The general perception of people about car wheels is that, they are completely straight and lie flat on the ground. It may appear to us that straight wheels provide more stability and also prevent the wear and tear of tyres. But the reality is completely different. The 4 wheels of a vehicle are designed to remain in particular angles depending upon the utility of the vehicle. If any of these wheels get misaligned, it affects the overall stability of the vehicle. It will cause abnormal wear and tear of the tyre and would reduce its life. It would also make the vehicle unstable during cornering and steering.
The wheel misalignment may result from the wearing out of bushes and bearings in the steering system. It may also be caused due to sudden jerk on wheels while crossing speed breaker or a pothole. The vehicles having misalignment have a tendency to move towards a particular side when their handle is left untouched. Their tyres wear in an absurd manner and you need to replace them very soon. The wheel alignment is defined using three parameters
So let’s discuss these three parameters- one by one.
Camber is the angle which the vertical axis of the wheel makes with the vertical axis of the vehicle. This angle is very important for the cornering performance of the vehicles. Generally, a Camber around 0.5-2 degrees is given on the vehicles. Depending upon wheel orientation, Camber can be of three types.
1. Positive Camber
The Camber would be called positive when the top of the wheels lean outwards. Positive Camber is generally used in off-road vehicles. This is because it improves the steering response and decreases the steering efforts required for turning the vehicle. Positive Camber is also provided in load-carrying vehicles. This is because the heavy load on these vehicles cause outward leaning wheels to straighten up, improving vehicle stability.
2. Zero Camber
The vehicle is said to have zero Camber when the wheels stand perfectly straight on the ground.
3. Negative Camber
Negative Camber is encountered when the top of the wheels lean inwards.
Providing Negative Camber improves cornering performance. When the vehicle turns on a corner, it performs a circular motion. Hence, it experiences equal and opposite centripetal & centrifugal forces. The centripetal force is experienced in the form of friction on tyres. The centrifugal force experienced by the car tries to throw it away from the turning centre. This increases the normal reaction on the outer wheels.
Due to the increase in normal reaction, the frictional force on the outer tyres also increase. This friction acts as a centripetal force and tries to bend the outer tyres inwards. The tyres get deformed due to bending and the contact area between the wheels and the ground decreases. This in turns decreases the frictional force between the outer tyres and the ground causing the vehicle to drift during cornering. Hence a negative Camber is given to the vehicles. The negatively cambered wheels lean inwards. So during cornering when the frictional forces try to deform the outer wheels, they just simply get flat on ground increasing the friction with the road surface.
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Caster is the angle which the vertical axis of the wheel makes with the steering axis. To have a clear visualisation you can consider the example of a motorcycle shown below. The caster can also be of three types.
1. Positive Caster
The point where the steering axis touches the ground lies in front of the point of contact of the wheel with the ground.
2. Zero Caster
The point where the steering axis touches the ground lies at the point of contact of the wheel with the ground.
3. Negative Caster
The point where the steering axis touches the ground lies in behind the point of contact of the wheel with the ground.
|Contact Point Of Steering Axis In Case Of Positive Caster|
For rear-wheel-drive cars, a positive Caster is given on front wheels. A slightly positive Caster of 3 to 5 degrees provides good stability and reduces the steering efforts. Giving positive Caster provides self-balancing capabilities to the steering. It means the steering of the vehicles returns to their position if left untouched. This happens because giving positive Caster shifts the position of steering axis to the front of the wheel axis. When the vehicle is turned about the steering axis, the wheel experiences a frictional force at the point of contact with the ground. This frictional force provides the opposite moment about the steering axis. This opposing moment helps the wheel to return back to its original position. Excessive positive Caster makes the vehicle difficult to steer. Hence it is generally kept around 3 to 5 degrees.
Toe is the angle which the horizontal axis of the wheel makes with the horizontal axis of the vehicle This is a very sensitive parameter and is responsible for vehicle’s steering stability. The Toe angles are kept very small i.e around 0.05 degrees. It l can be of two types.
1. Positive Toe
Toe is called positive if the front end of the wheels are positioned inward and the rear ends are positioned outwards. It is also called Toe-in. Positive Toe is generally provided in passenger cars.
2. Negative Toe
The Toe angle is called negative when the front ends of the vehicle are positioned outwards and rear ends positioned inwards. It is also called toe-out. Negative Toe is generally provided in racing cars. The outward or negative Toe increases steering sensitivity. It means the vehicle can be steered very easily and with very low efforts. But such sensitive steering decreases the straight-line stability of the cars. Hence Positive or inward Toe is provided in passenger cars. However, in racing cars, the negative toe is provided as these vehicles need very responsive steering systems.
All these three angles are very important aspects of wheel alignment. Every vehicle needs to go for wheel alignment after a prescribed time period. The wheels may get out of alignment even before the prescribed period. We need to take our vehicle to service centre whenever we feel any symptom of misalignment. This not only maintains the stability of the vehicle but also increases the life of the tyres.