Coilover Spring Rate Calculator
Find the spring rate that suits your car’s weight and how you drive it, or check how a set of springs you already own will feel. Built around ride frequency, so it works for any car. Results in kg/mm, lb/in and N/mm. No signup.
New to spring rates? The 60-second version
Spring rate is how much force it takes to squash the spring by one millimetre. Higher is stiffer. Coilover rates are often written like 6k or 8k, which means 6 or 8 kg/mm. One kg/mm is about 56 lb/in.
Ride frequency is how fast the car bounces on its springs, in Hz. It is the fair way to compare stiffness between cars, because it accounts for weight. A comfortable road car sits around 1.0 to 1.5 Hz, a firm street car 1.5 to 1.9 Hz, and a track car 2 Hz or more.
Motion ratio is why the spring rate on the box is not what the tyre feels. The spring is rarely right over the wheel, so the wheel only feels part of it: wheel rate equals spring rate times motion ratio squared. That is why a multi-link rear often needs a much stiffer spring than a strut front.
Flat ride: the rear is usually set 10 to 20 percent higher in frequency than the front, so the car does not pitch over bumps.
How will you use the car?
Pick the closest match. This sets the ride frequency we aim for. Show car that only cruises? Choose Comfort.
Tap a card above, or type your own.
Your springs
Enter the rate printed on the spring or the coilover box. Rates like 6k and 8k mean kg/mm.
Your car
Use the weight from your spec sheet or registration papers. We add the driver and fuel for you.
Typical values for each layout. If you know yours from a set of scales, type it in.
Advanced: corner weights and unsprung weight
Leave these blank to use the estimates above. Unsprung weight is the wheel, tyre, brake and hub, which the spring does not carry. Heavy wheels raise it.
Your suspension
Where does the spring sit? This sets the motion ratio. It matters a lot, so pick the closest match and see the note under each box.
How to measure your own motion ratio
Jack the car so the wheel hangs at ride height, then raise the wheel 25 mm at a time (a block of wood and a tape measure works). At each step measure how much the spring and damper length changes along their own axis. Motion ratio is spring travel divided by wheel travel. If you get a different number at each step, use the one near ride height. Use this value in the box above for the most accurate result.
Ride frequency in Hz. F marks the front and R the rear. The coloured bands are typical front-axle targets for each use. The rear normally sits 10 to 20 percent higher, so it lands a little further right.
- Rates quoted as 6k, 8k and so on are kg/mm. Confirm with the maker, and check whether the rate is linear or progressive.
- Set ride height with the spring perch, not by winding on extra preload. Preload raises how hard the spring feels at the start of its travel.
- Check you have enough suspension travel. If the car sits close to the bump stops, a stiffer spring will not fix it. Raise the ride height or fit shorter bump stops.
- Match the dampers to the springs. A stiff spring on a soft damper bounces, and a soft spring on a stiff damper skips and feels harsh.
- Springs change handling balance. Stiffer on the front tends to push the car wide, so keep the front and rear in step, and revisit your alignment after changing springs.
Quick answers
What does 6k or 8k mean on a coilover?
It is the spring rate in kg/mm. A 6k spring takes 6 kg of force to compress it by 1 mm, which is about 336 lb/in. An 8k spring is about 448 lb/in.
Why is my rear spring stiffer than my front?
Because of motion ratio. On many multi-link rear suspensions the spring sits well inboard on the lower arm, so the wheel only feels a fraction of the spring rate. The spring has to be stiffer to give the same wheel rate. A measured Audi, for example, had a motion ratio of about 0.69 at the front and 0.53 at the rear.
Is a stiffer spring always better for handling?
No. Past a point the tyre loses grip on bumps because the car skips instead of following the road. On the street, balance and compliance matter more than raw stiffness. Most road cars are happiest between 1.5 and 1.9 Hz.
How accurate is this?
The maths is exact, but the inputs are not. Motion ratio is the biggest unknown: a 10% error in motion ratio is roughly a 20% error in spring rate, which is why each result shows a range for your suspension type. Treat the answer as a very good starting point, then fine tune with the dampers.
Where do the ride frequency targets come from?
Published race-engineering guidance puts street cars at roughly 1.0 to 1.5 Hz and sports and road-race cars at 1.5 to 2.5 Hz, with the rear 10 to 20 percent higher than the front for a flat ride. We split those ranges into four everyday uses.
This calculator gives estimates from the numbers you enter and standard suspension formulas. It is for guidance only. Always follow your coilover manufacturer’s recommendations and have suspension work done by a qualified fitter.