> A tire's pressure works along the same
> lines as a shock absorber. A firmer
> shock at the front would allow
> less weight transfer.
Actually, a tire works like a spring. The air-pressure in the tire is a somewhat adjustable spring rate, but the rate is so high in comparision to a regular suspension spring that its basically irrelevant for our discussion (> 1,500 lb inch).
Shocks do not affect the total weight transfer, they only affect the rate that the weight is transferred. Let's say you have a gas pressure shock that requires 50 lbs to get it to compress, once that 50 lbs is reached, shock will compress all the way. Addiing and additional 10 lbs will only increase the rate of compression.
> When the Contour is pushing it's
> because the front tires are being
> asked to re-vector too much inertia
> (ie. too much weight is being
> transfered to the front).
In essence the tire is overloaded. More weight on a tire gives it more traction, up to the point that it can't take any more. Air pressures affect a tires traction capacity to the extent that the tire's contact patch is optimized. It is better to error on the side of too much pressure because the capacity fall-off is shallower on the high side.
> But when the problem is too much
> weight tranfer (braking and turning
> on the autocross course) more
> pressure (to a point) will help
> the car turn.
High tire pressures give a tire a better turn in feel (firmer shocks have this affect also.) It can reduce corner entry understeer but mid-corner understeer is most affected by chassis balance and corner exit understeer is most affected by a driver's right foot.
All the above nit-picky stuff aside, Chris is probably going in the right direction.
In general I would mark the side of the front tires with shoe polish/chalk to check for rollover. Make sure that you are using all of the tread and not rolling the tire onto the sidewall. Look for scuffing to the tops of the treadwear indicator marks.
Pressures greater than what's needed to maximize the contact patch are used to reduce traction to some degree and balance the vehicle. Once I had the front tire pressures figured out, I would use rear tire air pressures to balance the car.
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John Coffey
johncof@veriomail.com
