Pneumatic trail or trail of the tire is a trail-like effect generated by compliant tire

Tire

A tire or tyre is a ring-shaped covering that fits around a wheel rim to protect it and enable better vehicle performance by providing a flexible cushion that absorbs shock while keeping the wheel in close contact with the ground...

s rolling on a hard surface and subject to side loads, as in a turn. More technically, it is the distance that the resultant force

Cornering force

Cornering force or side force is the lateral force produced by a vehicle tire during cornering.Cornering force is generated by tire slip and is proportional to slip angle at low slip angles. The rate at which cornering force builds up is described by relaxation length...

 of side-slip

Slip angle

In vehicle dynamics, slip angle or sideslip angle is the angle between a rolling wheel's actual direction of travel and the direction towards which it is pointing...

 occurs behind the geometric center of the contact patch

Contact patch

Contact patch is the portion of a vehicle's tire that is in actual contact with the road surface. It is most commonly used in the discussion of pneumatic tires, , where the term is strictly used to describe the portion of the tire’s tread that touches the road surface...

.

Causes

Pneumatic trail is caused by the progressive build-up of lateral force along the length of the contact patch, such that lateral forces are greater towards the rear of the contact patch (though less so when the rear of the contact patch begins sliding) and this creates a torque

Torque

Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....

 on the tire called the self aligning torque

Self aligning torque

Self aligning torque, also known as aligning torque, SAT, or Mz, is the torque that a tire creates as it rolls along, which tends to steer it, i.e. rotate it around its vertical axis...

. Because the direction of the side-slip is towards the outside of a turn, the force on the tire is towards the center of the turn. Therefore, this torque tends to turn the front wheel in the direction of the side-slip, away from the direction of the turn.

Variation

Pneumatic trail is at its maximum when the slip angle is zero and decreases as slip angle increases. Pneumatic trail increases with vertical load.

See also

• Bike physics
• Camber thrust
  Camber thrust
  Camber thrust and camber force are terms used to describe the force generated perpendicular to the direction of travel of a rolling tire due to its camber angle and finite contact patch...
  
  
• Castor angle
• Cornering force
  Cornering force
  Cornering force or side force is the lateral force produced by a vehicle tire during cornering.Cornering force is generated by tire slip and is proportional to slip angle at low slip angles. The rate at which cornering force builds up is described by relaxation length...
  
  
• Relaxation length
  Relaxation length
  Relaxation length is a dynamic property of pneumatic tires that describes the delay between when a slip angle is introduced and when the cornering force reaches its steady-state value....