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Suspension

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Castor





In multi-track vehicles, the caster influences, together with the steering axis inclination, the reset forces of the steered wheels and the camber when cornering. A low caster causes the steering to be more smooth and sensitive, at high speed driving however, it makes it less stable. A high caster stabilises very strongly, it does however, cause the steering to be heavy and sometimes at low speeds can even cause it to flutter.

The caster generally describes the distance that the tyre travels after it's track point. The track point is the spot at which the swivel axis of the wheel contacts the road. With the very often used McPherson strut, the swivel-axis is the line through the crown bearing at the top and the ball bearing in the lower wishbone. The swivel axis can be at a slant, or vertical in front of the wheel center, as in a shopping cart. The latter is also called caster-offset. Often both are effective (e.g., in bicycles - see above figure).

If an angle is given instead of a distance, this is the tilt-angle of the swivel axis. It influences the camber of the inside-curve-wheel negatively. However, the influence of the spreading, which increases the camber on the curve-inside-wheel and reduces it on the curve-outside-wheel, must be taken into consideration. To ensure that the latter does not become positive, the caster (e.g. with 10°) is never smaller than the spreading (e.g. 6°). The high given values of the caster, show that a so called fore-running is practically non-existent.

The greater the angle of the swivel axis is to the perpendicular, the more difficult it becomes to move the steering from the center position. Shifting the swivel axis forward improves the straight-running stability, (almost) without the disadvantage of heavier steering. A further possibility of a caster occurs with every tyre, as long as it's not mounted on a driven wheel. If the wheel is drawn, then the tyre to road surface contact area is shifted slightly to the rear. This could also be called tyre-trailing. This in addition, also causes the steering to return to the center position after cornering.

To properly understand the caster, one should examine it on a two-wheeled vehicle. Here, both types of the above described caster are found together. The caster-ofset is shown by the distance between the red and the blue line at the bottom of the picture. If the swivel axis (red line) would be vertical, the relationship would be similar to that in a shopping cart. The wheels would always trail by this amount. The camber, when cornering, would not change.

On a bicycle, the caster-offset is reduced due to the forward curve of the front fork. This is also necessary, because although basically, too great an offset would stabilize, indeed, together with the slanting position of the swivel axis, it causes the front end to be lowered when cornering sharply, which again has a de-stabilizing effect. Therefore, the slant and the offset must have a balanced relationship to each other.

Under normal circumstances a standard bicycle will hardly dip at all, up to a certain steering angle. This effect, by the way, cannot be noted in two-track vehicles because the two sides stabilize each other through the steering connection. If however, the adjustment values on the right are different from those on the left, it will certainly be noticed in the steering. By the way, bicycles without a caster -(offset) are only used by cycle-artists. These bicycles are difficult to ride in a straight line. 11/12

The caster is measured as camber when the steering is turned.


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