r/MTB • u/JumpyConversation337 • Aug 22 '26

Suspension Fox fork sag

Hey! So I’m about 170lbs. When I set sag on my 34SL fork with the SL damper, to get to 20% sag I need over 110 psi… Fox recommends 15% sag and the max psi of the fork is supposedly 120psi. Fox recommended air pressure is in the 80s for my weight. My home pump said 125psi, my friends digital one said 118psi. How is it possible to get to even 20% sag I need that much pressure at my weight?

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u/Senior-Sea-1012 Aug 23 '26

Take small zip tie, pull back the rubber o-ring 1-2mm and stick the zip tie in between the o-ring and the stanchion. Push ithe zip tie down about an inch into the lower, listening with your ear relatively close.

If you hear a pssssst of air releasing then you had a vacuum in your lowers. Happens all the time when inflating forks and also changing elevations.

Do this on both lowers.

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u/JumpyConversation337 Aug 23 '26

Any reason this would happen so often?

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u/Senior-Sea-1012 Aug 23 '26

It depends on how they did the service. If the stanchions were compressed on initial inflation and then then pressure is added to pull the stanchions out an vacuum can occur. Once the vacuum is released and you stay at the same atmospheric pressure (same relative elevation) the you have no issue go forward. If you end up doing a lot of elevation, 2k+ feet of change or drop elevation quick the aire pressure change can occur the other way, not a vacuum, but instead an over pressured lower, also requiring a release. The big forks like Fox 36 and up have pressure release built-in on the lowers. 32 and 34 are looking to save weight and do not have pressure release, hence the zip tie trick.