The MGA With An Attitude
The Magic SHOE POLISH - SP-105
Small pressure adjustments for temperature changes and pavement conditions.

By now you've totally blackened most of your test lot, you've been running in several competitive autocross events, and you've completely fried that old set of tires you've been tinkering with. Along with the new set of tires, it's time to put the final edge on the racing advantage. This is where only experience and seat time can give you the last bit of expertise you need to win. When win or loose can be a matter of +/- a few thousandths of a second, the tiniest of changes can make the difference.

You have long since noticed that your tire pressure can increase after just one or two fast laps around the track. When the tires get hot the air inside warms up, and pressure increases. You can see up to 5 psi increase due to the temperature change, and this effect is more pronounced on hotter pavement and with longer courses. The prudent thing to do is to check the tire pressures in between runs, and let a bit of air out if necessary to maintain the optimum pressure for each successive run.

Also you may notice that when the tires are warmer they may give you a better grip on the pavement. Better grip means faster cornering, which means higher side loads on the tires, which may result in a bit more roll over of the tire, which may require a bit more pressure to compensate. On a cooler day, or with more cool down time between runs, the opposite may be true.

And, if the tires get too hot, the tread rubber may get too soft, and the grip may diminish, and the side load would be reduced. You may then find that you can reduce the tire pressure a bit to get a larger footprint and get back some of the lost grip, without the tire rolling over too much.

Then there's the problem of varying surfaces. Different racing sites will have different kinds of pavement. You may encounter seasoned concrete (very good grip), new concrete (smooth, and not so good for grip), old and spalling concrete (still a good grip but hard on the tires), seasoned asphalt (quite nice if it's smooth), freshly sealed asphalt (pretty damned slick by comparison), or deteriorating old asphalt (can be pretty bad to drive on). Like with the temperature changes, when the grip is better you may need more pressure to hold the tires to the proper shape. And when the grip is less, you may drop the pressure a bit for a larger footprint.

This bit about changing the pressure depending on the surface and temperature conditions can apply to any type of car, even those with fully adjustable suspension. Once you've been making the rounds for a while, and you've been there before, and you've seen those surface and temperature conditions before, you should be able to get the tire pressures extremely close to optimum on the first try. After that, you may find other things more important to spend your time and concentration on, at which time you may consider that you have won the war on tire pressure.

It's a little like the question "When is the restoration finished?". It's finished when you don't do it any more. But then when you get a new project car, the restoration thing starts again. And when you get a new and different set of tires, ...... Well, like the next restoration, with prior experience, the job of finding the best tire pressure should be easier and quicker the next time around.

Now you can just poke the little house below once or twice to get you back to home page.

Barney Gaylord

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