|The MGA With An Attitude
The MAGIC SHOE POLISH - SP-102
How to put the car through its paces to thoroughly exercise the tires.
Now the real fun starts. Get in the car, fasten your seat belt, start it up, and THRASH THE SUCKER. What you will be doing here is driving in such a way as to apply the maximum possible side load on the tires. Take a few hot laps around the lot, some right turns, some left turns. Start with broad turns and fairly high speeds. Push it fast enough for the tires to squeal, like a LOT.
You may notice one of three conditions when you reach the limit of adhesion. None of these conditions should concern you at this point (not just yet anyway). You're just testing to see how it goes.
1.) The front tires start to lose their grip, skid sideways, and don't want to turn the corner. This is very common with a production car. This is called understeer or pushing. Turning the steering wheel farther will not make it steer more, it will just cause the front tires to skid sideways more. Straightening up the steering a bit will relieve the side load on the front tires and allow them to hook up again (regain their grip). Slowing the car down will have much the same affect as straightening the steering, it just takes longer.
2.) The rear tires start to lose their grip, skid sideways, and leave you wondering why you're going sideways or backwards. This is called oversteer or pulling in. Not to worry, just approach it a little more gradual the next time. In extreme conditions, just before it lets loose completely, you may be in a left turn with the tail end hanging out a foot or more to the right, and the steering wheel tilted a little to the right of center. Dirt track racers call this turning right to go left. In effect, the front wheels are pointed in the desired direction of travel, while the rear wheels are skidding sideways. Incidentally, this is what you're supposed to do to regain control if you skid on a slick pavement.
3.) All tires lose their grip at the same time, and you do a really nice four wheel drift around the corner (fat chance). This is called neutral steering, and is the passionate desire of all road racers. It is also a little unpredictable as to what will happen next. Just a little change in adhesion or balance somewhere can cause it to transition into either over steer or under steer.
When you get the feel of drifting around with one or more tires in a skid, start tightening up the turns (at proportionately slower speeds of course) until you get the steering wheel around to about 1/2 turn from straight ahead. Keep pushing it fast enough to make it lose the grip occasionally, some left turns and some right turns. Then turn it into full lock at slower speeds and see what happens. Odds are that the front skids and you don't make the turn. If you then press the throttle hard in low gear, the inside rear wheel will spin rather dramatically. You may notice that it's not even touching the pavement.
Ready to get a little more wild now? Start at one end of the lot and head for the other end at about 30 mph. Start swerving left and right like you were changing back and forth from one lane to the other on a two lane highway, and driving in the spaces between the dashed lines without running on the paint. Push the speed up and/or make the turns tighter until the tires squeal (a lot) and start to lose their grip. If you hit the brakes or take your foot off the gas suddenly in one of these turns, you may find yourself going backwards. This is caused by a little weight transfer from the rear tires to the front tires. The front gets a little better grip while the rear gets looser.
One last test. Run it up to 30-40 mph in a straight line, then turn the steering wheel about 1/4 turn to one side, or enough to make the tires squeal just a bit, then step on the brakes just hard enough to make one or more tires lose the grip, but not hard enough to lock up the tires. Well, OK, it's alright to have one tire lock up, usually the inside rear, but too much of that will leave flat spots on the tires (not good). Repeat this for left and right turns. This is where the front tires get the worst of it.
Test drive is over. Back to the pits for inspection of the white shoe polish marks, and consideration of adjustments to the tire pressures.
Shoe Polish 103 tells you how to read the scuffed up shoe polish marks and how to adjust the tire pressure for maximum grip.