Simply put, wheel alignment refers to the adjustment of the wheels angle perpendicular to the ground and parallel to each other. Well aligned wheels will ensure stability and a vehicle that tracks straight along a flat road. To ensure maximum stability and tyre life, wheel alignment must be adjusted relatively frequently and can be affected by bad road surfaces or a hard knock. Below, we have listed some factors of relating to wheel alignment and their effects.
Camber: This is the angle of the wheel, measured in degrees, when viewed from the front of the vehicle. If the top of the wheel is leaning out from the centre of the car, then the camber is positive; if it’s leaning in, then the camber is negative. If the camber is out of adjustment, it will cause tyre wear on one side of the tyre’s tread. If the camber is too far negative, for instance, then the tyre will wear on the inside of the tread.
Caster: Turning the steering results in the front wheels turning around a pivot. Castor is the angle of this steering pivot when viewed from the side of the vehicle and measured in degrees. If the top of the steering pivot leans rearward, it has a positive caster and the opposite is true for a negative caster. Poor adjustment will cause the vehicle to track toward the side which has a less positive caster and if the caster on both sides is too negative, the steering will be light and the vehicle will wander around its lane. A positive castor will result in heavier steering that may kick over bumps.
Many front-wheel drive cars have non-adjustable caster and it has little effect on tyre wear. If the caster is not aligned properly, it may indicate a crash, or heavy knock, and bent parts must be replaced.
Toe-in/out: Toe is the measurement between the front and rear of the two front tyres. Viewed from the top, toe-in refers to the front of the wheels pointing slightly inwards while toe-out implies the opposite. Excessive toe in or toe out will cause accelerated tyre wear called feather wear. Rear toe angles are sometimes adjustable on a vehicle’s rear wheels too.
Steering Axis Inclination (SAI): This is the angle of the steering pivot when viewed from the front. The SAI angle along with the camber angle combine to create the included angle which causes the vehicle to lift slightly when steering. The vehicle’s own weight will encourage the wheels to go straight again, re-centering the steering. If the SAI is different on one side, it will cause the vehicle to pull to one side at very slow speeds.
Included Angle: The angle formed between the SAI and the camber. The included angle is not directly measurable. To determine the included angle, you add the SAI to the camber. If the camber is negative, then the included angle will be less than the SAI; if the camber is positive, it will be greater. The included angle must be the same from side to side even if the camber is different. If it is not the same, then it is most likely that the steering knuckle is bent.
Ride height: Changes in ride height will affect camber and toe, so that if springs are replaced or torsion bars are adjusted, the wheel alignment must be checked to avoid the possibility of tyre wear. It is important to note that the only symptom of weak coil springs is a sag in ride height.
Thrust Angle: The direction that the rear wheels are pointing in relation to the centre line of the vehicle. If the thrust angle is not zero, then the vehicle will “dog track” or ‘’crab’’, and the steering wheel will not be centered. An example of this may be an old bus or truck with a misaligned rear axle, driving forward with the entire vehicle at an angle. The best solution is to adjust rear toe during a four-wheel alignment check, but a bent or misaligned axle may also be a cause.