Pneumatic trail

[Tony]

We have a reasonable coverage of the Castor trail here in wim so i thought it's time to move onto the pneumatic trail.... This area generally is expressed as the failure of the tyre but rarely observed in a matter of fact format to why traction failed.

 

Pneumatic trail

 

 

The Geometric trail is assisted by the pneumatic trail. The forces that build up in the tyre print during cornering are initially greatest toward the rear of the tyre. At low slip-angles no part of the tyre is sliding on the road. Rather, the circular hoop of tread is skewed relative to the wheel, which allows the wheel to 'crab' sideways.

 

The forces are due to the elastic deformation of the tyre - more sideways deflection at the rear of the tyre, so more force. The vector sum of the distributed tyre-print forces acts behind the centre of the undistorted tyre-print. This distance is the pneumatic trail. As the cornering force increases the rearmost sections of the tyre-print reach their traction limit and start to slide.

 

These sections of the tyre-print are at (or just below) their maximum axial force, while the forward sections are still increasing in force, thus the vector sum of the forces is still increasing, but it's point of action is still moving forward. When the tyre-print is fully sliding forward the tyre-print forces are all equal and the resultant force is approximately at the wheel centre.

 

Tyre behaviour can also be presented as an axial force plus a 'tyre self aligning torque' that both act at the undistorted wheel-print. The tyre self-aligning torque is zero at zero slip-angles (ie. No axial forces), it grows rapidly at moderate slip-angles as the axial forces increase at the rear of the wheel-print, then it decreases to zero at larger slip-angles as the axial forces at the front of the wheel-print catch up to the forces at the rear of the wheel-print.

 

The reduction in pneumatic trail as the tyre approaches peak axial force is one of the main signals to the driver that the front tyres are starting to slide. Manipulation of the aspect ratio and indeed the air pressure can formulate predictable replies from the pneumatic slip-angle.