When we started this discussion, one of the things I said was I basing my
opinion on was experience with my Midget. I now see, due to Scott Fisher's
fine explanation, what some of the differences are.
Scott Fisher writes >
And, damn, I'm glad you are back!
>They are correct that increasing the front anti-roll bar will
>*reduce* understeer (and never mind what Fred Puhn says, it's
>because MGBs, as much as I love 'em, are antiques in the front
>suspension department). The Moss 7/8 front bar is in fact a
>great addition to an otherwise stock chrome-bumpered MGB.
This works to a certain extent, depending on how bad the camber is on front.
With some of the McPherson (I 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. But trust me, if you leave the rear barless
and keep putting incrementally stiffer front bars on an MGB, (or just about
anything else, for that matter), you will certainly reach a point were
steady-state understeer will increase dramatically, and terminally. The reason
is simple. You will have two tires doing the work on the rear, and only one on
the front. The lone upside is that wheelspin due to rear-wheel lift will
become impossible. Cold comfort, as you have to ease off the power to stay on
course. I was WAY past this point on the Midget while I tried to stay in stock
class. The addition of a rear bar to match, adjusted just before the point of
wheelspin, was magic.
>One problem with the B is the intense body roll.
Rear bars help with body roll, too. Since the B is fairly rigid for a
roadster, it is logical to expect a rear bar to lesson the camber change due to
body lean.
>Recall that the M. G. Car Company's motto was always "Safety Fast!"
>The B was designed in the early 1960s to be a great car for an
>inexperienced driver to pretend he was Stirling Moss. In an absolute
>extreme of bad judgment, an MGB will nearly always leave the road
>nose-first. It is possible to spin an MGB, but you either really
>have to be trying or you really have to be stupid.
>The Syd and John show (messrs. Enever and Thornley, respectively) did
>a fine job of ensuring this by two means: first, they retained the
>antediluvian front suspension geometry, which causes the outside
>front tire to tilt its upper end outboard under body roll
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.
>The second way that E and T ensured this was to build some passive
>steering into the rear suspension by manipulating the angle of the
>rear springs. The rear ends of the leaf springs are higher than
>the fronts. The result here is that when the B's chassis rolls
>in a corner, the outboard wheel points in toward the center of
>the turn. This enhances the stability, though at some cost in
>ultimate grip.
And here all these years I thought MGBs felt more sluggish than Midgets just
becouse of the extra weight and slower steering!
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. The B has a fairly flat
spring, and can believe it slopes up from the front mount to the axle.
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.
The Spridget shackle is about he same height as the front mount. The springs
have a lot of arch to them, and they slope down from the front mount to the
axle. The result here is that when the Midget's chassis rolls in a corner, the
outboard wheel (both of them actually, they are on a rigid axle, after all)
points away from the center of the turn. This lessens the stability, though at
some gain in maneuverability.
In short, there is a roll-oversteer effect on a semi-elliptical Spridget that
causes you to turn in RIGHT NOW. Be careful when you lift, as you can swap
ends pretty easily. The good news is that the car has very fast precise
steering to help catch such things. Getting on the power (such as it is)
helps hook you back up again.
>So how does this all interact with front and rear anti-roll bars?
>There are several ways. We've covered a little about how this
>works at the front end: increasing roll stiffness reduces the
>camber change, which keeps the tires planted better and gives more
>grip because the contact patch stays nice and large. An increase
>in front anti-roll bar thickness will reduce low-speed understeer
>while maintaining the high-speed tendency to leave the road nose-
>first if you completely overcook the turn. The only real solution
>is to move the suspension pickup points, a subject beyond the scope
>of this discussion.
>At the rear, the situation is comparable. Adding rear roll stiffness
>without changing the geometry means that the car has a little more
>initial oversteer, for two reasons: first, the change in weight
>transfer increases the slip angle on the outside rear tire, and
>second, the reduction in body roll at comparable speeds means there
>is less of the rear-steer effect.
>What this means is that rear anti-roll bars don't have to be built
>like Arnold Schwarzenegger on an MGB because the two effects are
>synergistic.
Right. 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.
>On a final note: my last (hmm, better make that "most recent," it
>doesn't sound so grim) autocross in the B made me decide that I
>don't want to change the front-rear balance much, I just want to
>reduce the body roll. This means I either need to add a rear bar
>while increasing the stiffness of the front bar proportionately,
That's what I'd do. Brian Erickson and others have had great success with
that. And they used rubber-mounted bars such as ADDCO, not heim-jointed and
nylon-bushed monsters like I had. Give the Ron Hopkinson set a try. Brian
made no changes to the suspenson other than sway bars when he went Street
Prepared.
>or I need to increase the spring rates at front and rear in equal amounts.
I think you are better off using the bars and leaving the springs alone. You
get more bang (camber control) for the buck (ride harshness and skipping over
bumps) with bars than with springs.
>--Scott "Safety Fast, or even a little faster!" Fisher
Phil "It's hip to be octagonal!" Ethier.
Phil Ethier, THE RIGHT LINE, 672 Orleans Street, Saint Paul, MN 55107-2676
h (612) 224-3105 w (612) 298-5324 phile@pwcs.stpaul.gov USA
"The workingman's GT-40" - Anthony Colin Bruce Chapman
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