Thanks for bringing in the whole scoop, TeriAnn. As I suspected,
your own memory was never in question, but the Moss description
of handling modifications is pure Usenet: so near and yet so wrong...
They are correct that increasing the front anti-roll bar will
*reduce* understeer (and nevermind 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.
As for the rear bar, where they're near is in the two observations
that a rear anti-roll bar will increase oversteer (the car's
desire to put the red plastic things in front of the clear glass
things as you exit a corner). The stock MGB chassis is very
stable, yet with just enough responsiveness built into the rear
suspension's geometry that it's a lot of fun to drive, even with
skinny tires (maybe especially with skinny tires).
One problem with the B is the intense body roll. Recall that they
softened the B's spring rates compared to the A's, down about 25%
at both ends. They could do this because the B's chassis was so
dashed rigid, and it allowed them to make the car more compliant in
ride while maintaining crisp handling.
The truth of the matter is that while the basic suspension design
is antiquated, simplistic and laughable in these days of fully-
independent dual-A-arms on econoboxes, the attention to detail and
the attention to balance on the MGB was, given the decomposing sow's
ear that Enever and Thornley had to work with, second to none.
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. (The
technical term for this is that that camber curve increases in a
positive axis.) WHat this means is that the faster you go through
a turn in a stock MGB, the more the outside front tire rolls onto its
sidewall. This reduces grip, but it reduces it where you can feel
it through the steering wheel. It also causes the car to slow down,
and it helps ensure that if you lift off the gas sharply the car
just tracks where you wanted it to go. There are few safer cars in
which to make a sudden emergency change in direction at high speed
than in an MGB.
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.
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. However, there are other ways you can tune the rear
end to take out some of the roll understeer built into it. Those
will have to wait till our next session.
And once again, the bit about using more unsprung weight to make
up for the added oversteer from using too stiff a rear bar is just
way out in left field. Larger tires and wheels *would* allow you
to use a larger bar because you'd have more grip, and therefore
the increase in slip angle would happen more slowly; the bit about
unsprung weight appears to be someone at Moss trying to impress the
readers by using words he doesn't quite understand (see comment
about Usenet, above).
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,
or I need to increase the spring rates at front and rear in
equal amounts. The simplest way to do this appears to be to
replace the tourer springs with GT springs, which are something
like 22% stiffer in front and 27% stiffer in the rear; the balance
isn't quite identical, but I think I can fix that either with tire
pressures or with modifications to the anti-roll bar. And I haven't
even begun to discuss the effect of damping rates on the different
stages of a corner...
--Scott "Safety Fast, or even a little faster!" Fisher
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