Fox 36 Evol air spring curve calculator- Mtbr.com
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  1. #1
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    Fox 36 Evol air spring curve calculator

    Has anyone tried modeling the Air Spring in the Fox 36 Evol fork with excel or some other software calculator? The positive side should not be too hard to do but getting an accurate volume versus stroke for the negative side is a little more tricky because I believe some volume is provided by the inside of the air shaft itself?

    I just got a DSD Runt for my 2019 Fox 36 Grip 2 fork with (170mm) and really wanted to be able to visualize what the air spring curves look like with different combinations of high and low positive chamber pressures to help me understand what I'm feeling during set up. The first step to this is obviously modeling the standard Fox Air spring and then modifying that with the change in volume and second positive air chamber the Runt brings to the table. I have seen some of the Manitou curves on the internet but want something specific to my fork that I can actually manipulate the numbers on to see how the curve changes.

    Has anyone else on here geeked out on these numbers already or do I need to do this all myself?

  2. #2
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    I've thought about it but have not done it. Other than taking the air spring apart and measuring things, IDK. There are the Fox drawings, so you might just scale off the dwgs, though that's never the best. Looking at the drawings, I don't see where the negative volume communicates with the interior of the hollow shaft. If it doesn't, connecting them would seem to be an easy way to increase neg volume if so desired. Dwgs are toward the bottom of the following page. Click on the thumbnail to the right to get a large dwg.

    https://www.ridefox.com/fox17/help.php?m=bike&id=956
    Do the math.

  3. #3
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    If anybody does it, please, feel free to share : D
    Shaft has a hole, that is behind the top out bumper.

    EDIT: andre XTR created excel spreadsheet for simulating air springs for shocks.
    https://www.youtube.com/watch?v=0-l2l2kca0A

    under the video, there is a link to download the file. If you know dimensions, it should probably also work for forks.

  4. #4
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    I made one in excel using Boyle’s law which is useful for comparing air springs and volume changes, but you would need to use adiabatic processes to have a better approximation of how it would actually feel. Boyle’s law alone doesn’t really doesn’t really correlate with an equivalent coil spring at all for example

    The maths isn’t too difficult though especially if you’re using excel to do the iterations. A Runt would be a little trickier though since the rate of volume change is different depending on the ratio of pressures in each chamber and the activation point of the second chamber. Wouldn’t be too difficult though

    Fox springs have a port under the piston to access the negative chamber

  5. #5
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    I don't recall seeing a hole into the air shaft, but it's been a while. Where is the hole in the air shaft? In the dwg below, there's a cross hole for a roll pin that retains the piston and the top of the shaft is covered by the piston.

    Fox 36 Evol air spring curve calculator-36-air-shaft.jpg
    Do the math.

  6. #6
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    You can see it in this video:
    https://youtu.be/yKfy5R7FuUM

    At around 2min 10sec.

  7. #7
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    Quote Originally Posted by Lone Rager View Post
    I don't recall seeing a hole into the air shaft, but it's been a while. Where is the hole in the air shaft? In the dwg below, there's a cross hole for a roll pin that retains the piston and the top of the shaft is covered by the piston.

    Click image for larger version. 

Name:	36 air shaft.jpg 
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    Theres groove on the inside of the piston so air can easily flow around and in through the top or down the split in the roll pin. Unscrew the foot stud one day then try inflate the fork and see how it goes!


    That's a really nice calculator there from Andre, I had to flick him a few dollars for that one!
    www.thesuspensionlab.nz
    Servicing in Rotorua, NZ/Vorsprung Elite Tuning Centre/DVO service centre/Insta @thesuspensionlab

  8. #8
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    Quote Originally Posted by Xorrox View Post
    Has anyone tried modeling the Air Spring in the Fox 36 Evol fork with excel or some other software calculator? The positive side should not be too hard to do but getting an accurate volume versus stroke for the negative side is a little more tricky because I believe some volume is provided by the inside of the air shaft itself?

    I just got a DSD Runt for my 2019 Fox 36 Grip 2 fork with (170mm) and really wanted to be able to visualize what the air spring curves look like with different combinations of high and low positive chamber pressures to help me understand what I'm feeling during set up. The first step to this is obviously modeling the standard Fox Air spring and then modifying that with the change in volume and second positive air chamber the Runt brings to the table. I have seen some of the Manitou curves on the internet but want something specific to my fork that I can actually manipulate the numbers on to see how the curve changes.

    Has anyone else on here geeked out on these numbers already or do I need to do this all myself?
    I've got calculators running for other forks, just haven't got to the F36 yet.

    Pretty much all of them have a spring-rate curve that looks like a bath-tub.
    Owner of www.shockcraft.co.nz, Mech Engineer, Tuner, Manitou, Motorex, Vorsprung EPTC, SKF, Enduro
    www.dougal.co.nz

  9. #9
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    Quote Originally Posted by JohnnyC7 View Post
    I made one in excel using Boyle’s law which is useful for comparing air springs and volume changes, but you would need to use adiabatic processes to have a better approximation of how it would actually feel. Boyle’s law alone doesn’t really doesn’t really correlate with an equivalent coil spring at all for example

    The maths isn’t too difficult though especially if you’re using excel to do the iterations. A Runt would be a little trickier though since the rate of volume change is different depending on the ratio of pressures in each chamber and the activation point of the second chamber. Wouldn’t be too difficult though

    Fox springs have a port under the piston to access the negative chamber
    First step is definitely going to be bolyes law but you certainly are correct that the actual suspension action will be somewhere between that and the true adiabatic process condition so I may try graphing both to give a operating area approximation.

  10. #10
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    I had an old 2019 170mm air shaft lying around so I did some measuring on that yesterday but still have to take some measurements of the positive chamber height with the airshaft installed in the fork and look at how much volume the normal top cap assembly takes up.

    Here is what I've got so far - if anyone sees a error please let me know. Also, please take these with a grain of salt and do your own measuring if you need 100% accuracy.

    Negative Air chamber volume at topout is around 36.7 mL (I would give that a +/- of around 1.5 mL). Of that about 9.6 mL is inside the hollow airshaft itself (238+/-1 mm long in a 170mm travel airshaft with an ID of 7.15+/-0.05mm). The trickiest part to measure is the topout bumper assembly which I did with some pretty crude volume displacement measurements and came up to the conclusion that this assembly reduces the volume of the negative chamber by about 8ml.

    The outer diameter of the air shaft appears to be 10.00+/-0.05mm and from measuring the seal head I would guess the ID of the fork stanchions to be 33.00 mm (but haven't measured that yet).

  11. #11
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    Quote Originally Posted by JohnnyC7 View Post
    Theres groove on the inside of the piston so air can easily flow around and in through the top or down the split in the roll pin. Unscrew the foot stud one day then try inflate the fork and see how it goes!..
    I knew it wouldn't be airtight, but ideally the air should be able to move in and out of the shaft freely at the speed the fork moves in order to actively contribute to negative volume. The holes in the air shaft covered by the top out bumper wouldn't seem to permit that. I'm not aware of the groove in the piston. I'll have to look when I get a chance.
    Do the math.

  12. #12
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    I've created a spreadsheet for the Pike and was planning on doing one for the Lyrik with Runt. From the ones I've done so far though, they tend to underestimate the stiffness (i.e. you have to use a base pressure 20-30% higher to match real world sag behavior). I assume this is due to the force developed by compression pressure in the lower legs that I have not accounted for but I haven't run the numbers to verify that.

    With the Runt in there, if I'm not mistaken, you will have an indeterminate system though so you'll have to iterate on to the solution. That gets a bit more tricky with keeping a spreadsheet that can be setup with a variable high side pressure since it will change the point where the Runt begine impacting the spring curve. Also, friction is likely to have a pretty big real world impact here as well.

    Probably just easier to build a fork spring tester and characterize it instead of finding an analytical solution that gives realistic results.

  13. #13
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    Quote Originally Posted by mike156 View Post
    I've created a spreadsheet for the Pike and was planning on doing one for the Lyrik with Runt. From the ones I've done so far though, they tend to underestimate the stiffness (i.e. you have to use a base pressure 20-30% higher to match real world sag behavior). I assume this is due to the force developed by compression pressure in the lower legs that I have not accounted for but I haven't run the numbers to verify that.

    With the Runt in there, if I'm not mistaken, you will have an indeterminate system though so you'll have to iterate on to the solution. That gets a bit more tricky with keeping a spreadsheet that can be setup with a variable high side pressure since it will change the point where the Runt begine impacting the spring curve. Also, friction is likely to have a pretty big real world impact here as well.

    Probably just easier to build a fork spring tester and characterize it instead of finding an analytical solution that gives realistic results.
    Yup that sums it up well, pressure in the legs is a big contribution, plus seal friction adds a large amount of "damping" and hysteresis to the real world response

    Modeling the runt spring curve would mean calculating it for the small volume until the pressure reaches the same as the second chamber, and then changing the chamber length/volume for the remainder of the stroke.

    I've got a plan for a test jig that should give much more useful experimental data but it will be a few months before I have time to build it, so its spreadsheets for now!
    www.thesuspensionlab.nz
    Servicing in Rotorua, NZ/Vorsprung Elite Tuning Centre/DVO service centre/Insta @thesuspensionlab

  14. #14
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    I'm doing some heavy mods to my Jekyll's Dyad, including a triple chamber setup (IRT-like), so I also made a spreadsheet to try various settings...
    Fox 36 Evol air spring curve calculator-grafico-triple-air.jpg

  15. #15
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    I have my own too for tokens and IRT, but fox 36 evol grip2 is first fork that I dont care about spring curve. Its just good enought.

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