The obvious answer here would be, “shock absorbers exist to absorb shock,” but the truth is, the name is quite misleading. 

The shock absorber actually exists to control the vertical motion of your vehicle’s wheels, affecting suspension travel and rebound, with the job of the shock absorption actually being handled by the springs.

Indeed, on their own, shocks don’t really do all that much. They don’t provide any lift and they would not technically be necessary in a standard vehicle as long as you were driving on a smooth surface, in a straight line. Once you tried to brake or turn a corner though, you’d certainly notice something was up.

This is why fitting replacement shock absorbers to a vehicle with worn-out springs will over-work your new shocks and lead to reduced performance or premature failure. 

Conversely, upgrading only your springs but retaining worn-out shock absorbers can lead to insufficient spring control and body roll.

In a 4X4, in practice, shock absorbers are effectively hydraulic pumps that smooth out your ride and ensure your tyres remain in contact with the ground at all times. They don’t actually support the weight of the vehicle, which is another common misconception.

Types of shock absorber

There are two basic types of shock absorber: mono-tube and twin-tube, with the majority of 4×4 upgrade shock absorbers falling into the twin-tube category. 

Twin-tube shocks – These are made up of two separate cylinders with the outer cylinder set inside the shell case and the inner cylinder containing the piston valve. 

In this design, there’s no piston or barrier between the oil and gas in the outer cylinder. These shocks are cheaper to produce and can operate with lower gas pressure, which explains why they are more popular.

Mono-tube shocks – Here, all of the internal components are contained within a single tube, with a piston used to separate the oil chamber from the gas chamber within the shell case. 

Mono-tube shocks use higher-pressure gas and have a larger oil capacity, which provides more efficient temperature control for improved performance in harsh conditions. 

They also generally have an improved life cycle, as the oil and gas are separated by the piston so they can’t combine and foam or degrade.

Even premium shocks from brands like Old Man Emu and Terrain Tamer will (generally speaking) be of a twin-tube design but make up for this with high-quality outer casings and internal components. Indeed, twin or mono is far from the only factor to consider when researching shock absorbers. 

Besides component quality and casing thickness, shaft diameter, type of valving and type and quality of end-fittings are all important considerations. 

What about bypass shocks?

Very much the new kid on the block, at least in terms of the 4WD market, these shocks are more commonly used in cycle and motorbike racing. 

These use a hose (or something similar) to attach a second cylinder or reservoir to the main body to allow for improved cooling and increased  oil and gas capacity.

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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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.