Toe & Total Toe
Toe is the angle of the wheels in relation to the vehicles centreline (an imaginary line straight down the centre of the vehicle, when viewed from above).
Just as the name suggests, imagine the pair of wheels (front or back wheels) as a pair of feet.
When you turn your toes inwards and your heals out, this is what is know as Toe-in . And yes, you’ve guessed it; when you turn your toes out and heels in, this is Toe-out .
When this happens to wheels, you end up wearing the edges of your tyres excessively:
- Toe in or Positive Toe causes your car to wear the outside edges
- Toe out or Negative Toe your car will wear the inside edges
Individual toe is measured from the vehicle centre line. Individual toe can be thought of as a ‘per wheel’ value.
Total Toe is the sum of two individual toe angle readings added together.
Think of Camber as the amount of ‘tilt’ of the wheel, one way or the other.
- Positive Camber is when the top of the wheel leans away from the car
- Negative Camber is when the wheel leans in at the top
The amount of tilt doesn’t have to be much and not always noticeable to the eye, but it is enough to cause undue tyre wear and make the car pull
The easiest way to understand Caster is to look at a bike (when viewed sideways on).
If you draw a line down from the centre point on the handlebars down the forks to the ground (on a car this would be the suspension strut), this indicates the ‘pivot point’.
Then draw a second line vertical through the centre of the wheel and where this touches the ground shows the ‘tyre contact point’.
The top angle that has been created is what is known as the Caster Angle.
If this top angle is reduced then stability is less, therefore it is important to get the Caster set to its optimum angle for the best possible control.
Is literally just that! An imaginary line running down the centre of the car, from the front to the rear and equidistant from the sides.
Thrust angle is the direction the rear wheels are pointing in relation to the centre line. If the thrust angle is not zero the vehicle will ‘crab’ – move sideways from the back!
This is simply to ensure the steering wheel is set straight when the vehicle is travelling along a straight and flat road
Other Phrases You May Hear
Often overlooked and often resulting in vehicle pull.
Cross camber is the difference in camber from one side to the other, if each wheel is itself correctly in tolerance, but at opposite ends of the scale, then the cross camber will be high and might need attention. Some manufactures now specify this value.
The car will tend, as a general rule, to pull to the side with the least negative or most positive camber.
SAI & Included Angle
Steering Axis Inclination (SAI) is the inward or side to side tilt of the suspension strut top towards the centre of the vehicle, and in all cases is positive.
SAI is not altered in the aftermarket, only returned to the factory setting, perhaps following accident damage. SAI gives driver feel to the steering and assists in unwinding the steering after steering lock has been applied, bringing the steering back to the straight ahead.
The Included Angle is calculated by adding the SAI and Camber Angle together.
Set back is defined, where one of the front wheels is sitting back (relative to the wheel on the other side).
- Negative setback is the left wheel back (relative to the right)
- Positive setback is the right wheel back (relative to the left)
Set back would be identified when measuring the SAI and caster angles as the values would be too low and out of specification. Which side is set back or set forward can be established by reviewing which wheels castor angle is within specification.
The height at which the car is from the ground and essentially is measured from the ground to the underside of the car. Although some motor manufacturers ride hide height stipulations vary, eg BMW specify (for some models) from bottom of wheel rim to top of wheel arch.
This is to check there is the same ride height around the car, as set in accordance to the manufacturers specifications and ultimately an accurate alignment measurement.
WEIGHTING / LOADING – A number of cars need weights to be loaded into the car to meet the car manufactures requirements – e.g. Simulating the average weight of driver and passengers and/or load carried. Weights can also be used to help achieve the specified ride height.