Technical page: Suspension and steering

Wheel tracking geometry

Wheels are three-dimensional items and so they can turn in many positions. Their relative position to the car or tracking depends on four different angles.

In the images, the angles have been increased to be easily seen. The red arrows point to the outer of the car (right or left sides), the green ones always to the front of the car and the blue ones to the top of the car.

Caster angle (a) Angle between the wheel orientation axis and a vertical plane in the right-left direction. In McPherson struts it's given by the shock absorber position, so not easily adjustable. Sometimes it's modified by moving the lower suspension arm silentblocks. Value: 1°30' - 3°30' for underbody heights between 50 and 130 mm. Sign: The caster angle is positive when the bottom of the shock absorber is closer to the front of the car. Effect: It gives stability to the steering when straight, and makes it return to the centered position. Less caster angle makes the car turn better into corners, but has an effect of understeer when exiting. It also makes the steering more 'lively' in bumps.

Camber angle (b) It's given by the wheel inclination with a vertical plane in the front-rear direction, and even not being easily adjustable, it's sometimes modified by working on the suspension strut holes used to bolt the stub axle carrier. Value: -1° ±30'. Sign: The camber angle is positive when the top of the wheel is closer to the outer part of the car. Effects: Positive values were used to keeps the wheel pressed to the axle and reduce the forces in the hubs, but nowadays negative values are used to increase traction in bends and stability.

Steering axis inclination (c) Angle between the wheel orientation axis and the vertical plane in the front-rear position. In McPherson struts it's given by the shock absorber position, not easily adjustable and seldom modified. Value: 13°10' ±30' Ph1 / 9° ±30' Ph2. Sign: The steering axis inclination is positive when the bottom of the shock absorber is closer to the outer part of the car. Effects: It can compensate the lack of camber angle. It makes the steering easier to turn and, like caster angle, eases the wheel returning to the centered position. It also compensates straight deviation.

Toe-out (d) Angle between the vertical planes in the front-rear direction on each wheel. It's adjusted by the steering track rods length, and so it's the only easily adjustable angle in a wheel alignment routine procedure. Value: 0°10' ±10'. Sign: The toe-out is positive when the front of the wheels is farther away than the rear. Effects: It compensates the front wheel tendency to close when the rotation torque of the engine is applied to them. It lightens the steering, making it more aggressive, but centering them becomes more difficult.

Wheel decoding

Wheels are identified with some number, for example for the standard GTT one, of the kind:

The wheel width (5.5) and diameter (13) are given in inches. The rim shape (J) is a code and the offset (36), in mm, that is usually NOT written, is the distance between the central plane that would divide wheel tyre width in two equal parts and the parallel plane of the hub surface. A positive number here means that the hub surface is more external than the middle wheel plane, so as ET decreases and even becomes negative the track width increases.

Renault wheels (included the GTT standard one) usually have ET 36 mm. The Cup version wheels enlarged 40mm the track width being the same width (5.5 B 13) because they had ET 16 mm.

Another data in the wheel is the bolt data or PCD. For the GTT:

The bolt location is the diameter of the circle made with the bolt centers, in mm. The last parameter that you can specify on a wheel is the centre bore diameter. It's 60.1 mm for the GTT hubs (And Renault in general).

Tyre decoding

Tyres are also identified by a number, for example for the standard GTT tyre, like:

The tyre width (175) is given in mm, while the inner tyre diameter (13) is given in inches. The tyre profile (60) is specified as the % of the tyre width. The speed (H) codes are chosen from a table:

The charge (76) codes are also chosen from a table: