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RE: Tire Pressures

Subject: RE: Tire Pressures
From: Bill Babcock <>
Date: Tue, 31 Jan 2006 16:56:35 -0800
24 pounds, and if you use that you'll be three seconds a lap slower than you
could be.

That's not the answer you want. The long diatribe below is closer to the
right one.

It's important to develop your own tire pressure settings since the ideal
pressure depends on your car setup. Tires are springs, so your corner
weighting, brake bias, nature of the track, suspension setup and how skinny
you are all play a big part. 

Yokohama A008s are radial tires with low sidewalls and overhang, so your
camber is very important too. You'll need 2.5 to 3 degrees of camber (though
again, your mileage will vary--best way to set that up is with a skid pad).
If you start with 20 pounds cold you'll have enough pressure to keep the
tire firmly on the rim. Run a couple of laps and do tire temperatures. You
need a needle pyrometer to do this properly. Push the needle under the
surface of the tire at an angle, deep enough to just bury the tip. Measure
about 1 inch inside from each edge and the center. Have a helper do it all
very quickly after you've stopped in the hot pits. Read the tire pressure
and the rim temperature at the same time. 

The center of the tire should be about the same temperature as the inside.
Depending on how tight the track is you may find the outer edge is quite a
bit colder. This can be too much camber, but it's likely also cooling from
running down the straight (that's why a skid pad is better). If the center
of the tire is substantially colder than the inside edge, add a pound and
try again. Lather, rinse, repeat until the center is within a few degrees of
the inner edge. By this time you will have built up some rubber deposition
on the tires. If you have a smooth surface from edge to edge then your
camber is great. If there is nothing on the outside edge and lots on the
inside then consider reducing camber a little. Look also at the tread
blocks. If they show scrubbing either inwardly or outwardly then your toe
might be excessive either way. I like zero toe, but that's me.

If you can find a place to use as a skid pad (a place you can run in a
100-200 foot or so circle for ten or more laps at a time without getting
arrested) then you can set your camber and tire pressures exactly. On a skid
pad you set camber until your temperatures on the inside and outside are the
same, and tire pressure so the center is the same as both edges. 

After racing or at the end of your test and tune day put the car on jack
stands and let it cool overnight. Record the tire temperatures in the
morning (anywhere) and the pressure for each tire. That's your cold
inflation temperature as long as the tire temperature is close to the
morning temp. If it's a lot colder or hotter you'll have to guess a little
bit, but keep it in proportion. What you find generally is that the driver
side tires take a little more pressure.

Most people run too high a pressure. After some study you can look at the
cars in the pits and see it. They have a hot band in the center of their
front tires. I find it hard to go from too high a pressure to the right
pressure. It just seems to make the measurements less sensitive.  If I wind
up with a center temperature that's even slightly hotter than the edges I
find it necessary to reduce the pressure by a two pounds and start over. One
pound seems to have no effect going downwards, where a half pound has
substantial effect going up.   

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