2.2 Front Suspension Springs and Struts

Front Suspension Springs and Struts

Front suspension springs and dampers support the vehicle's weight, maintain correct ride height, and control vertical wheel movement. Understanding their operation, failure modes, and adjustment procedures is essential for diagnostic accuracy.

Coil Springs and Ride Height Diagnostics

Coil springs are made of tempered spring steel wound in a spiral. They compress under load to absorb road shocks.

Spring Rate Characteristics

• Linear Rate Springs: Feature constant wire diameter and uniform spacing between coils. The force required to compress the spring increases linearly with compression distance.
• Progressive Rate Springs: Have variable spacing or varying wire diameters. They offer a softer ride under light loads but stiffen as suspension travel increases, preventing bottoming out.

Ride Height Measurement and Sag

Correct ride height is critical because it directly determines suspension alignment angles. A sagging spring alters caster, camber, and toe:

• Diagnostic Symptom: Vehicle pulls to one side, exhibits uneven tire wear, or bottoms out over bumps.
• Measurement Procedure: Always measure ride height with the vehicle on a level surface, tires inflated to placard specs, fuel tank full, and no passengers or cargo. Measure at the manufacturer's specified points (e.g., from the lower edge of the fender lip through the center of the wheel to the ground, or from specific frame/crossmember points to the ground).
• Side-to-Side Comparison: Compare measurements to specification. Typically, a side-to-side variation of more than 0.5 inches (12.7 mm) indicates sagged springs that require replacement. Always replace coil springs in pairs across the axle to maintain balance.

Inspection

Inspect coil springs for paint chipping, severe rust pitting, or cracks. Deep rust pitting creates "stress risers" where the spring is likely to snap. Look for aftermarket spring spacers, which are temporary fixes for sagging springs and indicate the spring has failed.

---

Torsion Bar Operation and Adjustment

Torsion bars serve the same purpose as coil springs but operate by twisting instead of compressing. One end of the bar is anchored to the vehicle frame, and the other is attached to the control arm.

Directional Stressing

Torsion bars are pre-stressed during manufacturing to twist in a specific direction. They are marked with an "L" or "R" for left or right side. Swapping torsion bars side-to-side will result in rapid fatigue, sagging, or catastrophic snapping under load.

Ride Height Adjustment Procedure

Unlike coil springs, torsion bars allow ride height adjustment:

1. Locate the adjusting bolt on the torsion bar key at the frame crossmember.
2. Tightening (turning clockwise) the adjusting bolt increases the twist on the bar, raising the vehicle's ride height.
3. Loosening (turning counterclockwise) the bolt lowers the ride height.
4. Diagnostic Check: After adjusting, bounce the vehicle several times to settle the suspension before re-measuring the trim height (often referred to as Z-height or D-height depending on the manufacturer).
5. Perform a full wheel alignment after any torsion bar adjustment, as ride height changes directly alter caster and camber.

---

MacPherson Strut Systems and Upper Mounts

A MacPherson strut is a structural suspension design that combines the coil spring, damper (shock absorber), and steering pivot into a single unit. It eliminates the need for an upper control arm.

The Upper Strut Bearing Plate and Mount

The upper strut mount connects the top of the strut assembly to the vehicle body. Because the strut rotates when the wheels are steered, the upper mount contains a thrust bearing.

Diagnostic Symptoms of Bearing Failure

• Popping, Groaning, or Creaking: Occurs when turning the steering wheel, particularly at low speeds or while stationary. This is caused by dry, rusted, or binding bearings.
• Memory Steer: The vehicle tends to pull in the direction of the last turn. The driver must actively pull the steering wheel back to center because the binding upper bearing prevents the wheels from naturally returning to center.
• NVH Transmission: A worn rubber isolator within the mount transmits road noise and sharp thuds into the passenger cabin over bumps.

Service and Safety

To service a MacPherson strut, the coil spring must be compressed using a heavy-duty spring compressor.

[!WARNING] The coil spring is under thousands of pounds of stored energy. Never remove the strut center shaft nut without fully compressing the spring. Failure to do so will cause the spring and mount to fly off with lethal force.

Ensure the upper mount is properly indexed (aligned) to the lower strut housing according to manufacturer marks during assembly to prevent spring binding or alignment issues.

---

Short-Long Arm (SLA) Suspension Geometry

The Short-Long Arm (SLA) or double-wishbone suspension utilizes two control arms. The upper arm is shorter than the lower arm.

Camber Gain and Track Width Management

• Equal-Length Arms (Theoretical): If the upper and lower arms were the same length, the wheel would maintain its static camber angle during suspension travel, but the track width (distance between tires) would change significantly, causing tire scrubbing and lateral instability.
• SLA (Unequal Lengths): As the lower arm travels upward (jounce), the shorter upper arm pulls the top of the steering knuckle inward. This creates negative camber gain during suspension compression. Negative camber gain is desirable during cornering because it tilts the outside tire into the turn, keeping the tread flat against the road surface as the body rolls.

---

Shock Absorbers (Dampers)

Shock absorbers do not support vehicle weight; their sole purpose is to control spring oscillations. They convert kinetic energy from suspension movement into thermal (heat) energy by forcing hydraulic fluid through calibrated orifices.

Diagnostic Failures and Inspection

• The Bounce Test: Forcefully push down on a corner of the bumper and release it. A functional shock absorber should stabilize the vehicle within 1 to 2 cycles. Continued bouncing indicates worn dampers.
• Fluid Leakage: Inspect the shock body. A light film of oil mist (weeping) on the upper housing is normal and aids seal lubrication. Active wet oil dripping down the sides of the shock indicates seal failure; the unit must be replaced.
• Tire Wear Patterns: Worn dampers allow the tire to bounce repeatedly on the road, causing cupping or scalloping (alternating high and low worn spots across the tread).
• Physical Damage: Check for a bent piston rod, dented outer housing (which binds the internal piston), or torn mounting bushings.