At 04:00 PM 6/18/98 -0500, Bill Eastman wrote:
>.... When cornering the car rolls to the outside.
True.
>Almost all cars are designed such that a tire gains negative camber under
bounce.
Nada. Definitely not all. Unequal length a-arm front suspension may be,
but not always. Swing arm rear suspension yes to the extreeeem. Any solid
axel, no. Most MacPherson struts, no. Front suspension of any MG, not
nearly enough to compensate for body roll.
>.... By adjusting the pressure, you are matching how much the tire bends
with the amount of roll induced negative camber designed into your
suspension and available grip.
Well, no. In essence, all that you do with the tire pressure is to adjust
the size of the footprint. Too much pressure, too small a footprint, not
enough grip. Too little pressure, to much tire squirm, inside part of the
tread lifts off the pavement, outside can roll over on to the sidewall,
gets real mushy feeling, and not enough grip. Correct pressure is a real
balancing act, but has very little to do with camber.
>On the rear end of solid axle cars you don't have any camber gain
True, until the inside tire lifts off the pavement because the rear
suspension is too stiff.
>so you have to run higher pressures (relative to supported weight) to keep
the tires from rolling over.
Mostly wrong. Sorry.
>I run autocross on race tires with 33psi front and 27 psi rear (average,
depending on many conditions). With less pressure the tires would scrub
the sidewalls. With more pressure there is significantly less grip. And
my MGA is about 50/50 weight distribution. Running autocross on street
tires I use a few psi higher pressure, but about the same difference front
to rear.
Stock MGs have a serious problem with body roll in hard cornering, to the
point that the outside front wheel has a nasty negative camber problem.
Anti-sway bars to reduce body roll do wonders to improve that situation.
SCCA allows any front stabilizer bar in Stock class, but no change at the
rear. Common knowledge says an increase in front stabilizer will induce
understeer by overloading the outside front tire. But with an MG just the
opposite happens, and an increase in front stabilizer induces oversteer
(reduces pushing). Reason in the real world is that the reduced body roll
eliminates a lot of the induced camber on the outside front wheel and keeps
the tire tread flatter on the ground. And as a bonus, reduced body roll
will give a nice improvement in steering response.
OBTW, for street use I like 25 psi front and rear, with an extra 2 psi in
the rear for trailer towing to prevent tail wagging.
>On swing axle cars like the Spit**re prayer works better than tire
pressure to keep the rear end in line.
Early Spits and early Corvairs _require_ rear camber compensators to enter
an SCCA autocross event. One of the few exceptions to the general rules
for stock class, due to safety considerations, to prevent the rear wheels
from tucking under.
Barney Gaylord
1958 MGA with an attitude
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