Below is some very useful information explaining the different wheel alignment terms and angles and what they are all about.

Toe in and Toe out (front and rear)

Toe-in and toe-out are generally the most understood of all the wheel alignment terms. When measuring toe we are looking at how parallel the wheels are in relation to each other, when viewed from above.

If the wheels are closer together at the front then positive (+) or toe-in is apparent. If the wheels are closer together at the rear then negative (-) or toe-out is apparent.

The reason why we have toe-in or toe-out is that the position of the wheels change between the vehicle being stationary (static) or moving (dynamic). This movement is due to flexing in the suspension mountings when the vehicle is in motion.

Toe-in or toe-out can normally only be set when the vehicle is stationary, therefore, this movement must be taken into account.

Generally, for rear wheel drive vehicles the front wheels are set with a small amount of toe-in. This is because the vehicle is being 'pushed forward', which flexes the front wheels backwards or out, on their mountings, as the vehicle is being driven. This causes the front wheels to run paralles in the moving or dynamic condition.

Likewise, front wheel drive vehicles generally have the front wheels set with a small amount of toe-out. This is because the vehicle is, in effect being 'pulled along'. The front wheels flex forward on their mounting to a parallel position. 

Incorrect toe can cause the following :

• Wear on the outside edge of the tyre : incorrect toe-in
• Wear on the inside edge of the tyre : incorrect toe-out
• 'Feathering' across the tyre thread
• Mis-aligned steering wheel
• 'Poor' handling
• Increased fuel consumptin due to tyre friction

Possible causes of incorrect toe :

• Incorrect adjustment
• Incorrect vehicle specifications used
• Incorrect ride height
• Bent steering components (track rods/steering arms)
• Wrongly calibrated alignment equipment

Note : Toe is always the last angle to be adjusted.

Toe-out-on-turns

When a vehicle travels around a corner, the wheel on the inside of the turn has a tighter turning circle or angle than the wheel on the outside. If the wheels had the same angle they would try to converge. This would cause excessive tyre wear and poor handling due to understeer. By making the inner wheel turn through a greater angle than the outer wheel, a much smoother turn will be possible, leading to improved handling and reduced tyre wear. The difference in angles is achieved by inclining the steering arms towards the centre line of the vehicle.

Incorrect toe-out-on-turns can cause the following :

• Excessive tyre wear : 'feathering'
• Tyre squeal on turns, even at low speeds
• Unresponsive steering,  i.e. understeer

Possible causes of incorrect toe-out-on-turns :

• Incorrectly adjusted toe / individual toe
• Bent steering arms
• Bent track rod arms

• Incorrect ride height

  

Camber

Camber is the inward or outward angle or lean of the wheels as seen from looking at the front of the vehicle and is defined as follows :

• Camber is positive (+) when the wheel leans out at the top
• Camber is negative (-) when the wheel leans in at the top
• Zero (0) camber is when the wheel is vertical

Camber applies to both front and rear wheels of the vehicle.

Camber is used to distribute the vehicle load evenly across the tyre / road contact point, in order to minimise tyre wear. Camber can also be used to alter the handling characteristics of a vehicle, sometimes at the expense of tyre wear. Negative camber can improve cornering, by ensuring a good tyre / road contact point during cornering.

Incorrect camber can cause steering pull, as well as excessive tyre wear. Generally, there should be no more than 30' (minutes) between the left and right side camber angles of a vehicle. Also, a vehicle will tend to pull to the side with the hightest positive or least camber.

Incorrect camber can cause the following :

• Wear on the inside edge of the tyre : excessive negative camber
• Wear on the outside edge of the tyre : excessive positive camber
• Pulling to one side
• Excessive wheel bearing wear (uneven load)
• 'Poor' handling and steering

Possible causes of incorrect camber :

• Bent stub axle
• Bent MacPherson strut / hub or wishbones
• Incorrect adjustment
• Incorrect ride height

Castor

Castor is used to give straight-line stability and directional control at vehicle speed. Castor also assists self-centring of the steering after a steering turn has been made. Castor can simply be described as the backward or forward tilt of the MacPherson strut or hub assemble when the vehicle is viewed from the side. Positive (+) castor is when the strut top leans towards the rear of the vehicle and is most commonly used on modern vehicles.

Negative (-) castor is when the strut top leans towards the front of the vehicle. Positive castor provides better directional stability and self-centring action than negative castor. Castor is achieved by placing the theroetical pivot point in front hte the actual tyre contact point. Therefore, if the wheel is steered to one side it will always want to return back to the straight-ahead position.

If you were to look at the front wheel and forks of a bicycle you would notice that a positive castor is formed between the two. It is this positive castor that enables the bicycle to be ridded without touching the handlebars. If negative castor is introduced the wheel has no point to follow, causing wander and no self-centring action.

The more positive the castor angle, the heavier the steering will become.

Because most modern vehicles are capable of high speeds, a positive castor angle is required to enable high speed stability. Power-assisted steering is therefore commonly fitted to overcome heavy steering. Generally, castor angles from the left to right side of a vehicle should not vary by more than 30' (minutes) otherwise the vehicle may pull to one side. The vehicle will pull to the side with the least angle.

Incorrect castor can cause the following :

• Heavy steering : excessive positive
• Road bumps / shocks felt through the steering : excessive positive
• Steering vibrations / shimmy : excessive negative
• Lack of steering self-centring : excessive negative

Possible causes of incorrect castor :

• Top MacPherson strut mounting moved backwards or forwards
• Bottom suspension arm or mounting moved backwards or forwards
• Vehicle not checked on a level surface
• Incorrect vehicle inclination
• Wheel not locked during alignment check
• Incorrect adjustment (if adjustable)

Kingpin Inclination

Kingpin Inclination (KPI) can also be know as Steering Axis Inclination (SAI) - an American term commonly used on some wheel alignment equipment. KPI is the angle that is formed between a verticle line and the inward tilt of the MacPherson strut or hub assemble when viewed from the front of the vehicle. KPI is normally a built-in angle, by the manufacturer and is non-adjustable. However, some manufacturers utilise the change of angle of KPI for the benefit of adjusting camber. It is common for many manufacturers not to give KPI angles. However, the angles should be approximately equal on both side. KPI is always positive as it always tilts inwards.

KPI together with camber can greatly reduce or eliminate the leverage effect on the steering, contributing to steering stability. If no KPI angle was given, road shocks, bumps and uneven braking would cause the wheel to turn about the moment A - B, making the vehicle unstable and causing violent steering pull. Excessive KPI angles can load components such as ball joints. KPI also aids self-centring of the vehicles steering, since the vehicle tends to be lifted as the front wheels are turned. The weight of the vehicle thus tends to make the front wheels run straight ahead. As can be seen from the diagram, changing a wheel to that with a different offset can have serious consequences, by changing the leverage effect A - B.

Incorrect KPI can cause the following :

• Excessive road feedback
• Reduce steering self-centring
• Excessive suspension components wear
• Incorrect camber angle
• Vehicle going off-line when encountering road shocks and bumps

Possible causes of incorrect KPI :

• Lower wishbone arm bent or location point moved in or out
• Top wishbone arm bent or location point moved in or out

Scrub Radius

When compared at ground level, the distance between the KPI line (drawn through the steering pivots) and the tyre centerling is called the Scrub Radius. When the scrub radius is towards the inside of the tyre thread the vehicle has a Positive Scrub Radius. When the scrub radius is towards the outside of the tyre thread, the vehicle has a Negative Scrub Radius.

Note : Scrub radius along with front and rear wheel drive configurations determines the requirement for Toe-in or Toe-out.

Thrust Angle

The rear suspension of a motor vehicle has a great influence on the vehicles handling and ride quality. Excessive rear tyre wear can also result from incorrect rear toe and camber settings. The only two measurements that can normally be checked on the rear of the vehicle are toe and camber. On an increasing number of modern vehicles it is possible to adjust the rear toe, and sometimes camber. If the rear wheel toe on a vehicle is incorrect it will have a steering effect on the vehicle, thus causing the driver to compensate by turning the steering wheel in order to make the vehicle travel in a straight line. This is known as 'crabbing'. Incorrect rear toe and uneven rear toe can negatively influence the vehicle's thrust angle, which ideally should be zero, or as close to this as possible.

Incorrect thrust angle can cause the following :

• Mis-aligned steering wheel
• Excessive rear tyre wear : 'feathering'
• 'Poor' vehicle handling, 'nervous' feel
• Understeer or oversteer
• Vehicle 'crabbing'

Possible causes of incorrect thrust angle :

• Rear individual toe incorrectly set
• Rear suspension bent or damaged
• Rear subframe mis-aligned
• Vehicle body mis-aligned
• Rear suspension mountings worn 

Setback

Setback is the term that describes the position of the right wheel in relation to the left wheel. Setback can apply to the front or rear of the vehicle. Positive (+) setback occurs when the right wheel is to the rear of the left wheel. Likewise negative (-) setback is when the left wheel is to the rear of the right wheel. Generally, setback will not cause problems affecting steering or handling as long as it is not excessive. 

Included Angle

Included Angle is the sum of the angles of KPI and camber. Some wheel alignment equipment will calculate included angle. Included angle together with KPI is very useful for determining bent or damaged suspension parts. Generally, the difference in included angle between the left and right side of the vehicle should not exceed 0.5° to 1.0°.