Shock Absorbers and Struts

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General background
Many ‘factory’ shocks have 30mm diameter pistons with relatively poor sealing around the piston head and a restricted length of travel. The volume of oil is limited, increases in temperature rapidly, losing viscosity, hence less damping effect. The poor seal allows ‘bypass’ – hot hydraulic oil under pressure leaks past the piston head that contains the valve stack. The shock soon looses control of the spring – known as ‘fade’.

Larger internal piston diameter and longer travel results in a substantial increase in the total hydraulic fluid oil capacity. Increased oil capacity results in greater heat dissipation, leading to higher retained viscosity which means greater effective damping action for longer – there is cooler, thicker oil porting through the valve stack in the piston head.

Additionally, multiple stage velocity sensitive valving improves response to varying road and track conditions. The valve stack is velocity sensitive, sensitive to the velocity of shock absorber movement. Two obvious different events are – a pot hole at 100 km/hr vs a slow undulation on road at 60 km/hr is accommodated by different valving in the piston head. The greater the number of valves the smoother the transition from speed to speed, hence smoother more controlled ride.

4WD Systems has 2 shock absorbers in the range:

  • 35mm Nitro-Gas shock
  • 41mm Foam-Cell shock (often known as Cellular Gas)

Both have a 14 valve in the stack, with an effective multi-lipped Teflon seal around the piston head to minimise piston head by-pass.



The increased diameter and stroke of the 35mm shock over the original does provide a noticeable improvement. However the 35mm gas shock does not have the capacity to work as hard as the 41mm shock, so its application tends towards lighter vehicles and leaf sprung vehicles where the friction between the leafs assists in the damping of the spring.

The size (35 or 41mm) represents the piston diameter, so the greater that number the greater the oil capacity, hence oil temperatures are lower. Cooler oil results in greater viscosity, and importantly – less Fade.

We usually suggest you consider the 41mm Foam Cell shock. While the 35mm Nitro Gas will do for a while, they wont last like the 41mm’s.

All shocks have oil in them, this is what does all the work, as the oil moves through the valving in the piston head it is restricted by the valving.

Shocks work at their hardest on corrugated dirt roads at moderate speeds or faster. The repetitive rapid movement of the piston results in heat build up, hence oil viscosity drops and the shock fades.

Slowly articulating over big rocks etc requires great articulation and balance – but doesn’t make a shock fade.

The ‘Foam Cell’ takes up the space of the gas (in Nitro-Gas shocks) or air in standard OE shocks. The ‘Foam Cell’ is comprised of a multitude of minute sealed gas chambers (called cells, hence Cellular-Gas) that prevent the oil and gas mixing, unlike the gas in Nitro-Gas shocks or Air in OE shocks – which can become aerated from this mixing.

Aeration also occurs due to the hydraulic oil boiling or bubbling as the shock extends rapidly after compression. The ‘vacuum’ or pressure reduction created as the piston head moves quickly away from the hydraulic oil allows the oil to boil at a lower temperature than when under ‘normal’ pressure.

The compressed gas chambers in the Foam Cell expand as fast as the piston head moves (after compression), forcing the oil back through the valve stack, hence maintaining a more normal pressure on the oil. The likely-hood of ‘low’ pressure – low temperature boiling is reduced.

Original shocks and 35mm Nitro Gas shocks both aerate in extended severe conditions due to oil-gas mixing and low pressure boiling of the hydraulic oil.

That said, it depends on the vehicle set-up and more importantly the use of the vehicle to determine which shock is best.

For example, If your ‘thing’ is beach fishing, then smaller capacity shocks are fine, as driving on soft sand doesn’t develop heat, therefore viscosity loss, leading to shock fade. Driving for hours-on-end over outback corrugations with 2 weeks worth of supplies on board is really tough on shock-absorbers.

Also of note:

  • Vehicles with Leaf springs require less damping. The friction between the leafs assists the damping process and there is less reliance on the shock. This is particularly true for lighter vehicles.
  • Heavy loaded leafs will last longer with the protection afforded by larger capacity shock-absorbers.
  • Independent Suspension (IFS & IRS) vehicles have particularly short bodied shocks with minimal oil capacity so they improve significantly when larger capacity shocks are fitted.

35mm Nitro Gas shocks have 2 Year Unlimited Km Warranty
41mm Foam Cell shocks have a 3 year Unlimited Km Warranty