ALT-1 How to Control your Steering during a Skid
In general terms a skid is a momentary (we hope) loss of traction and therefore loss of control. The key, then, is to regain traction and thereby regain control. To understand how to recover from a skid one must understand what causes skids.
A tire’s interface with the road surface does not follow the classic physics concept of static and kinetic friction, but it is similar. Because a tire flexes and deflects during each revolution, as well as is distorted when the driver steers to change direction, a more detailed analysis was required. The result of that work can be found in “Mechanics of Pneumatic Tires” by S. K. Clark.
Within Mr. Clark’s work he discusses traction in terms of the coefficient of road adhesion. What is important in that work, and a discussion of skids, is the difference in adhesion between a rolling tire and one skidding. For example, the reason Anti-Skid Braking Systems (ABS) improve braking performance is because they prevent wheel lock up and skidding. The coefficient of road adhesion drops from 0.9 for a rolling tire to 0.7 for a skidding tire. The bottom line: stopping distance is increased when the brakes are locked. Steering control is also absent when the brakes are locked.
No discussion of traction or vehicle handling can be complete without considering the “traction circle” concept as discussed by Mark Donahue in his book “Unfair Advantage”. In his book, racing driver Donahue explains how there is a finite amount of traction present within the contact patch. When performing a maximum effort stop straight ahead all of the traction is consumed by the stopping effort. If the driver also turns the steering wheel he is adding a cornering load to the equation; if he continues to use maximum braking he will overload the contact patch and a skid will ensue.
The final physics tool to be included is the vehicle’s center of gravity (CG). This is the point within the vehicle where, for calculating purposes, the mass of the vehicle is located. Assume, for the moment, that the CG is in the exact center of the vehicle and about one foot above the road surface (the CG for an SUV will be higher off the road surface). As long as the vehicle is stopping straight ahead it will continue in that direction even if the brakes are locked. If the driver had intended to swerve away from the object he is braking for but has locked the brakes, the vehicle will continue on the path it was on and not respond to the steering command.
One other point about the CG should be discussed. The front engine, rear wheel drive automobile has a CG somewhat forward of the vehicle center so the front tires carry more of the vehicle weight; front engine, front wheel drive moves the CG even further forward, again the front wheels carry more weight. Rear engine, rear wheel drive will have a CG somewhat rearward of the vehicle center so the rear wheels carry more weight. This impacts how a vehicle responds when operating at the limit of traction.
Let us set up a scenario to explain the cause of a skid and the how and why of a driver’s reaction.
A front engine, rear wheel drive vehicle is entering a left turn at a relatively high speed. As the driver enters the turn he realizes the curve has a decreasing radius and he must turn the wheel more. He also realizes he is going too fast and may lose traction. At mid-turn the front wheels are left of the CG and the rear wheels are to the right of the CG. Because the CG is about one foot above the road surface the vehicle is rolled somewhat to the right; the right side tires are carrying more of the load. The inertia of the vehicle acting on the CG is trying to make the vehicle travel straight but the traction present at each of the four wheels is containing that force and keeping the vehicle on the desired path. If the driver hits the brakes hard the rear brakes would be the first to lock up because the front wheels already carry more of the load and braking action causes the weight to shift even further forward. With traction lost at the rear wheels the inertia acting on the CG is now free to attempt to go straight, causing the vehicle to pivot on the front two wheels where traction still exists. We are now in the classic sideward skid, often called“oversteer”.
It is in the above scenario where ABS would come in very handy. Failing that, the driver must take immediate command of the situation himself, and regain rear wheel traction. Turning into the skid, in this case turning to the right, will relieve some of the cornering load on the rear wheels. Releasing the brakes momentarily in combination while turning into the skid will permit the rear wheels to regain traction and stop the sideways skid. The vehicle is now under control but still in jeopardy of going off the road. The driver must resume his turn to the left to stay on the road and pump the brake to slow the vehicle while not permitting brake lock-up to recur. Often the attentive driver can detect the beginning of a skid early enough to correct for it with steering input and only slightly relieving brake pressure.
A front engine, front wheel drive vehicle handles differently because the drive wheels are in the front and the front wheels carry more weight; the CG is further forward. In the same situation in a decreasing radius left turn the rear wheels will lock up earlier when brakes are applied. With this vehicle configuration turning into the skid, or turning right, would be the wrong approach. As long as the front wheels have traction they may be used to pull the vehicle through the turn, albeit with the cars tail feathers hanging out. As the vehicle begins to pivot about the front wheels the driver continues to steer in the desired direction, which may mean to reduce the amount of left turn but certainly not a true turn into the skid. Because the front wheels are the driving wheels it is possible to cause a skid in a turn by simply letting off the throttle. Engine compression will act to slow the front wheels like a brake and if the vehicle is at or near the traction limit that may be sufficient to cause a skid.
The rear engine, rear wheel drive configuration poses another problem. In the decreasing radius left turn employing the brakes shifts some of the load to the previously lightly loaded front wheels creating almost a neutral weight bias. This reduces the propensity to skid, but if the driver has pushed the limits as have the others the aft CG makes controlling the skid more difficult. There is a reason why Porsche 911’s are found having gone off the road backwards, a skid as described often reaches the point of no return very quickly, referred to as snap oversteer.
One additional skid scenario should be discussed at this point, that is the forward skid. In this case, whether going straight ahead or approaching a turn, the driver has panicked and locked up all four wheels while the CG is in alignment with the direction of travel. As mentioned previously locked up brakes will cause an increase in stopping distance.
This can also happen in a decreasing radius turn if the driver maintains excessive speed and continues to turn the steering wheel in an attempt to force the vehicle to make the turn. At some point the side loading on the tire exceeds the traction available and the vehicle will now follow a path dictated by the inertia acting on the CG, and run off the road. This type of skid is called“understeer”. In the case of the straight ahead skid or the understeer skid, pumping the brake to regain traction is the only available solution.
It should also be remembered that the coefficient of road adhesion quoted is for rubber on dry concrete. Wet surfaces, asphalt or other road surfaces carry with them different coefficients. The vehicle dynamics will be the same on slick surfaces, they will just occur at lower speeds. Regardless of how the driver got himself into a skid, taking proper action can avoid a painful ending.
