Before explaining the Ackerman’s Principle of Steering students must have an idea about Pure Rolling, Pure Sliding and a combination of Rolling and Sliding. In the fig. 1 three arrow heads are shown through X, Y and Z. Travel in X-X direction indicates pure rolling and Y-Y direction indicates pure sliding of the wheel on the travel surface. When the wheel is turned the travel of wheel is along the Z-Z, the result is a combination of pure rolling and pure sliding.
When a vehicle with four wheels is negotiating a curve the third condition as explained above with a combination of pure rolling and pure sliding prevails. But there should be only rolling motion on the wheels while taking a turn. To ensure that all the wheels only roll but do not slide on the travel surface, the kinetic linkages of all four wheels should be arranged in such a way that the centre of rotation of all the four wheels in plan coincide. In other words, the centre of rotation should be common to all the four wheels. This is the Ackerman’s Steering Principle.
The Ackerman’s steering geometry is shown in the fig. 2. In this linkage, which is a kinematic four bar chain, the two short links are equal in length and two long links are unequal in length. When the vehicle is moving straight ahead on the road, two long links are parallel to each other and all the four links form a trapezium [fig. 2]. The shorter links make an angle, ɵ with the wheel base line as shown in the diagram.
While taking a turn, in order to satisfy the condition that the axes of all four wheels coincide at a common centre, O as shown in the fig. 3, the links in the mechanism should have a proper proportion for a given angle, ɵ.
● It has been found that the point of intersection g, of the two short arms in the linkage as shown in fig. 2, should be at a distance of about 0.7 times the vehicle wheel base from the common axis of the front wheels. This condition and proportion offers very good results for steering with minimum sliding of the steered wheels.
● It is also observed that for a given relationship between angle, ɵ and location of point g, wheel base Lb and wheel gauge (track) Lg, there will be a single value of α which will give the best result. Normally, the ratio of wheel gauge to wheel base in most of the passenger cars is approximately 0.4.
