The Limits of Grip

A skid happens when one or more of your tyres are pushed beyond their ability to cling to the road surface. When the bond is broken, you go from gripping to skidding, and it can happen fast.

The Limits of Grip

A skid happens when one or more of your tyres are pushed beyond their ability to cling to the road surface. When the bond is broken, you go from gripping to skidding, and it can happen fast.

There are two types of skid: ones affecting the front tyres and those affecting the rears.

When the front tyres skid, the car tends to go straight on towards the outside of a corner despite your efforts to convince it otherwise. The car understeers.

When the rears let go, the car rotates into the corner far more than you asked it to in terms of the steering angle. It oversteers. Trust me, you will know the difference between understeer and oversteer.

Preventing a skid is a far better strategy than working to correct one. The dominant forces at play are braking and acceleration. These will always win the arm wrestle at the expense of the lateral grip required for cornering. That is why you should always apply them carefully. The skills of smooth driving and cornering technique should put you in good stead on that account.

Nonetheless, if you arrive at the point where the car is out of control, there are actions you can take to avoid a crash. The first thing to know is what are the common causes of a skid?

You might encounter a slippery surface, but usually you create the skid by doing something that pushes the tyres beyond their limit such as braking too late and heavily, carry too much speed for the corner, use rough or excessive steering, or accelerate too early or hard in the middle of the corner.

The good news is that understeer is a piece of cake to deal with, which is why most carmakers design their cars to do it. A front-wheel drive car is naturally prone to this because the front tyres have to contend with the jobs of steering and accelerating simultaneously. You flirt around the edges of understeer on a daily basis pulling out of tight junctions. If you push the gas pedal too hard then you will lose some steering. If you lose steering ability, you just back off the gas pedal, recover some steering and be on your way.

However, there is more to it than that.

Applying the throttle has a dynamic effect in the way it transfers weight within the car. Acceleration plants weight onto the rear tyres and loads them with grip at the expense of the fronts. In a sweeping corner with the rears squatting, the fronts may reach a point where they start to lose grip. If the driver responds using the corrective model for understeer by lifting off the throttle, it will effect a weight transfer back onto the front tyres.

Done gradually, this will restore balance to the universe and the car will negotiate the curve smoothly. If the driver snaps his foot off the throttle, the weight lurches forwards, giving the fronts more bite to steer just as the weight shifts off the rears, reducing their grip and causing them to skid wide. This is called lift-off oversteer.

With a rear-wheel-drive car, if you put excessive driving force through the rear tyres they will break traction, and the car will oversteer. This problem is often compounded by premature acceleration out of a corner, causing the rear to sit and provide grip at first. It feels good, so you accelerate harder without noticing that the fronts are skidding slightly up to the moment when the rear tyres spin. The extra steering angle causes the rear to snap sideways very quickly.

To counter this, you need to remove the cause by releasing pressure on the throttle and steering into the slide to control your sideways angle, or yaw. If the problem is extreme you can cut all drive to the rears by dipping the clutch.

Whatever the cause, when your car suddenly rotates more than you’re expecting and skids sideways, it will scare the life out of you.