> One draws an imaginary line from
> the center of the contact patch, up through the axle pivot, repeats it
> on the other side, and where the two lines intersect that is is the roll
> center of that suspension. That, and ONLY that, is what determines the
> roll center of a swing axle suspension.
*******************************
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.
********************************
> You don't have to take my word for it, I'm only an engineer. By all
> means pick up Fred Puhn's suspension book, Vizzards, Jackie Stewarts, or
> any of the hundreds of books writen on suspensions. They will all tell
> you this. It's basic engineering.
********************************
I'm also a mechanical design engineer, been doing this for over 20 years,
designed the very systems all the big auto and tire manufacturers used to
measure dynamic loading fore/aft, vertical, and horizontal acceleration as
well as pitch, roll, and yaw - there are all kinds of engineering, what has
that got to do with it? Hundreds, well I don't know about hundreds, but
there are several good books as mentioned -
********************************
> Secondly, roll stiffness has a GREAT deal to do with the way a swing axle
> jacks around.
********************************
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.
********************************
> With that in mind, the purpose of the swing spring (which simply pivots
> at its center) was to... REDUCE ROLL STIFFNESS, not increase it. Which
> it did, by about 80% as I recall. With a reduction in roll stiffness,
>the
>inner wheel isn't pushed down like it is with a non pivoting spring, < snip >
********************************
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
********************************
< snip >. However,
> roll center has NOT changed because of this spring, or a camber
> comensator.
*******************************
Yes it has, as stated before the weight transfer has changed (because of the
new action of the spring) Hence the roll center. Remember were talking
dynamic not static conditions here.
*******************************
< snip > you'll discover there's a
> hell of a lot of problems with a suspension with a roll center set below
> the surface of the pavement. Starting with a tendancy to throw
> themselves sideways when they roll. Not a good way to go around a turn,
> and why this idea of yours is not used on any race car I can think of.
*******************************
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 will also learn about the problems and dangers of having a car
> posess the flat horizontal roll axis you seem to think is ideal for some
> strange reason.
*******************************
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. . .
Barry Schwartz
Bschwartz@encad.com (work)
Bschwart@pacbell.net (home)
(San Diego)
70' Spitfire (major surgery almost done) , 72'-V6 Spitfire (daily driver)
70'GT6+
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