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Suspension System | Independent vs Rigid Axle Type

The suspension is located between the wheels of the vehicle and the body. The suspension system includes springs, shock absorbers and their mountings. The purpose of a suspension system is to improve driving comfort, reduce the amount of vibration and impact forces that are transmitted to the body. It also ensures that the wheels are always firmly in contact with the road surface and regulates the inclination of the body in order to improve the stability of the vehicle in any possible driving condition including acceleration, braking, and cornering.

The suspension system can be divided into two major types based on the design i.e. rigid axle and independent type suspension systems.

Rigid axle suspension system

In the rigid axle suspension system, the left and right wheels are connected with a single axle and the load directed to the wheels is supported by this system. The rigid axle suspension is more effective when it is desired to maintain a large suspension stroke in vehicles which exhibit large variation in load as a result of changes in cargo weight or passenger numbers. Thus, it is mainly employed in large or medium sized trucks and buses. The drawback of this type of suspension system is that driving comfort and stability are inferior to that of independent type.

Independent type suspension system

In the independent type of suspension, there is no axle connecting the left and right wheels, hence the load directed to the wheels is supported by the suspension arms. Thus, each wheel can move independently in response to its specific road condition. Though this type of suspension system is more complicated in terms of design than the rigid axle type, yet the unsprung mass is lighter and wheel-to-ground contact is better. Accordingly, driving comfort and stability are superior when this type of suspension is adopted.

The sprung mass refers to the weight, which is supported by the suspension spring, and the unsprung mass refers to weight, which is not supported by the suspension spring. The unsprung mass includes the weight of wheels, rear axle, steering linkages, and some suspension components. A low unsprung mass has a large effect in improving driving stability.

The independent suspension system has become almost universal in case of front wheels. The following types of independent suspension are generally used in automobiles:

1. Double wishbone

A double-wishbone suspension system is comprised of an upper arm, lower arm, and a damper assembly (coil spring and shock absorber) among other components. Each arm is connected to the knuckle and body via bushes. The double wishbone suspension is capable of reducing damper friction and offers a large degree of freedom in design. It is very effective in providing excellent travel and riding comfort.

2. MacPherson strut

The MacPherson strut suspension system is comprised of a lower arm and damper assembly. The strut referred contains the damper and a coil spring. The ends of the lower arm are connected to the body and the knuckle via bushes. The upper end of the damper assembly is mounted to the body in such a way that it is allowed to move relative to the body. The lower end is secured to the knuckle.

This design is effective for space saving. The spring is offset in relation to the strut to compensate for bending force imposed when the wheel is driven upwards and downwards. It allows more positive damper action resulting in less noises and smoother ride under all driving conditions.


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