One other thought about tire pressure. If you have ever been to an
autocross, you have doubtless seen the hot shoes chalking their tires to
see how much is contacting the ground during cornering. Too much pressure
and the entire tread doesn't touch. Too little and you start wearing
sidewall. AHA- you say- here is proof that pressure effects tread shape
and whether the entire tread touches the ground. If you said that you
would be wrong. What is at work here is the interaction between force,
tire pressure, and suspension design. When cornering the car rolls to the
outside. Almost all cars are designed such that a tire gains negative
camber under bounce. This keeps the tread in contact with the road while
the sideways force bends it over. 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. On the rear end
of solid axle cars you don't have any camber gain so you have to run higher
pressures (relative to supported weight) to keep the tires from rolling
over. On swing axle cars like the Spit**re prayer works better than tire
pressure to keep the rear end in line.
Regards,
Bill Eastman
61 MGA whose next date with an autocross track is 19 July!
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