Ackerman's Principle of Steering: Working & Applications

Have you ever noticed that the two front wheels of a passenger car turn at different angles when the vehicle is making a turn? If not, check it now. There is a unique mechanism called the Ackermann steering mechanism to control the steering angle of two front wheels. Read further to understand what is Ackermann steering and why it is needed.

What Is Ackermann Steering?

Ackermann steering was first invented by Georg Lankensperger and was adopted by Rudolph Ackermann for his horse-drawn carriages. He kept two wheels at different points connected using a tie rod to make the two wheels turn at different angles. So that one wheel can turn independent of the other wheel. Also, the linkages for this steering system resemble the shape of a trapezium with one fixed link and three moving links. This is Ackermann's principle of steering, which makes the car turn without any slip. The Ackermann steering mechanism is a modified four-bar linkage mechanism. The Ackermann steering kinematics is defined by the motion of links and the angle to which the wheels are turned. 

Need for Ackermann Steering Mechanism

There is a fixed turning radius for every vehicle, which is the space it requires to complete a circle. The radius is calculated from an imaginary centre point called the centre of turning radius. If you observe your car’s wheel while turning, you can notice that the inner wheel is closer to the centre of the turning radius when compared to the outer wheel. Also, the inner wheel has to take a shorter turn, and the outer wheel has to take a long turn when compared to the inner wheel. In this case, if both wheels are turned at the same angle, there may be a possibility of slipping. But why the slip happens? 

In order to understand this, you must first understand how a wheel works. A wheel has two different types of velocity. One is the rotational velocity, and the other is the translational velocity. They both have equal magnitude but are opposite in direction; hence they cancel each other. When the wheel has turned, the direction of the rotational velocity changes and there may be a possibility of slip. To avoid that slip, the direction of the translational velocity has to be changed. But, there arises an issue. While turning your car, the inner wheel travels a small distance and need less translational velocity compared to the outer wheel. So, both wheels must have different translational velocities and different directions for rotational velocity. This difficulty could be overcome by having different steering angles for two front wheels.

Working of Ackermann Steering

When you turn the steering wheel, the steering column translates the motion to the steering axis. The steering axis is connected to the moving link of the steering mechanism, which is at the centre. The moving link, in turn, changes the direction of the wheels by transferring motion to the link that is attached to the wheels. This is how Ackermann's steering kinematics works. It is referred to as kinematics because Ackermann's principle of steering doesn’t get influenced by any external forces. It involves only the relative motion between force links and doesn’t involve the study of the effect of forces. The Ackermann steering geometry is designed in such a way that the two front wheels are always aligned towards the common centre of the turning radius.

Applications of Ackermann Steering 

You could find Ackermann's principle of steering being employed in passenger cars. There are some robotic vehicles using the Ackermann steering mechanism built for the purpose of the experiment. We also have rack and pinion steering mechanism, parallelogram steering mechanism, and recirculating ball steering mechanism, to name a few. You can learn more about automotive and its subsystem by enrolling on mechanical engineering courses at Skill-Lync.