I have probably had over a dozen different front forks over the years and never experienced this issue with air in the lowers until this ‘26...
I have probably had over a dozen different front forks over the years and never experienced this issue with air in the lowers until this ‘26 Fox Factory 34SL. Can anyone explain why? I know it’s not just a Fox thing. What is new/different about these forks that make them susceptible?
Pressure in the lowers is almost always an air leak - the 34 SL/SC only has a single quad ring in the negative sealhead to keep...
Pressure in the lowers is almost always an air leak - the 34 SL/SC only has a single quad ring in the negative sealhead to keep the pressure in, but it is unusual for it to leak, especially in a new fork. If it comes back I would get Fox to look at it under warranty as its not normal.
The other place it can leak is the footstud to the air shaft - there is a small o-ring there but it also needs loctite to help make a perfect seal. Again its weird to leak there but it can happen if its loose or the o-ring has damage
So I think the footstud on my fork could have been the issue as it was barely more than hand tight. That would suck air in...
So I think the footstud on my fork could have been the issue as it was barely more than hand tight. That would suck air in during extension but compression would make it difficult for the air to escape right?
Even if the exterior bolt is hand tight the foot stud itself is a press fit with the lowers, and pretty much always under compression, so I doubt that would be an issue. Even if it was, the lowers wouldn’t be able to exceed atmospheric pressure.
Or did you mean the foot stud is loose from the air spring shaft?
Those forks don’t have bleed valves, correct? Pressure is always going change in the lowers due to temperature, altitude, or even gradual migration of air across the quad ring sealing the air shaft.
As an 130 XC fork one would assume the uppers would be as short as possible. Surprised casting ramp is that much of a problem on those.
Pressure in the lowers is almost always an air leak - the 34 SL/SC only has a single quad ring in the negative sealhead to keep...
Pressure in the lowers is almost always an air leak - the 34 SL/SC only has a single quad ring in the negative sealhead to keep the pressure in, but it is unusual for it to leak, especially in a new fork. If it comes back I would get Fox to look at it under warranty as its not normal.
The other place it can leak is the footstud to the air shaft - there is a small o-ring there but it also needs loctite to help make a perfect seal. Again its weird to leak there but it can happen if its loose or the o-ring has damage
So I think the footstud on my fork could have been the issue as it was barely more than hand tight. That would suck air in...
So I think the footstud on my fork could have been the issue as it was barely more than hand tight. That would suck air in during extension but compression would make it difficult for the air to escape right?
Even if the exterior bolt is hand tight the foot stud itself is a press fit with the lowers, and pretty much always under compression, so...
Even if the exterior bolt is hand tight the foot stud itself is a press fit with the lowers, and pretty much always under compression, so I doubt that would be an issue. Even if it was, the lowers wouldn’t be able to exceed atmospheric pressure.
Or did you mean the foot stud is loose from the air spring shaft?
Those forks don’t have bleed valves, correct? Pressure is always going change in the lowers due to temperature, altitude, or even gradual migration of air across the quad ring sealing the air shaft.
As an 130 XC fork one would assume the uppers would be as short as possible. Surprised casting ramp is that much of a problem on those.
My fork came that way for whatever reason. I have a few rides on it since bleeding the air off and it’s still feels great. I’m hopeful it was a one time thing.
I know this has been talked about a bunch but its kinda wild experiencing it yourself. Its amazing how much ramp the air in the fork...
I know this has been talked about a bunch but its kinda wild experiencing it yourself. Its amazing how much ramp the air in the fork lowers causes. I had my air cap off to change some volume spacers and I pushed down on the fork out of curiosity and it was difficult to get the fork to full compression with just the air in the lowers.
Was this on your 38? Even on the original Zeb which ramps up pretty hard compared to most, you should be able to bottom it pretty...
Was this on your 38? Even on the original Zeb which ramps up pretty hard compared to most, you should be able to bottom it pretty easily. With that fork off the bike just pushing down on the crowns with your hands is enough to bottom it out.
With my 38 on the bike bottoming it doesn’t take much effort at all. there’s added resistance for sure the last 25mm but I’d stop short of describing it as excessive.
It was my 38, I was doing back to back testing of my TopHat and regular volume spacers.
I would describe the resistance as “notable but not excessive.” I was able to bottom out the fork, it was just more force than I was expecting. I did press the bleeder valves before doing that to equalize everything.
So I think the footstud on my fork could have been the issue as it was barely more than hand tight. That would suck air in...
So I think the footstud on my fork could have been the issue as it was barely more than hand tight. That would suck air in during extension but compression would make it difficult for the air to escape right?
Even if the exterior bolt is hand tight the foot stud itself is a press fit with the lowers, and pretty much always under compression, so...
Even if the exterior bolt is hand tight the foot stud itself is a press fit with the lowers, and pretty much always under compression, so I doubt that would be an issue. Even if it was, the lowers wouldn’t be able to exceed atmospheric pressure.
Or did you mean the foot stud is loose from the air spring shaft?
Those forks don’t have bleed valves, correct? Pressure is always going change in the lowers due to temperature, altitude, or even gradual migration of air across the quad ring sealing the air shaft.
As an 130 XC fork one would assume the uppers would be as short as possible. Surprised casting ramp is that much of a problem on those.
My fork came that way for whatever reason. I have a few rides on it since bleeding the air off and it’s still feels great. I’m...
My fork came that way for whatever reason. I have a few rides on it since bleeding the air off and it’s still feels great. I’m hopeful it was a one time thing.
OK yeah it might have just been the way it was assembled - if theres a big enough change in temperature or altitude it can cause an increase in pressure. Also if the damper side has the nut tightened while fully extended, it will have some positive pressure from compressing it down to the air spring.
And @Ride886 had it right - the press fit should be air tight and not able to move, plus the air spring is basically only in compression (because thats the part pushing the fork back out). If it was moving there would be a horrendous knocking and probably wind all the way loose as well
The big contributors to lower leg ramp up are total travel and how much bath oil you're running. It's exactly the same as compression ratio in a piston engine. Small changes in dead volume can make for big changes in total compression ratio and ramp up.
Even if the exterior bolt is hand tight the foot stud itself is a press fit with the lowers, and pretty much always under compression, so I doubt that would be an issue. Even if it was, the lowers wouldn’t be able to exceed atmospheric pressure.
Or did you mean the foot stud is loose from the air spring shaft?
Those forks don’t have bleed valves, correct? Pressure is always going change in the lowers due to temperature, altitude, or even gradual migration of air across the quad ring sealing the air shaft.
As an 130 XC fork one would assume the uppers would be as short as possible. Surprised casting ramp is that much of a problem on those.
My fork came that way for whatever reason. I have a few rides on it since bleeding the air off and it’s still feels great. I’m hopeful it was a one time thing.
It was my 38, I was doing back to back testing of my TopHat and regular volume spacers.
I would describe the resistance as “notable but not excessive.” I was able to bottom out the fork, it was just more force than I was expecting. I did press the bleeder valves before doing that to equalize everything.
OK yeah it might have just been the way it was assembled - if theres a big enough change in temperature or altitude it can cause an increase in pressure. Also if the damper side has the nut tightened while fully extended, it will have some positive pressure from compressing it down to the air spring.
And @Ride886 had it right - the press fit should be air tight and not able to move, plus the air spring is basically only in compression (because thats the part pushing the fork back out). If it was moving there would be a horrendous knocking and probably wind all the way loose as well
The big contributors to lower leg ramp up are total travel and how much bath oil you're running. It's exactly the same as compression ratio in a piston engine. Small changes in dead volume can make for big changes in total compression ratio and ramp up.
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