Illustration 3: This graph...
Illustration 3: This graph shows the speed at which one driver turns the steering wheel versus another driver. Both turn the wheel 90 degrees, but the driver represented by the dashed line takes more than one half of a second, while the driver shown by the solid line takes just more than one tenth of a second for the same motion in the same turn. The first, slower-moving driver is considered smooth. The dashed-line driver's angle is abrupt and will most certainly upset the handling balance of the car more than is desirable. It is important for the driver to use smooth motions and applications of the controls to minimize upsetting the balance of the car due to abrupt motions.
Total Weight
Lighter is better. Most classes have minimum weight rules. You want to be at minimum weight. If there is no minimum weight rule, run as light as you possibly can. If your class has a minimum weight rule without driver and you weigh more than 140 pounds, fight to have the rule changed, because you are at a disadvantage, and the higher your weight, the bigger the disadvantage. If you weigh less than 140 pounds, don't let the big guys and track officials see this article--you will lose your advantage.
There are two big reasons why minimum weight is important. First, the engine must accelerate the extra weight. You have probably noticed that your street car does not accelerate as well with three passengers on board as it does with just you in the car. Second is the factor we looked at above--tire traction versus vertical load on the tire. If you add 500 pounds to the car to improve handling, you have added 500 pounds to the tire's workload, but only about 450 pounds of additional traction force. That makes the car slower under braking and cornering. It's not a good trade-off.
Tire-Slip Angle
The tire-slip angle, which is actually the amount of twist in the tire sidewall that causes the tire contact patch to turn at a smaller angle than the wheel centerline (the difference is the slip angle), determines the lateral force of the tire. At a given slip angle, the tire will create the maximum cornering force. At a smaller slip angle, the tire will create less cornering force. The same holds true at greater slip angles. The goal for the driver is to keep the tire at the optimum slip angle for maximum cornering force at all times in a corner--not an easy task. The front-slip angles versus the rear-slip angle determine the handling balance of the car. If they are equal, the car is neutral. If the fronts are bigger than the rear, the car will push or understeer. If the rears are greater, the car will be loose or oversteer (Illustration 4).
Illustration 4: This graph...
Illustration 4: This graph shows that maximum cornering force occurs when all of the tires operate at 8 degrees of slip angle in a turn. Any greater or smaller slip angle means the car is not going around the corner at the maximum speed. Very high cornering force is available from 7 to 9 degrees. The driver at 9 degrees is going the same speed as the driver at 7 degrees, but will overheat the tires sooner. Cornering force will deteriorate sooner for the driver at 9 degrees.
Weight Transfer
During cornering, weight transfers from the inside to the outside, under braking from the rear to the front and under acceleration from the front to the rear. Weight transfer hurts overall vehicle traction. In cornering situations, weight moves off the inside tires to the outside tires. This changes the vertical load on all four tires. Two tires (the inside) lose vertical load while the other two gain vertical load. The inside tires lose traction while the outside tires gain traction. Sounds OK so far. But remember, the relationship between vertical load on a tire and the traction force of that tire is not linear. The weight coming off the inside tires causes them to lose traction faster than the outside tires gain traction from the newfound additional vertical load. So the net total traction of the tires is reduced, compared to the same situation if no weight transfer occurred. Since it is not possible to eliminate weight transfer in a corner, we at least want to minimize it so the overall traction remains as high as possible.
Under braking, the same thing occurs, but is less pronounced. Under acceleration on a rear-wheel-drive car, weight transfer actually helps accelerate the car because the drive wheels are gaining traction while the tires losing traction are not driving the car. The opposite is true for a front-wheel-drive car, which makes the elapsed times of the front-wheel-drive import cars all the more impressive. Even though we gain some acceleration traction from more weight transfer, if you have to turn and slow down for corners, weight transfer hurts lap times, so our goal is to minimize weight transfer as much as possible.