Slip angles are very often an overlooked or misunderstood parameter in race car handling. Review this page for a better understanding of slip angles, their impact on handling and how computer software can help you take control of this very important aspect of handling tuning.
Slip angle is a term used to describe a particular type of flex in tires. Even though the name uses the word "slip," this characteristic has nothing to do with slipping or skidding. Slip angle is a measurement of how much the tire's contact patch has twisted (steered) in relation to the wheel. A good way to demonstrate this characteristic is to stand beside your car and turn the steering wheel. If you watch the left front wheel, you will see that it steers a few degrees before the tire's contact patch starts to turn. It is not uncommon for this slip angle with heavy cars to be as large as 6 to 10 degrees on the race track.
This characteristics is important, because as long as the tires have traction, the car tries to go where the tires are pointed. So, even though the wheels may not be steering, the tires can be steering due to the slip angle.
On a small oval track (3/8ths mile), the tires only need to steer slightly over 3 degrees for the car to travel through the turns (see Ackerman and Racing for more details). On larger race tracks, the tires may need to steer only 2 degrees or less. However, some of that steering can also occur at the rear tires due to the slip angles.
Often times, in a corner, you will turn the steering wheel a certain number of degrees initially and then need to reduce the steering angle before you reach the apex of the turn. This is because the rear tires are generating a slip angle and some of the steering is taking place at the rear tires. It is even possible to travel through the turn with the steering wheel in the straight forward position because the rear tires are doing all of the steering. Automotive engineers call this condition the "critical speed." On bigger race tracks (1 mile), this turn-in and turn-back characteristic is scary when it occurs and you'd better be paying attention, 'cause you're about to go for a ride!
Increasing the slip angles at the rear of the car tends make the race car looser (oversteer). Likewise, reducing the slip angles at the rear will often be interpreted by the driver as making the race car tighter (understeer). Armed with this knowledge, you now can tune your handling with this parameter.
TAKING CONTROL OF SLIP ANGLES
Weight transfer from the inside tires to the outside tires affects the slip angles. When your race car is in the turns, equally loaded tires will run at smaller slip angles. Conversely, a large difference between the inside tire's load and outside tire's load will cause the slip angle to increase.
Many items on the car (such as springs, spring location, roll centers etc.) have an impact on the weight transfer during a corner. This is where a program such as Computerized Chassis Weights is a great tool. You can experiment with changes in the computer and see how the weight transfer will be affected on the car.
Remember, changes that increase the weight transfer are going to increase the slip angle. Changes that reduce the weight transfer will reduce the slip angle. Increasing the slip angles at the rear makes your car looser, and reducing the slip angles (at the rear) will make your car tighter in the corners.
Also, keep in mind that these causes and effects are true as long as the tires have traction. If you drive on oil/water, or for some other reason your tires are already sliding, then you can pretty much forget the items above. Remember, racing can be da ngerous. Always use your best judgment and the best tools!
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