Volume Spacing Calculator & Explanation
MTB suspension setup is crucial for performance. Working with clients all over the world we hear a wide range of questions and theories. In this blog we take a look at setup offering a Volume Spacing Calculator & Explanation.
Let’s get some quick disclaimers and notes out of the way. We measured a 2019 Fox 36 Float Grip 2 29″ fork with 160mm of travel for the calculator. Measurements are fairly accurate – but I would not claim them to be exact. They are however, certainly close enough for the comparison theory we want to present.
Volume Spacing, The Marketing Angle
Lower air pressure means improved small bump compliance while creating more ramp across the mid and late stroke. And that’s mostly true.
Volume Spacing, In Application
“Lower Air Pressure” – kind of.
Let’s start at the beginning.
MTB suspension is aired up without rider weight on the bike. The rider mounts the bike to measure sag (how far the suspension “settles” under load). Air pressure changes are made until the rider finds the proper unloaded PSI for the percentage of sag desired.
This calculator works to illustrate the air pressure differences available when working from a particular sag setting. The force of the spring in any moment equals the PSI x Piston Surface Area. This means that to set a particular sag the PSI will be the same independent of air spring volume.
In this example we use Fox’s 20% (32mm) “Plush” sag setting.
With the calculator below you can change the PSI (in the white box) and it will calculate what the 20% sag would be with no volume spacers. It then transfers the 20% sag to the columns with additional volume spacers recalculating the pressure at 0, 40, 60, 80 and 100% travel for review.
What you will notice is the smaller volume air springs require less initial charge at 0% stroke. From 0 to 20% is in fact a lower PSI, which means lower piston (spring) pressure. So from 0 to 20% the suspension will feel softer by reducing volume.
Once you cross the sag setting the smaller volume air springs ramp more aggressively. From 20% sag on the reduced volume provides additional support in the midstroke. This additional support comes from greater PSI which means it takes more pressure to continue to compress the air spring.
A point of confusion when tuning is defining what constitutes “small bump” to a rider.
If we use the marketing jargon that volume spacers have improved our small bump compliance they must mean small bump is only up to to 20% (or your sag setting).
Given the capacity of modern bikes, the speeds, the terrain, etc most riders refer to small bump compliance deeper into the stroke than 20% (I know I do).
Take a minute and play with the calculator. Change the number in the white cell and hit enter to see the modified PSI as well as piston force.
Calculator this works best on a computer. It will work on a mobile device however the data will stack vertically.
Volume Spacing Calculator
Ok, you’ve got an idea of how the the different volume spacers effect ramp rate by seeing the PSI changes.
If your idea of small bump compliance, like mine, is say 3 to 5″ brake bumps or the equivalent, you’re better suited looking into a BikeCo Pro Tune to balance all of the performance than to simply add volume spacers. Pro Tune Suspension improves the fitment of the fork’s upper and lower assemblies, damper flow rates and control, has a wider range of functioning temperature, etc, etc, etc.
Volume spacing is much more effective in its role to modify mid and end stroke performance.
You can see in this graph how the different ramp rates appear.
Remember, these numbers are for conceptual review. Differences may create variances i.e. exact air spring sizing, pump gauge readings, etc may create variances. We’ve provided this to help riders understand and visualize how volume tuning changes each portion of suspension travel.
This is based on the Fork negative spring being charged at full extension thus a static PSI from gauge to air chamber.
Want to get a bit deeper? Check out our next calculator comparing volume spacing as well as sag modifications to review ramp rate, etc. Fork Volume Spacing Calculator with Sag.
Learn more about controlling ramp rate with compression tuning basics here.
Rear suspension is a bit more sophisticated to calculate. Typically the negative air spring is charged at approximately 25% travel. This means the neg air charge is higher than the shock pump’s reading and slightly lowers the positive air chamber psi depending on negative air spring volume.
Here are some of the formulas used in these calculations for reference.
Pressure change – Pressure 1 x Volume 1 = Pressure 2 x Volume 2
Spring “Force” at moment – Pressure x Piston Surface Area