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Steering Offset

When a wheel is steered, it turns on its steering-swivel axis. If the wheel is held by only one ball-head on top and one at the bottom e.g., as in the double wishbone axle, one can imagine this steering-swivel axis geometrically seen, as a straight line through these two ball-heads. In the case of the McPherson-strut, (see figure) it is determined on top, through the center of the strut bearing.

If the steering-swivel axis would be extended to the ground, it would show the track point. The steering offset is the measurement of the lateral deviation, from the middle of the tyre contact surface, to the trace point. With a negative steering offset, (see figure) the track point lies outside of the center, and with positive, inside. If the trace point lies dead center, its value is zero.

Positive steering offset nowadays, is found in the motor car only if the construction makes it necessary. Its value is exactly zero if an impact free steering is required, a negative offset is found, if various forces, e.g., when braking on surfaces with different grip right and left, should be compensated by automatic countersteering. This interpretation is also preffered in connection with the diagonal brake circuit scheme. Therefore, it is also called track-correcting steering offset.

The negative steering offset leads to interesting effects. It can be substantiated without exact measuring, e.g., by laying a sheet of carbon paper between the tyre and a piece of white cardboard on the floor. When steering the wheels, the carbon paper forms clear streaks on the cardboard, the centre of these streaks is then clearly recognizable as being off-center.

If the steering offset is sufficiently negative, you can also mark the uppermost point of the front wheel in the straight position with chalk. If you now turn the front of the wheel inwards, the point unexpectedly moves backwards. The wheel does not roll on an outside arc forwards, but on an inside arc backwards. This is the actual effect. With stronger friction on the left than on the right, automatic steering forces develop to the right and thus, countersteer.

In a sense, widening the track is always popular. This has, of course, an influence on the steering behaviour. The greater the track widening, the less negative steering offset. If this is exceeded, it even turns positive. Thereby, more road-impact is possibly transferred to the steering. However, an even more important aspect is, that then, all precautions taken against differing friction on the right and left, act oppositely.

As a rule, the normal driver will not notice the negative aspects, except for the fact that the steering is more sensitive. Only in the case of particular road conditions or in winter, the problem suddenly, appears. Therefore, most manufacturers of track widenings restrict themselves on the front wheels, more than the rear, which of course, for the alleged improvement of the handling characteristics of a front-wheel-drive, makes very little sense.

Naturally, a brake circuit collapse occurs very seldom. If diagonal distribution is combined however, with exhorbitant track widening, an emergency situation would be not at all amusing. The moral of the story is: The modification of the modern chassis requires special knowledge and a lot of experience. Best of all, when tuning, one should first take a look at the higher performance models from the same manufacturer. 08/09

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