Suspension Technology Glossary
Suspension, wheel alignment and bushes come with many technical terms. This glossary explains them briefly and clearly.
Wheel alignment
- Camber
- Angle between the wheel centre plane and a vertical line to the road, in degrees. Negative camber (top of the wheel leaning inwards) increases cornering force but causes inner-edge tyre wear. Road cars usually run 0 to -2°. Camber is a “tyre-wearing angle”. On double wishbone axles it can be adjusted via the upper arm; on MacPherson axles via slotted holes (usually only about 1.5°).
- Caster (static and dynamic)
- Angle of the strut or steering axis to the vertical (side view). It determines self-centring: more caster means heavier but more stable steering, more feedback and more negative camber on the outer wheel when turning. Higher caster delays understeer but can lead to earlier oversteer. Dynamic caster cannot be measured in an alignment: in corners the rear control arm bush twists towards the vehicle centre and caster is lost – softer bushes amplify this. Stiff polyurethane bushes and eccentric bushes counteract it.
- Toe (toe-in / toe-out)
- Angle of the wheels to each other seen from above. Toe-in: the front edges point slightly inwards; toe-out: outwards. Too much toe-in wears the outer tread edge, too much toe-out the inner edge. Toe influences straight-line stability and turn-in.
- Steering axis inclination (SAI)
- Angle between the steering pivot axis and a vertical line through the wheel centre (front view). Not adjustable; used for diagnosis and helps self-centring. Typically below 10° on rear-wheel drive and above 10° on front-wheel drive.
- Included angle
- Sum of camber and SAI. A diagnostic value only: if it is off, a component is bent or the suspension is damaged.
- Scrub radius
- Distance between the point where the extended steering axis meets the road and the tyre centre. Positive: the axis meets below the tyre centre (typical for rear-wheel drive). Negative: above it (typical for front-wheel drive), which reduces steering kickback and improves braking stability.
- Toe-out on turns (Ackermann)
- The inside wheel must turn more than the outside wheel. If the geometry is wrong, tyres scrub and squeal in turns.
- Thrust angle / set back
- The thrust angle describes the direction of the rear axle relative to the vehicle centreline and is corrected via rear toe. Set back (wheelbase difference) points to a suspension problem.
Suspension types
- MacPherson strut
- A lower wishbone plus strut connect wheel carrier and body. Bushes can influence geometry only via the rear bush of the lower arm (caster); camber via slotted holes or adjustable top mounts.
- Double wishbone
- Upper and lower arms offer more adjustment: camber via eccentric bushes or length-adjustable upper arms, caster as with MacPherson via the rear bush of the lower arm.
- Roll centre
- The imaginary point around which the body rolls in corners. Lowering moves it down and the car rolls more. Ball joints with a longer stud bring the arms back to a more parallel position and raise the roll centre again.
- Bump steer
- Unwanted steering of the wheels as the suspension moves up and down without body roll. Causes include unsuitable tie-rod position, bent steering parts, incorrect toe with an off-centre steering rack, or soft or worn bushes. Bump steer kits with modified tie rod ends and ball joints fix it.
Bushes and materials
- Polyurethane and rubber
- Rubber is comfortable but relatively soft and sensitive to oil, petrochemicals and UV. Polyurethane is harder, deforms less and lasts much longer – about 5 to 6 times as long with SuperPro.
- Polyether polyurethane
- SuperPro uses polyether-based polyurethane. Polyester polyurethane, used by many others, absorbs water and hardens over time – up to crumbling. Polyether keeps its hardness.
- Shore hardness
- Measure of hardness for elastomers after Albert F. Shore, here Shore A. Control arm bushes are typically made in 80 Shore (road) and 90 Shore (track). Values from different makers are only partly comparable.
- Progressive material
- Restoring force rises with applied force. Comfortable under low load, very direct under high load.
- NVH
- Noise, vibration, harshness – typical drawbacks of inferior polyurethane bushes. Products not marked “RACE” in the catalogue are designed to avoid NVH.
- Eccentric bush
- A bush with an offset bore that changes camber, toe or caster by a fixed amount. The greater the offset relative to the diameter, the greater the effect. Continuous adjustment is not possible.
- Crush tube
- Steel tube pushed through the bush, against which the arm bolt clamps. Fitted with grease.
- Duroball
- Hybrid bush of metal shell, polyurethane and uniball joint; pivots very freely and is available with centred or eccentric bore.
- Chemlock
- Process that chemically bonds polyurethane to metal (e.g. steel shell, uniball).
- SuperPro grease
- Transparent, silicone-based grease developed specifically for SuperPro polyurethane. It does not dry out and must be used on all contact surfaces between polyurethane and metal.