The distance from the ‘Steering Axis’ to the centre of the tyres’ footprint on the ground is called ‘Trail’ or ‘Caster Offset’.
The larger the trail, the more steering torque is needed.
Under acceleration or braking Trail can be broken down into:
- ‘Mechanical Trail’
- ‘Pneumatic Trail’
‘Mechanical trail’ is set by the dimensions of the upright and Caster angle.
‘Pneumatic trail’ is due to distortion of the tyres (Longitudinal Slip).
Mechanical trail maybe small compared to the Pneumatic trail from the tyre.
Pneumatic trail will approach zero as the tyres reach the slip limit. As this happens there will be a loss of self-centering torque, signalling to the driver that the tyres are about to break traction. Ideally this break in traction will be gradual. If the mechanical trail is large compared to the pneumatic trail this “breakaway signal” may be lost and the breakaway will happen appear to happen much more sharply.