|The MGA With An Attitude
The MAGIC SHOE POLISH - SP-104
How to CREATE oversteer, understeer, or neutral handling.
So you've gone through all the formalities, you've got the biggest, stickiest footprint you can get at all four corners, and it goes around corners pretty fast. But, you say, it still gets a little squirrely when it gets right up to the hairy edge. One end of the car just doesn't want to hang on as well as the other.
Well, for safety in casual street driving, you would like the front end to lose the grip first (but only by a small margin). For all out road racing, you would like the handling to be near neutral to get the fastest speed in the turns. And for autocrossing, with a lot of tighter turns, you may want the front tires to grip better than the rear tires, so you can get around the tighter corners quicker.
At this point, the best way to modify the over steer/under steer characteristics would be to leave the tire pressures at this optimum setting and tinker with adjustments on the front and rear anti-sway bars. OH? You don't have any? Or you only have one? Or the one(s) you have are not adjustable?
Well, now it is possible to fine tune the handling with some more changes in tire pressures. Bear in mind though that the tires are already at their optimum pressures for large footprint and best grip, so anything you do to change the pressure, up or down, is going to diminish the grip somewhat. First you figure out which end of the car is too loose (loses the grip too soon), and since you can't improve on it any more, you then go to the other end of the car and intentionally screw it up a bit so it loses some grip. This is very easy. Just make any change you want from the optimum tire pressure, and it will lose grip.
If you drop the pressure a bit, the tire will get softer and squirmy, might rollover on the sidewall too much, and might lift part of the tread off the ground. Any of these things will reduce the grip. The softer tires may also impart a somewhat mushy feeling to the handling.
OTOH, if you add pressure, you get a smaller footprint. This will also reduce the grip. At the same time, the higher pressure may give you more stiffness in the sidewalls, which can improve the steering response and make the car feel a little more nimble. I personally prefer the more nimble feeling. Others may prefer the slightly mushy feeling because it's easier to drive that way if you over do it in a turn, having a more gradual transition, less likely to break away suddenly.
If the tire gets too stiff, it can overcome the damping action of the shock absorbers on a rough surface. And if the tire then bounces off the ground, your grip will disappear completely for the duration. This whole problem turns into a study of something called "tire compliance", or how to keep the tire tread in contact with an irregular surface. Softer tires will flex more and stay in contact longer with a rough surface.
When you get into this stage of tinkering with the tire pressure, go at it real easy, like only 1 or 2 psi difference at a time, and only at one end of the car. And any change of pressure at this stage merits a fresh application of white stuff to the shoulders. If you're dropping the pressure, you want to be sure you don't end up scrubbing on the sidewalls. If you're increasing the pressure, the size of the white mark remaining on the shoulder is a good reference for being sure the pressure stays right, and for getting the pressure right the next time.
OK. You've done some more tire pressure tinkering, and you've re-marked the tires. So now it's time to get back to 102 and give it another test run. After which, of course, you come back to 104 and maybe fine tune it some more. When your continuing changes in pressure no longer make any noticeable improvement, or worse, the handling starts to deteriorate, it's time to quit tinkering.
The last step in this process is to wash the white stuff off of the tires before you head back out onto the streets. It's just as well if you never have to explain to a cop what those funny white marks on the tires are for.
And just one more thing. For normal street driving, it would be a good idea not to exceed the pressure rating molded on the sidewall of the tire. Prolonged running with excess pressure (or low pressure) can result in overheating and catastrophic failure of the tire.
Shoe Polish 105 covers small pressure adjustments for temperature changes and pavement conditions.
(And you thought we were finished.)