Subject: Automobile Engineering

Topic: Suspension, Wheels & Tires

Difficulty: Medium

1) Torsion bar:
The torsion bar is used with independent front suspension system. Two such units are there on either side of the frame. each suspension housing is pivoted at its front end to the tabular cross member(front axle) & carries at its rear end a wheel carrier arm mounted on the forward end of wheel carrier arm. Combined torsion bar & torsion tube splined together at their inner ends provide main spring load. It is made of heat treated alloy spring steel.

One end of the torsion bar is connected by splines with the wheel carrier arm. While the other end of torsional bar assembly is rigidly connected to the steering knuckle. The length of the combine torsion bar is composed of the length of the tube & rod to make softer suspension. It is lighter than the leaf spring & occupies less space.

2) Spring & Unsprung mass:
The total mass of the vehicle, for the purpose of the suspension design is broken up into two parts spring & unsprung. Unsprung weight consists of tyres , wheels & suspension components that attach these items to the main body - chasis structure . The rest of the vehicle is sprung weight. In dynamic terms , all items that move with the wheel are unsprung. This also include portion of the shock absorber & sway bar which are attached to the suspension. The portion that are attached to the frame & body are sprung. Light components can be kept in contact with the ground more easily than heavier ones. Motion of the wheels & related assembly must be stopped before the spring can return the tyre to the surface. Therefore, the lighter the component, the less momentum there is to control. With greater unsprung weight, the suspension has to allows the use of softer spring, which make for a smoother ride.

Low unsprung mass thus allow the use of softer springs, which move for a smoother ride. With less unsprung weight, more weight can be controlled with spring & shock absorber, thus giving a better ride quality. Resonance due to the sprung & unsprung weight adversly affect ride, handling & steering. The higher the unsprung weight, the higher the hop & tramp amplitudes & the lower the ride quality, road holding & stability.