Shock Absorber Replacement
The average car is likely to need at least one set of replacement shock absorbers during its lifetime in order to restore vehicle handling, safety and ride comfort to original levels.
Specifying OE quality ensures that the replacement part will be an exact fit for the vehicle and that it will match the vehicle’s handling and ride comfort characteristics. The Boge Automatic range comprises oil-filled or gas pressure units that exactly match the specifications of factory-fit OE shock absorbers.
For more sporty driving, Boge Turbo gas pressure shock absorbers are available in both mono and twin-tube versions. These provide firmer damping, giving a noticeable improvement in handling, steering response and vehicle stability under demanding driving conditions.
Function and Technology
The basic principle of a shock absorber is that as the unit 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 perform the same tasks but differ in design. A monotube gas shock is filled with oil and gas at 25-30 bar pressure, and a movable separator piston separates the two substances. A piston valve attached to the piston rod controls oil flow and damping effect.
Left: Monotube shock absorber schematic
A twin-tube shock absorber has two concentric chambers: the oil-filled working chamber housing the piston rod and piston valve; 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 6-8 bar pressure replaces the air. The piston valve and a valve in the base of the working chamber control oil flow and damping effect.
Left: Twin-tube shock absorber schematic
Where varying load conditions are anticipated and ride comfort would be difficult to maintain, Boge Vario technology can achieve a multi-stage damping action with both monotube and twin-tube shock absorbers. At a predetermined position during its stroke, the piston traverses one or more tapered vertical control grooves in the wall of the shock absorber, acting as a metered hydraulic bypass to reduce damping force.
Left: Boge Vario technology with control groove/hydraulic bypass
Diagnosing Common Faults
The most common faults with shock absorbers are oil leaks, side loading and worn mountings. During each stroke the piston rod transfers a small amount of oil from the working chamber to the piston rod seal to keep it lubricated so a light misting of oil on the outer tube of the shock is normal. Larger volumes of oil or obvious streaks 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, for example with the wheels hanging free while on a ramp. When the vehicle is lowered to the ground, the shock mountings cannot move to accommodate the angular change between axle and body so the shock absorber tries to bend, placing it under great stress. The condition may go unnoticed until sufficient wear occurs to cause an oil leak.
Normal wear and tear, consequential damage due to the operating environment and fitting errors can cause the rubber mountings to deteriorate. This is normally accompanied by noise or vibration.
Shocks should be regularly inspected for wear and renewed together (with the service kit of spring assister and gaiter) whenever the shock absorbers are replaced.
Replacement – Always Fit in Pairs
There is a real risk involved in only replacing this safety-critical component on one side of the vehicle so advice to replace shock absorbers in axle pairs should not be dismissed as a tactic to sell more shocks. The gradual deterioration in a shock absorber’s performance generally goes unnoticed by the driver, but a potential difference of 25% damping force between a worn shock and a new one on the same axle will almost certainly make itself felt under heavy braking or other avoidance manoeuvres, with potentially disastrous consequences.
Remember: Always replace shock absorbers in axle pairs!