Ackermann Linkage

Ackermann Principle as Applied to Steering

Ackermann Linkage

The self propelled motor vehicle almost from the beginning, used the double pivot wheel steering system. This was invented for horse drawn vehicles in 1817 by George Lankensperger, a Munich carriage builder. In England, Rudolph Ackermann acted as Lankensperger's agent and a patent of the double-pivot steering arrangement was taken in his name.



With this layout of the linkage the track rod arms are set parallel to each other and a track rod joins them together. In the straight ahead position of the steering, the linkage and axle beam forms a rectangle, but, as the stub-axles are rotated about their king pins, the steering arrangement forms a parallelogram. This linkage configuration turns both wheels the same amount. Figure 27.26A illustrates the parallel-set linkage positioned to provide both a 20 degrees and a 40 degrees turn for the inner and outer wheels.



Charles Jeantand in 1878 introduced an improvement to the Ackermann linkage layout in which inclined track rod arms form trapezium (Fig. 27.25). This trapezium linkage configura­tion allows the inner wheel to rotate about its king-pin pivot by a greater amount than the outer wheel, which is necessary for providing semi-true-rolling (Fig. 27.26B and C). True rolling is obtained in the straight ahead position and on the left and right hand turns (locks). In between these three positions, only partial true rolling occurs. The degree of departure from true rolling and hence the amount of tyre scrub occurrence depends mainly on the ratio of track-rod to track-rod-arm lengths, and on the track-rod-arm angular inclination or set. In case the steering linkage dimensions and settings are carefully selected, a very little misalignment takes place for angle of turn up to about 15 degrees, beyond which the error increases rapidly.



 Also the deviation of the linkage from the theoretical true-rolling angles can readily be corrected by the tyre's side-wall flexibility and tread distortion, provided the angular error between the steered wheels is small. Since the rear wheels turn on a smaller radius than the front wheels, it is easier to manoeuvre a vehicle in reverse than in the forward direction when parking.

Fig. 27.26. Ackermann linkage geometry.

A. Parallel set track-rod arms, outer wheel turned 20 and 40 degrees.

B. 10 degrees set track-rod arms, outer wheel turned 20 degrees.

C. 10 degrees set track-rod arms, outer wheel turned 40 degrees