Phil Ethier welcomes me with a hearty:
>And, damn, I'm glad you are back!
Good to be here, too. Maybe it'll keep me motivated to fix the
green car...
Phil goes on, driving me to clarify my comments:
>This works to a certain extent, depending on how bad the camber is on front.
In a word, bad. :-)
>With some of the McPherson (hope McPherson is rotting in Engineer's Hell next
>to Phillips) designs, even on front-heavy FWD cars, a SOMEWHAT stiffer front
>bar will help. The (backwards in my opinion) double whishbone setup on the MGB
>would react in a similar fashion.
Exactly so. You want to add just enough front bar so that the car rolls
less, without adding so much that it lifts the inside wheel. The GTI is
exactly the same way, and is one of those rare instances where a stock-
class autocrosser will understeer *less* with a somewhat larger bar in
front than the factory included. (For the record, the Miata is the other
way round -- adding a stiffer front bar does just make it plow.)
>I expect Donald Healey et al were responsible for the Spridget front geometry.
>They didn't do the above, but the overall compensation of the unequal-length
>control arms is not enough to prevent too much positive camber. Putting in
>more static negative camber via offset bushings helps. Where legal, a shorter
>upper arm helps, too.
Okay: the "right" way to fix an MGB's suspension is to move the inboard
ends of the lower A-arms, something that is legal only in Prepared and
(of course) in Production category road racing. John Ross' car (my old
car) has a fine example of how to do this. You relocate the spindle to
which the lower A-arms are connected, moving it outboard and up about an
inch and a half in each direction. This causes the lower control arms
to be lower at the outboard end than at the inboard end (unlike the stock
MGB setup, which is just about flat if memory serves).
One reason you don't want to lower an MGB at the front, BTW, is that you
will end up with the lower control arms tilted UP at the outboard end.
This gives you static positive camber and puts you right into the fat part
of the camber gain curve with the first little upset of the chassis. Add
a locked rear diff to this and you will plow right smack off course into
an earthen bank at the outside of turn 7, requiring several hundred
dollars worth of bodywork to your friend's MGB-GT (hi Sam! You still
on this list? :-)
>And here all these years I thought MGBs felt more sluggish than Midgets just
>becouse of the extra weight and slower steering!
Heh. No, it's because the B was a faster car than the 948/1098 Spridgets,
so they built in more "confidence-inspiring understeer." Fix the suspension
and they're every bit as nimble as a Spridget -- famous H Production Bugeye
driver Jeffrey B. Zurschmeide pronounced the black car as having a little
*more* tendency toward oversteer than Blaze.
>It is not really the rear shackle height that counts, it is the height at the
>spring and the height at the front mount that counts.
Well... yeah but it's easier to fabricate a new shackle and some lowering
blocks than to remount the front end of the spring. You can control the
overall spring angle that way.
>The B has a fairly flat
>spring, and can believe it slopes up from the front mount to the axle.
The rear end is slightly higher than the front, yes. The easy way to fix
this is to add a 1" lowering block between the axle and the spring, then
extend the rear shackles to get the ride height and more importantly the
spring angles right again.
What Phil and I are talking around, I just realized, is this, and
not everyone may know it:
On a leaf spring, the axle moves forward and aft as it moves up and
down, due to the curvature of the spring. Think of a bow and arrow:
when you pull back on the arrow, the ends of the bow get closer
together. A half-elliptic leaf spring works the same way, except the
front end is fixed to the chassis, so the axle (your fist) will of
necessity move with respect to the car's frame. In essence, the
wheelbase changes as the axle moves up and down.
When the car goes over a bump, nothing really changes much as both
ends of the axle move the same amount. But when only one side of
the car compresses -- or, as happens in body roll, when one side
compresses and one side extends -- you can run into the situation
where the wheelbase on the right side of the car and the wheelbase
on the left side of the car aren't equal. The two factors that
affect this are the arch of the spring and the angle of the spring.
A greater curvature to the spring will mean that the axle will move
forward as the spring arches further and backward as it goes flatter.
As an experiment, hold your hands in a T with the right on top. Now
bend your left fingers. Notice that they no longer touch your right
hand. Now move your right hand down to touch your fingers again, then
straighten your left hand. It pushes your right farther away. The
rear axle of your car does this if you've got leaf springs.
So the factory built into stock MGBs the tendency for the rear axle
to point *into* a turn when the body rolled. They did this by making
sure that the angle and arch of the spring move the axle forward under
bump and aft under rebound. To visualize this, the line that describes
the axle's range of motion is tilted slightly toward the front of the
car.
You can modify this slightly by the simple expedient of dropping the
rear end of the spring; this keeps the arch the same but changes the
angle, so that the path the axle takes under compression is more vertical
and less inclined toward the front of the car. The catch is that when
you do this, ride height goes up, about half as much as you lift the
rear end of the spring because the axle is about halfway down the leaf
spring. So if you raise the car 1-1/2" at the rear, you need to put
a 3/4" lowering block between the axle and the spring (which passes
under the axle on the B).
I'm still working out the problems of varying spring length as the
car rolls, but it seems that for real neutral steer you want a flat
spring with very little arch. I welcome discussion on the matter.
>I don't know who designed the semi-elliptical geometry in the rear of my
>Spridget. The quarter-elliptical that preceded it was surely Donald Healey et
>al. The opposite effect from the B as described by Scott exists.
Exactly so. On the 1/4-elliptic Spridgets, the axle actually moves in
such a manner that the rear end of the car steers to the left when the
front end steers to the right. This is why it's so snappy to drive a
Spridget, even with very little power.
>If you find it is too strong, you can soften a bar on the rear of a B
>by moving the mounts on the trunk panel closer together. Put in enough holes,
>and you can adjust to your heart's content. Brian Erickson had several sets
>of holes on his last (hmm, better make that "previous," it doesn't sound so
>final) B. He found the best distance, so his present chassis only has one set
>of holes.
The other way to make a bar adjustable, of course, is to slide the
end mounts back and forth along the longitudinal links. I may actually
give some thought to doing that if I don't just give up and buy a Miata. :-)
--Scott "Well, at least it's not a TRIUMPH" Fisher
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