When we turn the car while driving, no matter the type of steering wheel, it always returns to the centre or original position automatically. This does not happen due to any complex mechanism, but due to the caster angle at the front wheels. The caster angle is one of the wheel alignment angles in addition to the camber and toe angles.
Let’s find out what is a caster angle and how the steering wheel returns to the centre.
Science Behind Caster Angle
In order to understand how the caster angle makes the wheel and steering wheel return to their original position, we have to take a look at the steering wheel mechanism.
A rack and pinion system connects the steering wheel of a car to the front wheel. The base of the steering wheel column has a gear wheel known as pinion that works along the bar of straight teeth called rack.
When you rotate the steering wheel, it turns the pinion, which makes the rack move from side to side. At the ends of the rack are the connecting rods leading to the front wheels.
The wheels of the vehicle turn at a particular axis referred to as the steering axis. You may think the steering axis will be perpendicular to the tyres. However, that is not the case. In practice, the steering axis inclines from the vertical or normal angle.
This inclined angle is what we call the caster angle, which is primarily the reason steering wheel returns to the centre.
If you find the mechanism of the steering wheel interesting, also have a look at how electric power steering system functions.
Understanding Steering Wheel Returnability
To get an idea of how the steering wheel returns to the centre while driving, we have to understand the contact patch area of the wheels and the pivot point.
Patch area is basically the area of the wheel in contact with the road. The area is the only link between road and wheel. The pivot point is the meeting point of the steering axis with the road.
This is where the physics of caster angle comes into play. Without the caster angle, the contact patch area will be aligned with the pivot point. When the steering axis is at a caster angle, depending on the positive or negative caster angle, the pivot point will be away from the pivot point.
The positive caster refers to the angle with the steering axis tilted towards the driver. When you tilt the axis away from the driver, it is referred to as a negative caster.
With an understanding of contact patch area and pivot point with respect to caster angle, we can now take a look at the exertion of different forces leading to the returnability of steering wheels.
Impact of forces on caster angle
When you rotate the steering wheel on a turn, it generates reactive forces at the wheel. These frictional forces at the wheel contact patch area provide the centripetal force required to make the turn or a circular motion. At a positive caster angle, the patch area is behind the pivot point. When we release the steering wheel, a restoring torque helps align the wheel automatically.
To understand better, consider the zero caster angle where the pivot point aligns with the patch area. Since the centripetal force is acting on the pivot point, there won’t be any restoring torque and your steering wheel will not return automatically.
Caster Angle in Wheel Alignment
During the wheel alignment of a vehicle, a positive caster angle allows the steering wheel to self-centre and increases the directional stability of the vehicle. On average a caster angle can vary between 2 to 8 degrees, depending on the manufacturer requirement.
Apart from the caster angle, when we talk about steering wheel alignment, adjustments might be needed for camber and toe angle as well.
Camber angle is usually visible from the front of the car and impacts the cornering performance by adjusting friction and traction of wheels. The toe angle impacts the sensitivity of the steering wheel for safety concerns.
You don’t have to dwell much on the science behind the caster angle. All you need to understand is that the steering wheel does not return to the centre through external means but just due to a difference in angle. This results due to the manipulating reactive forces on the wheels. Also, find out all the signs of a failing steering system to take timely action.
For automotive enthusiasts, you will have a clearer idea of the science behind the caster angle. If your car faces regular maintenance issues including wheel alignment, you are better off buying a used car in the UAE.
Stay tuned to our auto blogs for more information on car parts, market trends, repairs and more.
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