Self aligning torque, also known as aligning torque, aligning moment, SAT, or Mz, is the

torque In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of th ...

that a

tire A tire (American English) or tyre (British English) is a ring-shaped component that surrounds a Rim (wheel), wheel's rim to transfer a vehicle's load from the axle through the wheel to the ground and to provide Traction (engineering), t ...

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 In vehicle dynamics, slip angle or sideslip angle is the angle between the direction in which a wheel is pointing and the direction in which it is actually traveling (i.e., the angle between the forward velocity vector v_x and the vector su ...

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

and the distance behind the wheel centre at which that force acts. This distance is known as the

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. More technically, it is the distance that the resultant force of side-slip occurs behind th ...

. The steering torque around a non-vertical steer axis with non-zero mechanical trail is given by : (trail + pneumatic trail) · cos( caster angle) · ''F''_''y''. Even if the slip angle and

camber angle Camber angle is one of the angles made by the wheels of a vehicle; specifically, it is the angle between the vertical axis of a wheel and the vertical axis of the vehicle when viewed from the front or rear. It is used in the design of steering a ...

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 depends on many variables) and falls to zero as the tire reaches its maximum lateral force capability.

References

*Milliken and Milliken "Race Car Vehicle Dynamics" Chapter 2 {{Tires, state=collapsed Automotive safety Tires Motorcycle dynamics

See also

References