Self aligning torque
Encyclopedia
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. In the presence of a non-zero slip angle
, this torque tends to steer the tire toward the direction in which it is traveling, hence its name.
The magnitude of this torque can be calculated as the product of the lateral force
generated at the contact patch
and the distance behind the wheel centre at which that force acts. This distance is known as the pneumatic trail
. The steering torque around a non-vertical steer axis with non-zero mechanical trail is given by (trail+pneumatic trail)*cos(caster angle
)*Fy.
Even if the slip angle and camber angle
are zero, and the road is flat, this torque will still be generated due to asymmetries in the tire's construction and the asymmetrical shape and pressure distribution of the contact patch. Typically for a production tire this torque reaches a maximum at 2-4 degrees of slip (this figure is very dependent on a lot of things) and falls to zero as the tire reaches its maximum lateral force capability.
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...
creates as it rolls along, which tends to steer it, i.e. rotate it around its vertical axis. In the presence of a non-zero slip angle
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...
, this torque tends to steer the tire toward the direction in which it is traveling, hence its name.
The magnitude of this torque can be calculated as the product of the lateral 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...
generated at 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...
and the distance behind the wheel centre at which that force acts. This distance is known as the pneumatic trail
Pneumatic trail
Pneumatic trail or trail of the tire is a trail-like effect generated by compliant tires rolling on a hard surface and subject to side loads, as in a turn...
. The steering torque around a non-vertical steer axis with non-zero mechanical trail is given by (trail+pneumatic trail)*cos(caster angle
Caster angle
thumb|250px|θ is the caster angle, the red line is the pivot line, and the grey area is the tire.Caster angle or castor angle is the angular displacement from the vertical axis of the suspension of a steered wheel in a car, bicycle or other vehicle, measured in the longitudinal direction...
)*Fy.
Even if the slip angle and camber angle
Camber angle
thumb|100px|From the front of the car, a right wheel with a negative camber angleCamber angle is the angle made by the wheels of a vehicle; specifically, it is the angle between the vertical axis of the wheels used for steering and the vertical axis of the vehicle when viewed from the front or...
are zero, and the road is flat, this torque will still be generated due to asymmetries in the tire's construction and the asymmetrical shape and pressure distribution of the contact patch. Typically for a production tire this torque reaches a maximum at 2-4 degrees of slip (this figure is very dependent on a lot of things) and falls to zero as the tire reaches its maximum lateral force capability.