Get in Line

Get in Line

With safety critical components such as tyres, it is vital technicians understand the potential causes of tyre wear. Hankook Tyre UK Ltd outlines how inflation pressure and wheel alignment can affect heavy-duty vehicles.

One of the most important aspects of tyre maintenance is correct tyre inflation – necessary to carry the load and avoid damage. Driving with improper inflation (particularly grossly under-inflated or over-inflated tyres) is dangerous, and can cause critical damage or sudden failure. Hankook suggests that inflation values should be checked at least once a week, and always before a long-distance drive. Indeed, in the space of one month, a tyre can lose up to 10 pounds of air pressure.

It is also advisable to take into account the axle load and driving conditions when setting inflation pressures. Compensation for heavier loads can be made by increasing inflation pressures – though make sure not to exceed the maximum inflation rates for the tyre or maximum load axle.

Under-inflation is the worst enemy your tyre can have. It causes increased tread wear on the shoulder edges and generates excessive heat, reducing durability. Soft tyres also make your vehicle work harder, meaning that fuel efficiency is reduced due to increased rolling resistance.

Over inflation, on the other hand, is also detrimental, as too much air pressure causes the centre of the tread to bear the majority of the truck’s weight, leading to faster deterioration and uneven wear.

There is actually more to alignment than initially meets the eye. Steering axle alignment alone is not sufficient, as the vehicle and all of its tyres are needed to be travelling in the same direction. Here, Hankook outlines the various different components needed to achieve total wheel alignment.

■ Camber is the inward or outward tilt of the steering axle tyres when viewed from the front
■ Positive camber is the top of the tyre tilted out
■ Camber becomes more negative as the load increases

The angle that a centre line of the wheel is inclined from, the vertical centre line perpendicular to a flat road, is called camber angle. If the top of the wheel leans out from the perpendicular then it is positive camber. If the top of the wheel leans in from the perpendicular then it is negative camber.

Camber is meant to compensate for the downward forces of added loads. Correct camber settings help the tyre maintain a firm and even tread contact with the road while the vehicle is travelling under loaded conditions. Often, wear at the outside or inside edge of the tyre may indicate incorrect camber setting.

■ Toe is the inward or outward pointing of the wheels when viewed from the top of the vehicle
■ The goal is to have zero toe when the vehicle is loaded to its normal operating condition

Toe-in refers to the inclination of the wheels of a vehicle so that the pair of front wheels are closer together at the front than at the rear of the wheels.

The purpose of toe-in is to relieve or counteract some of the force which pulls wheels outwards as they roll along the road. Correct toe-in will ensure the rotation direction and direction of travel are as similar as possible at driving speed.

Insufficient toe-in settings will result in steering instability. The opposite is considered toe-out. If toe-in or toe-out is insufficient or excessive, the tyre wear will be affected and appear as feathering at the edges of the tread.

■ Caster is the forward or rearward tilt of the king pin of the steering axle when viewed from the side
■ Caster is generally not considered to have a great effect on the tyre wear

Caster is the condition where the king pin is inclined with the top of the pin angled rearward, similar to the front forks of a bicycle. Caster angle is meant to compensate for the resistance that the tyre(s) encounters as a result of drag forces against the road. Caster angle should be the same for both wheels on a given axle, or the result will be vibration and abnormal tyre wear.

Too much caster will more than compensate for the amount of drag, but it will also create additional difficulty in steering. Too little caster makes steering become lighter, but also unstable. The caster angle should be checked as it can be distorted by impacts on the tyre or by driving in rough conditions.

■ The Ackermann Principle shows that, in any turn, the inside tyre needs a sharper turn angle than the outside tyre
■ The difference in turn angles between the tyres is determined by the actual turn angle and the vehicle wheelbase
■ Improper Ackermann causes side force, excessive scuffing and fast or irregular wear

Thrust Angle
■ Thrust angle is the difference between the line perpendicular to the axle and vehicle centre line
■ Each drive axle has its own thrust angle
■ The target is to have zero thrust angle

Tandem Scrub
■ Tandem scrub is the difference in the thrust angles of the drive axles
■ The target is zero
■ Tandem scrub errors cause constant side force on the steer tyres, which leads to irregular wear

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