Lots of us like to special tune our engines for a bit of extra zip. And
tires and springs and sway bars are frequently kicked around this list in
the interest of better handling. So let's see if suspension tuning can
tickle a serious note here.
This recent discussion of lever shocks and suspension travel reminded me of
a problem I have encountered with my MGA new leaf springs. Bear in mind
that the front rebound buffers and the rear rebound straps are necessary to
prevent damage to the shocks from over extension on airborn bounce.
I've been autocrossing my MGA rather seriously for several years now.
Stock class in SCCA allows only a select few changes to original equipment,
among these are:
(1) any DOT approved tires (has a tread patteren - not slicks)
(2) any front anti-sway bar
My current setup uses road racing tires with soft rubber compound on the
track, and a 3/4" front sway bar packed tightly with polyurethane bushings.
A couple years ago I had this package doing marvelous things, was winning
regularly with SCCA, sometimes taking the event index trophy (ask later).
The car had neutral handling on the road with standard radial tires (ala VW
Beetle), and a little oversteer on the track with the race tires, very nice
for autocrossing.
Early spring last year I was preparing the car for Brit Run to the Sun -
Alaska '97, and I thought it was a good idea to install new leaf springs to
regain some ground clearance lost to nearly four decades of gradual sagging
of the rear springs. So I did. And the trip was fantastic. But when the
car returned to the track it had a rather dramatic change in personality.
So here's the problem.
The new springs make the body sit higher at the rear, as was intended. So
now the rear axel has more upward travel available to the rubber frame
bumpers, but less downward travel available before it hits the maximum
length of the rebound straps. Sticky race tires on the car will induce a
certain amount of body roll in hard cornering, in spite of the heavy front
sway bar. As the car is approaching the limit of adhesion in hard
cornering, the body roll gets to the point where the inside rear fender
lifts until the rebound strap is stretched completely tight. At that point
the rear roll stifness suddenly transitions to infinity. Then with just a
little more body roll the inside rear wheel gets lifted right off the
pavement.
Now the car is running around on three wheels, and no matter what the
spring rates, stiffness, shocks or sway bars are doing, the one outside
rear tire is carrying about half the weight of the vehicle. That relative
extra loading of the tire reduces its grip efficiency a bit, and the rear
end lets loose and swings wide putting the car into a sever case of
oversteer. Now I can generally get a handle on it (hang it sideways) and
keep it from looping, but, it cannot corner any faster because of the loss
of latteral traction, and it cannot accelerate because one drive wheel is
in the air (open differential). Grrrrrrrr.
This is precisely why most of our beloved LBCs will benefit substantially
from a large front sway bar, the rear suspension is WAY too stiff. I was
thinking that a LARGER front sway bar might help, but some friends tell me
that the inside rear wheel is sometimes four inches off the pavement!
Yikes! Stiffer front springs and softer rear springs may be in order, but
in Stock class one is not allowed to change the springs from the stock
part. Right about now I am seriously considering reinstalling the old
sagging rear springs. Maybe nobody will notice and file a protest about
changing the ride height. If they do, my car could get kicked into Street
Prepared class, and I'm definitely not "prepared" for that.
So waddaya think folks? Is this stuff worth a few cells af gray matter?
Barney Gaylord
1958 MGA with an attitude
PS
If you want to get a good handle on handling, read How To Make Your Car
Handle, by Fred Puhn.
BG
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