Suspension Springs Guide: Types, Spring Rate, Sag, and Diagnosis

Spring Rate and Wheel Rate

Spring rate is measured in pounds per inch (lb/in) in the US system — the amount of force required to compress the spring one inch. A 300 lb/in coil spring needs 300 pounds of force applied to compress it one inch, 600 pounds for two inches, and so on. Spring rate is a linear relationship for most automotive coil springs through their operating range.

Wheel rate is different from spring rate and is often confused with it. Wheel rate is the effective stiffness as experienced at the tire contact patch — it accounts for the mechanical advantage (or disadvantage) of where the spring mounts relative to the control arm pivot. If a spring mounts at the midpoint of a control arm, one inch of wheel travel produces only half an inch of spring compression — the motion ratio is 0.5. The wheel rate equals the spring rate multiplied by the motion ratio squared (a consequence of the geometry). A 300 lb/in spring with a 0.5 motion ratio produces a 75 lb/in wheel rate — significantly softer than the spring rate alone suggests.

This matters when choosing replacement springs. A stiffer spring does not necessarily produce a stiffer ride if the motion ratio is low. And a "sport spring" that claims to be stiffer than stock may have a higher spring rate but the same wheel rate if the motion ratio is equal.

Coil Springs

Coil springs are the dominant spring type on modern passenger cars and most light-duty trucks with independent suspension. They are helical-wound steel wire (or increasingly, composite materials on some performance applications) that compress axially. The design is simple, compact, easily manufactured in a wide range of rates, and easy to package in a strut assembly (coilover) or alongside a separate damper.

Coil springs can be designed with uniform pitch (equal spacing between coils — constant spring rate) or variable pitch (tighter at one end — progressive spring rate). A progressive rate spring starts soft for small bumps and becomes stiffer as it compresses further. This gives a compliant initial feel on minor road inputs while preventing bottoming out under heavy load or hard cornering.

Coil spring failure is typically either fracture (the coil cracks and breaks, usually at the end of the spring where stress concentrations are highest) or sag (the spring takes a permanent set and loses free length). Both cause ride height drop. A broken coil spring on a MacPherson strut is particularly important to catch because the broken section can escape the spring perch and contact the tire — a dangerous condition.

Leaf Springs

Leaf springs are stacks of flat steel strips (leaves) of progressively shorter length, clamped together at the center. The main leaf is the full-length leaf that attaches to the vehicle at each end (typically via an eye at the front and a shackle at the rear to allow length change during compression). Additional leaves are added for higher load capacity and rate.

Leaf springs are primarily used on rear axles of trucks, vans, and older rear-wheel-drive cars. They are simple, durable, and can handle high vertical loads. The solid axle typically mounts directly to the center of the spring pack — the spring does double duty as both the spring element and the axle locating device, controlling axle position fore-aft and side-to-side (in the simplest designs with no separate track bar).

Leaf spring failures: the main leaf cracks (usually near the center eye mount), inter-leaf friction causes a harsh, choppy ride (especially on older multi-leaf packs without inter-leaf liners), center bolts break allowing the axle to shift position, and shackles wear or bushings collapse. Modern mono-leaf or composite designs address most of the harsh ride issues of older multi-leaf stacks.

Inspect leaf springs by looking for cracks in any leaf (usually visible as a kink or gap), checking that the main eye and shackle bushings are not collapsed or cracked, and verifying that the axle is centered under the vehicle (equal measurement from the axle to the body on each side). An asymmetric measurement means a shifted axle from a broken center bolt or main leaf.

Torsion Bars

A torsion bar is a long, straight steel bar that provides spring force by twisting rather than compressing. One end anchors rigidly to the frame. The other end connects to the lower control arm. As the suspension compresses, the control arm rotates and twists the bar. The bar's resistance to torsion provides the spring force. The farther the arm rotates, the more twist in the bar, the more spring force resisting further movement.

Torsion bars are compact in the lateral dimension — they run longitudinally along the vehicle rather than occupying strut tower space. They were common on front SLA suspensions of domestic trucks (GMT800/900 platforms, Ram 1500 prior to coil conversion) and are still used on some European designs. An adjustment nut at the frame anchor allows ride height adjustment — a significant maintenance advantage over coil springs that require complete spring replacement to change ride height.

Torsion bar failure: actual fracture is uncommon. More common is spring sag — the metal fatigues and the bar takes a set, dropping ride height. The adjustment nut can compensate for minor sag, but a bar that has sagged significantly needs replacement. Check the adjustment nut threads for corrosion — on high-mileage trucks, the adjuster bolt can be completely frozen and must be cut off before height adjustment or spring replacement is possible.

Air Springs

Air springs (air bags) replace conventional steel springs with reinforced rubber/fabric bellows filled with compressed air. The spring rate is determined by the air pressure and the effective bellows area — increasing pressure increases the spring rate and ride height, decreasing pressure softens the ride and lowers the vehicle.

Active air suspension systems (Lincoln Continental, Range Rover, Mercedes Airmatic, BMW EHC) use an onboard compressor, height sensors at each corner, and a control module to continuously adjust air pressure. The system maintains constant ride height regardless of load, can vary ride height for improved aerodynamics at speed or increased clearance off-road, and can adjust spring rate for different driving modes.

Air spring failures: the bellows creak, crack, or develop leaks as the rubber deteriorates. A leaking air spring causes the corner to slowly sink — the compressor runs more frequently trying to maintain height and eventually can't keep up. Height sensor failures cause incorrect spring pressures. Compressor failures leave the system unable to add pressure. Control module faults cause incorrect height commands.

Diagnosing air suspension: listen for the compressor running at abnormal times (should only run briefly after start or when height adjustment is commanded). Check for codes in the air suspension module. Use live data to monitor height sensor readings vs. target heights. If one corner is consistently lower than commanded, that corner has a leak. Spray soapy water on the bellows seams and air line connections with the system pressurized to find the leak location.

Spring Sag: Diagnosis and Measurement

All steel springs eventually develop permanent set — a gradual shortening of their free length as the metal fatigues over years of compression cycles. This is spring sag, and it's a natural end-of-life failure mode that isn't sudden — it happens gradually over high mileage.

Symptoms of spring sag: the vehicle sits lower than it used to (or lower than spec). Because sag rarely happens symmetrically, the vehicle may appear to lean to one side. Alignment may be off because ride height affects caster and camber on most suspension designs. The suspension may feel like it bottoms out more easily.

Measurement: check ride height at all four corners per the OEM procedure — this usually involves measuring from a specified point on the body or frame to the ground, or measuring the distance between the lower control arm and the body at a specified location. Compare to the OEM spec. If a corner is more than 1/2 inch below the minimum spec, spring replacement is indicated. Always compare the two front measurements to each other and the two rear measurements — a difference of more than 1/2 inch side to side indicates one spring is sagged more than the other.

Broken Spring Diagnosis

A broken coil spring is usually visually obvious once you look for it — there's a gap in the coil stack, often at the top or bottom where the spring contacts its perch and stress concentrations are highest. The corner sits noticeably lower. On a MacPherson strut, the break is sometimes near the bottom of the spring where it contacts the lower strut spring seat.

The danger with a broken spring on a strut: the broken end of the coil can shift outward and contact the tire sidewall. This causes rapid tire damage and in a worst case, the broken spring end punctures the tire. Any time a vehicle comes in with unusual tire damage on the inner sidewall or a corner that's sitting low, check the spring immediately before road testing the vehicle.

Broken leaf springs are harder to miss on a visual — the axle may be shifted laterally, the rear of the vehicle sits lower on one side, or the vehicle has an obvious lean. Find the fracture by inspecting each leaf from eye to eye. Cracks often start as corrosion pitting that concentrates stress until the leaf fractures completely.

Matching Rates Side to Side

When replacing springs, always replace in axle pairs — both front or both rear at the same time. Never replace a single spring unless the companion spring has been verified to match the replacement rate exactly and has adequate remaining life. Mismatched spring rates create a vehicle that handles differently left vs. right: it will lean more on the softer side during cornering, settle differently after bumps on each side, and may have a persistent ride height difference that cannot be corrected by alignment.

The same logic applies to choosing replacement springs. Do not mix brands or grades side to side. If you're installing performance springs on one side, the same springs go on the other side. Rate matched pairs, same manufacturer, same part numbers. This seems obvious but I've seen it done wrong — one side gets upgraded and the other side keeps the old spring because "it still looks fine." That's not the way to do it.

Frequently Asked Questions