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Rear Shock Spring Rate Calculator

Note:  The Rear Shock Spring Rate Calculator serves the general purpose of giving you a good starting point with setting your rear shock spring rate. Ultimately, the best spring rate is the one that best suits your personal preferences for your riding style.

Enter your input parameters and click "Submit" to calculate your spring rate, or "Reset" to start over.


  • Total Riding Weight (pounds): includes the rider and anything else additional; e.g., hydration backpack, spare tube/tools, etc.
  • Rear Weight Bias (percent): the standard formula is based on a 60:40, rear-to-front total weight distribution. A more accurate way would be to use a bathroom scale under the rear wheel and a 2x4 under the front wheel, to determine the true sprung weight at the rear wheel. Position your body according to your riding style, and then enter this value for your total weight. Note that the default weight bias is sixty (60) %. Alternatively, you can use 55% for X-Country, 65% for Freeride, and 70% for Downhill.
  • Rear Wheel Travel (inches)
  • Shock Stroke (inches)
  • Shock Sag (percentage of shock stroke): the standard formula uses the Fox Racing Shox-recommended 25% of shock stroke. A zero-preload spring rate is determined, then preload is applied to calculate the corresponding sag. Choose a sag appropriate to your riding style. If you are an extreme rider (jumps, drops, big hits, etc.) you should run less sag and a correspondingly higher spring rate.
  • Preload Adjuster (number of turns): a minimum of 1.0 is required.
  • End-Coil Effect (percent): closed/ground compression springs have a typically lower spring rate in their initial deflection range. The canned value is a best guess of the effect ECE will have on preload adjustment. It is a percentage of the overall spring rate used in calculating the spring tension for the specified turns of preload.


Input Parameters

Total Riding Weight (lbs)
Rear Weight Bias (%)
Rear Wheel Travel (in)
Shock Stroke (in)
Shock Sag (%)
Preload Adjuster (n.n)
End Coil Effect (%)


Output Parameters

Leverage Ratio (Travel/Stroke)
Calculated Spring Rate (lbs/in)
Suggested Spring Rate (lbs/in)
Suggested Spring Rate Preloaded Sag (%)

Optional Spring Rates

Spring Rate (lbs) Shock Sag (%) Shock Sag (in)
*** *** *** *** *** ***

Preload Precautionary

Preload adjusters apply about 1 mm or .039" per turn. If the spring is not marked as to its real stroke, never use more than 2.0 turns on the preload adjuster, to avoid coil binding. If the spring is marked as in 600# x 1.63" or 600# x 41 mm, calculate the available preload by spring stroke minus shock stroke.

CAUTION:  Be careful! Although you may be well within limits for avoiding coil binding, using excessive preload will overly stress the shock, leading to possible premature failure.

Correction For Suspension Progression

Anything plus or minus (±) 7% can be ignored.
  • Rising Rate (7% - 15%): subtract 50 lbs.
  • Extreme Rising Rate - rare (15% - 25%): subtract 100 lbs.
  • Falling Rate (7% - 15%): add 50 lbs.
  • Extreme Falling Rate - common (15% - 25%): add 100 lbs.

Or, simply ride with the calculated spring rate and let personal preferences make your final determination.

Correction For Stable Platform Shocks

If you have already corrected for suspension progression, try a 50 pound lighter spring, otherwise try a 100 pound lighter spring. Some suggest as much as 40% lighter, but you risk the chance of blowing through your travel when the stable platform compression filter opens up.

Or, simply ride with the calculated spring rate and let personal preferences make your final determination.

Reference Links

http://www.engineersedge.com/spring_terms.htm

http://www.engineersedge.com/spring_general.htm

http://www.engineersedge.com/spring_comp_calc_k.htm

http://www.engineersedge.com/spring_comp_calc.htm


use proper tools, supplies & kits · read all warnings · know oil volumes and inspection guidelines · know disc brake specifics for all Fox forks · know air shaft travel specifics for FLOAT and F-Series forks · use correct fork torque specifications