MRP Suspension - Mountain Bike Spring Rate Calculator

Total Riding Weight (pounds):

  • The rider's weight with riding gear (average 5lbs for riding gear weight)

Rear Weight Bias (percent):

  • The standard formula is based on a 60:40, Rear to Front weight distribution
  • Add 5% for a heavy rider (+200lbs.)
  • Subtract 5% for a light rider (150lbs-)
  • Add 2% for a recreational or beginner rider (more rearward bias)
  • Subtract 2% for a pro-level or highly experienced rider (less rearward bias)

Rear Wheel Travel & Shock Stroke: (inches or millimeters)

Shock Sag: (percentage of shock stroke)

  • The standard for downhill is 30% sag
  • The standard for extreme free-riding is 27% sag
  • The standard for all-mountain riding is 25%

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 millimeters

Rider Weight with gear (pounds)

Weight Distribution at rear wheel (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


Calculated suggested spring rate

( based on 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


Verification of calculated spring rate under maximum preload

( 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


Corrections 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

Progressive design, from high motion ratio (+3) to lower ratio (2.4-) > Subtract 50 lbs.

( may require more preload )

Extreme Rising Rate

Highly progressive design, from high motion ratio (+4) to lower ratio (2.4-), rare > Subtract 100 lbs.

( may require considerably more preload, possibly with a longer stroke spring )

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Falling Rate

Digressive design, ends with higher motion than the start (ex: from 2.4 to 2.8) > Add 50 lbs.

( may require less preload )

Highly digressive design (common with old designs), ends with higher motion than the start (ex: from 3 to 3.5) > 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

(already factored in formula for MRP Suspension 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 Precautions

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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.


 

© 2013 MRP