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Shock absorbers

2007-03-22 14:41

Colin Mileman

The shock absorber may be generally out of sight and mind, but it's one of the most important safety-critical components in any vehicle.

Shock absorbers are often referred to as dampers for a very simple reason - they have a dampening effect on suspension movement, which helps maintain control of the vehicle under acceleration, braking, cornering and driving over bumpy surfaces.

Without shocks, and using a simple spring set-up the suspension would be free to move in response to inputs such as driving over a bump.

The energy created by this movement will result in the spring oscillating or moving at its natural frequency until the energy is eventually dissipated. Considering that a spring is designed principally to store rather than dissipate energy, this will produce an excessively bouncy ride and severely impact on the controllability of the car - traits that are equally evident with worn-out or failed shock absorbers.

In basic terms the modern shock absorber is an oil pump mounted between the vehicle and the suspension, and works in conjunction with the spring set-up.

The top section of a common twin-tube hydraulic shock absorber is mounted to the vehicle body, and linked to a rod and piston operating in a pressurised hydraulic cylinder with a reserve tube and base valve that forms part of the lower section connected to the suspension or axle.

The piston incorporates small holes, or orifices, that allow the hydraulic fluid or oil to pass through at a controlled rate and very high pressure - a process that serves to slow down, control and dampen spring and suspension movement.

As the shock effectively 'absorbs' suspension movement and its associated kinetic energy, it is exposed to a high level of heat build-up - especially during demanding use when driving hard, braking heavily or towing. This heat is dissipated through the hydraulic fluid.

When driving over a bump, accelerating, hitting the brakes or turning, the shock is exposed to a compression cycle where the piston is forced downwards, pressurising the oil in the pressure tube and reserve cylinder, with a predetermined proportion flowing through the perforated piston. This cycle controls the movement of the suspension and wheels ? what is commonly referred to as the vehicle's unsprung weight.

Once the downward stroke is complete, the piston begins moving upwards again for the extension cycle and the fluid returns in the opposite direction, albeit at a slower rate to control the movement of the vehicle body, or the heavier sprung weight.

Most shock absorbers are also designed to adapt according to the driving conditions, with a greater rate of resistance being applied for an increased level of suspension movement. Ultimately, the amount of resistance the shock absorber develops is dependent on the speed of suspension travel, the number of holes in the piston and the grade of oil used.

A recent development of the twin-tube shock absorber is the gas-charged design, which incorporates nitrogen in the outer reserve tube, pressurised at around 100 to 150 psi.

The pressure of the nitrogen gas compresses air bubbles in the hydraulic fluid and prevents the oil and air mixing and creating foam under demanding conditions, which cannot be compressed. This allows the shock absorber to operate faster and more predictably, giving greater control and reducing fade.

A worn shock absorber cannot control the bounce experienced over bumps, roll or sway when cornering, suspension dive under braking or the squat when accelerating, leading to a vehicle that is far less controllable, is unstable in all facets of driving and will result in far greater stopping distances.

Worn shocks should be replaced without delay, and can be identified visually by fluid leaks, or by wayward antics when driving such as excessive bounce on rough surfaces, diving heavily under braking, leaning or swaying too much in corners, vague steering or uneven tyre wear.


Keep it cool

2007-05-04 14:34

Inside Wheels24

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