Spring Rate Calculator
Enter your current spring rate, the sag you measured, and your target sag. The calculator returns the spring rate you need. Output is shown in both N/mm and lb/in; the conversion uses 1 lb/in = 0.17512683524647 N/mm.
How sag-scaling works
Spring rate relates force to deflection: force = rate * deflection. When you set race sag by adjusting preload, the rider load stays constant and only the equilibrium position changes. That means rate and sag are inversely proportional under a fixed load: if measured sag is larger than target, you need a stiffer spring (lower deflection per unit force); if smaller, a softer one.
The scaling identity follows directly: k_new = k_current * (measured_sag / target_sag). This method, covered in suspension-tuning references including Race Tech's Motorcycle Suspension Bible and Sport Rider setup guides, lets you size a replacement spring from one set of measurements rather than iterating through spring choices.
Two caveats from the same class of references: first, the derivation assumes a linear spring and constant preload between measurements. Valve shimming, linkage stiction, or a non-linear rate will introduce error. Second, the fork calculation is close (most forks use near-linear springs); the rear shock is approximate because most rear linkages are progressive, so the effective spring rate at the wheel is not constant across the travel. Use the result as a starting point and confirm with a sag measurement on the new spring.
The formula, term by term
k_new = k_current * (measured_sag / target_sag). k_current is the rate of the spring currently installed, in N/mm or lb/in. measured_sag is the race sag you measured with your current spring, in mm. target_sag is the race sag your discipline requires, in mm.
Unit conversion: 1 lb/in = 4.4482216152605 N / 25.4 mm = 0.17512683524647 N/mm. To convert lb/in to N/mm, multiply by 0.17512683524647. To convert N/mm to lb/in, divide. The calculator shows both.
Worked example: current spring 4.8 N/mm, measured race sag 40 mm, target race sag 33 mm. k_new = 4.8 * (40 / 33) = 4.8 * 1.212 = 5.82 N/mm. In lb/in: 5.82 / 0.17512683524647 = 33.2 lb/in. The next available spring rate up from the nearest stock increment is the practical choice.
Rate scaling grid
Recommended spring rate grid computed at build time: rows are current rates (N/mm), columns are measured-to-target sag ratios. Values show how much the rate shifts for a given sag overshoot or undershoot.
| Current rate (N/mm) | x0.85 ratio | x0.90 ratio | x0.95 ratio | x1.00 ratio | x1.05 ratio | x1.10 ratio | x1.15 ratio |
|---|---|---|---|---|---|---|---|
| 4.0 | 3.40 | 3.60 | 3.80 | 4.00 | 4.20 | 4.40 | 4.60 |
| 4.4 | 3.74 | 3.96 | 4.18 | 4.40 | 4.62 | 4.84 | 5.06 |
| 4.8 | 4.08 | 4.32 | 4.56 | 4.80 | 5.04 | 5.28 | 5.52 |
| 5.2 | 4.42 | 4.68 | 4.94 | 5.20 | 5.46 | 5.72 | 5.98 |
| 5.6 | 4.76 | 5.04 | 5.32 | 5.60 | 5.88 | 6.16 | 6.44 |
| 6.0 | 5.10 | 5.40 | 5.70 | 6.00 | 6.30 | 6.60 | 6.90 |
| 6.5 | 5.52 | 5.85 | 6.17 | 6.50 | 6.83 | 7.15 | 7.47 |
FAQ
What spring rate do I need for my weight?
Set race sag first: adjust preload until race sag is in your discipline's target range, then measure static sag. If static sag is outside 25-40 mm with race sag correct, the spring rate is wrong for your weight. Use this calculator with your current rate, your measured sag, and the target to find the replacement rate. Suspension tuning references (Race Tech, Sport Rider) consistently recommend this measure-then-scale approach over weight-based charts because rider gear weight, bike weight, and riding posture all affect the number.
N/mm vs lb/in: which unit should I use?
European and Japanese OEMs typically publish rates in N/mm; US aftermarket suppliers often list lb/in. One N/mm equals about 5.71 lb/in; a typical motocross rear spring of 5.0 N/mm is roughly 28.6 lb/in. The calculator accepts either and shows both. Match the unit to whatever your spring supplier quotes so you can order directly from the output.
Is this calculation exact for my rear shock?
For the fork, the linear-spring assumption is close: most fork springs are near-linear through normal sag range. For the rear shock, the result is approximate. Most rear linkages are progressive: the motion ratio changes through travel, so the wheel rate is not constant even with a linear spring. The sag-scaling method gives a good first estimate, but verify with a sag measurement on the new spring before committing to a race setup.
What race sag targets should I use?
Race sag targets by discipline, from suspension tuning references: motocross rear 100-105 mm, off-road/enduro rear 105-110 mm. Fork sag targets vary more by model; 30-35 mm is a common published starting point. Static sag in the 25-40 mm range with race sag on target indicates the spring rate matches the rider. Use the Suspension Sag Calculator to take the three measurements and check your numbers.
Sources
Sag-scaling formula k_new = k_current * (measured / target): standard suspension-tuning method described in Race Tech's Motorcycle Suspension Bible (Bremmers, multiple editions) and Sport Rider setup guides. Assumes linear spring, constant preload.
Unit conversion: 1 lb/in = 4.4482216152605 N / 25.4 mm. Derived from SI base units (1 lbf = 4.4482216152605 N, 1 in = 25.4 mm). Implemented as nmmPerLbIn = 0.17512683524647 in formulas.LbInToNmm and formulas.NmmToLbIn.
Progressive-linkage caveat: standard disclaimer in suspension-tuning references. Fork linearity observation: consistent across OEM fork-spring specifications.