MacPherson Struts

The MacPherson strut is the most widely used front suspension design globally, combining the shock absorber and a structural load-bearing member into a single unit that also locates the wheel hub. Unlike a conventional shock absorber which only damps spring movement, the strut's outer tube is a structural component — the wheel hub knuckle is attached directly to the bottom of the strut body, and the strut bears the lateral and braking forces at the top through its upper mount. This integration simplifies the suspension geometry and reduces the number of components compared to double-wishbone designs, making it the dominant design for front-wheel-drive vehicles. The strut assembly consists of the outer spring perch and bump stop, the coil spring (retained on the spring perch), the hydraulic damper insert inside the body, and the upper strut mount that attaches to the inner wing. The upper mount typically contains a bearing (thrust bearing) that allows the strut body to rotate as the wheels steer, isolating the body from strut rotation. The complete strut assembly (spring, damper, and mount) is commonly replaced as a unit for efficiency, though the damper insert can be replaced independently on some designs.

The MacPherson strut performs three functions simultaneously in a single assembly: it supports the coil spring that bears vehicle weight, it damps spring oscillation through its internal hydraulic mechanism, and it provides the rigid structural member that locates the wheel hub laterally and longitudinally relative to the chassis. The lower end of the strut is rigidly connected to the steering knuckle — the knuckle cannot move independently of the strut body. The upper mount provides the pivot point around which the entire assembly rotates when steering is applied, and the strut bearing within the mount reduces the friction of this rotation. When the wheel is steered, the strut body and spring rotate together about the axis of the strut — the spring's installed position and any preload must be oriented correctly for proper operation. Incorrect spring seating during assembly causes noise and handling imbalance. The strut's rigid connection to the knuckle means that any wear or damage to the upper mount directly affects wheel alignment — a failed mount with play allows the camber angle to vary, causing handling instability and tyre wear. The bump stop inside the strut limits maximum compression travel, protecting the damper and chassis from metal-to-metal contact under severe inputs.

Upper strut mount bearing failure: The rotating bearing in the upper mount wears with age and mileage, developing roughness or seizing. Symptoms include a grinding or scraping sensation during parking manoeuvres when the wheel is at full lock, a clunking noise over bumps as the worn mount allows play, and in severe cases a complete seizure that prevents smooth steering. The top mount rubber element also deteriorates, transmitting more road noise and vibration into the body.

Damper insert failure: The hydraulic damper inside the strut body degrades in the same way as a conventional shock absorber. Oil leakage around the piston rod, excessive body bounce, and cupped tyre wear are the primary indicators. Damper insert-only replacement is possible on some strut designs but requires the same spring compressor tooling as complete strut replacement.

Strut body corrosion: In high-salt environments, the steel strut body develops surface rust that can progress to structural wall thinning in severe cases. Inspect for any perforation or deep pitting in the spring perch area, where the spring's end load concentrates stress on the body.

Level 1 — Monthly Steering Feel Test: Turn the steering wheel slowly from lock to lock while stationary and listen for any grinding, grating, or clunking sounds from the front suspension. These sounds originate at the upper strut mount bearing and indicate wear requiring inspection. Any change in steering feel — increased effort, notchiness, or vibration transmitted to the steering wheel — warrants professional inspection of the strut mounts and bearings.

Level 2 — Inspection at Service: Visually inspect the upper strut mount area through the bonnet or inner wing for corrosion, distortion, or cracking at the mounting points. Bounce test all four corners and inspect for oil weeping from the strut body. Inspect bump stop condition when the vehicle is on a lift with wheels hanging free — the bump stop should be intact and not compressed from permanent deformation.

Level 3 — Professional Replacement: Complete strut assembly replacement requires a spring compressor, proper upper mount nut tooling, and torque wrench access to the mount bolts. Replace in axle pairs. After replacement, a four-wheel alignment check is mandatory — the camber and caster angles are partially determined by the strut's position in the body mount. Where possible, replace the complete assembly (spring, damper, and mount) to avoid disturbing a recently replaced spring when the damper fails later.