Skid Prevention & Control

When you participate in a Ride Drive advanced driving course you will find we spend a lot of time and effort helping you to understand how to maximise your car control potential. Our aim is for you to learn better car control skills so as not to get into sticky situations, such understeer, oversteer, or any other skid or slide.

But what happens if you do get into a skid? How do you deal with it? Come to that, what causes the car to skid in the first place?

Usually, and this is said with absolutely no disrespect to anyone who may be reading this page, a tricky situation is made worse by doing the wrong thing, doing the right thing wrongly or doing either at the wrong time.

Hopefully, after reading this edition of the Ride Drive advanced driving tips and advice series, you will be less likely to get into a skid, or you may be more able to cope with skidding if you do get into a skid situation. Certainly you will improve your car control skills.

Tyre Grip Trade–Off

There is a term that you may well not have heard of, and that is, Tyre Grip Trade–Off. If you think about all four tyres on your car having 100% grip potential, for every task that you ask of that tyre, it has to use up some of its grip allocation to complete that task.

If a tyre uses 80% of the available grip allocation to cope with a period of braking, whilst that is happening there is only 20% available for anything else. However, if you don’t brake whilst steering, for example, the tyre will give you all of its performance for the one task. If you expect otherwise you are expecting too much and the car will not cope with what you ask of it.

The System of Car Control does not allow for the vehicle to multi–task and that is one of the reasons why it goes you the ulimate level of car control.

The Four Causes of Skidding

Skidding can be defined as the involuntary movement of the vehicle due to the grip of one or more of the tyres on the road becoming less than a force or forces acting upon that vehicle. In other words, a vehicle skids when one or more of the tyres loses normal grip on the road.

What may amaze you is that officially there are actually only four causes of skidding, and these can occur either singularly or as a combination.

The above four items are perhaps best summed up by saying that a skid occurs where the driver has expected more from the vehicle than it was capable of delivering under the conditions at that time. The vehicle only has so much performance that it can give to you, and if you demand more, you are exceeding its capability.

Working on the principle that if you know what causes a skid it is easier to avoid one, or you know how to deal with one if it was not so avoided, let’s now look at each of the four causes in turn.

1 – Excessive speed for the existing circumstances

This means that you are trying to do something, such as drive a 40mph bend at 50mph. It just ain’t going to happen. Or, it could be that you are driving according to a dry road, but when the road is wet, or in snow. Like it or not, when you drive a car, you will always be bound by the rules of physics relating to moving bodies.

If you have read the chapter about cornering forces within this series of articles on advanced driving tips and advice you will remember we talked about how, when making a vehicle steer around a bend, you are asking it to do something unnatural. We used the analogy of a conker (horse chestnut) tied on a piece of string to demonstrate this, whirling it around like a helicopter rotor above your head, and how it would behave if you let it go.

The model was used to demonstrate why, when the grip of tyre on road was lost during cornering, the affected end of the car will slide in a direction that is at a tangent to the curve of the bend. Unfortunately, and according to another unbreakable law (sod’s law), this will normally involve contact with a tree, or other hard object, such as another vehicle, or entry to a ditch.

2 – Coarse steering in relation to a speed, which in itself is not excessive

This means that you are still entering your 40mph bend, but now at 35mph, and yet you still get a skid situation. If this happens on a good road surface it is that instead of feeding in the steering progressively, you have grabbed a big armful of steering wheel and given it a big tug with no empathy for the car. To illustrate the effect of this we have another analogy.

Fred and George are twin brothers who work in a warehouse. They are both of the same build, level of fitness and height. Fred is carrying a heavy box that he needs to pass over to George. If Fred stands holding the box, and allows George to take hold of it by bearing the weight in his own time, George is able to take the box from his brother and carry it equally as well.

However, if Fred had thrown the box, George would drop it, only because of being asked to cope with too much too quickly and too soon in terms of bearing its weight. The same applies to steering. Throw in too much too quick and the tyres won’t cope, but feed the steering input in progressively and the car will respond up to its full cornering potential.

3 – Harsh, excessive acceleration

You can apply the above heavy box story to this one as well, because if you are harsh with the loud pedal, and⁄or when releasing the clutch, the tyres at the driven wheels may not manage to maintain grip. This is when you encounter wheel spin under power. By applying power progressively, and being smooth with the driving controls, better car control is maintained.

4 – Excessive or sudden braking

Again, the heavy box story applies. Brakes, like the accelerator and steering, are variable devices, and not an on⁄off switch. Apply the brakes gently at first, then ease in the pressure smoothly and progressively, and you will get the best stopping power available at that time. Give the pedal a poke, or stamp on it, and you will get a poor response.

Notice how none of the causes of skidding are given as ice on the road, oil or mud. This is because it isn’t the ice that makes you skid as such. Ice, oil and mud etc. on the road is just another variation in road surface, perhaps a temporary surface condition on a piece of road that would more normally provide better grip.

The reason you slide here is because you are not taking into account the low coefficient of friction properties of the road when you apply acceleration, braking or steering. Therefore, you are doing this in a manner that is inappropriate for the conditions and therefore asking more of your car than it is capable of coping with at that time.

Correcting a Skid

If you get into a skid situation, how you deal with it has one generic answer – remove the cause.

If you have applied your brakes and your wheels lock into a skid, to remove the cause you need to lift off the brake pedal. For a better effect you need to immediately go back on the brake to it again, but this time a little more gently to try and avoid re–locking wheels.

Always remember that if you are driving a car that is not equipped with Anti–Lock Brakes (ABS), and you brake heavily enough to get front wheel lock up, whilst you are skidding along the road you can spin your steering wheel from lock to lock and you will go nowhere else but straight on.

If you experience skidding under braking when you are heading for the back of a vehicle stopped in front, and there isn’t enough room for you to pull up before hitting it, you will have to go against all your instincts that keep you rooted to your brake pedal. You are going to have to get your foot off so you can then steer away from the obstruction.

When you take part in an advanced driving course with Ride Drive there is a particular point we try to get across to you, and that is you do not brake and steer at the same time. Completing an emergency lane change, for example, will require initial application of the brakes, release of the brakes, steer away from the obstruction, straighten up and re–apply the brakes.

The Difference Between Understeer and Oversteer

Staying with bends just for a moment, if tyre grip at the front of the car is lost during cornering, this is called understeer. If it is the rear tyres that lose grip in a bend, this is called oversteer, and we will now deal with how you tackle each individual situation.

Correcting Understeer

Understeer, as the name suggests, is where you apply steering and the response from the front of the vehicle is disproportionately less according the amount of steering applied.

You will probably have induced understeer by putting in too much steering, or by putting in too much too soon. To counter this you will have to take some or all of the steering input out again before re–applying more sympathetically.

Straightening the steering
in a bend is not easy

It takes real courage, so it is most important that you don’t get to that stage to begin with. In our experience, customers find it almost impossible to take steering out when in an understeer situation, even on an open expanse of aerodrome where there is nothing in front of them. It goes against every human instinct there is, but it does work if you can do it.

Correcting Oversteer

Oversteer is where the amount of direction change is disporoportionalely greater than the amount of steering input applied. This is the one where the back of the car swings out and we have deliberately left this next one to last. This is because we want to talk about how the standard suggestion of always removing the cause of a skid can actually get you into more trouble.

Rear wheel drive cars

The most familiar cause of oversteer is harsh or excessive application of power (acceleration) in a rear wheel drive car.

For example, rear wheel drive cars, without any form of traction control equipment, will be provoked into oversteer if you apply sufficient power to cause the rear tyres to lose their grip on the road, and when driving anything other than a straight line.

When this happens the normally recommended course of action is to immediately counter the direction of the slide by steering in the same direction. Therefore, if the back of the car swings to the right, you have to steer to the right to try and catch it. You may have heard this referred to before as Opposite Lock, and to apply it at this point is good advice.

Opposite lock is a technique used to regain control,
but does not remove the cause of a skid

To remove the cause of the oversteer skid, which in this case is the excessive or aggressive application of engine power, you are going to have to lift off the gas pedal. Most will do this instinctively anyway, but it is how you do it that can actually cause more of a problem than it will solve.

When you snap your foot off the accelerator you can easily convert power induced oversteer into power–lift oversteer. This is due to your new direction of steering (opposite lock) and is likely to cause the tail of the car to swing in the opposite direction. The steering input needs to be reversed again, but now you have a pendulum effect buidling at the rear of the car, and getting into a spin is almost inevitable.

How to Neutralise Power Induced Oversteer

If you experience power induced oversteer in a rear wheel drive car, this is because the driven wheels are driving (rotating) at a speed greater than the car is travelling. The speed of the car we shall refer to as Road Speed. If this occurs, and you snap your foot off the gas pedal, without power to the driven wheels, they will slow until they match the road speed of the car. This point we shall refer to as the Bite Point of the tyres.

As the rear wheels match the speed of the car (Bite Point), just for a fleeting moment, full grip is restored, but then lost again as they are dragged along by the remaining momentum within the car.

As you have applied opposite lock with the steering, in your attempt to correct the initial skid, this is the point where the back of the car kicks round in the opposite direction.

When the rear of the car is into the second slide the effect is even more violent than the first. This is so difficult to counter because the slide has begun that second phase from beyond a central point, and therefore has a bit of a run up at it. That is the one that normally throws you off the road.

Never snap your foot off the accelerator

What you have to do is find another method by which to remove the cause of excessive acceleration, to make the Bite Point of the rear tyres the successful end to the situation. To do so you will have to act very quickly.

If the rear end of the car steps out of line, because you are applying power in a bend, you still need to steer in the direction of the slide (opposite lock), but don’t snap your foot straight off the accelerator. Instead, feather the power back gently until the driven wheels have slowed down to the point where they again match the road speed of the car (Bite Point).

Keep your right foot steady

By keeping your accelerator foot steady at this point, you can smoothly re–adjust the steering back to your intended course. It is vital that you do not make any change in power setting, at least until the car has been fully stabilised.

There is another option, which is a whole lot easier to apply, and which is very effective. In this case, do exactly the same correction process with the steering as above, but this time just slam the clutch pedal down to the floor. The effect this has is to instantly create a situation where the driven wheels are neither driven nor dragged, but will assume a pure rolling action that matches the road speed of the car. Now you should be able to straighten the steering and smoothly take up the drive.

When we allow customers to demonstrate this for themselves on an airfield they are usually flabbergasted at how quickly the car will change from being an unruly wild animal into a state of complete obedience. The change in vehicle attitude really is very quick.

Power Lift Oversteer

Notice how this one does not appear in the list of causes of skidding. Power–lift is a more common cause of oversteer on bends than power–on oversteer, and is more of a danger, because people are so much less aware of it.

If you talk to those who drive powerful rear wheel drive cars they will eagerly tell you in a sage–like manner how accelerating in a bend, particularly if the road is wet, will cause you to lose the back end. However, few will give warning of the effects of power–lift in a bend.

Power lift oversteer is caused by a driver lifting off the gas pedal in a bend, and as a consequence, the rear tyres lose grip on the road due to the braking effect upon the drive wheels caused by the drag of the engine.

The way to prevent power–lift oversteer is never to lift off the power whilst cornering. However, if you do experience it, the way to cure it is to apply some gentle opposite lock at the same time as re–applying engine power. Not too much, however, as that will cause further issues.

Front wheel drive cars

Power lift oversteer is not reserved prurely for rear wheel drive cars, as it will also affect front wheel drive cars, but for a slightly different reason. We have said before that a vehicle in motion is at its most stable state whilst travelling in a straight line and at a constant speed. This is because the weight of the vehicle is evenly shared as possible between all four wheels and tyres.

Bearing in mind that grip is created through the tyre being pushed onto the road surface by vehicle weight, when accelerating the available grip offered by the front tyres is reduced, but increased at the rear. Under deceleration the situation is reversed, as it will be the rear tyres that have less grip in that case.

Lifting off the power mid–bend
in a front wheel drive car is not good

Lifting off the gas whilst the car is travelling straight shouldn’t be an issue, but doing so during cornering is a different mattter. With the rear tyres becoming unloaded (reduced grip), and with down force at the front becoming greater (increased grip), there is an imbalance in the amount of available grip front to rear. In this situation the rear of the car will try to orbit the front.

Power–lift oversteer affects front wheel drive cars quite badly and is a situation that very difficult to recover from. The best that you can hope for is that you have lightening quick reactions, because once this situation starts to develop, unless you can counter it in a nano–second, the moment of redemption will have passed. For most, it will be the beginning of the inevitable.

Removing the cause of the skid here isn’t going to be enough

If you experience oversteer with a front wheel drive car, particularly if it is power–lift oversteer, the old remedy of removing the cause is still good advice, but in this case it isn’t going to be enough. Therefore you still have to do something more.

Bsically, you have two options. Either go for immediate power–on (if you have the road space) or to de–clutch. The power–on option may give you the opportunity to pull the car straight, but you have to remember that in this case you are increasing speed whilst in a deepending control loss situation.

The de–clutch option will allow all four tyres to resume a pure rolling action, being neither driven nor dragged, and if you are quick enough, everything may settle down. However, for the public motorist, it is most unlikely that recovery will happen.

Julian Smith
Ride Drive Limited

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Part–1 Skid Prevention and Control
Part–2 Clutch Drag Oversteer
Part–3 Skid Pan Driving Tuition

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This page was last updated
Thursday, 27-Jan-2011

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