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I was looking into the same thing while building a dirt jumper a few months ago. My first instinct, like yours was to turn to a 26" fox 36 and modify an air spring down to 100mm. Surprisingly, those were pretty hard to find in salvageable condition and by the time I fussed around with sourcing rebuild kits, messing with the air spring and my time, it started to seem like a dead end. You may be able to find one, but I got impatient after looking for 45 days and headed in another direction.
Regarding the other direction... obviously, that sorta depends on the hub standard you have or are targeting.
-15x100: The two options I came down to were a 100mm travel 27.5 fork (ideally pike) and cut the air spring down to 80mm. You can snipe forks like this in the 100-150 range on ebay, pinkbike or fb marketplace. Separately from that, sourcing a period correct fox 34. There is a modern rockshox reba that is 26" and 15x100, but it spendy. The modern pike DJ would fit the bill but it's incredibly expensive. Premium option if you can find one used... but that's a diamond in the rough.
-20x110 non-boost: This leads you to the manitou circus line as the obvious choice. I was able to source a straight steer 1-1/8" manitou circus expert in 100mm travel for 250 bucks. I used a crown race adapter to install this in my tapered steer tube frame. The gotcha with that fork is that you cannot adapt the 130mm fork down to 100mm, so you need to be careful not to get tempted by the ultra cheap 130mm travel variants. The circus comp is also worth a look, I don't think the expert provides any performance that the comp doesn't... I'd view the comp/expert as the same and only really pay a premium for the pro edition, which is sick... but totally un-necessary for my absolutely shite dj skills.
Thanks for the tip, I managed to find a 2016 36 on pinkbike that might work, do you remember where you found any info on shortening the air spring? I can't seem to find anything
Fox uses a very insane thread spec and pitch on the air spring foot nut that you will not find a tap for. I went down this road many years ago, actually consulted steve from vorsprung and he happened to be in the middle of developing the secus. He shared with me that sourcing the tap for cutting the foot nut stud would be a fools errand. I did once try cutting the air spring shaft down from the roll pin/piston side, but it is difficult, neigh impossible to drill the roll pin hole exactly centered and you create a binding element in the air spring if it's not perfect. This is however something a real machine shop can do for you...
That leaves the most obvious solution of getting a proper length air spring from a different fox product, and moving air piston, bottom out assembly and the negative plate/seal head over to that proper length air spring. Technically, the best way to do this is to get the 10mm shaft clamps, and the foot stud bullet tool (can be bought for cheap on ebay, bought for expensive through fox, or 3d printed if you have access) and move the associated air spring parts over from the foot stud size of the shaft. If you try to do it from the piston side, you'll need a roll pin tool (same tool used to install a vorsprung luftkappe) but you risk screwing up the oring, and the glide bushing as you slide them over the roll pin hole. You can sorta work around this by putting the air shaft in the freezer, and then praying... but it's not a great idea.
In my case, I did this to a 2018 fox 36 to convert that 36 to 130mm travel long before fox offered any ability for the 36 to be built shorter then 140mm. I used an air spring from a fox 34, pulled the foot stud and moved everything over.
Damn.
That’s sounds awful, thanks for the word of warning
I've got very little DJ skills and the fork is only there to save me when casing jumps, so I've gone for the jank but easy way of putting a spacer on the airshaft of a longer travel fork (35mm 27.5" Revelation I got for a hundred bucks used). Works OK, and I've got a strong sub 2000g fork for cheap.
I did consider trying to fit a pike DJ shaft, which could have worked from my research, but never actual go through with this. Not too familiar with Fox forks so cant help with those.
If its an NA2 or Evol fork with the equalisation dimple, you can swap the air piston of a 36 on to any shorter shaft from another Fox fork from recent years - the threads and roll pin size is the same across all of them
The older NA air spring used thread-on travel spacers on the neg plate and an adjustable centre rod which might need some fiddling to make work, although they did produce an 831 version of that fork. Second option for that type is tracking down an MRP fulfill kit - which converts it to an adjustable negative spring which means you could just stack up a bunch of spacers behind the air piston to change the travel, and you fill up the neg chamber manually
Love the janky solution, but I have more questions. How did you make the spacer, with 3D printing or with tooling? And how does it ride? I'm curious if it does anything weird at top out.
Is that a Fox air shaft going in to the Rockshox fork? That might technically function but there won't be any negative spring pressure
If you can get an old non-boost 36, that would be a good starting point.. I had an old 27.5 from 2015 (only the lowers and stanchions). I aimed for 80mm for my son's build (lowers are 27.5, wanted to keep the bike low)..during the process, I found some issues,.. dremel works wonders..
summary:
* 2015 fox36 rc2 non-boost 170mm.. with the fantastic convertible axle (with the inserts to accomodate 15 and 20mm), I had the 20mm axle but had to helicoil the thread on the lowers.. it had been bad since 2018..
* original air shaft was too difficult to modify (the ones you could adjust travel a few cms), so I used a newer one I had laying around (2017-2018 I believe).
* measured desired travel, marked it on the shaft, measured the distance of the port, removed the seal, cut the shaft, drilled a new port, put the seal back..
* had to reduce the thickness of the lower metal part in order to seat properly inside the stanchion and have room for the retainer clip.. newer airshafts have taller end of this part highlighted in blue, I just grinded it down until the clip seated properly.
Bike is holding good https://www.vitalmtb.com/community/luisgutrod/banshee-amp-grom-edition
3D printed out of PETG with a good infill, been in there for over a year, hasnt failed yet.
Ride wise it does feel a bit funky at top out, I wouldnt put it on anything but a dirtjumper, my setup tend to be high pressure fast-ish rebound as I mostly ride asphalt pumptracks. I'll see if I can record any sort of meaningful video
Woops I grabbed the first pic that looked like it in my galery 😅 I tested the idea on a 34 Rythm I had laying around but never rode it (29" fork on 26er look a bit daft), that must be what the one pictured is
the '16 36 float can be internally adjusted in 10mm increments spacers. there were 2 versions of the air spring shaft for that era: 160 max travel and 180 max travel. the 160 can be adjusted down to 110mm, the 180 can only go down to 130mm.
here's the instructions:
https://tech.ridefox.com/bike/service-procedures/576/2015-2017-36mm-float-internally-adjusting-fork-travel
the spacer(s) you'll need are p/n 234-04-627. might be a little hard to source these days.
i was wrong. google turned them up pretty readily. $5 a pop. here's a video that shows the process:
https://www.youtube.com/watch?v=TTUcov4UlJA
surprised i hadn't seen this before, is there any way you could "modify" the 180mm down to 100 instead of 130?
A good suspension shop can machine a 100mm air shaft for you. I've had a couple done over the years. I have an '18 36 831 that I wanted a 140 shaft, the shop didn't have any so I had one modified for a 160mm. And last year I had a Fox 40 shaft modded to be 180mm travel.
Technically feasible, yes, but as mentioned above requires some machining and modifications most home mechanics aren't equipped for. IIRC the 180 has a longer shaft and isn't configured in a way that more spacers will work.
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