"John D." wrote in message
...
On Sun, 11 Apr 2010 10:23:22 -0400, "Ed Huntress"
wrote:
"John" wrote in message
. ..
On Sat, 10 Apr 2010 12:11:41 -0400, "Ed Huntress"
wrote:
"John" wrote in message
m...
On Fri, 9 Apr 2010 08:30:43 -0400, "Ed Huntress"
wrote:
"John" wrote in message
news:k82ur5dm3ukaoqqug7smtd1l00jf6f5ue6@4ax. com...
On Thu, 08 Apr 2010 07:29:27 -0700, Larry Jaques
wrote:
On Thu, 08 Apr 2010 08:20:07 -0600, the infamous Lewis Hartswick
scrawled the following:
I didn't read the whole tirade but Ralph Nadar
is/was an IDIOT. He lost all credibility when
he wrote " Unsafe at any Speed".
What a load of BS.
Yeah, he castrated GM while allowing the VW to go unpunished,
despite
the fact that they had more problems than the Corvair, including the
extreme tendency for several to set themselves on fire almost as an
almost daily practice.
No that is not correct. Nader wrote Unsafe at any Speed, which was
pretty much a hatchet job to the extent that totally incorrect
statements were made and even a sketch of how a swing axle works was
deliberately drawn showing the outside wheel in a turn tipping
inward
at the bottom, "tuck under" as Nader preferred to it when in fact
it
does exactly the opposite, although if you do not understand how the
suspension works it might appear to be correct.
Ah, John, no. I hesitate to interfere with your trashing of Nader g,
but
he was correct. It was commonly called "jacking" among the sports car
fraternity. Anyone who drove an early VW, Corvair, Triumph Spitfire,
or
box-stock Porsche 356 (including the original Speedster) will be glad
to
relate some horror stories about it for you. d8-)
Some older chassis books can explain and illustrate the same thing. If
you
were actively driving sports cars around, say, 1965, you were very
familiar
with it.
Well, actually the "sports cars" I was tinkering with didn't have
fenders and had wishbone suspension all around. But I did do some work
on swing axle cars and unless you were able to weld the chassis to the
axles body roll, and they all had it caused the inner end of the
inside axle to move downward. Of course the inboard end of the outside
axle moved up :-)
The formula VW guys didn't have problems with "wheel tuck tripping the
car" as Nadar claimed...
They would have if they weren't strapped down with stabillizer bars or
Z-bars. The lower CG made it less likely than in a bug, but you still
had
to
strap down the rear. I never saw a FV that didn't have a stabilizer bar
in
the rear, although someone probably tried it at one time or another.
No Ed, the whole "wheel tucking" is so much hogwash. Before you reply
draw a little picture. Differential in the middle, attached to the
chassis; axle going our either side firmly attached to the wheels. Now
imagine going around a corner - the chassis/body rotate around an
imaginary line called the "roll Center" that body rolls outward at the
top and the diff goes right along rolling the top of the diff toward
the outside of the corner which moves the axle attaching point down
which in turn causes the wheel to lean inward at the top and outward
at the bottom.
John, enough. Here's a photo of an early, swing-axle Triumph Spitfire
jacking:
http://forums.pelicanparts.com/uploa...1090328641.jpg
Here's a Triumph Herald -- same suspension, higher CG. This is the extreme
case: the inside wheel actually lifts:
http://herald-tips-tricks.wdfiles.co...ilt_herald.jpg
Here's an illustration that shows it:
http://www.rqriley.com/images/fig-17.gif
Your analysis is missing the primary forces at work here, which are the
inward force applied at the bottom of the tire, and the outward force of
the
car as it goes through the turn, applied from the pivot point through the
half-axle, to the center of the wheel hub. The couple's effect is to tuck
the tire under the car.
Forget body roll for a moment and just look at how that force couple is
resolved -- by the tire tucking under, and the car "tripping" over the
outside wheel.
That's what happens. Compression of the outer spring from body roll
counteracts it. When forces are low, the body roll usually dominates. As
cornering forces increase, the outside wheel snaps from negative to
positive
camber, the pivot point reacts by moving in the only direction it's free
to
move -- upward -- and the car jacks.
You can see it clearly in the photos above.
Yes, I can clearly see it in the photos and certainly the wheels are
both positive.
I've read your description a number of times and I think something
besides cornering force is effecting the car.
Ok. If you find it, let us know. d8-)
As you describe it
cornering force alone overcomes every other force and lifts the entire
back of the vehicle enough for the camber, which would have probably
been at least a degree or so negative as a result of body roll to
suddenly go, from your pictures at least 10 degrees positive.
To go through the whole thing, analyzing all of the vectors, is fairly
complicated. My chassis books are not handy and I'm not going to do it now.
I'm sure you can do it yourself, if you've analyzed chassis before and think
through all of the loads.
But one fact here may help: When the cornering force is first applied, the
downward force measured at the differential is zero, because the car springs
are supporting the weight of the car. So when that upward vector is
applied -- the resultant of the couple I described before -- it's lifting,
initially, against zero resistance. In other words, it takes very little
force to lift the rear end of the car a little bit. And doing some numbers
in my head, I'll guess that the lifting force, at maximum cornering of
around 0.6 g or so (the most you'll get out of those tires), is well over
100 pounds. Then the jacking reinforces itself because the bottom of the
outside tire is moving in toward the centerline of the car, and the vectors
result in more upward force for a given cornering force.
Also keep in mind that the body roll, while it's acting downward on the
outer spring, is dead neutral on the car's centerline -- and thus on the
diff. It is not supplying any downward force to oppose that lifting force
vector. The car is rolling *around* the roll center: one side depresses
while the other side lifts.
It gets complicated with further considerations of body roll and its effect
on camber, and with considering the offset between the car centerline and
the actual pivot point on each half axle (this is shown in the illustration
linked to above). I'll leave those things for you to work on.
On the off chance that we discuss this again, let's clarify one more thing,
so we don't get tangled in terminology. We've been loose in the use of the
term "oversteer." Oversteer refers only to the effect of slipping, which is
the result of tire-tread distortion in cornering. The tire is pulling in a
direction offset somewhat, angularly, from the rolling direction of the
tire. It does NOT refer to the effect of a car's rear end coming around
because of *sliding*. Most of what we're discussing here is the result of
sliding, and is not oversteer. Once a car starts to slide, other dynamics
take over.
It's important to keep that clear because the transition from slipping to
sliding is where a lot of the nastiness occurs. A rear-engined car will not
necessarily oversteer (later Porsche 911s do not oversteer). But a
front-engined car *can* oversteer. An out-of-the-box 289 Cobra, with full
independent rear, oversteers. So does a Bugeye Sprite, with its solid rear
axle. But a Corvair with a stiff front stabilizer bar understeers. Put on a
rear stabilizer bar, and it oversteers.
Suspension geometry and dynamics determine whether a car oversteers or
understeers. Front-to-rear weight bias *can*, and often does, determine what
a car does in that regard once sliding begins. But with swing axles,
traction on the rear tires diminishes so rapidly as the car jacks that it
can snap you from understeer to violent tail-end sliding with a snap of your
fingers. That occurs whether the car is rear-engined, like a VW bug, or
front-engined, like the Triumph Spitfire in the photos above.
If I haven't confused you yet, you may see that we're actually dealing with
two kinds of handling transitions. One is from understeer to oversteer, or,
conceivably (I can't think of an example), vice-versa. Some advanced
rear-engined performance cars transition from understeer to oversteer.
Actually, a Bugeye Sprite does that too, but the transition occurs at fairly
low speeds. It transitions into oversteer just driving it smartly around
town. (It's the result of the rear-spring configuration; later Sprites do
not do that.)
The other is from slipping to sliding, which can cause any car --
understeering or oversteering, front-engined or rear -- to transition from
any kind of handling to a tail-end slide. Or, in some instances (Allard J2;
some early Lotus race cars; modern front-wheel drive cars) to a front-end
slide. That has nothing to do with applying power and causing a power slide,
which is another factor that Bill mentioned.
It's complicated, but what's important here is that swing-axle cars can be
deadly because they're prone to snap transitions from any one condition to a
severe rear-end slide. Thankfully, no one builds them anymore.
But the
Internet is so slow here in the Marina that any research will have to
wait until I get back home to a faster connection.
I had damned Nadar for all these years when he was right......but what
the hell, I'm not going to start lauding him with phrase, I plead the
rights of RCM to continue my own cockamamie view point, evidence to
the contrary be damned :-)
Cheers,
John D. Slocomb
(jdslocombatgmail)
You aren't alone in damning Ralph Nader. I did, too, for years, until I got
the hang of what he was doing.
As for cockamamie points of view -- isn't it fun being human? d8-)
--
Ed Huntress