Car Parts and Accessories

When the front and rear suspension units are inter-connected, the result is known as a linked suspension system.

The primary advantage of connecting front and rear springing in this way is that it greatly reduces any tendency of the car to pitching (rocking in a fore-and-aft direction); this gives the occupants a more nearly level ride. Outstanding examples of linked suspension on cars of recent times are the Moulton Hydrolastic and Hydragas suspensions used by Austin- Morris and the spring-linkage system used by Citroën on their smaller models.

The main difference between the two is that the Hydrolastic and Hydragas systems are hydraulically operated, while the Citroën system works mechanically.

Auto Hydrolastic system

In the hydrolastic system, each wheel has a `displacer‘ which fulfils the functions of spring and damper. The displacer is mounted on the car body and inside it, at one end, is a bonded-on conical rubber spring.

Closing the other end of the displacer is a flexible diaphragm, in the middle of which is a piston connected to the wheel- suspension linkage. The chamber between spring and diaphragm is divided by a metal separator plate containing a rubber two- way valve. A pipe connects the front end rear chambers on each side of the car; chambers and pipes are filled with fluid.

When the front wheel rises over a bump, the diaphragm moves inwards, forcing fluid through holes in the separator plate past the two-way valve; the resistance of which provides the damping effect.

The diaphragm movement reduces the chamber volume and increases pressure, displacing some fluid along the connecting pipe. This causes the diaphragm of the other displacer to be pushed outwards, extending the rear suspension. Any lifting of the front end of the car over the bump is thus matched by a rise of the rear end, and vice versa. The Hydragas system is a further development, using pressurized nitrogen gas as a damping and springing medium instead of rubber.

Auto Gas Compression provides Springing Effect

ALTHOUGH not available on currently produced South African cars, hydro- pneumatic suspensions warrant some mention. On their larger cars, Citroen, among others, use this design.

Each wheel has its own independent suspension unit. In each unit, nitrogen is contained under pressure in the upper half of a metal sphere above a fluid-resistant diaphragm clamped in position.

The lower half of the sphere is connected to a hydraulic cylinder. Sliding inside this cylinder is a piston with a push-rod joined to a suspension arm by a pivot pin.

When a wheel moves up to ride over a bump in the road, this movement is passed on by the suspension arm to the piston. Movement of the piston exerts pressure on the hydraulic fluid, and this in turn compresses the gas, which acts as a spring. Downward movement of the wheel causes the piston to travel downwards also, reducing pressure on the gas.

Height adjustment is achieved by the flow of fluid into or out of the cylinder. The fluid is kept under pressure in a reservoir supplied by a pump, and its flow is controlled by a slide valve.

If the load on the car is increased, the body at first sinks in relation to the wheels and suspension arms. Each arm then operates a linkage that opens the slide-valve connection to the reservoir, and fluid enters the cylinder to push the body up. The linkage returns to neutral when the body regains its original height.

The reverse happens if the load on the car is decreased. The body rises, the valve linkage opens the return pipe to the reservoir, and excess fluid flows back until the original body height is reached.

A damper is fitted at the top of the cylinder. It consists of a valve restricting the flow of fluid into and out of the sphere.

Auto Self-levelling suspension

Self- levelling suspension systems prevent a car from ’squatting down’ on its suspension when it has a full load of passengers and luggage. They are used on some cars with limited suspension travel and on luxury models like the BMW 7 series. These systems automatically compensate for changes in load by modifying the springing characteristics of the car keeping it the same height off the road.

This ensures that, even when the car is fully laden, the full upward travel of the suspension system is available for absorbing bumps. It also means that the headlights are kept in proper focusing alignment, whatever the distribution of load. A weakness of conventional suspension systems is the way in which the addition of rear-seat passengers, or luggage in a rear boot, makes the car ’sit down’ at the back and so tilts the headlight beams upwards.

Any form of self-levelling is operated by one or more load-sensing devices which measure the vertical distance between, say, the floor and a point on a suspension arm. The greater the load, the smaller this distance tends to become. An initial variation in the distance operates a valve which controls the height-adjusting system. Usually, height adjustment takes place at both ends of the car, but it can be at the rear end only.

Auto Air Suspension

Control of the riding height of a car is simple with air suspension, a system that has been used with considerable success on the Mercedes 600.

It could become more widely adopted in the future, because of the ease with which the spring characteristics can be suited to individual vehicles. Air suspension is simple in principle. Collapsible, pressurized air containers take the place of conventional springs; the upward movement of the wheel reduces the volume of the air spring, raising its pressure, so that it tries to extend itself again.

If the air spring is inflated more, it can carry a heavier load before contracting to a given height.

A height-control valve connects the air spring to a high-pressure air reservoir when the load is increased; and pressure is released through the valve to the atmosphere when the load is reduced.

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