What is a
wheel alignment?
The alignment of the wheels consists in adjusting the angles of the wheels so that they are parallel to each other and perpendicular to the ground, while ensuring that the tracks of the front and rear tires are correctly positioned. Often, a tire shop or mechanic will notice abnormal tire wear and will check the wheel alignment to determine if it is a cause of inappropriate wear
. Sometimes a component of the suspension wears unusually and may even be a sign of an "out of wheel alignment" vehicle. If a customer complains of improper handling or a "tug", the mechanic will check the wheel alignment to see if that is the reason they are worried. Wheel alignment angles can also be changed beyond the manufacturer's specifications to achieve distinct characteristics. Running, moving up and down the vehicle (in appearance or handling) and off-road applications often mean that the different wheel alignment angles will be separated from what is "normal" for the vehicle to meet the customer's needs.
Types of Wheel Alignments
In a two-wheel or front end wheel alignment only the two front whee
ls are aligned. This does not take into account the position of the rear axle. Mechanical gauges are typically used for this type of wheel alignment. Almost the entire modern orientation is designed for full thrust or four-wheel alignment.
Thrust wheel alignment is generally performed on vehicles with a massive rear axle. Vehicles with a fixed rear axle usually have no provision for adjusting the orientation. The wheel alignment angles of the four wheels are read by the alignment equipment, but only the front axle is adjusted. The front wheel alignment angles are parallel to the pressure line of the rear wheel. It's better than two wheel alignment, but there can still be drag and steering problems.
Full four-wheel alignment is performed on independently adjustable rear suspension vehicles on the front and rear axles. Both axles are tuned to the vehicle axle. This ensures that the vehicle follows and the steering wheel is centered. For all types of alignment, the front and rear suspensions may require additional parts to make the appropriate wheel alignment adjustments.
Primary Angles
The primary wheel alignment angles are the basic angle alignment of the wheels relative to each other and to the car body. These adjustments are the camber, caster and toe. On some cars, not all of these can be adjusted on every vehicle.
These three parameters can be further categorized into front and rear (with no caster on the rear, typically not being steered wheels). In summary, the parameters are:
- Front: Caster (left & right)
- Front: Camber (left & right)
- Front: Toe (left, right & total)
- Rear: Camber (left & right)
- Rear: Toe (left, right & total)
Secondary Angles
The secondary wheel alignment angles include serveral other adjustments, such as:
- SAI (Steering Axis Inclination) (left & right)
- Included angle (left & right)
- Toe out on turns (left & right)
- Maximum Turns (left & right)
- Toe curve change (left & right)
- Track width difference
- Wheelbase difference
- Front ride height (left & right)
- Rear ride height (left & right)
- Frame angle
- Setback (front & rear)
Setback is the difference between right side and left side wheelbase length. It can also be measured as an angle. Setback less than the manufactur
er specified tolerance (for example about 6mm) does not affect car handling. That's because, when the vehicle is turning, one wheel is ahead of the other by several centimetres and therefore the setback is negligible. There are even some vehicle models with different factory setting for right and left side wheelbase length, for various design reasons. An off-spec setback may occur because of a collision or a difference between right and left caster.
Rake is the difference between the front ride heights and the rear ride heights, a positive number when the rear ride height is larger.