Volume Spacing Calculator & Explanation

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9 1 18 Fork Volume Spacing Calculator BikeCo

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

Starting PSI 72
160mm (full extended) 20% sag - 128mm 40% sag - 96mm 60% sag - 64mm 80% - 32mm 100% (bottomed out)
No Spacers PSI 72 81.5715 94.0781 111.1142 135.6845 174.5217
No Spacer Force 90 101.9644 117.5977 138.8927 169.6056 218.1522
1 Spacer PSI .61 71.5428 81.5715 94.8703 113.3500 140.7706 186.0695
1 Spacer Force 89.4285 101.9644 118.5879 141.6876 175.9633 232.5869
2 Spacers PSI 1.22 71.0397 81.5715 95.7697 115.9520 146.9120 200.8937
2 Spacers Force 88.7996 101.9644 119.7121 144.9401 183.6400 251.1171
3 Spacers PSI 1.83 70.4835 81.5715 96.7995 119.0181 154.4752 220.6190
3 Spacers Force 88.1043 101.9644 120.9994 148.7727 193.0939 275.7737
4 Spacers PSI 2.44 69.8652 81.5715 97.9904 122.6847 164.0183 248.1576
4 Spacers Force 87.3315 101.9644 122.4880 153.3558 205.0229 310.1970
5 Spacers PSI 3.05 69.1739 81.5715 99.3834 127.1471 176.4362 289.2983
5 Spacers Force 86.4674 101.9644 124.2293 158.9339 220.5453 361.6229
6 Spacers PSI 3.66 68.3959 81.5715 101.0347 132.6962 193.2582 357.4237
6 Spacers Force 85.4949 101.9644 126.2933 165.8703 241.5727 446.7797
Sizing For Formulas cu in, w/ out spacers 13.38 11.81 10.24 8.67 7.10 5.52
Volume Reducer cu in .61 Piston SA sq in 1.25

 

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.

9 1 18 Fork Volume Spacing Calculator BikeCo

 

Drawing Conclusions

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

 

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