>Not entirely true, given two vehicles with the same suspension and track, in
>this case swing axles, one a top heavy truck and the other a low slung
>go-cart, can you guess which one will roll first in a turn at limit? Yep
>it's the truck. What you fail to take into account is weight transferring
>during dynamic conditions not static. (read that book a little more) Weight
>transfer among other things determines a cars roll center at speed. I
>guarantee that in the above example the roll centers are different
>regardless where the center of your pivot points are (which by the way is a
>static roll center, not dynamic). There are a LOT of factors in determining
>roll centers, way too much to condense all of them here and I'm not about to
>get into a big debate about it.
I never failed to take weight transfer into account. I just didn't
missaply it, which is what you're doing here. Of course a truck or any
other vehicle with a higher weight transfer moment will _tip_ first.
However, this has no effect on the basic mathematical technique of
determining the position of the roll center. That position _is_
dictated by suspension geometry only, and not the weight of the
vehicle. Again, roll center for a swing axle vehicle is dictated by
tread contact and pivot point. And of course the position of the roll
center changes around as the position of the components changes around.
That's why one lowers the rear end of a swing axle car. You get a bit
of benefit from the negative camber itself with regards to tread
position on the pavement in curves, and a bit from lowering the roll
center, and some from lowering the center of gravity. But in no case
did you change the basic math of measuring where the roll center is.
Please don't go and hide behind "I refuse to debate this" statements.
You made a wrong, and hairballed claim about swing axle suspensions, and
other wrong and hairballed claims about suspensions in general. It's a
very feeble minded thing to refuse to admit a mistake or explain
oneself.
>Yes, never said it didn't, that's why they reduced it. But you will note
>that they also increased it at the front, again for WEIGHT TRANSFER. Maybe
>I should qualify that roll stiffness alone doesn't determine a cars roll
>center.
No, not for weight transfer. You don't want weight transfer all
absorbed by one set of wheels if you can avoid it. That simply causes
those wheels to exceed the traction limit faster and you start sliding
earlier. Which is not a particularly quick way through a curve.
However, with a swing axle, you are pretty much forced to increase the
front end to keep the rear from being so loaded.
What you can get by stiffening a suspension end is a flatter position in
a curve through reduced body roll. The stiffer the suspension, the less
roll can take place. You can take this to it's extreme like a go-kart
and eliminate the suspension entirely. Works well, under certain
conditions. Roads not being one of them.
You take a negative hit for increasing the weight transfer to the front.
You get a positive hit by reducing body roll, weight transfer and wheel
angle change.
If you're lucky and do it all right, your positive outweights the
negative, and you go around turns faster.
If you're going to qualify things, you should really qualify that roll
stiffness doesn't change the math of determining a cars roll center...at
all. Regardless of what position a swing axle car is it, tail up, tail
down, rolling onto its side, the roll center is determined by the
intersection of two imaginary lines extending from the tire contact
patch up through the axle pivot point.
>Whoa, hold on here, back up. The inner wheel certianly is pushed down, by
>the outside LOADED wheel "pivoting" about its new spring center effectively
>transferring some of the load (weight) to the inside wheel! Stop and think
>for a moment just exactly what's going on here. Take the spring out an
>substitute a bar. As I stated before the roll center has changed. If it
>hadn't what would be the purpose? With the spring firmly clamped (no
>pivot) the outer loaded wheel tries to push the car up (no weight transfer
>to the inner wheel) as well as the inner wheel pushing up hence the
>'jacking' problem
You may have stated the roll center has changed, but that still doesn't
make it right. Your statements have no effect on the basic engineering
principles and math of a swing axle suspension. The roll center is
determined by tire contact patch and axle pivot location. No other
factors.
When you allow a swing axle suspension to swing and move it's spring you
gain a more supple suspension, and one that reduces roll stiffness (you
even said it yourself before). Doing so reduces the tendency to jack
because the car can now more easily pivot about its roll center, among
other things. The ultimate refinement of this is the zero roll
stiffness suspension of course. Which you can do it a Spitfire fairly
easily if you are willing to give up the use of the trunk.
I do believe you misunderstand the jacking problem of a swing axle
suspension from your words. You seem to think a swing axle suspension
goes into a a turn, and suddenly jumps up on the outside tire. A very
rare occurance actually. For a swing axle suspension actually does
quite well in the first turn, what with a wheel getting canted over to
negative camber and all. The only way to get it to jack on a first
curve is to go into the curve extremely fast, possess extremely sticky
tires, and keep or reposition the center of gravity above the roll
center. Then it is possible to get the suspension to jack. An
extremely rare occurence, for obvious reasons. More likely, you'd tip
first.
The swing axle suspension tends tojack on the second curve, not the
first. And only if you switch directions so rapidly that the body rolls
over to the opposite side before the suspension moves. Ie, switching
back and forth through things like slaloms, or dodging things on the
road. That's the jacking. Now, if you reduce the body roll and or
increase suspension suppleness through reduced roll resistence, you have
a better chance of getting the car down before the body rolls over and
up.
>Funny, every race car I've ever seen loose traction in a turn does exactly
>that! I don't know about you but I prefer to loose traction and slide on my
>tires, not my roof, a little easier to recover from -
You have seen very few race cars, or any other cars, do that actually.
Lets study what you said. You said the ideal roll center would be below
the surface of the ground. I said not so, because that makes the car
throw itself sideways when it rolls. You don't seem to understand the
difference between a car sliding in a turn, and a suspension that
physically throws the car sideways.
Lets take the clasic old leaf spring solid axle. It's got a roll center
dead center. Ie, in the differential. If you go into a turn and induce
5 degrees body roll, the body pivots about the differential, having no
effect on wheel location.
Now, lets position that roll center say 10' below the ground through an
independent suspension by positioning its arms awkwardly. Inducing 5
degrees body roll in that car results in shoving it sideways 10 inches!
That is _NOT_ a good thing in anyones book.
>I'm not sure where you got that, but I'll rephrase it in different terms.
>The Ideal situation is to have as low a roll center as possible, with
>neutral steering and equal loading at all four corners. A situation seldom
>achieved, but strived for. . .
I'd like to hear how you propose to have equal loading at all four
corners while racing through a curve...
Look, I agree completely that high sitting, wallowing cars are not
exactly the neatest thing around, unless you are a little old lady, and
not from Pasadina. However, there are limits to all this, and they are
not precise. Body roll provides feedback for the driver. Same
understeer and oversteer. Some do well with one, some well with the
other. Most americans need some understeer to drive fast. A truly
neutral car is a beast because you have no idea what it's doing.
Yes, a low roll center, coupled with a low center of gravity are
basically good things. But you can't take it that lower is always
better, and that the lower you go the better is works. Engineering
wise, it will fail eventually, and driverwise it will fail as well. A
vehicle that feels horrid cannot be driven as well as a car that feels
good. If it was all precise computer calculations, the perfect car
would have long ago been built. It hasn't, because it cannot.
Compromises, conditions, drivers. All these are variables, and the car
that wins must be built to address these variables. Which is why some
of us drive Triumps, some Fords, and some even Fiats. <G>
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