2.5 Shock Absorbers and Strut Replacement
Key Takeaways
- While shocks only control spring oscillation, struts serve a structural role, acting as the upper suspension locating member and carrying lateral loads.
- Diagnostic testing includes the bumper bounce test, visual inspection for active oil leaks vs. normal misting, and checking for physical damage.
- Spring compressor safety is paramount: hooks must be fully seated, safety pins locked, and the compressor aligned parallel to the spring, never standing in line with the spring ends.
- MacPherson strut replacement requires checking and adjusting wheel alignment (camber and caster are often affected because the strut acts as the steering axis).
- Torque specifications are critical, particularly for the mount locknut and spindle-to-strut bolts, which must be tightened at normal ride height to prevent bushing pre-load.
Shock Absorbers and Strut Replacement
Dampeners are critical safety-related components that control the rebound and compression speed of vehicle springs, maintaining tire contact with the road. While shock absorbers and MacPherson struts perform similar dampening functions, their structural roles and service procedures differ significantly.
Hydraulic Dampening Principles and Component Differences
Dampeners convert the kinetic energy of suspension motion into thermal (heat) energy through hydraulic resistance.
- Operating Mechanism: Inside the dampener body, a piston is attached to a polished shaft. As the suspension moves up and down, the piston pushes through hydraulic oil. The oil is forced through calibrated orifices and spring-loaded valving discs in the piston. This restriction creates resistance, damping spring oscillations. Most modern dampers are gas-charged, utilizing pressurized nitrogen gas (typically 100 to 200 PSI) to put pressure on the oil. This prevents aeration (foaming), which causes "fade" (loss of dampening efficiency under rapid cycling).
Shocks vs. Struts: Structural Comparison
- Shock Absorbers: Do not carry vehicle weight or locate the suspension. The shock absorber is mounted parallel to the suspension spring. Removing a shock absorber does not cause the suspension to collapse and has no direct effect on wheel alignment (camber and caster).
- MacPherson Struts: Serve as a structural locating member of the suspension system. The strut assembly integrates the coil spring, damper, and upper steering pivot mount. The strut housing forms the upper control arm link and steering axis. Removing a MacPherson strut assembly will cause the wheel spindle/knuckle to flop outward, and replacing it requires a wheel alignment because it directly affects camber and caster angles.
Diagnostic Inspection and Wear Indicators
Technicians must use a combination of physical tests and visual inspections to diagnose worn or failed dampeners.
The Bumper Bounce Test
This test evaluates overall damping control but is subjective and best used as an initial indicator.
- Push down firmly on the vehicle bumper at one corner three times to induce maximum suspension travel.
- Release the bumper at the bottom of the stroke.
- Observe the vehicle body recovery. A functional dampener should stabilize the vehicle within 1 to 1.5 cycles (one full upward stroke, one minor downward settle, and stop).
- If the vehicle continues to bounce (oscillate) two or more times, the dampeners are severely worn.
Visual and Physical Inspection
- Fluid Leakage: Differentiate between "misting" and "dripping."
- Misting/Sweating: A light film of oil mixed with road dust on the upper body of the shock/strut is normal and does not require replacement. It occurs when a tiny amount of oil lubricates the shaft seal during normal operation.
- Active Dripping: Wet, flowing oil running down the damper body indicates a failed shaft seal. The unit must be replaced (always in pairs across the axle).
- Physical Damage:
- Bent Piston Rod: Causes binding during travel and destroys the shaft seal.
- Dented Housing: Restricts piston travel internally, leading to harshness or sudden locking of the suspension.
- Pitted/Corroded Rod: A pitted shaft damages the rubber shaft seal as it enters the housing, causing fluid loss.
- Torn Bushings: Lower mount bushings can crack or tear, creating a distinct metallic clunking noise over bumps.
Strut Disassembly and Replacement Procedures
Replacing a MacPherson strut cartridge or unit requires spring compression, which presents significant safety hazards.
The Spring Compressor: Operation and Safety
A compressed coil spring stores thousands of pounds of mechanical energy. A sudden release can cause catastrophic injury.
- Inspection: Inspect the spring compressor threads. Clean and lubricate them with high-pressure grease. Never use a tool with worn or cross-threaded components.
- Mounting: Place the compressor hooks on the spring coils, spacing them 180 degrees apart (directly opposite each other) to ensure even compression. Ensure all safety pins or locking jaws are fully engaged.
- Compression: Tighten the compressor bolts evenly in alternating steps. Use manual hand tools (sockets and ratchets) rather than impact wrenches, unless the tool manufacturer explicitly permits impact use. Compress the spring only until the tension is relieved from the upper strut mount bearing plate (test by checking if the upper mount rotates freely). Do not over-compress.
- Directional Safety: Never stand directly in line with either end of the strut shaft or spring. Point the ends away from yourself and others.
- Disassembly: Once tension is relieved, use a strut shaft holding tool and a pass-through wrench to remove the upper piston rod locknut. Never use a standard impact wrench on the shaft nut without holding the shaft, as the shaft will spin, destroying the internal seals of the damper or spinning the piston off the shaft inside.
- Inspection of Associated Parts: Inspect the upper mount bearing (should rotate smoothly without roughness or binding), rubber spring isolators (replace if torn or dry-rotted), and the bump stop/dust boot (replace if disintegrated or torn).
Cartridge vs. Strut Assembly
- Strut Cartridge (Replaceable Inner): Some older struts allow the technician to unscrew a gland nut on top of the outer housing and slide out the inner damper cartridge, replacing only the internal unit.
- Complete Strut Assembly (Unit): The entire strut assembly (housing, spring, mount) is replaced as a unit.
- Quick-Strut/Loaded Strut: Pre-assembled units that include the spring, mount, and damper. These do not require a spring compressor, making them faster and safer to install.
Torque Specifications and Bushing Pre-load
Reassembling and installing suspension components requires strict adherence to torque specifications.
| Fastener Location | Typical Torque Range | Rationale |
|---|---|---|
| Strut Shaft Locknut | 35 – 50 lb-ft (47 – 68 Nm) | Prevents shaft movement without striping threads or crushing internal seals |
| Upper Mount-to-Chassis Nuts | 20 – 35 lb-ft (27 – 47 Nm) | Secures upper plate; over-torqueing breaks studs |
| Strut-to-Spindle Knuckle Bolts | 80 – 120 lb-ft (108 – 163 Nm) | Structural joint; carries braking and cornering forces |
Preventing Bushing Pre-Load (Wind-Up)
When tightening the lower strut mounting bolt or control arm pivot bolts, the suspension MUST be at curb height (normal ride height with weight on wheels).
- The Issue: If these fasteners are torqued while the vehicle is raised on a frame hoist (suspension hanging in full droop), the inner metal sleeve of the rubber bushing is locked in a twisted state once the vehicle is lowered to the ground.
- The Consequence: The rubber bushing is subjected to continuous torsional stress, causing a harsh ride, slightly elevated ride height, and premature tearing of the bushing rubber.
- Correct Procedure: Lower the vehicle onto drive-on ramps or support the lower control arm with a screw jack to compress the suspension to normal ride height before final torqueing.
A technician is replacing a front MacPherson strut cartridge. When removing the upper strut shaft locknut, which of the following is the correct procedure?
While inspecting a vehicle's shock absorbers, a technician notices a dry, light layer of oil film mixed with road dust on the upper section of the shock absorber body. The vehicle has no history of ride control complaints. What is the correct action?
A technician tightens the lower strut mount bushing bolts while the vehicle is supported on a frame-contact lift with the suspension hanging down in full droop. What is the most likely result of this action?