Ackermann Principle

The Ackermann Principle as Applied to Steering

The Ackermann Principle

To achieve true rolling for a four wheeled vehicle moving on a curved track, the lines drawn through each of the four wheel axes must intersect at the instantaneous centre (Fig. 27.23). The actual position the instantaneous centre constantly changes due to the alternation of the front wheel angular positions to correct the steered vehicle's path. Since both rear wheels are fixed on the same axis but the front wheel axles are independent of each other , the instantaneous centres lies somewhere along an imaginary extended line drawn through the axis of the rear axle.

The Ackermann principle is based on the two front steered wheels being pivoted at the ends of an axle-beam. The original Ackermann linkage has parallel set track-rod-arms, so that both steered wheels swivel at equal angles. Consequently, the intersecting projection lines do not meet at one point (Fig. 27.24.). If both front wheels are free to follow their own natural paths, they would converge and eventually cross each other. Since the vehicle moves along a single mean path, both wheel tracks conflict continuously with each other causing tyre slip and tread scrub. Subsequent modified linkage uses inclined track-rod arms so that the inner wheel swivels about its king-pin slightly more than the outer wheel. Hence the lines drawn through the stub-axles converge at a single point somewhere along the rear-axle projection (Fig. 27.25).

Fig. 27.24. Side-pivot steering with parallel-set track-rod arms.

Fig. 27.25. Side-pivot steering with inclined track-rod arms