Better Driving: Skidding

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SKIDDING

Skidding is one of the major causes of accidents arising out of mechanical faults, adverse road conditions and/or human error. We analyse the various causes and what to do in the event of a skid.

Skidding is one of the major causes of accidents, most of them arising from a lack of anticipation and knowledge of skid prevention resulting in dangerously excessive speeds relative to prevailing conditions. A very small percentage of skids are due to circumstances that no reasonable driver could have anticipated.

Skids are difficult to correct but fortunately are easy to avoid, which is the key. Preventing a skid will save your hide, not correction once you are in one. Corrective action can sometimes enable a return to control but since time and space to act are so often restricted or absent, escape from severe skids without accident is usually due only to luck. Skids are caused due to either mechanical defect, road cause or human error. More often than not, in fact most times, a skid is caused or aggravated by human error.

There are three types of skids you could find yourself in: skids when braking hard, skids when over-accelerating or the side-slip skid usually called a 4-wheel drift in which all four wheels may drift or slide off the steered course.

Skids when braking hard:
Skids when braking hard are caused by one or more wheels locking up. A wheel (or wheels) has locked when it stops rolling and momentum of the moving car thereafter carries it along with that wheel or those wheels sliding.

Let's analyse the phenomenon of aquaplaning. On wet roads, traction gradually lessens as speed increases until the tyres instead of gripping, are sliding on a film of water. On wet roads at speeds up to 120kmph, good tread can cope with almost unlimited road surface water by squelching it out behind, and depending on design, a certain amount is squeezed away to the sides. The tread squeezes out the water by the revolving wheel bringing a constant fresh supply of dry tread. As each area of tread meets the road surface it sucks up the loose water. This the tread stores during the fraction of a second before again leaving the tarmac when it sprays it out behind.

In a rear wheel drive car, the front wheel can actually stop revolving as it starts to aquaplane. In front wheel drive cars, too much acceleration at around 100kmph can result in wheelspin resulting in loss of contact between tyre and road surface as a water film intervenes. Once the front wheels aquaplane, all steering control vanishes, which is frightening, to say the least. In this situation even a mild crosswind is capable of pushing the car off the road.

In all types of cars, aquaplaning on a damp surface becomes serious over 110kmph or even at lesser speeds in fact, if tyres or road surface are bad. On a straight open road with no crosswinds, bereft of traffic and no likelihood of being forced to swerve or stop quickly these speeds could be construed as being in the safe zone. But if there is any possibility of being forced to swerve or stop quickly, forget about it. Your margin of safety must be extended to counteract the possibility of locked brake aquaplaning.

If you lock brakes on wet roads the tread is robbed of its mechanism for clearing away the water. The locked wheel will tend to aquaplane until again allowed to rotate. The wheel can continue to skid up to 15kmph or less if stuck locked. The water which is then a torrent between the tyre and road surface, because it can't get away, works like a continuous wedge into the angle between the tread (in front) and the road. This lifts the wheel into aquaplaning. While aquaplaning continues there is almost no effective stopping of the car and you have no steering control.

Prevention is better than cure:
Prevention is better than cure. Ensure that the car is in top mechanical condition. Using worn tyres is not the way to economise. Ensure that the brakes are in perfect condition and check tyre air pressure weekly.

Experienced drivers maintain vigilance on all changing road surfaces. Any suspect surfaces are always tested, speed being dropped till a test confirms it can safely be increased. Test brakes under any extra load so that you know and appreciate the vast additional stopping distance required and the increased possibility of losing balance and control.

Slow punctures can be detected by heavier steering or bumping noises. Wrongly adjusted brakes usually pull a car to one side, progressively more so with rising speeds. Testing brakes at 60kmph may not serve much of a purpose. It is often only at 100kmph on dry roads that a fault is sufficiently magnified to be noticeable.

Page No: 1 2

Author: Sirish Chandran

Source

March 2002

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