> Let's kinda set the record straight. The problem is not roll stiffness in
> the rear on early Spitfires, otherwise the factory simply would have bolted
> on a hefty rear roll bar and been done with it. The problem with all swing
> axles is roll "center" or the imaginary point above the rear axle that the
> car wants to roll about. The ideal would be somewhere below the ground. On
> swing axle cars the roll center is a few inches above the top of the
> differential. The front roll center is much lower so that if you were to
> draw an imaginary line between the two centers you would have a line at an
> angle, with the front lower than the rear. Thats why the rear has a
> tendency towards pitching its self in the air (it wants to roll first) with
> the axles hanging in their arcs (being "swing" axles they have no where else
> to go) The purpose of the "swing spring" is to RELOCATE the roll center
> somwhat lower, transferring more of the cars weight back to the rear and
> preventing the rear from "jacking up". The ideal setup is to have both roll
> centers at the same point somewhere below the ground, seldom achieved. Roll
> bars don't change the roll center, just the tendency for the car to roll, or
> its roll stiffness.
Yes, lets set the record straight.
First off, a swing axle roll center is determined by the tread contact
with the pavement and the location of the pivot points on the axles,
called
u-joints in the case of a spitfire. 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. You will notice no mention was
made of the spring. That's because it doesn't have anything to do with
roll center, regardless spring mounting hardware, spring height, or
anything else to do with the springs. The springs are totally
irrelivant to the roll center.
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.
Secondly, roll stifness has a GREAT deal to do with the way a swing axle
jacks around. Those same books will also tell you about this, complete
with
pictures and diagrams. You can also learn about this by simply looking
at a race car with a swing axle, and noticing what has been done.
That's the very reason you see so many winning formula V's
running...ZERO ROLL STIFFNESS setups with their swing axles. Again,
don't take my word for it, just do a little basic research.
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,
achieving much the same affect as what a camber compensator can achieve
with
a rigidly mounted spring. Ie, the inner wheel in a hard curve isn't
being
shoved away from the car so hard, and the car can more easily switch
directions and get the now outer side of the car back down. However,
roll center has NOT changed because of this spring, or a camber
comensator.
All that has changed geometry wise is a reduction in the tendancy to
jack.
And as long as you're reading thes books, doing some basic investigation
and learning about suspensions in general, 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.
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.
Again, don't take my word for it, get some books on suspensions, and
read them.
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