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I have just finished a full service of my 2010 Revelation Team U-Turns and am now having trouble getting them to behave.
Prior to the service, if I inflated +ve then -ve chambers, the fork would only start to suck down when the -ve pressure exceeded the +ve.
After the service they start to suck down much earlier, meaning I would have to have the -ve about 20psi lower than the +ve to avoid loosing a good chunk of travel.
Does anyone have any ideas what would cause this? Perhaps it is just the reduced stiction caused by new seals and oil? I was very careful with the damper oil volumes.
Cheers!
Dan
Are you sure there was no damage internally(as miniscule as it maybe), surprising that after a service they would be worse.
Sucking down usually means one thing O rings gone, but as Northwind found out there can be damage internally that only someone with extensive knowledge could spot.
have you put the negative piston on the correct way round?
^^^^^ listen to this man!!!
I was pretty meticulous with the service and replaced all of the seals. No visible damage or scratches anywhere. I left the +ve inflated to 150 psi and the negative empty over night and there does not seem to be any air migration or loss.
I've got the same fork which went back to SRAM under warenty as the u turn failed
It came back with a new uturn assembly and a full service by SRAM and it behaves like your fork. I always run around 10psi less in the neg chamber or the fork sucks down and dives too much .
Its different to my other rs forks which all have matched +- pressures yet its the plushest fork I own
Loco, when you say negative piston do you mean the blue block in this picture?
I did not remove the actual black piston unit from the end of the shaft. I removed the bottom out bumper but that is back on the correct way as in the picture.
dvatchmark - that is really interesting, perhaps it is just the reduced stiction?
When I first got my fork back I thought it was still knackered
What you describe just sounds normal to me? Mine have always sucked down (fractionally) with the negative pressure lower than the positive. I tend to set them up that way- air up the positive, then add negative til it twitches which typically is about 20psi below.
The confusing thing is that before the service I was running -ve at 5 psi above +ve with only a few mm of suckdown. Won't the performance of the fork be drastically altered if I drop 20psi from -ve?
Doesn't the manual recommend keeping the -ve and +ve within 10psi of each other or something?
I am also wondering if it could be to do with the way I re-assembled. I had the Uturn wound to max travel when installing the air unit and lowers. When adding the oil to the lowers and tightening the bolts on the ends of the lowers the forks were pretty much full compressed (as they had no air in them).
As soon as I had finished I pumped up the positive to 100psi which extended the fork. I then pumped the negative and it started sucking down VERY early - after about 50psi. I emptied both chambers and pumped them up again a couple of times over and things improved. Very confused by this behavior!
Meant the blue sealhead bit as need the circlip on it.
The pressures should be balance in pos. and neg chambers for initial setup and the fork may compress and few mm, more than 7/10mm with pressures balance there maybe issue.
Run my dual air revs (reduced to 110mm) with 10psi extra in the posative chamber
I'm pretty sure it was the right way around - I guess the circlip would not have fitted if I had it the other way?
Just to drag up this thread and to help anyone with the same issue.. yesterday I had another go at getting the fork working and succeeded.
As I said, I originally let all of the air out of both chambers and refilled with +ve first. I did this several times over but always suffered the same suck down.
This time I put all of my weight on the fork while holding the valve pin in, first on +ve, then on -ve side. I then turned the fork upside down and released the -ve valve while pulling up hard on the lower legs. I repeated this cycle several times and each time a tiny bit more air would release from the -ve side.
After this the fork seems to function more normally. With balanced +ve and -ve I get about 5mm suck down.
Can anyone enlighten me as to how this fixed it? Obviously there was air trapped somewhere or some kind of pressure imbalance. Could it be to do with how compressed the fork was when I re-assembled it? I still can't make sense of how the -ve started causing suck down at about half +ve pressure.
Anyway, hope this helps anyone else who finds this thread!
Cheers
Dan
Ok an update to help anyone else in with the same issue.. I did as suggested and ensured that the air piston was fully extended before pushing in the air piston base cap and circlip. I also wound the Uturn down to min travel before installing the top cap. Afterwards I wound out to max travel again and put the lowers on. Result: perfectly working fork again!!!