Coil Springs

Coil springs are helical steel springs that support the vehicle's weight, absorb road impacts, and determine ride height. On modern independent suspension designs, each wheel has its own coil spring — either mounted concentrically around the strut (combined strut-spring assembly on MacPherson strut designs) or positioned separately between the lower control arm and chassis on double-wishbone designs. The spring's fundamental characteristic is its rate — the force required to compress it by one millimetre, typically 15–35 N/mm for passenger car front springs and 10–25 N/mm for rear springs. A higher spring rate produces firmer, more controlled handling at the expense of ride comfort; a lower rate provides a more compliant ride but allows greater body roll. Spring rate is determined by the wire diameter, coil diameter, and number of active coils. Springs are manufactured from high-tensile spring steel alloy, heat-treated and shot-peened to introduce compressive surface stresses that resist fatigue cracking. Progressive-rate springs — which use variable coil spacing to provide a softer initial rate that stiffens as compression increases — are common on passenger vehicles to balance everyday comfort with handling under load.

Coil springs serve two simultaneous functions: supporting the vehicle's sprung mass against gravity, and absorbing kinetic energy when the suspension compresses over road irregularities. The spring's force at its installed length must equal the weight it supports at that corner of the vehicle — a correctly specified spring holds the vehicle at the correct ride height, placing all suspension components in their designed geometric positions. When the wheel encounters a bump, the spring compresses, absorbing the upward impact force and preventing it from transmitting directly as a shock jolt to the chassis. The compressed spring stores this energy and attempts to return to its natural length, driving the wheel back down. Without a shock absorber to control this return, the stored energy would drive the wheel down past the rest position in an oscillation that would continue indefinitely. Springs do not wear in the conventional sense — they do not have friction surfaces or fluid seals — but they do fatigue and permanently shorten (settle) over time and high-cycle use. A spring that has settled reduces the corner's ride height, changing alignment geometry and tyre contact angle, and reduces suspension travel clearance above the bump stop.

Spring sag or permanent height loss: High-stress cycling over decades causes the spring steel to yield slightly, permanently reducing the spring's free length. This manifests as one corner of the vehicle sitting lower than its specification, causing asymmetric handling, uneven tyre wear, and bump stop contact on moderate road inputs. Springs cannot be restretched or adjusted — replacement is required. Replace in axle pairs to restore balanced ride height.

Spring fracture: A clean break through one coil, typically at the coil end where stress concentration is highest. A fractured spring allows the affected corner to drop suddenly, bringing the coil end into contact with the tyre or surrounding components. First symptom is often a sudden change in ride height or a scraping noise from the wheel arch. This is a safety-critical failure — the vehicle should not be driven until the broken spring is replaced.

Corrosion cracking: Surface corrosion on the spring wire progressively reduces the effective cross-sectional area, creating stress concentration points that initiate fatigue cracks. Pre-treatment of the wire and post-manufacture coating protect against this, but both deteriorate over time in harsh environments. Inspect spring surfaces for rust penetration at every strut service.

Level 1 — Ride Height Monitoring: Check vehicle ride height quarterly by measuring from the wheel arch to the centre of the wheel at all four corners. Compare measurements side to side — a difference greater than 15mm on the same axle indicates spring sag and requires workshop inspection. Listen for unusual clunking at the end of suspension travel that may indicate a spring coil contacting the bump stop — this suggests either a settled spring or an overloaded suspension.

Level 2 — Inspection at Strut Replacement: Always inspect springs when replacing struts — the spring is removed as part of the strut disassembly. Check for corrosion pitting (particularly at the lower coil end and the spring seat), cracks in the coil, and compare free length to specification. Replace any spring showing pitting, cracking, or settling beyond specification. Spring replacement cost is small relative to the labour already performed for strut replacement — replace preventively if condition is marginal.

Level 3 — Professional Replacement: Spring compression during strut disassembly requires a dedicated spring compressor tool. Coil springs store significant energy under compression — approximately 1,500–3,000 joules in a typical passenger car spring at ride height. This energy can be released violently if a spring compressor slips or fails. Spring replacement is not a hand-tool repair. After replacement, four-wheel alignment must be verified — new springs change ride height and therefore suspension geometry.