Shock Absorbers Explained


Shock absorbers are fitted to virtually all passenger vehicles, as a means of controlling suspension travel and rebound.  In their own right, shocks do not provide any amount of lift.  A vehicle with coil or leaf springs fitted, but without shock absorbers, might handle acceptably when travelling in a straight line on a smooth surface, but would dive and roll during braking or cornering, and be unpleasant and unsafe to drive.  A vehicle fitted with shock absorbers but no coil or leaf springs would behave very differently, as the shocks would remain fully compressed or ‘bottomed out’, giving no suspension performance at all and potentially distorting or breaking the shock absorbers.

There are two basic types of shock absorber construction: mono-tube and twin-tube.  The majority of 4×4 upgrade shock absorbers are twin-tube.  However, this is not the only factor to consider when selecting the right shocks for your build.  Outer casing thickness, shaft diameter, type of valving and type and quality of end-fittings are all important considerations. 

Premium quality upgrade shock absorbers from brands such as Old Man Emu and Terrain Tamer are (mostly) twin-tube design, but they benefit from having high quality casings and internal components.  In some cases, different valving options are available to suit specific spring rates; this makes a set of Old Man Emu shock absorbers a natural choice when fitting Old Man Emu springs.  Terrain Tamer offer fully integrated kits for some vehicles, which contain everything from springs and shocks to fasteners, fittings and bushings, all engineered to work together as a complete package.

Fitting replacement shock absorbers to a vehicle with worn-out springs (or other suspension issues) will over-work your new shocks and can lead to reduced performance or premature failure.  Likewise, upgrading only your springs but retaining worn-out shock absorbers can lead to insufficient spring control and body roll.

Some manufacturers offer varying lengths of shock absorber for lifted (or lowered) vehicles.  In normal use, there should be no point at which the shock absorber fully extends (‘tops out’) or fully compresses (‘bottoms out’) as this can cause premature component failure.

Twin-Tube Shocks

In a twin-tube shock absorber there are two separate cylinders.  The outer cylinder is set inside the shell case, and the inner cylinder contains the piston valve.  With a twin-tube design, there is no piston or barrier between the oil and gas in the outer cylinder.

Twin-tube shocks are cheaper to produce and are able to operate with lower gas pressure, which may benefit ride quality.  However, aeration of the outer cylinder oil can occur under harsh conditions and this can lead to ‘shock fade’ or premature failure.

Mono-Tube Shocks

In a mono-tube shock absorber, the internal components are contained within one tube. A mono-tube system consists of an outer case and a cylinder containing the piston, valve, oil and gas.  A mono-tube design utilises a free piston which separates the oil chamber from the gas chamber within the shell case.

Mono-tube shocks may give a firmer ride due to containing high-pressure gas, and they are also more expensive to manufacture.  However, their larger oil capacity provides more efficient temperature control for improved performance in harsh conditions, and improved life cycle.  As the oil and gas are separated by the piston, they cannot combine and foam or degrade.

Bypass Shocks

A relatively new innovation in the 4WD market, bypass shock absorbers are best-known for their use in cycle and motorcycle racing.  Typically, a secondary cylinder or reservoir is attached to the shock absorber main body by means of a hose, and this design allows for increased oil and gas capacity together with improved cooling.  Design principles vary between manufacturers.

Many bypass shocks (particularly Old Man Emu BP-51’s, Terrain Tamer Pro Shocks, and some Fox Shocks) offer some form of on-body ride adjustment and are often fully rebuildable.  These premium products are recommended for punishing environments, competition use, or for obtaining the best ride quality and handling within the design limitations of any given suspension setup.

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4×4 Works are one of the UK’s best known premium 4WD upgrade specialists.  We import and sell the best products from around the world.

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