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Triumph TR3 FAQ page

Finding the correct tyre pressure for your (race) car

    This is a reply from Bill Babcock to a question about what tyre pressures other people are using for a certain race car tyre

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.

   Bill

 

  

  

 

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