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Fox fork air pressure loss

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Fox fork air pressure loss

Old 07-23-20, 02:01 AM
  #1  
Amt0571
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Fox fork air pressure loss

After receiving a Canyon Grand Canyon with a damaged frame, a leaking front caliper and a creaking bottom bracket, my good luck with the bike keeps going on.

The bike has a Fox Rhythm 34 fork. I weight 75kg, but found I was unable to use the whole travel at the suggested 72psi, so I tried with 50psi and it felt well. Today I noticed the fork felt harsh on bumpy trails, and when arriving home I tried to jump on the pedals and noticed It was really easy to bottom out and also rebound was really slow without having touched the dial. I took my pump and noticed there were only 25psi in it. I checked that when removing the pump it only loses 5psi, so the pressure definitely went somewhere.

I have read something about equalization of the positive and negative chambers and, TBH, I did not do anything special when initially setting the pressure. Is it possible that the chambers equalized and the pressure I had put in the fork lowered because of that? or it's a definitive leak?

What's exactly the procedure to equalize the chambers when setting the pressure? I read that there's a point where air should "rush" from the positive to the negative air chamber, but I can't feel this when cycling the fork.

Thanks!
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Old 07-23-20, 10:32 AM
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bikeme
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When you say you can't use full travel, what do you mean? Do you mean you can't bottom-out the fork when dropping off something? I have my bike set at 25% sag (pretty standard) and have never bottomed out--I'd have to jump off something scary to do so. I still have some travel left indicated by dust or the o-ring on the fork.

When a fork is new or serviced or put upside down, you will need to cycle it several times to get oil and air in sync. With the open/lock dial in full open, push down on your bars several times. Now, you might have an air leak based on the loss you're encountering. I suspect the cap to your air chamber might be loose or its o-ring damaged or missing, or the cap's schraeder valve itself is leaking to lose as much psi as you are.

As far as rebound, your dial might be set too slow. If it's not fast enough, the suspension gets "packed out "--it never fully rebounds before the next hits you take, thus losing more and more travel. The ride gets harsh. Try the setting in the middle, that should get you in rideable ballpark.

You will not lose air when disconnecting the shock pump. That little whoosh is the air from the pump hose. If you reconnect the pump, you then will lose some psi.
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Old 07-23-20, 02:53 PM
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Originally Posted by bikeme View Post
When you say you can't use full travel, what do you mean? Do you mean you can't bottom-out the fork when dropping off something? I have my bike set at 25% sag (pretty standard) and have never bottomed out--I'd have to jump off something scary to do so. I still have some travel left indicated by dust or the o-ring on the fork.

When a fork is new or serviced or put upside down, you will need to cycle it several times to get oil and air in sync. With the open/lock dial in full open, push down on your bars several times. Now, you might have an air leak based on the loss you're encountering. I suspect the cap to your air chamber might be loose or its o-ring damaged or missing, or the cap's schraeder valve itself is leaking to lose as much psi as you are.

As far as rebound, your dial might be set too slow. If it's not fast enough, the suspension gets "packed out "--it never fully rebounds before the next hits you take, thus losing more and more travel. The ride gets harsh. Try the setting in the middle, that should get you in rideable ballpark.

You will not lose air when disconnecting the shock pump. That little whoosh is the air from the pump hose. If you reconnect the pump, you then will lose some psi.
When I say I can't use full travel I mean that I use 90cm of the 120cm of the fork at the recommended pressure. At 50psi I easily use 110mm, maybe more.

Problem is I'm not sure If it's a leak or it has something to do with pressure equalization between positive and negative chamber. I fully deflated the fork before setting the pressure because the negative chamber seemed to have too much pressure and the fork didn't extend fully. I read somewhere that I should compress the fork several times while pumping air to equalize both chambers. At one point I heard a "whooosh" inside the fork and it seemed to me that it was it (I read it should do that), but now I'm not really sure if I didn't do it properly and pressure got lower because equalization was not fully complete, or if it's a leak.

Valve cap is ok. I've only noticed the different behaviour on the last 2 rides.
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Old 07-23-20, 07:04 PM
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Well hell, pump it it up and stop guessing
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Old 07-24-20, 01:05 AM
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Originally Posted by Amt0571 View Post
When I say I can't use full travel I mean that I use 90cm of the 120cm of the fork at the recommended pressure. At 50psi I easily use 110mm, maybe more.

Problem is I'm not sure If it's a leak or it has something to do with pressure equalization between positive and negative chamber. I fully deflated the fork before setting the pressure because the negative chamber seemed to have too much pressure and the fork didn't extend fully. I read somewhere that I should compress the fork several times while pumping air to equalize both chambers. At one point I heard a "whooosh" inside the fork and it seemed to me that it was it (I read it should do that), but now I'm not really sure if I didn't do it properly and pressure got lower because equalization was not fully complete, or if it's a leak.

Valve cap is ok. I've only noticed the different behaviour on the last 2 rides.
Sorry, I don't have enough knowledge about equalizing those pos/neg chambers to help. Quite frankly, all ever I've done is just filled mine to X-amount of PSI when it was new or after I've serviced oil and seals. No issues ever. As far as usable travel, if you're traveling 90mm of the 120mm available on most of your big drops, that's about right: 25% of the 120mm is left over for really big hits. You're not supposed to bottom out all of the 120mm but on a rare occasion. If you bottom out too often, that means you probably need to add more psi or at the very extreme buy a longer travel fork bike for jumping off big features. Those psi guidelines are off a bit. Rider weight is only one element. Rider aggressiveness and terrain play a big role. I just reread you had a 22 psi diff between the two rides (72 and 50 psi). With suspension, even two to three pounds change is a big amount. Try some psi in between those two, just 2-3 psi at a time. And check the rebound knob--try full fast and full slow to compare. I like mine from the middle plus a few clicks towards max fast.
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Old 08-18-20, 03:57 AM
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Well, I've finally found the issue. The fork is effectively leaking. Over the last 3 weeks it lost about 10 / 15psi.

The issue is, it's leaking inside the left leg. Yesterday I found out about "burping" the fork which consists of carefully inserting a zip tie in the seal to equalize interior and exterior pressure. Guess what? I did it on both legs. The right one only made a small "pscchhhh". The left one, which contains the air piston, started spurting high pressure air and a small quantity of oil.

It's now quite obvious to me that the negative chamber is leaking air into the leg.

Since the fork was bought in march and has less than 50h of use, I have contacted Fox and I'm waiting for their response.
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