10 Facts about car tyres every driver must know
What does the tread do?
In a perfect world, like a perfectly dry road, tread actually reduces a car's performance because it has the effect of reducing the contact patch area, and the forces that can be transmitted through the contact patch are correspondingly reduced.
But in a not-so-perfect world, on a wet road, the tread is vitally important.
The tread is designed to disperse water from the contact patch, thereby helping the tyre grip the road.
Without tread the tyre's ability to grip a wet road is severely limited, making it almost impossible to stop, turn, accelerate and corner.
What is the contact patch?
The contact patch is the area of the tyre that is actually in contact with the road.
It's a small area, only about as big as the palm of your hand, through which the cornering, steering, braking, accelerating forces are transmitted.
When is a tyre worn out?
Tread wear indicator bars are moulded into the tread grooves at regular intervals around the tyre to indicate when a tyre is worn to its limit of safety.
The minimum legal tread depth is 1.5 mm across the width of the tread.
When the tyre is worn to the legal limit the bars will be flush with the surface of the tread.
While that is the legal requirement some car manufacturers recommend you replace your tyres before they wear to that extent.
Check your owner's manual to find out what your car maker recommends.
Setting the inflation pressure
Maintaining the correct inflation pressure is one of the most important things you can do to look after your tyres.
A correctly inflated tyre should wear evenly across the tread, but one that's incorrectly inflated will wear unevenly.
An under-inflated tyre will wear more heavily on the outer shoulders, while an overinflated one will wear more in the centre of the tread.
The inflation pressure should only be set when the tyre is cold. The pressure increases when the car is driven, so setting it after driving for some distance will result in an incorrect pressure.
The correct pressure
The recommended inflation pressure is shown on a placard attached to the body – usually on the driver's door pillar – and also in the owner's manual.
Inflation pressures are given for normal driving, and also when loaded up with the maximum number people the car can legally carry and luggage.
When should I check the inflation pressure?
Tyres need to be checked regularly, at least once every two weeks.
They should also be checked before you go on a long journey, or before towing, when they might need to be set higher.
Remember to also check the spare.
Rotating your tyres
Tyres wear at different rates depending on their position on the car. On a rear-wheel drive car the rear tyres wear faster than the front tyres; on a front-drive car it's the front tyres that wear the fastest.
Rotating the tyres around the car can even out the wear on all tyres. That way they should all need replacement at the same time.
If you do rotate your tyres do it regularly, at 5000km intervals, so the disparity between those that are wearing the fastest and those wearing the slowest is minimised.
When rotating your tyres you can also include the spare.
When should I replace the spare?
The spare is almost always forgotten, left to sit in the dark in the boot of our car until needed in an emergency.
Spare tyres that are six years or older should only be used in an emergency.
A tyre that is 10 years old should be replaced.
Do my tyres really need replacing?
Some mechanics and tyre men will tell you your tyres need replacing by simply looking at them and telling you they're worn.
Don't take their word for it, check them yourself. Visually inspect them for wear and damage, and check the depth of the grooves.
To maximise your tyre life avoid spinning the wheels on acceleration, or locking them when braking.
Maintaining your car
Keeping your car in tip-top shape can help extend the life of your tyres and regular wheel alignment checks are a good idea.