The axis around which the wheel rotates as it turns to the right or left is called the “steering axis”. This axis is found by drawing an imaginary line between the top of the shock absorber’s upper support bearing and the lower suspension arm ball joint (in the case of strut type suspensions). This line is tilted inward as viewed from the front of the car and is called “steering axis inclination”( S.A.I) or “kingpin angle”. This angle is measured in degrees. Furthermore, the distance “L” from the intersection of the steering axis with the ground to the intersection of the wheel centerline with the ground is called the “offset”, “kingpin offset” or “scrub radius”.
Roles of Steering Axis Inclination
1. Reduction of steering effort Since the wheel turns to the right or left with the steering axis as its center and the offset as the radius, a large offset will generate a great moment around the steering axis due to the rolling resistance of the tire, thus increasing steering effort. This offset can be reduced in order to reduce the steering effort. Either of the following two methods can be used to make the offset small:
(1) Give the tires positive camber
(2) Incline the steering axis.
2. Reduction of kick-back and pulling to one side If the offset is excessively large, the force due to driving or braking generates a moment around the steering axis whose magnitude is proportional to the amount of offset. Also, any road shock applied to a wheel will cause the steering wheel to jerk or kick-back. These phenomena can be improved by reducing the amount of offset. If there is a difference between the left and right steering axis inclination angles, the vehicle will typically pull to the side of the smaller angle (having the larger offset).
3. Improvement straight-line stability The steering axis inclination causes the wheels to automatically return to the straight-ahead position after the completion of turning.
In front engine, front-wheel-drive cars, the offset is generally kept small (zero or negative) to prevent the transmission to the steering wheel of shock from the tires generated during braking or by striking an obstruction, and to minimize the moment created around the steering axis by the driving force at the time of quick starting or acceleration.
If there is a difference between the steering angle on the left and right, there will also be a difference between the moments around the steering axis on the left and right during braking and the braking force will be greater on the side with the smaller steering angle. Also, any difference between the left and right offsets generates a difference in the drive reaction force (torque steer) on the left and right. In either case, a force acts that attempts to turn the vehicle.