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Safer Driving Tips Page 8

Skidding - causes and cures

Risk of skidding road sign

What is a skid? This is the official definition:

A vehicle skids when one or more tyres loses normal grip on the road causing an involuntary movement of the vehicle. This happens when the grip of the tyres on the road becomes less than the forces acting on them.

The causes of skidding

  • Excessive speed for the circumstances
  • Coarse steering in relation to a speed which in itself is not excessive
  • Harsh acceleration
  • Sudden or excessive braking
The road surface is never the cause of a skid - it is always the driver.

Four types of skids

  1. Rear wheel (Over steer)
  2. Front wheel (Under steer)
  3. Four wheel
  4. Aquaplaning.

Rear wheel skids

In a rear wheel skid you feel the back of the vehicle swing out. On a corner or bend the swing is always to the outside of the curve. In many situations removing the cause - acceleration - may correct the skid without having to alter steering. But if the speed of the vehicle is excessive there may not be sufficient space to regain directional control whatever you do, physics will take over.

Front wheel skids

In a front wheel skid the front of the vehicle will carry on in a straight line. You will not be aware of the skid as quickly as in a rear wheel skid, so you will already have started to deviate from your intended path. Removing the cause - acceleration - may correct the skid, but if the speed is too high then again physics takes over, or you may not have enough room to recover control.

There will be no sensation of loss of directional control through the steering wheel. The natural reaction is to apply more steering in the direction you want to go. This will make the skid worse as the tyres will now have lost the ability to grip the road. Applying more steering will mean the tyres are now scuffing sideways, and they cannot grip in that position. For the tyres to grip the wheel needs to roll normally so the only chance of regaining control is to come off the accelerator and steer in the direction the car is skidding - straight ahead. Once the speed has decreased enough for the tyres to grip you can gently apply steering to regain course.

The problem is that on a road that is about 3 or 4 cars in width you will already have started deviating from your course by the time you become aware that something is wrong. On a left hand bend that means you are travelling into the path of oncoming vehicles. By the time you come off the accelerator and steer in the direction the car is now heading, and like it or not it will be going in a straight line, it is probably too late to do anything. Add some parked cars on the street and it is over before you know it even got into a skid.

Ice warning road sign

Four wheel skid

A four wheel skid is usually the result of sudden or excessive braking causing all four wheels to lose grip on the road. You feel lightness and loss of directional control as all four wheels lock up. This is most likely where the driver has to lose speed rapidly in an emergency. ABS does not always help. Excessive speed is often a contributory cause.

Aquaplaning

One of the most frightening experiences a driver can encounter is aquaplaning. This where a wedge of water builds up between the tyres and the road surface. This can be caused by excessive speed for the conditions or inadequate tyre tread depth. Neither steering nor braking respond. The only option is to remove the cause - acceleration - and allow the speed to reduce until grip is re-established and then apply gentle steering and/or braking. If brakes or steering are applied during aquaplaning then when the speed drops enough to allow the tyres to grip the vehicle will either go into a four wheel skid or steer in the direction that the front wheels are pointing.

Tyre grip trade off

Tyres can do one thing to a maximum 100% efficiency. If they need to do two or more things then they have to share that ability. This is called tyre grip trade off. Therefore, if the vehicle is travelling in a straight line the front tyres need only steer. If the vehicle needs to slow down then the tyres need only brake. If the vehicle is on a bend and you need to apply the brakes then clearly the tyre has to do two things - steer and brake. This means it cannot do either to maximum efficiency. If the speed is such that the tyres are starting to squeal then they are starting to lose grip. Application of more steering usually results in total loss of directional control.

Don't accelerate into a bend, get your speed off before, then maintain the speed in the bend and accelerate gently out. This applies to any hazard.

ABS

This is a safety device that allows a driver to steer and brake the same time and retain directional control whilst stopping in an emergency. Without ABS the wheels would lock and the vehicle would go into a skid and not respond to steering. ABS locks and releases the brakes up to ten times a second so allowing the wheels to continue to roll and respond to steering. However, to get ABS to work you must apply maximum pressure to the brake pedal immediately, and keep that pressure on the pedal until the vehicle has stopped. If the pressure applied to the brake is inadequate then the ABS will not function. If the pressure is reduced after the ABS has cut in then it will stop working. It is also likely that the overall stopping distance is increased, so braking too late from speed with nowhere to steer to will result in a collision, albeit at a lower speed. A good driver should never need to use ABS.

Traction Control

There are various names for this safety feature that will detect if a drive wheel is losing grip during acceleration. It reduces power to that wheel until it regains proper grip. On a rear wheel drive vehicle it is very effective and will often prevent a rear wheel skid. On a front wheel drive vehicle it is less effective, in fact it could cause problems. If a front wheel drive vehicle fitted with traction control is travelling in a straight line under maximum acceleration and one or both of the front tyres lose grip, then power will be reduced until normal grip is achieved. If the vehicle has steering applied then it will attempt to achieve normal grip but this will allow the vehicle to increase speed to a point where the speed may be excessive for the conditions and result in a front wheel skid. More advanced systems on the luxury market cars are much better at stabilising the car. Treat traction control as a warning device; if it cuts in then you are applying excessive acceleration for the conditions. Release the accelerator until normal grip returns.

Stopping distances

You must be able to stop in the distance you can see to be safe and on your own side of the road. The easiest rule-of-thumb is to adhere to the saying "Only a fool would brake to two second rule". Pick a reference point that the vehicle infront of you is about to pass then count to two seconds. If you pass that point before you get to two seconds then you are too close. This works at all speeds. Howver, when there are adverse road conditions you need to double your braking distance.

The width of the road also has an implication. On a wide road the 2-second rule applies, but as the width reduces, and this includes a wide road made narrower by parked vehicles or other obstructions, then you have to reduce speed and double the stopping distance. On a narrow road with no kerb look at the edges. If there are tyre marks on the verges then it suggests wide vehicles use that road. If you are doing 30mph and the unseen vehicle around the corener is doing 30mpg, that is a closing speed of 60mph. In these situations you must be able to stop in half the distance you can see to be safe.

All this assumes your vehicle is in a roadworthy condition and you are alert and anticipate the likelyhood of a vehicle coming towards you.

Front wheel drive- vs- rear wheel drive

  • A skid on a rear wheel drive vehicle will be apparent a lot sooner than a front wheel drive vehicle. Depending on the speed, it is also easier and quicker to recover from a rear wheel skid in a rear wheel drive vehicle.
  • When you accelerate in a front wheel drive vehicle the weight is thrown back, so the rear tyres grip more but the front tyres grip less because the shoulders (edges) of the front tyres are lifted off the road surface. Car manufacturers compensate by putting wider tyres on the vehicle, especially the more powerful cars. This does work, but it also means you will achieve a higher speed before the grip is lost, then you go into a front wheel skid so fast the results are often dreadful.
  • Front wheel drive vehicles understeer.

Clearly, rear wheel drive is safer than front wheel drive. Many cars are now made with four wheel drive, especially the more powerful ones. This allows the power to be applied in a more uniform manner through all four wheels.

Cars with electronic stability control systems.

Modern cars are equipped with effective electronic anti-skid systems. They do work very well. However, that does not mean you can drive without taking the advice above. Modern cars still skid and crash. Remember that if you are going too fast Sir Isaac Newton's laws of physics will beat any modern electronic systems on any vehicle. If you do experience a skid then ensure you have read the vehicle owners handbook on how the systems work on your car. It is often best to allow the system to correct the loss of control - provided your speed is not excessive.

Remember - skids are caused, they don't happen by accident.

Bob Isaac

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