Wheel Alignment

WHEEL ALIGNMENT THEORY Heavy Vehicles Basic Alignment Theory and Concepts

What is wheel alignment?

Wheel alignment is a broad based term which covers many types of vehicles and suspensions, all with their own applications and uses. On heavy goods vehicles there are two systems used:

In both cases wheel alignment remains the same. Wheel alignment is the process by which all wheels on a common chassis are measured and aligned or positioned correctly. There are 4 main angles or settings that are critical to tyre or tread life. These settings are also responsible for the vehicle's road holding and handling.

The important angles to align are as follows:

  • Caster Angle

    Caster is responsible for directional stability and returnability of the vehicle.
    Caster is the forward or rearward tilt of the steering axis (IE: the king pin).

    Positive caster is when the top of the steering axis is tilted rearward.
    Negative caster is when the top of the axis is tilted forward.

    With caster, when the steering is turned the vehicle's weight is being lifted as the spindle (stub axle) will be travelling up or down as the steering is turned. lt is this weight that returns the steering to straight ahead when the turn is completed.

    lf a vehicle has excessively high caster the steering will be hard. However with power assisted steering this is not felt.

    lf a vehicle has little or no caster it will tend to wander on the road and will make the steering wheel more sensitive to road surface imperfections.

    Caster is on some axles built into the beam itself. The end of the axle is cast and machined at the required angle. On other axles where the caster is not built in the caster is adjusted by fitting taper or wedge shims between the axle and the spring pack. This tilts the axle creating the angle or spec required.

    NB! When removing and refitting a front axle it is very important that any caster wedges/shims are replaced the same way that they were so that the caster remains correct.

    Caster is not a tyre wearing setting on its own. However in conjunction with Camber this can cause abnormal wear.

  • Camber Angle

    What is Camber?
    Camber is the inward or outward tilt of the wheel at the top.

    Positive camber is when the top of the wheel is tilted out.
    Negative camber is when the top of the wheel is tilted in.
    Zero camber is when the wheel is vertical with no in or outwards tilt.

    Camber is normally specified positive. Positive camber assists with road handling and transfer of the vehicle weight to the inner wheel bearing. lf camber is too much positive this will result in dropped shoulder wear on the outer edge of the tyre. Too much negative camber will result in dropped shoulder wear on the inner edge of the tyre.
    More than 1/2 a degree difference in camber from side to side can cause the vehicle to pull to the more positive camber side.

    Camber is a tyre wearing angle if out of the required specification. However camber is not adjustable on solid l-Beam front axles. Axle camber correction can only be carried out at a wheel alignment workshop.

    Camber can be effected by severe play in the king pin bushings, loose wheel bearings or by a bent spindle (stub axle).

    NB! Incorrect tyre sizes or pressures can give a false impression of positive or negative camber. These factors should be taken into consideration before sending a vehicle for an alignment.

  • Toe Angle

    Toe is the difference in measurement between the front and rear of the tyres.
    Toe out or negative (-) toe is when the front of the wheels are further apart than the rear.
    Toe in or positive toe (+) is when the rear of the tyres are further apart than the front.
    Zero toe (0) is when the wheels are parallel to each other.

    A vehicle with toe out or negative (-) toe will wear the tyres on the inner edge similar to negative camber but will have a saw tooth effect across the tyre as well.

    A vehicle with toe in or positive (+) toe will show similar wear to a positive camber wear but also will have a saw tooth effect across the tyre .This can easily be felt by running your hand across the tyre one way and then back.
    The tyre will feel smooth in one direction and rough in the other direction. With this method you can determine whether you have a toe or a camber problem.

    On heavy vehicles a common source of toe problems is bent track rods or tie bars. Vehicles are jacked up on the track rod or the vehicle gets stuck and a rope or chain hooked over the track rod to recover the vehicle, damaging the track rod in the process.

    NB! When replacing king pins or tie rod ends it is critical to have the wheel alignment set before sending the vehicle out. Severe toe settings can destroy a tyre within a very short time.

  • Toe Out on Turns

    When a vehicle makes a turn "even though they are set to a zero toe" the wheels have to follow different arcs.
    The wheel on the inner side of the corner taken, has to turn sharper than the wheel on the outer corner. Without this setting tyres will scuff during cornering and handling will be affected.

    The turning angle difference is determined by the steering arm design. This is not adjustable. lf a problem does occur with this setting there is a possible bent steering arm on the vehicle or the wheelbase has been changed drastically.

  • Rear Axle Alignment

    Wheel alignment does not only concern the front wheels of any vehicle. The rear axles have just as much importance when checking alignment.

    lf the rear axle does not track correctly with the front axle the vehicle will crab to one side. The driver then has to compensate by turning his steering wheel to the opposite side to maintain a straight line. This is tiring for the driver and creates unnecessary wear on the suspension components. The risk of a jack knife is greater in the event of an emergency stop. Rear axle alignment also affects drive line angles.

    Rear axle alignment is determined by the torque rods between the axle and the chassis. Adjustment can be made by placing shims of the required thickness between the torque rod and the hanger bracket as necessary. On air suspension fitted vehicles the alignment can go out if the locating dowel between the spring blade and axle is damaged or broken. This normally caused by an impact on the wheel or by loose u bolts.

    If the toe or camber is found to be out on a rear axle, this indicates that the axle is bent and must be either straightened or replaced.

    NB! If replacing rear torque rods/bushes or a spring blade or pack wheel alignment must be reset before sending the vehicle out.

  • Trailer Alignment

    Perhaps one of the most frequent problems concerning tyre wear on truck tractors is caused by mis-aligned trailer axles.

    If a trailer axle is out of alignment this will cause the truck to pull to one side. The driver will compensate to the other side. He is then effectively driving straight but with his front wheels slightly turned in a counter steer.

    This will cause tyre wear very similar to toe wear however both tyres will wear in the same direction. Most trailers go out of alignment to the left side as this is the short or tighter corner. Therefore the left side of the trailer axles will get most of the impacts from curbs, road edge etc. This will over time knock the trailer alignment out to the left side.

    Trailer alignment should be set whenever the suspension is worked on or at least twice a year.

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