Mountain Bike Spring Rate Calculator V6.0
Total Riding Weight: (pounds) includes the rider and anything else added to the bike: lock, water bottles, etc.
Rear Weight Bias: (percent) The standard formula is based on a 60:40, Rear to Front 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. Enter this value for total weight and 100 for Rear Weight Bias. Alternatively use 55=XC, 65=FR, 70=DH, all others use the standard 60.
Rear Wheel Travel & Shock Stroke: (inches or millimetres)
Shock Sag: (percentage of shock stroke) The standard formula uses the Fox Shox recommended 25% of shock stroke. However, this tends to calculate too high a spring rate for the majority of riders. A more appropriate starting point would be 30% to 35%.
Preload Adjuster: (number of turns) The standard formula ignores the effect of applying preload and tends to calculate too high a spring rate. Usually 1.0 is required to lock the spring to the shock body. Choose a preload value that will allow some plus or minus adjustment consistent with your required sag level and re-submit. A zero preload spring rate is found, then preload is applied to find an alternate spring rate taking into account the preload spring tension. Calculation is based on a linear progression.
End-Coil Effect: (percent) Closed/Ground compression springs have a typically lower spring rate in their initial deflection range. The suggested 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 number of turns of the preload adjuster.
Input Parameters: inches or millimetres
Total Riding Weight: pounds
Rear Weight Bias: percent
Rear Wheel Travel: in or mm
Shock Stroke: in or mm
Shock Sag: percent
Preload Adjuster: turns
End-Coil Effect: percent
Output Parameters: Travel Setting Three
Leverage Ratio: travel divided by stroke
Zero Preload Spring Rate: lbs per in
Alternate Preload Spring Rate: lbs per in
Suggested Spring Rate: lbs per in
Suggested Spring Rate Preload Sag: percent

Optional Spring Rates Constant Preload
( target preload as entered above )
( sag value is calculated from optional spring rate )
( never exceed spring preload limit - if unknown, never use more than 2.0 turns )
( more than 4.0 turns is not recommended )
Preload Travel Setting One Travel Setting Two Travel Setting Three
Travel:     Ratio:  Travel:     Ratio:  Travel:     Ratio: 
Rate Sag Sag Rate Sag Sag Rate Sag Sag

Optional Spring Rates Constant Sag
( target sag as entered above )
( optional spring rate is calculated from preload value )
( never exceed spring preload limit - if unknown, never use more than 2.0 turns )
( more than 4.0 turns is not recommended )
Preload Travel Setting One Travel Setting Two Travel Setting Three
Travel:     Ratio:  Travel:     Ratio:  Travel:     Ratio: 
Rate Sag Sag Rate Sag Sag Rate Sag Sag

Correction For Suspension Progression
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The calculator assumes a linear suspension progression.
For any deviation plus or minus <7% use the calculated spring rate.
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Rising Rate ( 7% - 15% ) - Subtract 50 lbs.
( may require more preload )
Extreme Rising Rate - rare ( 15% - 25% ) - Subtract 100 lbs.
( may require considerably more preload, possibly with a longer stroke spring )
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Falling Rate ( 7% - 15% ) - Add 50 lbs.
( may require less preload )
Extreme Falling Rate - common ( 15% - 25% ) - Add 100 lbs.
( may require as little preload as possible )
( extreme falling rate suspension bikes should be using an air shock )
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( or simply ride with the calculated spring rate and let personal preference decide )

Correction For Stable Platform Shocks
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If you have already corrected for suspension progression
try a 50 pound lighter spring, otherwise try a 100 pound lighter spring.
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One nameless publication suggests as much as 50% lighter but you stand the chance
 of blowing through your travel when the stable platform compression filters open up.
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( or simply ride with the calculated spring rate and let personal preference decide )

Preload Precautionary
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Preload adjusters apply about 1mm 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,
or install a spring with a longer stroke ( if you can find one that will fit ).
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Preload does not change the spring rate. It does change the load capacity,
hence the ability to vary sag levels with the judicious application of preload.
The calculator factors in the effect of preload to arrive at an alternate spring rate.
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Even though you may be well within the limits for avoiding coil binding,
( as in having installed a spring with a much longer stroke than the shock )
using too much preload stresses the shock, possibly leading to premature failure.

Reference Links
"A Bicycle Rear Suspension Analysis Method" by Ken Sasaki
http://www.mtbcomprador.com/pa/english/
"Linkage V2.0 Bicycle Suspension Simulation Program" by Gergely Kovacs
http://www.extra.hu/linkage/
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
http://www.rpmnet.com/techart/spring.shtml

Mountain Bike Spring Rate Calculator V6.0
 
Copyright (C) 2005 Michael R Young ( mrdy )
 
This program is free software; you can redistribute it and/or
modify it under the terms of the GNU General Public License as
published by the Free Software Foundation; either version 2
of the License, or (at your option) any later version.
 
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty
of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
See the GNU General Public License for more details.
 
http://www.gnu.org/copyleft/gpl.html

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