Coilover Glossary: 60 Terms in Plain English

Coilover. A damper with a coil spring mounted around it, usually with threads to adjust ride height. Our guide what are coilovers covers the basics.

Coilover kit. A full set of four coilovers, or front coilovers plus rear shocks and springs, built for one car.

Damper. The shock absorber. It controls how fast the spring compresses and extends by forcing oil through valves.

Monotube. A damper with one tube. Oil and high-pressure gas are kept apart by a floating piston. BC Racing uses nitrogen-charged monotube dampers in every series.

Twin-tube. A damper with an inner working tube and an outer reservoir tube. KW V1, KW V2 and KW V3 use twin-tube dampers. Our monotube vs twin tube guide compares the two.

MacPherson strut. A suspension layout where the strut also locates the wheel. Common on the front of many cars.

Inverted strut. A strut with the damper mounted upside down inside its housing. It is chosen for bending stiffness under cornering loads.

Top mount. The mount that bolts the upper end of the coilover to the car's body. It may use rubber or a spherical bearing.

Camber plate. An adjustable top mount with slots that let you move the top of the strut to set camber.

Threaded body. The threaded section of the damper body where the perch and lower mount adjust.

Lower mount. The bracket that bolts to the knuckle or arm. On full-length kits, it threads on the body to set ride height.

Spring perch. The threaded seat the spring sits on. On full-length kits, it sets preload.

Locking collar. A ring that locks the perch or lower mount in place. BC Racing describes its lower locking ring as a patented concave design.

Seal. The part at the top of the damper body that keeps oil in and dirt out as the shaft moves.

Dust boot. A rubber or plastic sleeve over the shaft that keeps grit away from the seal.

Bump stop. A cushion that stops metal from hitting metal at the end of compression travel.

External reservoir. A separate canister that holds extra oil and gas. It helps the damper shed heat under sustained track use.

Spring rate. The force needed to compress a spring a set distance. Higher rates resist roll and pitch more.

kg/mm and lb/in. The two common spring rate units. Multiply kg/mm by 55.9974, or about 56, to get lb/in.

Linear spring. A spring whose rate stays the same through its working travel.

Progressive spring. A spring whose rate rises as it compresses.

Main spring. The spring that carries the car's weight at ride height.

Helper spring. A light spring that keeps the main spring seated when the wheel hangs. It adds almost no rate at ride height.

Tender spring. A softer spring stacked with the main spring to give a dual rate until it goes solid.

Preload. Free length (unloaded length) minus installed length, measured with the wheel hanging. It does not change the spring rate. Our guide to coilover preload explains how to set it.

Coil bind. When the coils touch each other and the spring cannot compress further. It is set by spring design and travel.

Spring sag. Slow relaxation of the spring material. It shortens free length and drops ride height, but the rate barely changes.

Motion ratio. How far the spring moves compared with the wheel. Our spring physics guide goes deeper.

Wheel rate. The rate the tire feels. Wheel rate equals spring rate times the motion ratio squared.

Compression. Damping as the suspension squeezes together, such as over a bump.

Rebound. Damping as the suspension extends again after a bump.

Low-speed compression. Damping at slow shaft speeds. It shapes body roll, dive and squat.

High-speed compression. Damping at fast shaft speeds. It shapes how the car handles sharp hits and curbs.

Single adjustable. One knob changes damping. BC Racing BR has 30-click simultaneous compression and rebound.

Double adjustable. Separate knobs for compression and rebound. KW V3 adjusts rebound and compression independently.

Three-way adjustable. Separate rebound, low-speed compression and high-speed compression. BC Racing ZR and ZX work this way.

Clicks. The detents on an adjuster knob. Count and direction differ by brand, so start from your manual's setting.

Valving. The shims and ports that set how much force the damper makes at each shaft speed.

Digressive valving. Valving that builds force quickly at low shaft speeds, then levels off. Fortune Auto states it uses digressive valving.

Nitrogen charge. Gas inside a damper. Nitrogen is dry and inert, so its pressure behaves predictably as it heats.

Damper fade. Mainly a reversible drop in oil thickness as the damper gets hot, which softens damping.

Cavitation. Vapor bubbles that form when local oil pressure drops below the oil's vapor pressure. It is not caused by heat.

Hysteresis. Force lag between the opening and closing parts of a stroke at the same shaft speed.

Ride height. How high the car sits, usually measured from hub center to fender lip.

Full-length adjustment. Ride height is set at the lower mount, separate from preload, so the damper keeps its stroke.

Perch-only adjustment. Also called a sleeve kit. The perch sets ride height and preload together.

Droop. Suspension travel below ride height, as the wheel drops away.

Bump travel. Suspension travel above ride height before the bump stop.

Camber. The tilt of the tire seen from the front. Negative camber leans the upper edge of the tire inward.

Toe. Whether the fronts of the tires point in or out, seen from above.

Alignment. Setting camber, toe and caster to spec. Any ride height change needs a new alignment.

Corner balancing. Adjusting corner heights so the diagonal weights match. Our guide to corner balancing walks through it.

Cross weight. The share of weight on one diagonal pair of tires. The goal is about 50 percent.

Unsprung weight. Mass the springs do not carry: wheel, tire, hub, brakes, knuckle and part of the arms and damper.

ASP. BC Racing's adjustable spring perch kit, which keeps the car's factory electronic dampers. We do not list an ASP kit; call us to check your car.

HLS. KW's Hydraulic Lift System. It raises the front axle to clear driveways, on selected KW V3 and higher applications.

EDFC. TEIN's Electronic Damping Force Controller, a TEIN-only add-on for adjusting damping from the cabin. It is not a coilover kit.

DFV. Öhlins Dual Flow Valve, used on Öhlins Road & Track kits.

TÜV. The German technical inspection authority. Most KW kits carry TÜV certification.

SPEC-R. Swift Springs' upgrade spring line. It is the upgrade option on most BC Racing fitments and ships standard on HM and ZR kits on most fitments.