(nodeshell rescue)

When turning the steering wheel of a car, and then letting it go, the wheels turn back into a straight orientation. This is due to the way the front wheels are attached to the car: the kingpin and castor are inclined in such a way that when the front wheels are turned, the front of the car is slightly elevated. Thus, the weight of the car forces the wheels in a straight position when the steering wheel is free. However, the axis of rotation of the kingpin, when prolongated to the ground, does not in general intersect the point of contact of the wheel with the ground. This is what is called the steering offset.

In a front wheel drive car, when applying torque to the wheels, there is a force causing them to turn around the kingpin axis, which is proportional to the steering offset. Normally, this force is the same on both front wheels but on opposite directions, and since they are linked, it is cancelled out. However, if the friction is not the same on both wheels, which is in general the case when making a turn, then the torque steer does not cancel out. Since more weight is put on the wheel at the exterior of the turn, friction is higher on it and torque steer forces it to turn even more in the direction of the turn. Torque steer can also happen when braking.

  (View from above)

  kingpin   /|||||
      axis / ||||| 
          O  |||||
           \ |||||
  movement   ||||| wheel
  of the 
  wheel     /  |
        ---/   |
               V  force direction

It is to be noted that the kingpin inclination is not the cause of torque steer: without it (that is, if the kingpin axis is perpendicular to the ground), the steering offset is even greater, and no return to the straight position is possible. Augmenting the kingpin angle so that the steering offset is reduced is a solution to reduce torque steer, but then the shock absorbers transmit lateral forces to the car's chassis, thus causing it to shake from side to side and thus leading to instability.

 (Front view)

    |||||      / kingpin axis
    |||||     /
    |||||   /  |
    ||||| /
      |  |
      |  |
 steering offset