Shock-absorber

Cab shock absorbers limit relative movement between the cab and chassis. Together with cab springs, silent blocks, guides, and the tilting and locking mechanism, they reduce vertical movement, residual vibrations, body roll and shocks transmitted from the chassis to the cab. A correctly functioning shock absorber not only improves driver comfort, but also protects dashboard structures, wiring harnesses, HVAC components (Heating, Ventilation and Air Conditioning), cab locks and interior mountings against fatigue damage.

Hydraulic oil-filled and gas-pressure shock absorbers are available, usually designed as double-acting telescopic dampers. Gas-pressure types use pressurised nitrogen to limit foaming (cavitation) in the hydraulic oil at high stroke frequencies. The damping characteristics are determined by piston diameter, valve flow passage, oil viscosity, gas pressure and the balance between compression and rebound damping. Stroke length, end damping, mounting eye, bush diameter, installation angle and damping force differ according to the cab design.

Mechanically suspended cabs use hydraulic cab shock absorbers. Air-suspended cabs use cab shock absorbers with integrated air bellows. The design therefore differs according to the type of cab suspension.

For MAN F90, F2000, TGL, TGM, TGA, TGS, TGX and TG3, as well as Mercedes LN, SK, Atego, Axor, Actros, Antos, Arocs and Econic, several cab suspension configurations may be fitted within a single model series. This applies to day cabs, sleeper cabs, low cabs, medium-height cabs, high cabs and extra-high cabs. Therefore, always identify shock absorbers using the vehicle chassis number; visual comparison or selection based solely on the model name is never sufficient.

A faulty cab shock absorber typically causes a bouncing cab after speed bumps, rattling noises on poor road surfaces, increased steering column and dashboard vibrations, or noticeable fore-and-aft cab movement during braking and acceleration. On heavy tractor units, drivers may also report delayed cab stabilisation after a roundabout or uneven road surface.

First inspect the shock absorbers visually for hydraulic oil leaks, dented cylinder tubes, damaged piston rods, corrosion around the seal and torn rubber-metal bushes. A light oil film may result from preservation treatment, but wet deposits with dirt accumulation around the oil seal indicate an internal leak. Then inspect the mounting bolts, spacer sleeves and mounting brackets for elongated holes, insufficient tightening torque or metal-to-metal contact. Measure the cab ride height on the left and right sides on level ground; a height difference is more likely to indicate springs, air bellows, ride-height control or silent blocks rather than the shock absorber alone.

Unload the cab in accordance with the manufacturer's instructions and inspect the shock absorbers throughout their full stroke. The piston rod must be straight, dry and free from pitting corrosion. With the shock absorbers removed, resistance during compression and extension must be smooth and even, without dead travel, sticking or audible air displacement. Always compare the left and right sides; significantly lower resistance indicates a loss of oil or gas pressure. Do not use an open flame to loosen seized mountings: heat damage to rubber bushes and nearby cable insulation can cause subsequent faults.

Preferably replace cab shock absorbers in matching pairs on the left and right sides. Where necessary, replace both the front and rear cab shock absorbers. A new shock absorber combined with a collapsed or leaking counterpart causes uneven damping behaviour and may increase torsional stresses in cab brackets, tilt points and locking components. Also replace worn mounting bushes, spacer washers and self-locking nuts where specified by the manufacturer. Install the shock absorber in the correct direction and tighten the mountings to their final torque only when the cab is at normal ride height, so that the rubber bushes are not preloaded.

After installation, check the cab closure, free movement of wiring harnesses and lines, door alignment and operation of the tilting mechanism. Visible leakage, recurring knocking or a tilted cab after replacement requires inspection of the cab spring, integrated shock absorber air bellows, support bracket and chassis console. Correct fitment and rapid parts availability help minimise truck downtime.