SKIDDING occurs when the forces acting on a vehicle are greater than the friction between its tires and the road surface.  A skid occurs when changing speed or direction or both, such as when entering a bend too quickly, when braking or accelerating too harshly, or when steering and braking at the same time.

The causes of skidding are all driver-related.  Improving your smoothness and observation will greatly increase your safety.

The two most common types of skids which affect a car, especially when cornering, are described as ‘understeer’ and ‘oversteer’.  It is useful to understand these terms, which are often used to describe the way a car handles at the limits of its road-holding ability.

A tendency to understeer is built into many cars (particularly front-wheel drive cars) for example, as it is considered easier for the average driver to control.

UNDERSTEER (front wheel skid) is what happens when the slip angle of the front tires exceeds that of the rear.  The front tires lose adhesion, and the car takes a wider line than is steered.  Instead of turning in the desired direction, the car tries to go straight on.

Understeer is most common in front-wheel drive vehicles, particularly when wheel spin, due to excessive engine performance is combined with a steering input.  However, excessive speed, braking and steering can induce understeer in both front and rear-wheel drive vehicles.

REMEDY:  To recover the vehicle from an ‘understeering’ condition, reduce speed and steering angle until the front tires grip again and the cornering (turn) line is recovered.

OVERSTEER (rear wheel skid) is what happens when slip angle of the rear tires exceeds that of the front.  The rear tires lose adhesion and the car takes a much tighter line than is steered.  The back of the car slides out sideways.  Extreme oversteer can result in the vehicle rotating into a spin.  Most common in rear-drive vehicles, this loss of rear adhesion can be caused by excessive use of engine performance, causing wheel-spin, when combined with a steering input (i.e. when cornering).  Oversteer can also be induced in certain vehicles – including front-wheel drive cars – by sudden lifting of the accelerator during cornering and can be exaggerated by combining this accelerator release with brake application or increased steering angle.

REMEDY:  Successful oversteer control is much more difficult to achieve than understeer control and requires steering angle in the opposite direction to that originally intended – hence the term “opposite lock” and “steering into the skid”.  In other words, if the rear wheels slide to the right, steer to the right.

This steering response is instinctive in some drivers but is commonly applied excessively and/or too late.  This over-correction may check the skid abruptly, but because it is too harsh a steering input it often causes the vehicle to oversteer again, this time in the opposite direction – hence the term “pendulum effect” or “fish-tailing”.  Smooth recovery from oversteer requires great sensitivity towards vehicle behavior – a sense of balance, a delicate touch and feel, skill and lots of practice.

FOUR-WHEEL SKIDDING (loss of adhesion of all tires) can be caused by violent or late braking (causing the wheels to lock in non-ABS cars) or by excessive cornering speeds.  The vehicle will slide in whatever direction it was heading when the wheels lost adhesion.  If the car skids sideways, it may change direction suddenly when the wheels unlock or recover their grip.

REMEDY:  Recovery from a four-wheel slide requires speed reduction: the methods used in both understeer and oversteer control may also be needed.  In the dry, near maximum speed loss may be achieved by locking the wheels (non-ABS cars).  However in cars without ABS braking, this has the effect of losing all steering control, which can be regained by releasing the brake pressure and allowing the wheels to rotate again.  In cars without anti-lock braking (ABS), ‘cadence’ or ‘pulse’ braking (pumping the brakes on and off) will retain some steering control.

A short article like this can only be a very basic introduction to what is a complex subject.  If you are caught out by a skid, it is vital that you react very quickly and accurately and do not ‘freeze’ through sheer panic. 

Consider visiting a skid pan facility to take lessons in skid prevention and control: it’s not expensive and can be a lot of fun!  And in an emergency, you will find that theory is no substitute for practical experience.

Take care and be safe!