Skid Pad Exercises
Published: November 26, 2013
Skid pad is basically a large circle, typically marked with cones, that you drive around. When used for the purposes of teaching car control, skid pad is typically watered. A skid pad can also be used for setting up the car balance, in which case it will be dry.
Despite "only" being a circle, there are a lot of things one can practice on the skid pad.
All skid pad exercises begin with the car in the "steady state". This means the driver is holding constant steering angle and throttle position and the car is following a circular path around the skid pad. Corrections to either steering or throttle, if any, should be very small.
Starting with the steady state, hold steering angle constant and add throttle. In most street cars this will induce understeer, that is, the front wheels will lose traction and the car will tend to go in a straight line rather than turn, increasing the diameter of the circle it is driving in.
Cars with a lot of power or "sporty" cars that have more neutral handling may experience oversteer as throttle is increased. Many such cars can be pushed into understeer by increasing throttle very smoothly.
Correcting Understeer With Throttle
Drivers without track experience often try to correct understeer by increasing steering angle. In other words, if the car does not turn as much as they would like, they ask it to turn more. The problem with this logic is if the car is past its traction limit on the front tires, asking it to turn more only produces more understeer.
In other words, increasing steering andle on an understeerng car does not cure understeer.
A common first exercise on the skid pad then is to correct understeer by reducing throttle. Starting from the steady state, increase throttle until the car understeers. Then gently reduce throttle position (pressure on the gas pedal), while maintaining constant steering angle. The car should turn better, tightening the circle it is driving in.
Correcting Understeer With Steering
The second exercise, and the counterintuitive one, is to correct understeer by reducing steering angle.
We have established that increasing steering angle does not help with understeer. Reducing steering angle, on the other hand, reduces the grip demand on the front tires, and while they were not gripping at the higher steering angle, they may grip at the lower steering angle.
For example, if at 45 degrees of steering angle the car exceeded the friction limit of the front tires and understeered, in effect giving 10 degrees of rotation, at 20 degrees of steering angle the car may be under the friction limit of front tires and will give all 20 degrees of rotation. Even though the steering wheel is turned less, the car rotates more.
Starting with the steady state, increase speed until the car understeers. While holding throttle position constant, i.e. maintaining speed, slightly reduce steering wheel angle. The car should tighten the circle it is following.
Correcting Understeer With Braking
I have had good success asking students to brake with their left foot for this exercise, even if they have never had left foot braked before. If the student applied the brakes too hard, they were able to apply the brakes appropriately upon me telling them to brake much gentler.
So, starting with the steady state, increase throttle position until the car understeered. Then very gently get on the brakes. The car should tighten its circle.
This exercise is all about weight transfer. You are already exceeding the front tires' traction limit with cornering, and you now ask them to brake as well. How are they able to oblige? The answer is weight transfer: braking transfers weight from the rear tires to the front, giving front tires additional grip which not only permits the car to decelerate, but also to turn better. The weight transfer must be gradual, therefore the requirement that the driver gets on the brakes very gently.
Exacerbating Understeer With Sudden Braking
The contrasting exercise to the previous one is getting on the brakes abruptly. This works best after the driver has already tried using the brakes gently and felt the car tightening the circle that gentle braking provides.
Starting from the steady state, increase throttle position until the car understeers. This time get hard on the brakes. A car equipped with ABS should slow down without locking up front tires, but do so more or less in a straight line; the driver should note that the car no longer follows a circle around the skid pad. A car not equipped with ABS might lock up the front tires, which will cause it to plow straight ahead without turning at all.
Alternating between gentle and sudden braking should illustrate the difference between weight transfer and exceeding grip limit at the front tires.
Alternating Braking And Throttle
This is a good exercise to give to more advanced drivers.
Starting with the steady state, increase throttle position until the car understeers. Use left foot braking to eliminate understeer and tighten the circle the car is following. When the car is getting close to the cones indicating the skid pad boundary, use throttle to increase the radius of the circle the car is following. When the car gets too far from the cones, get off throttle and get on the brakes to tighten the circle again. Repeat for several revolutions.
This exercise is all about weight transfer: the car is not significantly changing its speed, but whether the driver is on throttle or on brakes determines where the weight is and consequently whether the front tires have more traction than the rear tires.
This exercise only works in rear wheel drive cars or all wheel drive cars with sufficient rear bias.
Starting with the steady state, get on the gas hard to get the back end of the car to lose traction. As the back end loses traction it will cause the car to rotate. Steer into the skid (in drifting parlance, countersteer) to recover the car.
For less experienced drivers, the biggest value in this exercise is feeling a rear wheel skid and practicing turning into the skid to recover it.
More advanced drivers, especially those doing or expecting to do track days, will benefit from attempting to hold a drift. It is much easier to handle oversteer in track driving environment when you already know how do deal with it. Roughly, the three phases of a drift are:
- Exceeding rear tires' traction - there is a multitude of ways this can be done, with power being the easiest one.
- Countersteer - also known as "turning into the skid", the driver must countersteer rapidly but not excessively. The first issue beginning drivers typically have is late countersteering which causes the car to spin in the initial direction of the skid. The second problem drivers typically have is excessive countersteering, which causes the car to spin in the opposite direction after the initial rotation is caught.
- Maintenance throttle - once the initial rotation is brought under control, the driver needs to add throttle to keep the rear tires spinning while maintaining appropriate countersteering angle.
It will help to practice drifting with an instructor.
Emergency Brake Oversteer
This technique is typically used to demonstrate oversteer in front wheel drive cars.
Starting with the steady state, pull the emergency brake. The car should rotate more than the steering input alone required. Countersteer to recover the rotation.
Cars with strong understeer in their setups may not be able to achive much rotation even with the emergency brake, especially on poor tires.
Trailing Throttle Oversteer
In a front wheel drive car without a hand-operated emergency brake this is probably the most effective way to induce oversteer.
Starting with the steady state, sharply take your foot off the gas pedal. If the car is sufficiently balanced, you should feel excessive rotation. Traction/stability control should be turned off to experience trailing throttle oversteer in modern cars.
This exercise should work in rear wheel drive cars as well, but in rear wheel drive cars it is typically easier to initiate oversteer via power over.
Dry Skid Pad
A dry skid pad is normally used for car setup rather than teaching drivers what understeer and oversteer is, although drivers can practice understeer and oversteer if they want to. It can often be found at autocross "test and tune" events, typically before the regular season starts.
Using your undrestanding of understeer and oversteer, use the dry skid pad to determine which of them applies to your car in dry conditions. Experiment with increasing and decreasing tire pressures to see if you can alter the oversteer/understeer tendency of your car. Road race cars typically want a very slight amount of understeer in steady state cornering, so that they can put the power down exiting corners.
Tire pressure can be changed on any car to alter its balance. Other suspension parts that can be altered, depending on the car, are:
- Sway bars (replace with stiffer or softer)
- Sway bar end link length (requires adjustable end links)
- Length of sway bar (adjustable sway bars have multiple holes on the ends for the end links to go through)
- Shock stiffness (compression/rebound)
Keep in mind that stiffness of a car affects the tires' ability to stay attached to the surface over bumps. Bumpy tracks will require softer suspensions. If you can adjust shock stiffness, it is a good idea to have several positions in the hardness range where the car has desired balance. For example, a "softest" setting would have one end of the car at full soft and the other end adjusted appropriately for neutral handling. Then, a "medium" setting would start with both ends of the car brought up, say, 4 clicks, and then increased/decreased as necessary to arrive at neutral handling again.