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.