Spring Rates in Plain Terms: kg/mm vs lb/in and How to Choose
Coilover spring rates tell you how much force it takes to compress a spring by a set distance. To turn kg/mm into lb/in, multiply by 55.9974, or about 56. Then pick a rate for your car's weight, its motion ratio and how you drive, not for the biggest number on the forum.
You are comparing two coilover kits for the same car. One lists 8 kg/mm front springs. The other lists 450 lb/in. Are they close? Yes, almost identical. Most of the confusion around spring rates comes from units. The rest comes from comparing numbers across cars that use their springs differently. Here is how we explain it on the phone.
What a Spring Rate Number Means
A spring rate is force per distance. An 8 kg/mm spring needs 8 kilograms of force to compress it one millimeter, 16 to compress it two, and so on. A 450 lb/in spring needs 450 pounds of force to compress it one inch. Unless a spring is sold as progressive, treat its rate as constant through its working range.
Higher rates resist body roll, pitch and squat more. Lower rates let the tire follow a rough surface with less force sent into the chassis. Neither is right on its own. The goal is enough rate to control the body without making the car skip over bumps. If you want the deeper material science, our guide to spring physics has it.
Two settings often get mixed up with rate. Preload does not change the spring rate. At ride height, the car's weight compresses the spring far past its preload anyway. Ride height does not change the rate either. On full-length kits, you set ride height at the lower mount and the spring perch sets preload. A locking collar holds each setting. Our guide to coilover preload covers that in detail.
Converting Coilover Spring Rates: kg/mm to lb/in
Many Japanese and Taiwanese brands list springs in kg/mm. Many US spring makers use lb/in. The conversion is fixed. One kilogram-force is 9.80665 newtons, one pound-force is about 4.448 newtons, and one inch is 25.4 millimeters. Put those together and 1 kg/mm equals 55.9974 lb/in.
For quick math, 56 is a fine rounded rule of thumb. To go the other way, divide lb/in by 55.9974. If you see N/mm on a European spec sheet, multiply kg/mm by 9.80665.
Common coilover spring rates converted
4 kg/mm = 224 lb/in = 39.2 N/mm.
5 kg/mm = 280 lb/in = 49.0 N/mm.
6 kg/mm = 336 lb/in = 58.8 N/mm.
7 kg/mm = 392 lb/in = 68.6 N/mm.
8 kg/mm = 448 lb/in = 78.5 N/mm.
10 kg/mm = 560 lb/in = 98.1 N/mm.
12 kg/mm = 672 lb/in = 117.7 N/mm.
14 kg/mm = 784 lb/in = 137.3 N/mm.
16 kg/mm = 896 lb/in = 156.9 N/mm.
Going the other way
300 lb/in = 5.36 kg/mm.
400 lb/in = 7.14 kg/mm.
500 lb/in = 8.93 kg/mm.
600 lb/in = 10.71 kg/mm.
800 lb/in = 14.29 kg/mm.
Spring Rate Is Not Wheel Rate
Here is why two cars on the same spring can feel nothing alike. The spring rarely sits right at the wheel. It mounts somewhere along the control arm, so it moves less than the wheel does. That ratio of spring travel to wheel travel is the motion ratio.
What the tire feels is the wheel rate. Wheel rate equals spring rate times the motion ratio squared. A spring mounted with a 0.7 motion ratio gives the wheel about half its rate, because 0.7 times 0.7 is 0.49. A spring with a motion ratio near 1.0 delivers close to its full rate.
So a stiff spring feels soft when the motion ratio is low. That is why a rate that works on a strut car usually feels softer on a double wishbone car, and the reverse. Compare wheel rates, not spring rates, when you move numbers between platforms. Our guide to motion ratio goes deeper.
How Car Weight and Use Change the Choice
Weight sets the baseline. A heavier car needs more rate to get the same body control as a lighter one. Weight on each corner matters too. A front-heavy car usually runs more rate up front than at the rear.
Use moves the number from there. A daily driver on broken pavement wants enough compliance to keep the tire on the road. A track car on smooth asphalt with sticky tires can carry more rate, because the tires hold more cornering load and the surface is kinder. Aero changes the picture again. Downforce scales with the square of speed, so a car with real wings needs rate to hold its height at speed.
Your dampers have to match. Damping force controls how fast the spring moves. If you go far from a kit's default rate, ask whether the valving still suits it. A soft damper on a stiff spring feels bouncy. A stiff damper on a soft spring feels harsh.
Rate is one tool in a performance suspension setup, alongside damping, alignment and tires. Tires and roads matter more than most buyers expect. If you are not sure, stay close to the default rate the kit was built with. It was chosen for your chassis.
Front and Rear Balance
The split between front and rear rates changes how the car turns. More front roll stiffness relative to the rear generally pushes the car toward understeer. More at the rear pushes it toward oversteer. Sway bars change that split too, so look at springs and bars together.
Change one end at a time and in small steps. If you raise the front rate and the car starts pushing in tight corners, you have your answer. Keep notes on rate, ride height and damper clicks for every change, so you can go back.
Drivetrain plays a part. A front-drive car leans on its front tires for both turning and traction. A rear-drive car needs the rear to put power down on exit. That is why default rate splits differ between platforms. When in doubt, ask us what the kit for your car ships with.
Default Rates, Custom Rates and Swift Springs
Every coilover kit ships with springs chosen for the car. For most street builds, that default is where you should start. Custom rates make sense when your car is far from stock weight or runs aero or sticky tires. They also help with a clear handling problem.
Is a Custom Rate Worth the Wait?
A custom rate adds lead time, because the kit is built or rebuilt with different springs. It is worth it when the car is far from stock weight or runs aero or race tires. It also makes sense for a handling problem the default rate clearly causes. On a first kit for a stock-weight street car, start on the default rates and drive it. Springs can be changed later. Our guide to buying coilovers online explains how custom builds affect timing.
BC Racing builds kits with custom spring rates. Swift Springs SPEC-R springs are the upgrade spring option on most BC Racing fitments. They ship standard on HM and ZR kits on most fitments. Swift springs and custom rates are ordered through the custom build option for BC coilovers. BC Racing North America introduced a custom build fee effective June 30, 2025, and we pass it on without markup. Our guide to BC Racing and Swift Springs covers that pairing.
We also list individual BC Racing springs and a half kit of two Swift springs for BC coilovers. Call us before you order loose springs, so we can confirm the size and rate for your kit.
Other brands handle rates their own way. Fortune Auto custom-valves its kits for your application, so tell us your rate plans when you order and we will pass them to Fortune Auto. KW and TEIN kits ship with springs matched to the kit. Ask us before changing rates on those. The spring is also part of the unsprung weight, but only partly. Most of that mass is the wheel, tire, hub and brakes, so rate choice matters far more than spring weight.
Fitment comes first. Confirm your kit on the product page, or call 1-800-460-9106 with your VIN and we will check it and talk rates.
Frequently Asked Questions
How do I convert kg/mm to lb/in?
Multiply kg/mm by 55.9974. For quick math, 56 is a fine rounded rule of thumb. An 8 kg/mm spring is about 448 lb/in. To go from lb/in to kg/mm, divide by 55.9974.
Does a higher spring rate improve handling?
Not by default. More rate cuts body roll, but too much makes the car skip over bumps and lose grip. The right rate keeps the tire on the road while controlling the body. For most street cars, the kit's default rate is the place to start.
Does preload change spring rate?
No. Preload is set with the wheel hanging. At ride height, the car's weight compresses the spring far past its preload. The rate and the force to absorb small bumps stay the same.
Why do two cars with the same coilover spring rates ride so differently?
Motion ratio. Wheel rate equals spring rate times the motion ratio squared. A spring on a low motion ratio gives the wheel much less rate than its label says. Weight, tires and damping change the feel too.
Should I run stiffer springs in front or rear?
It depends on weight split, drivetrain and how you want the car to turn. More front roll stiffness generally adds understeer. More rear roll stiffness generally adds oversteer. Start with your kit's default split and change one end at a time.
Can I change coilover spring rates without changing dampers?
Small changes, usually yes. Large jumps can leave the damper valving out of step with the spring. If you plan a big change, call us first and we will check what your coilover kit can handle.
Are Swift springs worth it on BC Racing coilovers?
Swift Springs SPEC-R springs are the upgrade spring option on most BC Racing fitments. They ship standard on HM and ZR kits on most fitments. On other series, you order them through a custom build. We are happy to talk through whether they suit your build.
Not Sure Which Rate Suits Your Build?
Tell us your car, its weight changes, your tires and how you drive. We will tell you what your kit ships with and whether a custom rate makes sense.
Call 1-800-460-9106 with your VIN. We carry BC Racing, KW, Fortune Auto, TEIN, Öhlins and more, and we can help with rates on any of them. Free shipping across Canada on most kits.
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