TYRE LAW

Tyres must be suitable (i.e. of the correct type and size) for the use to which the vehicle is being put and must be inflated to the vehicle or tyre manufacturers’ recommended pressures.

Tyres of different types must not be fitted on the same axle, i.e. cross ply / radial or directional / asymmetric. On a two axle vehicle with single rear wheels, radial tyres must not be fitted to the front if cross ply tyres are fitted to the rear.

No tyre must have a break in its fabric or a cut deep enough to reach the body cords. No cut must be more than 25mm or 10% of the tyres section width in length, whichever the greater.

There must be no lump, bulge or tear caused by separation or partial fracture of its structure, neither must any portion of the ply or cord structure be exposed.

Tread Depth

A new car tyre begins life with approximately 8 mm of tread, the minimum legal tread depth is 1.6 mm, most motoring organisations recommend changing at 2 mm while the majority of manufacturers recommend changing at 3 mm.

As the tread depth reduces, the tyres ability to remove water from the road surface also reduces, increasing the risk of aquaplaning and because the tyre is unable to transmit traction and braking forces to the road as effectively the vehicles stopping distances increase.

Tread depth new 8mm.

7mm 16% worn.

6mm 31% worn.

5mm 47% worn

4mm 63% worn.

3mm 78% worn.

2mm 94% worn.

1.6mm 100% worn.

Although the legal minimum tread depth is only 1.6 mm, research shows that Tyres with 3 mm of tread have an average 25% better performance than those at 1.6 mm— which represents an extra 8 metres stopping distance in wet conditions.

Reasons for Replacing Tyres

Over inflation: Tyres will wear more in the center of the tread.

Under inflation: Tyres will wear more on both tyre edges.

In both of the above situations efficiency and traction would also be lost due to the reduced area of tread in contact with the road. Over an extended period this will also result in increased fuel consumption.

Tracking: Incorrect wheel alignment can result in rapid wearing on the edges of the tyre and could affect the handling of the vehicle.

Valves

Valves play an important part in maintaining the correct pressure within the tyre.

A valve cap in good condition is essential to provide a primary seal and prevent dirt getting in.

As a result of centrifugal force the valve body is subjected to a force equivalent to 1.7 kg.

The valve should be changed every time a new tyre is fitted, or if the tyre is removed for repair or examination.

How to check tyre tread depth – the 20p test

Just take a 20p coin and insert it into the tread grooves on the tyre.

If you can't see the outer band on the coin, your tyres are above the legal limit.

However, if you can see the band and that section of the coin is still visible, your tyres could be unsafe.