Shock absorber

Shock absorbers control the compression and rebound movement of the axles and prevent trucks, buses and vans from continuing to sway, bounce or roll excessively after driving over an uneven surface. They do not carry the vehicle weight themselves; this function is performed by leaf springs, air bellows or coil springs. However, a properly functioning shock absorber keeps the tyres in continuous contact with the road surface and therefore supports the operation of ABS (Anti-lock Braking System), EBS (Electronic Braking System) and ESP (Electronic Stability Program). For MAN and Mercedes vehicles, shock absorbers must always be identified using the chassis number. Visual comparison, mounting distance or selection based solely on the model name is never sufficient, as axle version, permitted axle load, suspension system and chassis variant are decisive.

Depending on the vehicle configuration, hydraulic oil-gas shock absorbers, twin-tube shock absorbers or monotube shock absorbers are used. Twin-tube types have a working cylinder and reservoir tube, whereas monotube versions operate with a separating piston and nitrogen pressure. The gas pressure limits foaming and cavitation in the hydraulic oil at high stroke frequencies. Especially on heavily loaded MAN TGL, TGM, TGS, TGX and TG3 models or Mercedes Atego, Axor, Actros and Arocs vehicles, this thermal stability is essential during long journeys, on poor road surfaces or in intensive distribution work.

Shock absorbers differ in extended and compressed length, effective stroke, damping characteristics in rebound and compression, mounting with an eye bush, pin or bolt connection, and the diameter of the mounting bushes. For MAN Lion's City, Lion's Coach and Neoplan coaches, as well as Mercedes Citaro, Intouro and Tourismo, shock absorbers are also specifically matched to axle load, air suspension and body height. In vans such as the MAN TGE, Mercedes Sprinter and Vito, damping values are matched to the front axle design, drive configuration and permitted payload.

A leaking shock absorber can be identified by a wet housing with hydraulic oil around the piston rod seal. A light oil film may occur, but visible dripping, contaminated mounting points or oil traces across the entire housing indicate that replacement is required. Other fault patterns include internal wear of the piston, base valve or valve assembly, loss of gas pressure, a bent piston rod and torn rubber-metal bushes. Typical symptoms include continued oscillation after speed bumps, cab sway, unstable cornering behaviour, tyre cupping or sawtooth wear, and increased stress on stabiliser bars, tie rods and air bellows.

Place the vehicle on a lifting platform or over an inspection pit and first inspect the shock absorber housing, dust boot, bump stops, bolted connections and bushes. Then measure the mounting points and compare the compressed and extended length with the technical specification for the chassis number. Check the piston rod for corrosion, scratches and bending; damage to the chrome-plated surface quickly destroys the seal. Move the axle using a suitable axle jack and assess play in the eye bushes with a pry bar. A road test confirms the diagnosis: look out for continued swaying, pulling brake responses, knocking over short ridges and instability when changing lanes. At the same time, check tyre pressure, wheel bearings, stabiliser mounts and the air suspension ride height, as these faults can cause similar symptoms.

Replace shock absorbers per axle, always on both the left and right sides at the same time. Different damping forces on the same axle cause uneven tyre contact, brake reactions and instability. Use new self-locking nuts, specified bolts and spacer bushes where required by the manufacturer. Tighten the mountings to the manufacturer's specified torque and load the suspension before final assembly if required, so that rubber-metal bushes do not twist under preload.

Never fit a shock absorber with a different stroke or mounting fitment: a version that is too short can reach the rebound limit, while an excessively long shock absorber can overload the air bellow, leaf spring or axle guidance. On vehicles with air suspension, the ride height and operation of the level control system must also be checked after the repair. Suitable stock availability and technical support for chassis number identification help to minimise downtime and incorrect installation.