Professional Driver's Handbook

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The amount of control the driver can maintain over a vehicle depends upon the amount of traction between the tires and the road surface.

Tire pressure and tire conditions are important factors in safe vehicle operation.

About pressure

The rotation of the tires under load causes a flexing of the casing. This flexing causes internal friction which generates heat. The tires dissipate the heat to the atmosphere. If the correct tire size is used in accordance with the load carried and tires have the proper air pressure at the start of the trip, the heat build-up will reach a heat balance temperature for which the tire is designed. The cooling rate will balance the heating rate.

If the tires are under-inflated or over-loaded for their size, or are subject to too much speed, the flexing action will be increased. This will result in the heat build-up rate exceeding the cooling rate and the tire will overheat. As the heat builds up, it causes the air pressure within the tire to increase to pressures which are higher than that for which the tire is designed.

Bleeding pressure

If the tire has the correct pressure when it is cool, the heat build-up which is normal will cause the pressure within the tire to increase and reduce the amount of wall flexing, controlling the heat build-up. If the driver bleeds the pressure down on a warm tire, the cooling balance will be upset and the tire will generate more heat. Never bleed down a warm tire. Pressure should be checked and adjusted when the tires are cold.

Tire pressure and tire wear

Correct tire pressure is an important influence on tire wear and steering control. An over-inflated tire will result in centre tread wear. Over-inflation of a tire also results in less tread surface being in contact with the road surface, which reduces the amount of traction.

An under-inflated tire results in wear on the outer edges of the tread surface. On a wet road surface, an under-inflated tire will not squeeze the water out from under the tire surfaces as well as a correctly inflated tire will. If the tire is under-inflated, it has more chance of riding up on a film of water (hydroplaning). The amount of traction with the road surface would be greatly reduced, in turn reducing steering control.

Tire condition

Regulations define the limitations of tire wear and condition permissible for certain vehicle operation.

The professional driver must be familiar with the standards as defined in regulations for Vehicle Safety Inspection requirements.

Tire matching

Never mount bias and radial tires on the same axle. Always ensure tire diameters are within 12.5 mm (1/2 in.) of each other on a set of duals.