Strut Top Mounts

The strut top mount (also called the upper strut mount or strut bearing mount) is the assembly at the top of a MacPherson strut that attaches the strut to the vehicle's inner wing, provides the upper pivot point for steering rotation, isolates the spring and damper forces from the body, and houses the strut bearing that allows the strut body to rotate when the wheels are steered. The mount consists of a central metal plate bolted to the inner wing through three or four threaded studs, a rubber element bonded between the metal plates that isolates the strut's vibration from the body, and a thrust bearing or ball bearing at the centre through which the strut piston rod passes. The bearing allows the strut body and spring to rotate about the strut axis during steering without transmitting the rotational friction load to the body structure. The rubber compliance element absorbs the high-frequency road vibrations travelling up the strut piston rod. Without the mount's isolation, every small road texture input would transmit directly into the body panel as harshness and noise. Top mounts are wear items — the rubber element fatigues with age, and the bearing wears with the millions of steering movements and vibration cycles it experiences. Top mounts are almost always replaced together with the shock absorber or strut during suspension service, since accessing the top mount requires the same disassembly as the strut itself.

The strut top mount performs three functions: structural attachment, isolation, and rotation bearing. As a structural attachment, it must support the entire corner weight of the vehicle transmitted through the strut spring, plus the vertical forces from road inputs through the damper, plus the lateral forces that attempt to rock the top of the strut in its mount. The rubber compliance element in the mount allows a small controlled amount of angular movement, reducing the stress concentration at the mounting points in the body. As an isolation element, the rubber compound is carefully formulated to have high stiffness in the load-bearing (vertical) axis while maintaining compliance in the horizontal and rotational axes to absorb vibration. As a rotation bearing, the thrust bearing at the strut's piston rod provides the low-friction pivot around which the strut body and spring rotate during steering. Without this bearing, steering effort would be transferred through the spring into twisting friction in the rubber element, producing a notchy or stiff steering feel. Worn bearings produce a grinding or groaning noise during low-speed parking manoeuvres and can seize completely, making the steering feel notchy and heavy on slow turns.

Bearing wear producing grinding noise on steering: The strut bearing operates in a corrosive environment and under high loads, causing progressive wear. The first symptom is a faint grinding or creaking sensation felt through the steering wheel during slow parking manoeuvres when the strut body is rotating. This progresses to a distinct grinding sound. In severe cases, the bearing seizes and the spring must twist instead of the bearing rotating, causing heavy and notchy steering. Any grinding on slow steering lock should be inspected before the next long trip.

Rubber element deterioration: The rubber compliance element cracks and separates from its bonded metal plates with age. When the rubber fails, the strut piston rod can contact the metal housing, producing a sharp clunking noise over bumps. Road vibration transmission into the body also increases significantly. In extreme cases, the piston rod can punch through the deteriorated mount under a severe impact.

Loose or corroded mounting studs: The three or four studs that attach the top mount to the inner wing can corrode and loosen, allowing the entire mount assembly to move slightly. This produces clunking sounds from the engine bay over bumps and can cause the strut to move laterally at the top, affecting wheel alignment.

Level 1 — Steering Noise Assessment: Perform slow, full-lock-to-full-lock steering manoeuvres (such as in a parking scenario) and listen for grinding, creaking, or groaning from the front suspension area — this is the characteristic symptom of strut bearing wear. Any noise produced specifically during slow-speed steering at significant lock angle (rather than over bumps at speed) points to the top mount bearing as the primary suspect.

Level 2 — Inspection with Strut Service: Always inspect the top mount when any front suspension work requires strut removal. Check the rubber element for cracking and separation from its metal plates. Check the bearing for roughness by rotating the strut body by hand. Inspect mounting stud threads for corrosion. Replace the top mount as a matter of course whenever replacing the strut damper — the incremental cost is minimal compared to the labour of re-accessing the mount.

Level 3 — Professional Replacement: Top mount replacement requires strut removal and a spring compressor to safely disassemble the strut assembly. This is a professional operation. After replacement, confirm the spring is correctly seated on its perch and the mount's orientation is correct relative to the vehicle centreline before final torquing. Four-wheel alignment check after any strut top mount replacement.