How does the Steering Wheel automatically return to its center?
You might have noticed that after making a turn, when you release the steering wheel it will automatically return to the central position. You will be amazed to find out that this steering wheel returnability is not achieved by any
complicated mechanism using springs or valves. Instead, engineers have achieved the return motion of the steering wheel solely by giving the front wheels an angle called a caster angle. Let’s discover the interesting physics behind how providing an angle to the front wheels makes the wheels and the steering wheel come back to their original positions. First, let’s have a closer look at the steering wheel mechanism. It is clear that the rotation of the steering wheel is transferred to the rack and pinion mechanism which then turns both the wheels. If you observe closely, you can see that the wheels are turning along a particular axis. This axis is called the steering axis. In the current demonstration, you can see that the steering axis is perfectly vertical. However, this is not normally the case in practice. In practice, the steering axis will be slightly inclined to the vertical as shown. This angle is called the caster angle. You will get a clearer picture of this angle when it is viewed from the side.
Let’s see how this steering mechanism works.
At first, you might not notice much difference from the previous mechanism. However, when you understand the concepts of patch area and pivot point, you will notice a big difference. Patch area is the area where force is transferred to the wheels. It is clear that the wheel is pivoted to turn around at the meeting point of the steering axle and road. Let’s call this meeting point the pivot point. In the first mechanism, the patch area is in line
with the pivot point. However, in the second case, the patch area is way behind the pivot point. To understand the effect of this trailing patch area, let’s study more about the forces acting on the wheels during cornering. Assume that your car is making a perfect circular turn in a level plane.
To make this turn, what the car needs is a centripetal force. In a level plane turn, this centripetal force should come from the frictional forces at the wheel patch area and this fact is clear from this snapshot. Now let’s analyze in detail what this frictional force does to the front wheels. In the actual steering wheel geometry, we saw that the patch area is behind the steering axle meeting point or pivot point. If you no longer hold the steering wheel in this turned condition, the effect of the centripetal force on the wheels is obvious. It will produce a restoring torque and the wheels will automatically realign to the center. Let’s watch it from the top view also to get a better idea. However, for the initial geometry with zero caster angle, there won’t be any restoring torque since the centripetal force passes through the pivot point. In short, just by giving a positive caster angle to the steering mechanism, engineers were able to attain the restoring torque. We salute the brilliant minds that visualized such an ingenious idea and avoided the need for a complex mechanism. It is clear from these discussions just how critical the caster angle is for a vehicle’s straight-line stability. The caster angle is not adjustable on modern cars. But if during the wheel alignment operation any variation is found, due to wear and tear of the connected parts, the issue should be fixed.
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