The MGA With An Attitude
How to read scuffed shoe polish marks and adjust tire pressure for maximum grip.

So you think the fun is over. No way. You get to do the testing again later. Maybe several more times. But right now you need to find out how the tires did.

Get out and take a look at those white marks you put on the tires. What's that? Of course they're not all there any more. They're not supposed to be. Just part of each one. What you need to be looking at here is the shoulder of the tire, that transition point between the tread and the sidewall. You should notice by now that the corner is getting worn away and is rounding off some. That's actually good. When you get rid of that sharp shoulder you'll get a little better grip.

Now you want to check on just how much of that white mark is still there. It's likely that the mark will be scuffed off about halfway down that last tread block. And, it's OK if the white mark is nearly all gone. But, under no circumstances should there be any traces of scuffing on the tire sidewall past that raised ridge. And, ideally, the raised ridge should still have some trace of white still left on it. If you find any scuffing on the sidewall beyond the raised ridge, you need to add more air to the tire@ (Another big period).

Now suppose that some of the white mark is still left on the last tread block. In this case you could lower the tire pressure a little. What you want here is a large footprint for the tire on the ground. Generally speaking, the larger the footprint, the better the grip. Lower pressure lets the tire squat more, resulting in a larger footprint. You do not however want to be rolling the tire over onto the sidewall, so there's a practical limit to how low you can run the air pressure.

About the time the tire starts scuffing the sidewall beyond that last tread block, it will also start to lift the opposite side of the tread off the ground. Tires work best with all of the width of the tread on the ground, even if part of it is only pressing down with a little pressure. Tires have a lousy grip on open air. This is one of the reasons that anti-sway bars can have such a good effect on handling. If you can reduce body roll, the tires can stand up straighter, and the tread can lay flatter on the ground.

As another practical matter, until you have a lot of experience with this, you should not go below 25 psi pressure in any tire, no matter how tempting it looks. Tires with low pressure can squirm a lot, and that squirming at the contact surface diminishes the grip.

OK. So if it scrubs on the sidewall you add air. And if you have a lot of the white mark left on the last tread block you let some air out. Make small changes, not more than 2 or 3 psi difference at a time. When you get close to the optimum pressures, small changes can make a big difference in handling. And, if you're adding air you need to re-do the white marks on the tires. If you're letting air out, you can leave the marks alone -- they'll get smaller next time around.

Having made your pressure adjustments, you can now go back to Shoe Polish 102 and take another test run. When you're finished, come back to 103 and check out the white marks again. This process may take several iterations. When you get the pressures right, the remaining white marks should be very small or non-existent, but there should still be no scuffing on the sidewalls. Then you can go on to, you guessed it .....

Shoe Polish 104 tells you how to CREATE oversteer, understeer, or neutral handling, whatever suits your taste for the occasion.

Barney Gaylord

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