Barney,
Have you tried using lowering blocks to bring the rear end back down?
This would accomplish two purposes at the same time.
1. Increase the travel before the rebound straps go solid
2. get the CG closer to the roll axis, reducing body roll somewhat.
Additionaly, without actually seeing your car, it may be that bringing
the rear back down to the old range, may actually reduce the amount of
roll steer, by getting the spring mounts in a more optimal configuration.
Too, you have to remember that the new springs are "tighter" than the
old, sagged ones, and this increases the roll stiffness of the rear end.
Not unlike adding an anti-roll bar to that end.
Does the SCCA allow you to have the springs re-arched? Or could you get
this by tech if you don't tell? If it allowed (or can be sneaked in)
de-arching them may be a possible solution, although, the increase in
rear roll stiffness would still be present, no matter what you come up
with to utilize the new springs. Re-arching the old springs may be an
option, though, if you only go just enough to get the correct ride height
for the techies. Anything else would probably work against you anyway, by
raising the unsprung CG relative to the roll axis, especially the rear
roll center.
Good luck.
Rick Morrison
72 MGBGT
74 Midget
PS Another good book with a good suspension section is Paul Von
Valkenburgh's
"Race Car Engineering and Mechanics". There are two versions out; the
original and the "updated". The original has the better
suspension/handling sections.
RM
On Mon, 10 Aug 1998 01:33:30 Barney Gaylord <barneymg@ntsource.com>
writes:
>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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