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Increasing air volume thru schraeder valve?

4K views 23 replies 10 participants last post by  doralswheels 
#1 ·
I have a Fox RP2. Is it possible to increase the air volume by taking out the schraeder valve core and attaching a little chamber to it? Yes, I know there exists a large volume can.
 
#4 ·
Here's a Risse version that I've had for eons. You do it to gain more volume to alter the air spring characteristics. You really have to match this thing to a shock and bike that it will be compatible with or you'll have a very mushy mid-stroke. It has a volume adjuster built in to it.
 

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#6 ·
beanbag said:
How does the Risse unit screw onto the schraeder valve? Or does it replace the valve?
Can't find any such units on their web site right now.
My shock is a 6", and Fox doesn't make a high volume canister for that size.
Yeah, I doubt they make them anymore. This was more effective in the days when a normal shock was the size of yours. Yes, the unit replaces the schrader valve, and it has a schrader filler valve there in the pic...along with the volume adjuster.
 
#8 ·
beanbag said:
Looks like I will ghetto rig up my own unit.
How does the volume adjuster on the Risse work?
That will be the tricker part of a homemade rig but not impossible. Have you ever looked at a Fox DHX shock?...even just from the outside? It has an airtight screw-in-and-out affair quite similar to the Risse unit that changes the volume of the reservoir on the piggyback. Having an adjustable volume device that is airtight will be more of a challenge than just having an additional volume device plugged into the shrader valve.
 
#9 ·


This is the ghetto rig setup I made. The end cap with the schraeder is held in with a circlip and uses an oring as a seal. If I wanted to make the volume adjustable, I could move the schraeder valve somewhere else and just use a screw to push the cap with the oring further down into the chamber. For now, I will resort to stuffing things inside the canister.

One difficult spot was making the attachment to the schraeder valve on the shock itself. I couldn't find any such fitting as a female schraeder to npt pipe, compression fitting, or anything else, only to hose barb. I wanted to avoid using that system so I just used the hose barb verson and soldered the hose end of the connector into a drilled-out npt plug.

Does anybody know of this connector that I am looking for?
 
#12 ·

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#15 ·
So I was able to test this chamber these last few days. It worked like I expected, but not like I hoped. The initial problem was that I thought my current shock is too progressive, i.e. a lot of sag and hard to use the last few mm of travel. I started off with about 10 cc extra, but that made the shock bottom way too easily, even at higher pressures. I kept decreasing the volume more and more until I determined that the optimal setting for me is probably between 0-3 extra cc. At that point, there isn't much purpose to hanging all this extra hardware off the shock, and maybe just a mini-canister that just screws directly on the schraeder would be better.

This shock was recently pushed and I was planning to send it back to them anyway for a re-tune. Is there anything they can do about the spring characteristics?
 
#16 ·
beanbag said:
This shock was recently pushed and I was planning to send it back to them anyway for a re-tune. Is there anything they can do about the spring characteristics?
Personally I don't care for air suspensions. I have tried a half dozen air shocks and they all have similar negative characteristics. They have initial stichion, have mushy mid-strokes and just can't handle fast chop. Again personally I like a coil shock that has LSC / HSC adjustments. Last air shock I demo'd was supposed to be tuned w/ high compression to allow the damper to do the work and the air spring would follow - still felt like 99% of the air shocks out there.
 
#19 ·
PMK said:
You are looking for a 6" length with what stroke?

Also, by the sound of it you want less progression, do you realize that it may be possible to work this from the other end...increase the IFP chamber volume.

PK
I think the stroke is 1.25, although the frame can actually handle a little more both compressed and extended.

Based on how much force the shock exerts when compressed but deflated, I don't think reducing the contribution from the ifp is going to make that much difference compared to the main chamber.
 
#20 ·
Depends upon the ratio of shaft area to IFP area / volume. Fox RP's don't run a super small shaft, though not as large as a DHX.

As for length vs stroke, a good suspension tuner can most times easily fabricate the travel reduction or length reduction spacers for similar dimension units and revalve as needed for good performance.

Not sure if PUSH does much beyond shim shuffles and a few proprietary parts, but you could ask if they can build you something to your liking.

PK
 
#21 ·
Previously on this same bike, I was running a 6.5" high volume Triad shock with eccentric bushings and an extra o-ring stuffed into the air can to prevent full extension. It worked fine. When I called Push about it, the guy suggested against the eccentric bushings (they will rotate?) and mentioned how preventing the shock from fully extending messes up the negative air because the dimple on the outer body doesn't get a chance to vent.

As for asking Push anything, I have been failing to get a hold of them for the last few weeks.
 
#24 ·
Try this site if you are looking to add air vol. to a Fox float fork. I have this kit on 12 of my bikes and i just love the full travel i get with this kit. rollonthrottle.com and click on mtb fork ace. He just came out with a plastic can instead of the stanless model it shows. Believe me, this is the very best ride you can get with a Fox float (float models only). I weight 250 and i use 100 psi in a float rl 150.
 
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