At 07:04 PM 8/22/2000 -0400, Jim Stuart wrote:
>....
>.... It all seems to be a "black art" to me.
>
>Perhaps Barney & others can give a better explanation
Well, it's not exactly black art, but could at least be dark gray. Some of
the input parameters are quite subjective, such as:
You like soft ride or firm?
You want maximum grip or a nice ride?
For road use of tight track?
Road racing or autocross? (Different approach.)
Street tires or race tires?
Stock suspension or tinkered with?
And how stiff are the anit-sway bars?
In the end a lot of it is a matter of personal choice, once you figure out
how to "dial it in". Each to his own, so to speak. Even among
professional race drivers looking for the ultimate speed there is a
difference in preferences for setup depending on the drivers style.
Well, I'll try for an explanation in a nutshell, but this nutshell is
pretty big. In general, for soft ride use less pressure, and for crisper
feel use more pressure. for a good grip (on dry pavement) you want the
largest footprint possible while still keeping the tread on the pavement
and the sidewall off the pavement. Less pressure gives a larger footprint,
but too little pressure and it starts to rub the sidewall on the ground and
lifts part of the tread. There will be an optimum pressure where you get
the best grip, and any change from there, up or down, will spoil the grip
some.
Once you get close to the optimum pressure, minor changes in pressure will
change the force distribution across the footprint, which affects wear and
heat buildup either in the center ar at the edges, and camber changes
affect wear and heat from side to side of the footprint. Ideally you would
like even temperature across the width of the tread as an indicator of even
tread loading, but unless you have available suspension adjustments in
several directions that isn't going to happen, so forget it.
Now if you want the fastest cornering you start by obtaining the best
possible grip at all four corners. Then you evaluate the
oversteer-vs-understeer characteristics. With more grip in front and less
in back the rear drifts out and you get oversteer (wants to spin out).
With more grip in the rear the front drifts out and you have understeer
(wants to runn off a curve). To correct this and balance it out you have
to spoil the grip some on the end that has too much. Less pressure spoils
the grip and gives a soft mushy feel because the tread squirms. More
pressure also spoils the grip because it results in a smaller footprint,
but this gives a stiffer and more responsive feel.
That's the entire theory (according to barneymg) of how it works. Now for
those who have more time and interrest to see how you can actually go about
achieving your personal desired handling characteristics, check here:
http://www.ntsource.com/~barneymg/mgtech/tires/tire1.htm
The methods laid out there are quite generic and should work with any car
and any tires or suspension setup. The beginning assumption for all this
is that you have a fixed mechanical setup and can only use tire pressure
adjustments to obtain the desired handling traits.
Barney Gaylord
1958 MGA with an attitude
http://www.ntsource.com/~barneymg
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