Item | Function | Effects If Incorrect |
|---|
Caster The forward (-) or backward (+) tilt of spindle support (ball joints or king pin) at top. Positive caster is the rearward tilt of the steering axis from a true vertical as viewed from the side of the wheel. | Positive caster gives front wheels tendency to maintain straight ahead position and return to straight ahead in turn. Positive caster can be used to enhance stability. Uneven caster will cause car to pull to side of least caster. Negative caster can be used to reduce steering effort. Negative caster can be used to enhance ride.
| Can cause vehicle to pull to one side. Can cause vehicle instability and to wander and weave. Instability at high speeds and braking. High effort steering. Uneven braking.
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Camber The inward (-) or outward (+) tilt of the wheel at top. Positive camber is the outward tilt of the wheel at the top from a true vertical line as viewed from the front of the wheel. | Projects vehicle load towards the center of the tire. Provides easier steering by having the weight of vehicle borne by larger inner bearing. Reduces tire wear by allowing for the crown of the road. Reduces vehicle side skidding in a turn. Gives maximum tire tread contact with the road.
| Uneven camber from side to side will cause vehicle to pull to side with most camber. Cause tire wear and scuffing. Vehicle instability.
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Toe-Inward or Outward Pointing of Wheels Toe is defined as when a pair of wheels is set so that their leading edges are pointed slightly towards each other, the wheel pair is said to have toe-in. If the leading edges point away from each other, the pair is said to have toe-out. The amount of toe can be expressed in degrees as the angle to which the wheels are out of parallel, or more commonly, as the difference between the track widths as measured at the leading and trailing edges of the tires or wheels. | Reduces tire scuffing and wear. Ensures tires are pointing in straight line with vehicle when in motion. Enhances vehicle stability.
| Excessive tire scuffing and wear. Vehicle wander. Torque steering on front wheel drive vehicles.
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Steering Axis Inclination (SAI) - Kingpin Inclination The inward tilt of a line through the centre of top of a strut mount or ball joint and the centre of lower ball joint in relation to a true vertical line through the centre of the tire. | Helps project the load of the car towards center of tire. Enhances vehicle stability. Assists in returning the steering to straight ahead position after a turn. Can be used to increase steering effort.
| Adversely effects stability especially in braking. Steering wander. Increased transmission of road shock through steering. Torque steering on front wheel drive vehicles if different from one side to the other.
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Included Angle The included angle is the sum of the steering axis inclination angle (SAI) and the camber angle. A negative camber angle must be subtracted from the SAI to calculate the included angle. | Enhances vehicle stability.
| Difference from one side to the other will affect directional stability. Difference from side to side will affect the steering returning to neutral position.
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Total Toe Total toe is the sum of the toe, whether in or out, of the right and left tires of the vehicle. | Reduces tire wear. Enhances stability.
| Increased tire wear. Vehicle wander. Torque steering on front wheel drive vehicles.
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Setback Setback is the condition in which one wheel is moved rearward in relation to the other. | Ensures that the front wheels track straight with the vehicle chassis and the rear wheels. Steering wheel centered with vehicle direction.
| Tire scuffing and wear. Adverse vehicle handling if extreme.
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Turning angle The turning angle is also known as the Ackerman Angle. This is defined as a turning angle of a vehicle in a corner when the front and rear wheels turn around a common pivot point with respect to the turning radius of each wheel. This is the difference in steering angles of the right and left front wheels in turn. In a turn, the inside wheel must roll around a smaller circle than the outside wheel. Therefore, the inside wheel must be turned more if it is to follow the smaller inside circle. | Ensures the vehicle can negotiate a turn with minimal side slip. Enhances vehicle stability when vehicle is in a turn. Reduces tire wear associated with a turn.
| Tire scuffing in turns. Poor turning characteristics. Excessive side slip.
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Thrust angle The angle between centreline and rear wheel track. | Allows the rear of the vehicle to follow the front. Rear wheels run perpendicular to the front.
| Poor handing. Tire wear.
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