2015 Fox RC2 160/36 (650b) fork - negative air pressure problem

Question

As far as I can tell after speaking with 2 different Fox Service technicians, nobody seems to know why their forks have this well-known problem with negative air pressure increasing and migrating, and not re-equalizing.

I don't understand the whole thing, but there's only one port that allows air to move from the positive to negative chambers. I've now had 2 of the 2015 RC2's and both began doing the same thing after about 3 months of riding: the stanchions started not fully retracting when I am standing next to bike, compressing fork as far as possible and immediately lifting front tire off the ground. I tried "burping" with a ziptie between dust wipe and stanchion, but nothing there. I will next try removing the top cap and rod and slick honey it as directed and see if that fixes the problem, as well as do an oil change.

One year ago, with my first 2015 RC2 (160/36 - 26") Fox had me send it in under warranty - they ended up sending me a brand new fork, but it was never clear what the problem was. I hope I don't have to send this one in. Even one of the techs I talked to said he had asked an engineer about this who also didn't know what causes this problem(!) Hard to believe as it's a well-known issue, and I'd like to know how to prevent it. Btw, I have not been riding at high altitudes. I have bottomed out a few times - but why should that, if it has something to do with this, be the cause?? thanks!

Answer 1

Thanks, moderators, for asking contributors to answer the actual question! So different from the millions of unmoderated, often tangential and perfunctory answers found on MTBR.

This problem occurs on both new and serviced forks. Fox's 40 mm downhill fork has pressure release micro-bleeders near the top of the legs, just below the dust wiper seals that get pushed in to activate. Since these are on both legs in the same location, and since sliding a zip tie past the wiper seals on both legs can usually release the pressure and return the stanchions to full extension, I find this statement,

air negative spring becoming over pressurized due to air migrating past the seal separating the positive/negative sides of the FLOAT assembly,

confusing, at best. And again, it's not just Fox Floats having this issue, Rock Shox users do, as well. I'm doubting there's an answer to how to prevent this, but hope in near future that Fox puts those micro-bleeders on the 36 if they can't engineer the problem away.

Posts #104, 106 & 108 on the link below has a contributor who shows how to modify your fork with your own micro-bleeders:

http://forums.mtbr.com/shocks-suspension/air-pike-lowers-psa-929695-3.html

Answer 2

This issue has to do with the air negative spring becoming over pressurized due to air migrating past the seal separating the positive/negative sides of the FLOAT assembly.

I've had the same thing happen to me with my current 36 FLOAT and it can be fixed with a service, but in my case, it's doing it again.

I suspect some quality problems with this model, since it doesn't affect all. I've considered switching to the new TALAS, which has a different spring entirely, but it's not like I need the travel adjustment or weight, and I like the tunability of the FLOAT. Hopefully you can get someone from FOX to resolve the issue...

Have you heard anything else?

Answer 3

I also have this issue on 2016 Fox 36 Float 160mm forks. I tried emptying the negative chamber (Empty all air from top valve, remove 32mm nut on air side, removing negative transfer rod, clean, grease then replace). Cures temporarily but unfortunately within a ride my stanchions again wouldn't extend more than 145mm. Current in discussion with YT as to the answer. Will report back.