch is not constant. So either the linear velocity or the angular velocity of the rubber on the road has to vary - in other words, something's got to give. The sidewall probably absorbs most of the difference when the tire is properly inflated, but can only do so much. Keep in mind that underinflated tires wear more rapidly, which implies at least some scrubbing. Your example of a loosely strung wheel with a rigid (I assume) rim really isn't analogous since the rim only contacts the road at a point. or that the tire's radius has actually changed. If the axle is closer to the ground, hasn't the effective radius of the wheel been reduced? The heat is probably almost exclusively from the flexing, primarily in the sidewall. I'm skeptical, especially in a seriously underinflated tire. Ned Simmons My example was extreme for the point of illustration. Sure, the flexing in a radial tire is more complex. However, I've blown enough tires due to underinflation or overloading (boat trailer, radials) to have observed fairly intact tread with totally disintegrated sidewalls each time. These were 6-ply rated, with 50 psi pressure, and seem much more sensitive to load - inflation conditions than most passenger car tires. Here's another extreme example. I've seen rubber treaded caterpillar tractors in the last few years, like the one on the right he http://www.deere.com/en_US/ProductCa...ries/9020.html Now you've got a tread with no sidewall, so it runs fine. Angular velocity vs. linear velocity loses its meaning. When a sidewall is forced to follow a tread trying to run like a track, it flexes more and more, and at least in my experience, fails. You make a good point that the tread will flex some. The two are of course one piece, at least until it all comes unstuck. I've got to get to bed now, to catch a plane in the morning, and will be out of touch for a while. I enjoyed the thought games. Pete Keillor |
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In article ,
Pete Keillor wrote: My example was extreme for the point of illustration. Sure, the flexing in a radial tire is more complex. However, I've blown enough tires due to underinflation or overloading (boat trailer, radials) to have observed fairly intact tread with totally disintegrated sidewalls each time. These were 6-ply rated, with 50 psi pressure, and seem much more sensitive to load - inflation conditions than most passenger car tires. 6-ply rating means the sidewalls have 2-ply. Is ir any wonder that the 6-ply part lasts longer than the 2-ply part? -- Free men own guns, slaves don't www.geocities.com/CapitolHill/5357/ |
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On Fri, 19 Aug 2005 10:36:12 GMT, Nick Hull
wrote: In article , Pete Keillor wrote: My example was extreme for the point of illustration. Sure, the flexing in a radial tire is more complex. However, I've blown enough tires due to underinflation or overloading (boat trailer, radials) to have observed fairly intact tread with totally disintegrated sidewalls each time. These were 6-ply rated, with 50 psi pressure, and seem much more sensitive to load - inflation conditions than most passenger car tires. 6-ply rating means the sidewalls have 2-ply. Is ir any wonder that the 6-ply part lasts longer than the 2-ply part? Nope. |
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"Pete Keillor" wrote in message
... | On Fri, 19 Aug 2005 10:36:12 GMT, Nick Hull | wrote: | | In article , | Pete Keillor wrote: | | My example was extreme for the point of illustration. Sure, the | flexing in a radial tire is more complex. However, I've blown enough | tires due to underinflation or overloading (boat trailer, radials) to | have observed fairly intact tread with totally disintegrated sidewalls | each time. These were 6-ply rated, with 50 psi pressure, and seem | much more sensitive to load - inflation conditions than most passenger | car tires. | | 6-ply rating means the sidewalls have 2-ply. Is ir any wonder that the | 6-ply part lasts longer than the 2-ply part? | | Nope. The tread bands are wrapped circumferentially and are the best wrapped part of the tire due to the least amount of flex needed and highest pressure. When a tire fails the tread is rarely part of it. When tires explode (car or aircraft tires, even) they blow out sideways, which is why you see a roll in cage at places that do semi truck tires. |
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carl mciver wrote:
"Pete Keillor" wrote in message ... | On Fri, 19 Aug 2005 10:36:12 GMT, Nick Hull | wrote: | | In article , | Pete Keillor wrote: | | My example was extreme for the point of illustration. Sure, the | flexing in a radial tire is more complex. However, I've blown enough | tires due to underinflation or overloading (boat trailer, radials) to | have observed fairly intact tread with totally disintegrated sidewalls | each time. These were 6-ply rated, with 50 psi pressure, and seem | much more sensitive to load - inflation conditions than most passenger | car tires. | | 6-ply rating means the sidewalls have 2-ply. Is ir any wonder that the | 6-ply part lasts longer than the 2-ply part? | | Nope. The tread bands are wrapped circumferentially and are the best wrapped part of the tire due to the least amount of flex needed and highest pressure. When a tire fails the tread is rarely part of it. When tires explode (car or aircraft tires, even) they blow out sideways, which is why you see a roll in cage at places that do semi truck tires. The cage is to catch the split ring, not pieces of the tire. |
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For this kind of money, I feel a system could be developed to inflate
the tires while traveling. Hummer has had a central tire inflation system for almost 10 years! All tires can be inflated or deflated at the push of a button. They can be inflated as a pair (front/rear) or all together. It also has alarms for high and low pressure. |
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I think SUV's are soon (already?) required to have a "direct" method
(after the tire fiasco of a few years ago), an actual pressure sensor. This morning on the way to work a car came up beside making all kinds of funny tire noises. One front tire was almost running on the rim and the driver was oblivous to it all. |
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"Jeff Wisnia" wrote in message ... The "NPR "Car Talk" show's "Puzzler" a couple of weeks ago gave an answer stating that some car's computer "knew" a front tire was low on air because the ABS system noted that wheel was rotating "a heck of a lot faster" than the other wheels when the car was driven. I didn't buy that one. Sure, the rolling radius of a low tire is less than that of a fully inflated one, but the overall circumference, particularly on a steel belted tire, remains the same. Barring slippage, that circumference must lay its whole length on the road once per revolution, just like the circumference of a full tire does. From my TSD rallying days I remember that low tire pressures made some slight differences in odometer measurements, but these were in the second decimal place, hardly "a heck of a lot". Am I missing something here? What do the great minds on rcm think about this one? I'm not sure if this applies but on the newer Chrysler products they have a tire pressure monitoring system that works by having a transponder located in the valve stem. There is a sensor located in the fender. If you are not familiar with transponders, they are common in several applications like injectable little pellets for dogs and cats that can be scanned to help a lost pet. Newer cars also have them molded into the head of the key. if an unauthorized duplicate is made or the lock is forced the car will not start. Perhaps the car in question had a tire pressure monitoring system. -- Roger Shoaf About the time I had mastered getting the toothpaste back in the tube, then they come up with this striped stuff. |
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carl mciver wrote:
"Jeff Wisnia" wrote in message ... | Ned Simmons wrote: | | In article , | says... | | On Thu, 18 Aug 2005 13:09:28 -0400, Ned Simmons | wrote: | | | In article , | says... | | The "NPR "Car Talk" show's "Puzzler" a couple of weeks ago gave an | answer stating that some car's computer "knew" a front tire was low on | air because the ABS system noted that wheel was rotating "a heck of a | lot faster" than the other wheels when the car was driven. | | I didn't buy that one. | | Sure, the rolling radius of a low tire is less than that of a fully | inflated one, but the overall circumference, particularly on a steel | belted tire, remains the same. Barring slippage, that circumference must | lay its whole length on the road once per revolution, just like the | circumference of a full tire does. | | But if the circumference remains constant as the rolling | radius decreases there has to be slippage. Underinflated | tires run hot, and some of that heat surely comes from | excess flexing of the tire, but I imagine a large | proportion is a result of the rubber scrubbing against the | pavement. | | "a heck of a lot faster" may be exaggeration, unless the | tire is seriously under inflated, but I'm sure the effect | is measurable under controlled conditions even with small | changes in pressure. I guess the question is how sensitive | can the system really be without causing nuisance alarms? | | Ned Simmons | | Picture a spoked wheel with string instead of spokes, and the strings | 1/2" too long. Just because the axle is closer to the road doesn't | mean the tire is slipping, | | | I don't think it's the fact that the axle is closer to the | road that's causing the tire to slip relative to the | pavement. When the tire deforms the radial distance from | the axle to the ground across the length of the contact | patch is not constant. So either the linear velocity or the | angular velocity of the rubber on the road has to vary - in | other words, something's got to give. The sidewall probably | absorbs most of the difference when the tire is properly | inflated, but can only do so much. Keep in mind that | underinflated tires wear more rapidly, which implies at | least some scrubbing. | | Your example of a loosely strung wheel with a rigid (I | assume) rim really isn't analogous since the rim only | contacts the road at a point. | | | or that the tire's radius has actually | changed. | | | If the axle is closer to the ground, hasn't the effective | radius of the wheel been reduced? | | | The heat is probably almost exclusively from the flexing, | primarily in the sidewall. | | | I'm skeptical, especially in a seriously underinflated | tire. | | Ned Simmons | | | I didn't prased my OP post clearly. I know that that part of the ABS and | couputer sytem will report a difference in the revolutions of the wheels | after integrating the revolutions over some time period long enough to | let you make a few consecutive turns in the same direction without | trigering a warning. | | What I was incredulous about was the part of the puzzle's answer saying | the tire with low air pressure would be rotating "a heck of a lot faster". | | The specific wording of the answer, by Ray, of Bob and Ray's "Car Talk" | show was: | | *************** | | RAY: But when a tire loses air pressure and its diameter gets smaller, | when the car is going down the road, in order for that tire to keep up | with all the others and not get left behind, it has to turn faster. And | your car does have something that is constantly monitoring the speed of | all the wheels and comparing them to one another. | | What most modern cars have is ABS-- antilock brakes. And there's a | sensor at every wheel that's reading how fast each of the wheels is | turning. So, if it notes that the right front wheel is going a heck of a | lot faster than the other wheels, it can either assume that you're | making a lot of left hand turns or driving around a circle...or that | your right front tire is going flat. | | ************** | | It sounded to me like Ray somehow tricked himself into thinking that the | increase in rotations per unit distance would be in direct proportion to | the decreased rolling radius, and I don't believe that could be the | case, for the reasons I already stated. | | Jeff | | -- | Jeffry Wisnia | | (W1BSV + Brass Rat '57 EE) | | "Truth exists; only falsehood has to be invented." If the tire is low, the axle is therefore lower to the ground. That means the effective radius is shorter. Since the radius is shorter, the effective circumference must be smaller. Following the progression of basic geometry, more revolutions are required to move the same distance. Now Carl, that explanation is what I had trouble with in the first place. Imagine if you would that the tire had side to side notches on the tread like an inside out timing belt and the pavement had mating pitch grooves on it. (Sort of like the ones which make a warning sound if you start to wander off the side of the road?) That would create a "rack and pinion" configuration. Would you still say that the number of revolutions per mile that tire makes would vary with the air pressure in it, or as you put it "the effective radius". That's where my skepticism to the "Car Talk" answer stemmed from. I don't doubt that second order effects come into play to make the rotations per unit distance increase somewhat with lower tire pressure, but I'm willing to bet that the effect is nowhere near as large as being fully inversely proportional to the rolling radius, at least not until the tire jumps right off the rim. Jeff snipped -- Jeffry Wisnia (W1BSV + Brass Rat '57 EE) "Truth exists; only falsehood has to be invented." |
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On Fri, 19 Aug 2005 07:45:29 -0700, Jim Stewart
wrote: carl mciver wrote: "Pete Keillor" wrote in message ... The tread bands are wrapped circumferentially and are the best wrapped part of the tire due to the least amount of flex needed and highest pressure. When a tire fails the tread is rarely part of it. When tires explode (car or aircraft tires, even) they blow out sideways, which is why you see a roll in cage at places that do semi truck tires. The cage is to catch the split ring, not pieces of the tire. Even tubeless tires (no split ring) are required to be placed in a cage. |
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Andy Asberry wrote:
On Fri, 19 Aug 2005 07:45:29 -0700, Jim Stewart wrote: carl mciver wrote: "Pete Keillor" wrote in message ... The tread bands are wrapped circumferentially and are the best wrapped part of the tire due to the least amount of flex needed and highest pressure. When a tire fails the tread is rarely part of it. When tires explode (car or aircraft tires, even) they blow out sideways, which is why you see a roll in cage at places that do semi truck tires. The cage is to catch the split ring, not pieces of the tire. Even tubeless tires (no split ring) are required to be placed in a cage. Perhaps now, but not when I was in the business. In any case, the discussion is about semi tires which have a split rim that can do grievious bodily harm if they pop out and hit you. I've known of 2 people seriously injured that way, and nobody injured by flying rubber. |
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"Jeff Wisnia" wrote in message ... | Now Carl, that explanation is what I had trouble with in the first place. | | Imagine if you would that the tire had side to side notches on the tread | like an inside out timing belt and the pavement had mating pitch grooves | on it. (Sort of like the ones which make a warning sound if you start to | wander off the side of the road?) | | That would create a "rack and pinion" configuration. | | Would you still say that the number of revolutions per mile that tire | makes would vary with the air pressure in it, or as you put it "the | effective radius". | | That's where my skepticism to the "Car Talk" answer stemmed from. I | don't doubt that second order effects come into play to make the | rotations per unit distance increase somewhat with lower tire pressure, | but I'm willing to bet that the effect is nowhere near as large as being | fully inversely proportional to the rolling radius, at least not until | the tire jumps right off the rim. | | Jeff Believe me, I have a bit of trouble getting it, even visualizing it, but there's really no other way to see it. The difference in rotational speed has to be taken up in the wrinkling in the tread and all the sidewall flexing, which is why a low tire is a very bad thing, since all that action creates a lot of heat. |
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"Jim Stewart" wrote in message ... SNIP | | Even tubeless tires (no split ring) are required to be placed in a | cage. | | Perhaps now, but not when I was in the business. | In any case, the discussion is about semi tires | which have a split rim that can do grievious | bodily harm if they pop out and hit you. I've | known of 2 people seriously injured that way, | and nobody injured by flying rubber. If it was a long time ago, perhaps there was just bias ply, and now everything is steel belted, and they do deflate rather explosively. The rubber of course carries lots of steel wire with it, and for all those who've had wire brushes lose strands into your skin you get the picture. I'll agree that the risk on passenger tires is a lot lower, but there's lawyers everywhere, spoiling all the really cool accidents we can laugh about later. |
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In article ,
says... carl mciver wrote: "Jeff Wisnia" wrote in message ... | Ned Simmons wrote: | | In article , | says... | | On Thu, 18 Aug 2005 13:09:28 -0400, Ned Simmons | wrote: | | | In article , | says... | | The "NPR "Car Talk" show's "Puzzler" a couple of weeks ago gave an | answer stating that some car's computer "knew" a front tire was low on | air because the ABS system noted that wheel was rotating "a heck of a | lot faster" than the other wheels when the car was driven. | | I didn't buy that one. | | Sure, the rolling radius of a low tire is less than that of a fully | inflated one, but the overall circumference, particularly on a steel | belted tire, remains the same. Barring slippage, that circumference must | lay its whole length on the road once per revolution, just like the | circumference of a full tire does. | | But if the circumference remains constant as the rolling | radius decreases there has to be slippage. Underinflated | tires run hot, and some of that heat surely comes from | excess flexing of the tire, but I imagine a large | proportion is a result of the rubber scrubbing against the | pavement. | | "a heck of a lot faster" may be exaggeration, unless the | tire is seriously under inflated, but I'm sure the effect | is measurable under controlled conditions even with small | changes in pressure. I guess the question is how sensitive | can the system really be without causing nuisance alarms? | | Ned Simmons | | Picture a spoked wheel with string instead of spokes, and the strings | 1/2" too long. Just because the axle is closer to the road doesn't | mean the tire is slipping, | | | I don't think it's the fact that the axle is closer to the | road that's causing the tire to slip relative to the | pavement. When the tire deforms the radial distance from | the axle to the ground across the length of the contact | patch is not constant. So either the linear velocity or the | angular velocity of the rubber on the road has to vary - in | other words, something's got to give. The sidewall probably | absorbs most of the difference when the tire is properly | inflated, but can only do so much. Keep in mind that | underinflated tires wear more rapidly, which implies at | least some scrubbing. | | Your example of a loosely strung wheel with a rigid (I | assume) rim really isn't analogous since the rim only | contacts the road at a point. | | | or that the tire's radius has actually | changed. | | | If the axle is closer to the ground, hasn't the effective | radius of the wheel been reduced? | | | The heat is probably almost exclusively from the flexing, | primarily in the sidewall. | | | I'm skeptical, especially in a seriously underinflated | tire. | | Ned Simmons | | | I didn't prased my OP post clearly. I know that that part of the ABS and | couputer sytem will report a difference in the revolutions of the wheels | after integrating the revolutions over some time period long enough to | let you make a few consecutive turns in the same direction without | trigering a warning. | | What I was incredulous about was the part of the puzzle's answer saying | the tire with low air pressure would be rotating "a heck of a lot faster". | | The specific wording of the answer, by Ray, of Bob and Ray's "Car Talk" | show was: | | *************** | | RAY: But when a tire loses air pressure and its diameter gets smaller, | when the car is going down the road, in order for that tire to keep up | with all the others and not get left behind, it has to turn faster. And | your car does have something that is constantly monitoring the speed of | all the wheels and comparing them to one another. | | What most modern cars have is ABS-- antilock brakes. And there's a | sensor at every wheel that's reading how fast each of the wheels is | turning. So, if it notes that the right front wheel is going a heck of a | lot faster than the other wheels, it can either assume that you're | making a lot of left hand turns or driving around a circle...or that | your right front tire is going flat. | | ************** | | It sounded to me like Ray somehow tricked himself into thinking that the | increase in rotations per unit distance would be in direct proportion to | the decreased rolling radius, and I don't believe that could be the | case, for the reasons I already stated. | | Jeff | | -- | Jeffry Wisnia | | (W1BSV + Brass Rat '57 EE) | | "Truth exists; only falsehood has to be invented." If the tire is low, the axle is therefore lower to the ground. That means the effective radius is shorter. Since the radius is shorter, the effective circumference must be smaller. Following the progression of basic geometry, more revolutions are required to move the same distance. Now Carl, that explanation is what I had trouble with in the first place. Imagine if you would that the tire had side to side notches on the tread like an inside out timing belt and the pavement had mating pitch grooves on it. (Sort of like the ones which make a warning sound if you start to wander off the side of the road?) That would create a "rack and pinion" configuration. Would you still say that the number of revolutions per mile that tire makes would vary with the air pressure in it, or as you put it "the effective radius". I think Carl has explained the paradox. Imagine that as your inside out timing belt engages with the rack it develops a bubble in the center of the engagement such that there are x+1 pitches of belt between x teeth on the rack. The overall length of the belt hasn't changed, but its effective length has been reduced by one tooth. That's where my skepticism to the "Car Talk" answer stemmed from. I don't doubt that second order effects come into play to make the rotations per unit distance increase somewhat with lower tire pressure, but I'm willing to bet that the effect is nowhere near as large as being fully inversely proportional to the rolling radius, at least not until the tire jumps right off the rim. It seems to me no matter what the shape the tire is forced into, the distance traveled will be 2*pi*r per rev, where r is the distance from the ground to the axle. Think about it from the standpoint of torque. If the tire is driving the auto, the torque at the axle is clearly equal to F/r, where F is the force required to move the car. If the car travelled more than 2*pi*r per rev you'd have the basis for a perpetual motion machine. Ned Simmons |
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"Ned Simmons" wrote in message ... SNIP | Would you still say that the number of revolutions per mile that tire | makes would vary with the air pressure in it, or as you put it "the | effective radius". | | I think Carl has explained the paradox. Imagine that as your inside out | timing belt engages with the rack it develops a bubble in the center of | the engagement such that there are x+1 pitches of belt between x teeth | on the rack. The overall length of the belt hasn't changed, but its | effective length has been reduced by one tooth. Thank you for making it clearer than I did. I was so wrapped up in the big picture I missed the simple explanation! |
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On Fri, 19 Aug 2005 04:54:53 GMT, "carl mciver"
wrote: *snip* Hairy story: I was in a company Astrovan on a freeway in Dallas rush hour, inches from the zipper barrier, doing ~70 when a dumb bitch in front of me blew a tire. In the minivan, bolted to the floor, was 1000+ pounds of scale test weights (500 was too little for the way I liked to calibrate scales) and lots of tools, so when she slammed on her brakes, I about **** my pants 'cause traffic was asshole to belly button and FLYING. I stopped short of her by two feet and was surprised that no one hit me from behind. As I'm chewing her out for being so phenomenally stupid, I was removing her spare from the back seat (???) and changing it as fast as I could. I could barely touch the old rim, it was so damn hot! The sidewall was all but gone, and when the wheel flipped to the ground in front of me there was a pile of steaming rubber powder on the ground some four inches high and six or so inches around. I'm sure some of this was from the sidewall that ground away when she stomped on the brakes, but I can't see the tire providing any stopping effort given its condition, so I'm confident most of it was rubber that crumbled before the tire blew and was trapped inside the tread by centrifugal force. The tread, of course, was hotter than **** and intact. About the time I got her tire back on I started to "come down" and the transportation truck showed up to (more kindly) explain to her how to brake safely after a blowout. I looked back at the million car traffic jam and there, a few cars back, were a couple folks exchanging information and sour looks. The difference between a royal ****up of massive proportions was merely milliseconds at that speed and it could have all been averted if that $%^&*! had used her brain instead of her foot. Thinking back on it, I'm sure my normal scatterbrained self would have just run over her, but for some reason, at that very moment, I had my head screwed on right. Still get that intense feeling when I think about it. Or, perhaps this was a direct object lesson from a Higher Power, telling you to get smarter about driving defensively, and, leaving a COMFORTABLE stopping distance between you and the vehicle in front of you, and, a parable for the rest of us about it. Glad you survived that one...and I hope you (and everyone that reads this) will take a second to think about it, and, leave a bit more space the next time. Drive more defensively folks...keep the insurance companies from posting record profits in the coming years! Don't use the excuse that if you leave room someone will pull in front of you. Trust me...they are not making you take ANY significant amount of time more to get to your destination. Even if you are going 2 MPH slower than surrounding traffic, you are STILL moving along at a good clip. Which gives me an opportunity to post an interesting question that came to me. If I am driving through a congested area (say, downtown Atlanta, or, a construction zone) at the posted speed limit and YOU are blasting through at 10 MPH or 20 MPH over that...which is better - For ME to speed up to an illegal (and likely more dangerous) speed, or for YOU to slow down a little to approach the LEGAL speed for the area? Regards Dave Mundt |
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Ned Simmons wrote:
In article , says... carl mciver wrote: "Jeff Wisnia" wrote in message ... | Ned Simmons wrote: | | In article , | says... | | On Thu, 18 Aug 2005 13:09:28 -0400, Ned Simmons | wrote: | | | In article , | says... | | The "NPR "Car Talk" show's "Puzzler" a couple of weeks ago gave an | answer stating that some car's computer "knew" a front tire was low on | air because the ABS system noted that wheel was rotating "a heck of a | lot faster" than the other wheels when the car was driven. | | I didn't buy that one. | | Sure, the rolling radius of a low tire is less than that of a fully | inflated one, but the overall circumference, particularly on a steel | belted tire, remains the same. Barring slippage, that circumference must | lay its whole length on the road once per revolution, just like the | circumference of a full tire does. | | But if the circumference remains constant as the rolling | radius decreases there has to be slippage. Underinflated | tires run hot, and some of that heat surely comes from | excess flexing of the tire, but I imagine a large | proportion is a result of the rubber scrubbing against the | pavement. | | "a heck of a lot faster" may be exaggeration, unless the | tire is seriously under inflated, but I'm sure the effect | is measurable under controlled conditions even with small | changes in pressure. I guess the question is how sensitive | can the system really be without causing nuisance alarms? | | Ned Simmons | | Picture a spoked wheel with string instead of spokes, and the strings | 1/2" too long. Just because the axle is closer to the road doesn't | mean the tire is slipping, | | | I don't think it's the fact that the axle is closer to the | road that's causing the tire to slip relative to the | pavement. When the tire deforms the radial distance from | the axle to the ground across the length of the contact | patch is not constant. So either the linear velocity or the | angular velocity of the rubber on the road has to vary - in | other words, something's got to give. The sidewall probably | absorbs most of the difference when the tire is properly | inflated, but can only do so much. Keep in mind that | underinflated tires wear more rapidly, which implies at | least some scrubbing. | | Your example of a loosely strung wheel with a rigid (I | assume) rim really isn't analogous since the rim only | contacts the road at a point. | | | or that the tire's radius has actually | changed. | | | If the axle is closer to the ground, hasn't the effective | radius of the wheel been reduced? | | | The heat is probably almost exclusively from the flexing, | primarily in the sidewall. | | | I'm skeptical, especially in a seriously underinflated | tire. | | Ned Simmons | | | I didn't prased my OP post clearly. I know that that part of the ABS and | couputer sytem will report a difference in the revolutions of the wheels | after integrating the revolutions over some time period long enough to | let you make a few consecutive turns in the same direction without | trigering a warning. | | What I was incredulous about was the part of the puzzle's answer saying | the tire with low air pressure would be rotating "a heck of a lot faster". | | The specific wording of the answer, by Ray, of Bob and Ray's "Car Talk" | show was: | | *************** | | RAY: But when a tire loses air pressure and its diameter gets smaller, | when the car is going down the road, in order for that tire to keep up | with all the others and not get left behind, it has to turn faster. And | your car does have something that is constantly monitoring the speed of | all the wheels and comparing them to one another. | | What most modern cars have is ABS-- antilock brakes. And there's a | sensor at every wheel that's reading how fast each of the wheels is | turning. So, if it notes that the right front wheel is going a heck of a | lot faster than the other wheels, it can either assume that you're | making a lot of left hand turns or driving around a circle...or that | your right front tire is going flat. | | ************** | | It sounded to me like Ray somehow tricked himself into thinking that the | increase in rotations per unit distance would be in direct proportion to | the decreased rolling radius, and I don't believe that could be the | case, for the reasons I already stated. | | Jeff | | -- | Jeffry Wisnia | | (W1BSV + Brass Rat '57 EE) | | "Truth exists; only falsehood has to be invented." If the tire is low, the axle is therefore lower to the ground. That means the effective radius is shorter. Since the radius is shorter, the effective circumference must be smaller. Following the progression of basic geometry, more revolutions are required to move the same distance. Now Carl, that explanation is what I had trouble with in the first place. Imagine if you would that the tire had side to side notches on the tread like an inside out timing belt and the pavement had mating pitch grooves on it. (Sort of like the ones which make a warning sound if you start to wander off the side of the road?) That would create a "rack and pinion" configuration. Would you still say that the number of revolutions per mile that tire makes would vary with the air pressure in it, or as you put it "the effective radius". I think Carl has explained the paradox. Imagine that as your inside out timing belt engages with the rack it develops a bubble in the center of the engagement such that there are x+1 pitches of belt between x teeth on the rack. The overall length of the belt hasn't changed, but its effective length has been reduced by one tooth. That's where my skepticism to the "Car Talk" answer stemmed from. I don't doubt that second order effects come into play to make the rotations per unit distance increase somewhat with lower tire pressure, but I'm willing to bet that the effect is nowhere near as large as being fully inversely proportional to the rolling radius, at least not until the tire jumps right off the rim. It seems to me no matter what the shape the tire is forced into, the distance traveled will be 2*pi*r per rev, where r is the distance from the ground to the axle. Think about it from the standpoint of torque. If the tire is driving the auto, the torque at the axle is clearly equal to F/r, where F is the force required to move the car. If the car travelled more than 2*pi*r per rev you'd have the basis for a perpetual motion machine. Ned Simmons Well, you sure changed my thinking with that one Ned. I guess the circumference of a tire which is low on air must develop a large enough moving "ripple" in it to accept the decreased radiusin order to get its entire circumfrential length moved around the axle once per revolution. Excuse me while I go off and fall on my sword. G Jeff -- Jeffry Wisnia (W1BSV + Brass Rat '57 EE) "Truth exists; only falsehood has to be invented." |
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"carl mciver" wrote:
"Jeff Wisnia" wrote in message ... | Ned Simmons wrote: | | In article , | says... | | On Thu, 18 Aug 2005 13:09:28 -0400, Ned Simmons | wrote: | | | In article , | says... | | The "NPR "Car Talk" show's "Puzzler" a couple of weeks ago gave an | answer stating that some car's computer "knew" a front tire was low on | air because the ABS system noted that wheel was rotating "a heck of a | lot faster" than the other wheels when the car was driven. | | I didn't buy that one. | | Sure, the rolling radius of a low tire is less than that of a fully | inflated one, The distance from axle to ground is less, but calling that distance a radius (rolling or otherwise) is misleading. In the first place, that distance does not equal half of the deformed diameter, so it's not a radius in any conventional sense; secondly, and more importantly, the shape of the under inflated tire is not circular, so even the semi-diameter of the deformed shape does not have a 2 pi relationship to the tire's circumference. but the overall circumference, particularly on a steel | belted tire, remains the same. Barring slippage, that circumference must | lay its whole length on the road once per revolution, just like the | circumference of a full tire does. Yes. | But if the circumference remains constant as the rolling | radius decreases there has to be slippage. No. You're assuming that the relationship between the circumference and the axle-to-ground distance (what you call the "rolling radius") remains constant; it does NOT. under inflated | tires run hot, and some of that heat surely comes from | excess flexing of the tire, but I imagine a large | proportion is a result of the rubber scrubbing against the | pavement. I doubt there is much slippage at all. The car will tend to pull in the direction of the under inflated tire because there is more friction, not less, as would be the case if the tire were slipping. (Of course, the lean of the car also contributes to the pull.) | "a heck of a lot faster" may be exaggeration, unless the | tire is seriously under inflated, but I'm sure the effect | is measurable under controlled conditions even with small | changes in pressure. I guess the question is how sensitive | can the system really be without causing nuisance alarms? | | Ned Simmons | | Picture a spoked wheel with string instead of spokes, and the strings | 1/2" too long. Just because the axle is closer to the road doesn't | mean the tire is slipping, Nor does it mean that the radius has changed. | I don't think it's the fact that the axle is closer to the | road that's causing the tire to slip relative to the | pavement. When the tire deforms the radial distance from | the axle to the ground across the length of the contact | patch is not constant. So either the linear velocity or the | angular velocity of the rubber on the road has to vary - in | other words, something's got to give. Neither the linear velocity of the rubber nor the angular velocity of the hub has to vary. What "gives" is the mathematical relationship between the two, due to the departure from a circular form. The sidewall probably | absorbs most of the difference when the tire is properly | inflated, but can only do so much. Keep in mind that | underinflated tires wear more rapidly, which implies at | least some scrubbing. That is true, but the scrubbing occurs during turns, not during straight travel. Because the under inflated tire has a longer distance between the foremost and aftmost points of contact with the road, more scrubbing will be involved when turning, compared to a properly inflated tire. Hence, more wear. | Your example of a loosely strung wheel with a rigid (I | assume) rim really isn't analogous since the rim only | contacts the road at a point. It also isn't analogous because the rigid rim retains a circular shape. | or that the tire's radius has actually | changed. | | If the axle is closer to the ground, hasn't the effective | radius of the wheel been reduced? No. The only meaningful definition of "effective radius" in this context is c/2*pi. If the circumference doesn't change, the effective radius doesn't change. The question is, how much does the circumference change as the pressure changes? Will it change enough for the sensors to classify the associated change in rotation speed as significant? Anyone have a compressor and a tape measure handy to gather a little empirical data? | The heat is probably almost exclusively from the flexing, | primarily in the sidewall. And from scrubbing during turns. | I'm skeptical, especially in a seriously underinflated | tire. | | Ned Simmons | | | I didn't prased my OP post clearly. I know that that part of the ABS and | couputer sytem will report a difference in the revolutions of the wheels | after integrating the revolutions over some time period long enough to | let you make a few consecutive turns in the same direction without | trigering a warning. | | What I was incredulous about was the part of the puzzle's answer saying | the tire with low air pressure would be rotating "a heck of a lot faster". | | The specific wording of the answer, by Ray, of Bob and Ray's "Car Talk" | show was: | | *************** | | RAY: But when a tire loses air pressure and its diameter gets smaller, | when the car is going down the road, in order for that tire to keep up | with all the others and not get left behind, it has to turn faster. And | your car does have something that is constantly monitoring the speed of | all the wheels and comparing them to one another. | | What most modern cars have is ABS-- antilock brakes. And there's a | sensor at every wheel that's reading how fast each of the wheels is | turning. So, if it notes that the right front wheel is going a heck of a | lot faster than the other wheels, it can either assume that you're | making a lot of left hand turns or driving around a circle...or that | your right front tire is going flat. | | ************** | | It sounded to me like Ray somehow tricked himself into thinking that the | increase in rotations per unit distance would be in direct proportion to | the decreased rolling radius, and I don't believe that could be the | case, for the reasons I already stated. It could be that you're not giving enough weight to Ray's penchant for hyperbole. Maybe by "a heck of a lot" he really means "a little bit"... If the tire is low, the axle is therefore lower to the ground. That means the effective radius is shorter. Since the radius is shorter, the effective circumference must be smaller. Following the progression of basic geometry, more revolutions are required to move the same distance. No. See discussion above about effective radius. As for the "effective circumference", that's just the circumference, since each point will contact the road during each revolution. The circumference will change a little due to the change in pressure, and possibly a little due to compressive effects within the contact zone, but not because the axle is closer to the ground. When a tire is low, the contact patch is not necessarily larger, once you discount the lack of equal pressure in the middle of the contact patch. I'm not sure if I buy the notion that the area of the contact patch would remain unchanged, but in any case, the *length* of the patch will be longer, resulting in a greater deviation from a circular shape, and thus a greater deviation from the relation c = 2*pi*r, especially if you're using the axle-to-ground distance as r. The circumference is still the same, it's just not round, so there's a bubble in the middle of the contact patch. Anyone who has seen a flat (and mounted) tire sitting for a long time will see it clearly when it's rolled That's a static condition. I don't know if that bubble would be maintained under rolling conditions, since that would require each point on the circumference to travel up and over the bubble (kind of a standing wave thing). My hunch is that the bubble would be diminished when rolling, but I could be wrong... over. Since the tire's still rolling, that excess slack as it passes through the patch "humps up," and you will see the sides of the tread worn more than the middle, since the pressure is so much lower in the middle. Since a tire with normal pressure has a given diameter, it follows that a tire with lower pressure will have a slightly smaller diameter, although the bulk of the movement is taken up by the sidewall's expansion (due to the way the wires route.) There's obviously a lot of flexing, and you can see the sidewall flexing and wrinkling in a very low tire being driven slowly. This kind of flexing in rubber, strung with steel belts, gets really hot and the rubber starts to break down, even pulverizing itself. At some point, the flexing becomes so much that the bead wrinkles and breaks. At that time the tire deflates rather violently and at that point how smart or stupid you are determines the rest, and who lives and who dies. The stresses on a tire when it's way low are incredible and I thank God for steel belted radials every time I have a flat! Bert |
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On Sat, 20 Aug 2005 01:47:46 -0400, Jeff Wisnia
wrote: Ned Simmons wrote: In article , says... carl mciver wrote: "Jeff Wisnia" wrote in message ... | Ned Simmons wrote: | | In article , | says... | | On Thu, 18 Aug 2005 13:09:28 -0400, Ned Simmons | wrote: | | | In article , | says... | | The "NPR "Car Talk" show's "Puzzler" a couple of weeks ago gave an | answer stating that some car's computer "knew" a front tire was low on | air because the ABS system noted that wheel was rotating "a heck of a | lot faster" than the other wheels when the car was driven. | | I didn't buy that one. | | Sure, the rolling radius of a low tire is less than that of a fully | inflated one, but the overall circumference, particularly on a steel | belted tire, remains the same. Barring slippage, that circumference must | lay its whole length on the road once per revolution, just like the | circumference of a full tire does. | | But if the circumference remains constant as the rolling | radius decreases there has to be slippage. Underinflated | tires run hot, and some of that heat surely comes from | excess flexing of the tire, but I imagine a large | proportion is a result of the rubber scrubbing against the | pavement. | | "a heck of a lot faster" may be exaggeration, unless the | tire is seriously under inflated, but I'm sure the effect | is measurable under controlled conditions even with small | changes in pressure. I guess the question is how sensitive | can the system really be without causing nuisance alarms? | | Ned Simmons | | Picture a spoked wheel with string instead of spokes, and the strings | 1/2" too long. Just because the axle is closer to the road doesn't | mean the tire is slipping, | | | I don't think it's the fact that the axle is closer to the | road that's causing the tire to slip relative to the | pavement. When the tire deforms the radial distance from | the axle to the ground across the length of the contact | patch is not constant. So either the linear velocity or the | angular velocity of the rubber on the road has to vary - in | other words, something's got to give. The sidewall probably | absorbs most of the difference when the tire is properly | inflated, but can only do so much. Keep in mind that | underinflated tires wear more rapidly, which implies at | least some scrubbing. | | Your example of a loosely strung wheel with a rigid (I | assume) rim really isn't analogous since the rim only | contacts the road at a point. | | | or that the tire's radius has actually | changed. | | | If the axle is closer to the ground, hasn't the effective | radius of the wheel been reduced? | | | The heat is probably almost exclusively from the flexing, | primarily in the sidewall. | | | I'm skeptical, especially in a seriously underinflated | tire. | | Ned Simmons | | | I didn't prased my OP post clearly. I know that that part of the ABS and | couputer sytem will report a difference in the revolutions of the wheels | after integrating the revolutions over some time period long enough to | let you make a few consecutive turns in the same direction without | trigering a warning. | | What I was incredulous about was the part of the puzzle's answer saying | the tire with low air pressure would be rotating "a heck of a lot faster". | | The specific wording of the answer, by Ray, of Bob and Ray's "Car Talk" | show was: | | *************** | | RAY: But when a tire loses air pressure and its diameter gets smaller, | when the car is going down the road, in order for that tire to keep up | with all the others and not get left behind, it has to turn faster. And | your car does have something that is constantly monitoring the speed of | all the wheels and comparing them to one another. | | What most modern cars have is ABS-- antilock brakes. And there's a | sensor at every wheel that's reading how fast each of the wheels is | turning. So, if it notes that the right front wheel is going a heck of a | lot faster than the other wheels, it can either assume that you're | making a lot of left hand turns or driving around a circle...or that | your right front tire is going flat. | | ************** | | It sounded to me like Ray somehow tricked himself into thinking that the | increase in rotations per unit distance would be in direct proportion to | the decreased rolling radius, and I don't believe that could be the | case, for the reasons I already stated. | | Jeff | | -- | Jeffry Wisnia | | (W1BSV + Brass Rat '57 EE) | | "Truth exists; only falsehood has to be invented." If the tire is low, the axle is therefore lower to the ground. That means the effective radius is shorter. Since the radius is shorter, the effective circumference must be smaller. Following the progression of basic geometry, more revolutions are required to move the same distance. Now Carl, that explanation is what I had trouble with in the first place. Imagine if you would that the tire had side to side notches on the tread like an inside out timing belt and the pavement had mating pitch grooves on it. (Sort of like the ones which make a warning sound if you start to wander off the side of the road?) That would create a "rack and pinion" configuration. Would you still say that the number of revolutions per mile that tire makes would vary with the air pressure in it, or as you put it "the effective radius". I think Carl has explained the paradox. Imagine that as your inside out timing belt engages with the rack it develops a bubble in the center of the engagement such that there are x+1 pitches of belt between x teeth on the rack. The overall length of the belt hasn't changed, but its effective length has been reduced by one tooth. That's where my skepticism to the "Car Talk" answer stemmed from. I don't doubt that second order effects come into play to make the rotations per unit distance increase somewhat with lower tire pressure, but I'm willing to bet that the effect is nowhere near as large as being fully inversely proportional to the rolling radius, at least not until the tire jumps right off the rim. It seems to me no matter what the shape the tire is forced into, the distance traveled will be 2*pi*r per rev, where r is the distance from the ground to the axle. Think about it from the standpoint of torque. If the tire is driving the auto, the torque at the axle is clearly equal to F/r, where F is the force required to move the car. If the car travelled more than 2*pi*r per rev you'd have the basis for a perpetual motion machine. Ned Simmons Well, you sure changed my thinking with that one Ned. I guess the circumference of a tire which is low on air must develop a large enough moving "ripple" in it to accept the decreased radiusin order to get its entire circumfrential length moved around the axle once per revolution. Excuse me while I go off and fall on my sword. G Hi Jeff, This has been beaten to death, but maybe a few calculations or examples would help. Say the radius from the ground to the axle is 10 inches properly inflated. The seen/working circumference would be: 2*3.1416*10 = 62.8320 inches This would work out to 5280*12/62.8320 = 1008.40 revolutions per mile. Now soften the tire so that the radius drops a mere 1/2 inch and you get this: 2*3.1416*9.5 = 59.6904 inches This would work out to 5280*12/59.6904 = 1061.48 revolutions per mile. Try to visualize the distance from the axle to the ground. This seems to be the key part. The rest of the tire seems to be just a distraction. I did this kind of quick and didn't recheck my calculations all that well, but the theory should be okay... -- Leon Fisk Grand Rapids MI/Zone 5b Remove no.spam for email |
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What I was incredulous about was the part of the puzzle's
answer saying the tire with low air pressure would be rotating "a heck of a lot faster". The guys on Car Talk have a style of exaggeration that should be regarded as for listening enjoyment, not quantitative accuracy. What quantity is "a heck of a lot faster" anyway? I'm guessing that being 10 PSI low (out of say 30 or 35 PSI) is only a few percent of difference in rotation speed. It's certainly not 33% difference in rotation speed! But it'll be visibly low, bulging out to anyone who cares to look, you don't need ABS or pressure sensors to see that much. To an ABS system with wheel revolution counters that's easily detectable (if not "a heck of a lot"!) And anyway the indirect system won't tell you if all your wheels are equally deflated... Tim. |
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In article ,
Leon Fisk wrote: : :This has been beaten to death, but maybe a few calculations  r examples would help.: :Say the radius from the ground to the axle is 10 inches  roperly inflated. The seen/working circumference would be:: :2*3.1416*10 = 62.8320 inches : :This would work out to 5280*12/62.8320 = 1008.40 revolutions er mile.: :Now soften the tire so that the radius drops a mere 1/2 inch :and you get this: : :2*3.1416*9.5 = 59.6904 inches : :This would work out to 5280*12/59.6904 = 1061.48 revolutions er mile.: :Try to visualize the distance from the axle to the ground. :This seems to be the key part. The rest of the tire seems to :be just a distraction. : :I did this kind of quick and didn't recheck my calculations :all that well, but the theory should be okay... Your calculations are for circles of different radii. In case you hadn't noticed, a flat tire isn't even close to being a circle. What you are neglecting is that the circumference of the tire is formed by a steel belt whose length does not change significantly. For each turn of the wheel, that unchanging circumference also makes one complete revolution. When the tire is flat its shape changes, but not that total length. For the car to travel anything other than that fixed distance per revolution, something other than normal contact has to happen where the rubber meets the road. It has been suggested that a ripple forms under the tire, which is the only way I can think of that would allow 6" of tire circumference to travel along less than 6" of road surface.** Absent such a ripple, the car has to travel essentially the same distance per revolution regardless of whether the tire is inflated or flat. So the big question is, "Does such a ripple actually form?" On a _really_ flat tire I strongly suspect that the answer is, "Yes," but I have my doubts that such a ripple forms under a tire that is only moderately underinflated. ** A slipping tire could also do that, but a slipping tire running faster than the road surface beneath it requires a source of energy, and that would be absent on an undriven wheel. -- Bob Nichols AT comcast.net I am "rnichols42" |
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On Fri, 19 Aug 2005 20:27:08 -0400, Ned Simmons
wrote: It seems to me no matter what the shape the tire is forced into, the distance traveled will be 2*pi*r per rev, where r is the distance from the ground to the axle. Think about it from the standpoint of torque. If the tire is driving the auto, the torque at the axle is clearly equal to F/r, where F is the force required to move the car. If the car travelled more than 2*pi*r per rev you'd have the basis for a perpetual motion machine. r is not constant because the tire has a flat patch. C = pi *2 r comes from integrating dC = r(theta) d(theta) thru 2 pi radians with r constant. If r(theta) is not constant, then the formula for circumference of a circle (C = pi *2 r or C = pi * D) is no longer valid. Even a properly inflated tire has a flat patch. An underinflated tire just has a bigger flat patch. Circumference can remain unchanged, so revs/rolled_distance also can remain unchanged. |
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Robert Nichols wrote:
For each turn of the wheel, that unchanging circumference also makes one complete revolution. When the tire is flat its shape changes, but not that total length. For the car to travel anything other than that fixed distance per revolution, something other than normal contact has to happen where the rubber meets the road. It has been suggested that a ripple forms under the tire, which is the only way I can think of that would allow 6" of tire circumference to travel along less than 6" of road surface.** Absent such a ripple, the car has to travel essentially the same distance per revolution regardless of whether the tire is inflated or flat. So the big question is, "Does such a ripple actually form?" On a _really_ flat tire I strongly suspect that the answer is, "Yes," but I have my doubts that such a ripple forms under a tire that is only moderately underinflated. I would argue that that question is irrelevant, and here's why: I believe what has been proposed is a ripple that stays more or less centered within the contact patch. If that's the case, consider the front edge of the contact patch. This edge is always in contact with the road (by definition), and each point on the circumference has to pass through that edge during each revolution of the wheel. The other possibility is one or more "permanent" indentations that remain at fixed points on the tire (i.e., that rotate as the tire rotates). Such indentations could result in less than 100 percent of the circumference contacting the road each revolution, but they would also lead to a bumpy ride, and I can't think offhand of a physical reason why such indentations would form in a normal under-inflation scenario. ** A slipping tire could also do that, but a slipping tire running faster than the road surface beneath it requires a source of energy, and that would be absent on an undriven wheel. Yep. |
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On Fri, 19 Aug 2005 16:16:38 -0700, Jim Stewart
wrote: The cage is to catch the split ring, not pieces of the tire. Even tubeless tires (no split ring) are required to be placed in a cage. Perhaps now, but not when I was in the business. In any case, the discussion is about semi tires which have a split rim that can do grievious bodily harm if they pop out and hit you. I've known of 2 people seriously injured that way, and nobody injured by flying rubber. I don't know when you were in the business. Tubeless truck tires have been around for 30 years or more. Fact: Very few new big trucks have tube type tires (split ring/lock ring). Almost everything except roadable cranes have tubeless tires. Reason for placing the tubeless tire/wheel in a cage is to restrain the big pieces if one lets go. More and more truck tires have steel body plies; not just belts. When one is run under inflated or flat, there is a tremendous amount of flexing in the sidewall. This flexing is just like bending a piece of wire until it breaks. Except in this case the broken or weakened wire is concealed by rubber. When the tire is inflated, as pressure builds the sidewall lets go in what is known as a zipper failure. A ragged rip in the sidewall that is parallel to the tread. It may be a foot long. That sudden release of air can send a tire and wheel flying. Next month will mark 40 years in the business. Twelve with Goodyear and 28 for myself. |
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On Sun, 21 Aug 2005 02:10:50 -0400, Ned Simmons
wrote: In article , says... On Fri, 19 Aug 2005 20:27:08 -0400, Ned Simmons wrote: It seems to me no matter what the shape the tire is forced into, the distance traveled will be 2*pi*r per rev, where r is the distance from the ground to the axle. Think about it from the standpoint of torque. If the tire is driving the auto, the torque at the axle is clearly equal to F/r, where F is the force required to move the car. If the car travelled ^^^^^F*r more than 2*pi*r per rev you'd have the basis for a perpetual motion machine. r is not constant because the tire has a flat patch. C = pi *2 r comes from integrating dC = r(theta) d(theta) thru 2 pi radians with r constant. If r(theta) is not constant, then the formula for circumference of a circle (C = pi *2 r or C = pi * D) is no longer valid. Even a properly inflated tire has a flat patch. An underinflated tire just has a bigger flat patch. Circumference can remain unchanged, so revs/rolled_distance also can remain unchanged. Then where is the length of tread that compensates for the length that is lost to the flat spot? It seems to me that it either must be in an inward wave in the middle of the contact patch, or causing an outward bulge just outside the contact patch, or possibly both. If the radius is less at the flat spot, then it must be greater elsewhere. No length of tread is lost, it just isn't all at the same distance from the axel. Note that we all seem to be accepting the fact that the tread length is fixed. Do we really know this to be the case? I'm sure the belts are pretty effective at limiting the length of the tread in tension, but how do they really behave in compression? If the tread can compress slightly as it rotates into contact with the road that would resolve the entire controversy. In any case, I just can't accept that the car travels anything other than 2*pi*r per rev (as before, r is the distance between the axle and the road), regardless of what the tread does. This is true if you use the r that the tire had when it was circular in shape. To carry the torque and work argument further, consider that the horizontal reaction of the driving tire on the road is equal in magnitude to the horizontal force at the axle pushing the car forward, call it F. Work is equal to F * d, d being the distance the car travels. Work is also equal to Torque * angular displacement in radians, T * theta. The torque at the axle is F * r. So... F * d = T * theta = F * r * theta But if d per rev is greater than 2*pi*r, the work moving the car forward is greater than the work input to the system by the torque turning the axle. These formulae were derived for circular geometry. The tire covers once circumference per rev regardless of its shape, so work output = work input (minus losses that go to heat the tire). Torque is exerted on all partsof the periphery, not just the part that touches the road. The parts of the tire not touching the road still have torque due to "pulling" the tread around with circumferential tension. The total torque is the sum of the various moments (at various radii) around the axel. It is true that the *average* radius is always r, which is the (constant) radius of the tire when it is circular in shape. If you use that r in your assertion, then your assertion is correct. If r varies with theta, then the average radius is (1/(2*pi)) * integral ( r(theta) d theta) integrated over 2pi radians. If r is constant, as in a circle, this comes out to r, fancy that! |
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