CASTER

Caster

When the top of the kingpin is tilted slightly toward the rear of the vehicle, this inclination is referred to as caster. With caster, the centre line of the tyre is behind point where an extended centre line of the kingpin intersects with the road surface. Therefore, as the front wheels are encountering a rolling resistance to forward motion, the wheels are forced to trail behind the kingpin centre line, and automatically line up in the straight-ahead position of the vehicle. Due to this caster effect, the front wheels always return to the straight-ahead position after making a turn (known as recovery) and remain in this straight-ahead positive even when your hands are off the steering wheel.

The above figure illustrates caster as utilized on a bicycle and a caster roller used for a piano. When the piano is pushed forward (direction of arrow), the centre line of the wheel is behind the centre line of the kingpin so the wheel follows accordingly. This difference between the centre lines is called lead (or trial). In this example, the distance (lead) between the centre lines is called lead (or trial). In this example the distance (lead) between the centre lines of the kingpin and roller produces the caster effect, and, of course, the greater this distance, the greater the caster effect.

Although the construction is different, the front wheel of a bicycle also has caster similar to that of the piano roller i that the point where the front makes contract with the ground is behind the point where an extended centre line of the fork would intersect with the road surface. The reason why a bicycle can be ridden with “no hands” is because of this cater which always keeps the front wheel directed straight-ahead.

The caster provided for the automobile is exactly the same as for the bicycle except that the automobile uses a kingpin instead of the fork. Although the term “kingpin” is used here, for the ball joint type front wheel, the angle formed by a line connecting the upper and lower ball joints interesting with a vertical line is the caster. The amount of caster on automobiles is usually 103 degrees. Caster creates the recovery force which tends to always bring the front wheels back to the straight-ahead position. Caster also has an adverse effect, however, in that it increases the steering effort. the counteract this, some vehicles are made with no caster.

References

1. Automobile Engineering (Vol. 1 & 2)            -           K.M.Guptha
2. Automotive Mechanics                                   -           Joseph Heitner
3. Automobile Engineering                                 -           Harbans Singh Reyd
4. Automotive Mechanics                                   -           William H. Course