First, I must point out that I drove the Spitfire to work today top-
down. 'Twas a cold ride home but oh so satisfying! I'd forgotten
just how rev'y the engine was, and how much farther apart 2nd and 3rd
gears are compared to the GT6. I got sorta' used to the extra
torque. And you know, when the top is down I don't hear any of those
noises that worry me so much!
On 1 Nov 2006 at 15:57, Barry Schwartz wrote:
> The Swing Spring FIXES the problem that is inherent
> with Swing axles... period, and does so quite effectively.
Now don't you go a'bad-mouthing Joe ("fluif") Curry. He knows what
he is about!
Barry, you are absolutely right that the swing-spring fixes the
problem. However there is a cost. It increases the vertical loading
during a turn of the outside front tire and the inside rear tire, and
decreases the loading of the tires on the other diagonal. The
outside front tire is already the most loaded, so you'd really like
to transfer weight from the outside front to the inside front, not
the other way around. Because the effect of friction is not quite
linear with downward force, the more uniform you can distribute the
weight the higher total lateral force you can generate. So this
further shifting of weight to the outside front causes a net loss of
traction at the front, and the car understeers more. You can tweak
the dynamic balance by playing with tire pressures but the net total
lateral g's is still less.
This doesn't matter to me because even though I like driving
spiritedly I really don't care whether I've given up a few tenths of
a lateral g. Absolute performance specs, either laterally or in a
straight line, have never mattered much to me, which is how I can be
delighted at driving a Spitfire instead of a Corvette. But it
matters to anyone who competes.
Now with a GT6 the situation is not so clear. A Spitfire is
obviously sub-optimal with engine mass, a high c.o.m., and low roll
axis at the front, and at the back very little mass, a high roll
axis, and c.o.m. so low that it has almost no roll couple. So it
benefits from more roll stiffness at the rear to help out the front
by unloading that outside tire. On a GT6 the rear has more weight
and a higher c.o.m., and different geometry which lowers the roll
axis. The engine is bigger too, but I don't know how that extra
weight is distributed front to rear. So perhaps losing roll
stiffness at the rear doesn't hurt much, maybe even helps if it
actually has too much.
Back around mid-September 2003 I posted a detailed description (to
either spitfires or triumphs@autox.team.net) of a swing axle's
behavior and how the swing-spring works. (I made one error near the
end, choosing the wrong word for something, duhhh.) If you have
access to archives, see if you can find it. If not, drop me a note
and I'll send you a corrected copy.
--
Jim Muller
jimmuller@rcn.com
'80 Spitfire, '70 GT6+
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