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Replacing Shock Absorbers

05 Sep 2019


Product Quality

OE Matching quality ensures that the replacement part will fit the vehicle correctly, and that it will match the vehicle’s handling and ride comfort characteristics. The NAPA range comprises oil-filled or gas pressure units that match the specifications of factory-fit OE shock absorbers.
Function and Technology

As the shock absorber compresses or rebounds, valves within the oil-filled tube restrict the flow of oil to reduce the movement of the piston. This reduces oscillation of the road spring, keeping the tyre in contact with the road and improving ride comfort.

Monotube and twin-tube shock absorbers fulfil the same function but differ in design.

A monotube gas shock is filled with oil and gas at 1.6-2.6MPa, and a movable separator piston separates the two substances. A piston valve attached to the piston rod controls oil flow and damping effect.

Alternatively, a twin-tube shock absorber has two concentric chambers. The oil-filled working chamber housing the piston rod and piston valve, and the compensation chamber formed of the space between the working cylinder and the outer tube. This is filled with two-thirds oil and one-third air. In a gas-pressurised shock, gas at 0.3-0.9MPa replaces the air. The piston valve and a valve in the base of the working chamber control oil flow and damping effect.

Diagnosing Common Faults

The most common issues with shock absorbers include oil leaks, side loading and worn mountings.

A light misting of oil on the outer tube of the shock is normal since the piston rod transfers a small amount of oil from the working chamber to the piston rod seal to keep it lubricated on each stroke. However, larger volumes of oil or obvious streaks can indicate a failing piston rod seal.

Side loading on a shock absorber is usually the result of shock mountings being fully tightened without the vehicle weight acting on the suspension. Once the vehicle is lowered off the ramp onto the ground, the shock mountings cannot move to accommodate the angular change between axle and body. The shock absorber tries to bend, placing it under great stress, which may go unnoticed until sufficient wear occurs to cause an oil leak.

Normal wear and tear, consequential damage from the operating environment, and fitting errors can cause the rubber mountings to deteriorate. This is normally accompanied by noise or vibration.

Replacing in Pairs

Shocks should be regularly inspected, and replaced in axle pairs to ensure even wear. Uneven shocks are a valid safely concern.

The driver may not notice gradual deterioration in a shock absorber’s performance, but the damping force can differ up to 25% between a worn shock and a new one, which will almost certainly be noticeable when braking heavily or other avoidance manoeuvres, with potentially dangerous results.

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