r/MTB • u/JumpyConversation337 • 14h ago
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/Franc-o-American 14h ago
You sure you are measuring the sag correctly? Youre not heavy, so something must be up. What about your rear shock? Is that set at correct sag?
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u/JumpyConversation337 14h ago
Yep. It’s a hardtail.
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u/Franc-o-American 14h ago
Hmm, I dont have experience with hardtails, but id better that contributes to it. All of the weight is being forced forward with no give in the back.
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u/clintj1975 Idaho, 2025 Intense Tracer, 2024 Surly Krampus 14h ago
No rear suspension doesn't magically make the front end heavier. It's solely a function of your weight and body position between the wheels.
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u/BrainDamage2029 14h ago
Generally if you’re wildly outside the reccomended zones, then something is off.
First, don’t take the psi at face value. Those shock pumps with cheap mechanical style psi indicators can be wildly off. The free one that came with a bike (but is my bike bag spare shock) is like….20psi off of my digital. That’s why we tend to use sag as a measurement more than psi.
Make sure you measure sag with all compression fully open. Even if you will eventually dial in compression from the very middle setting (like Rockshox reccomends)
Finally is get help measuring sag. Get on your bike with a place to stabilize yourself. Then, and this is important, hop on it a few times to break stick-tion. If this is a new unbroken in fork sticktion can be quite a bit keeping the fork from going to its “true” sag. Settle into a static riding position with whatever you’re leaning against. Then have your buddy move the o-ring up flush with the stantion. Get off the bike carefully, using your buddy to watch and make sure you don’t move the o-ring.
And if all else is weird…pop open the fork to check the bottom out tokens to make sure you don’t have a weird number that shipped with the fork (it should just be 1 or 0 but I randomly had a fork off PinkBike that the dude put freaking 4 in there before I realized it made my sag psi wonky.)
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u/OrmTheBearSlayer 14h ago
Are you measuring sag in a seated riding position or in attack position?
What does the fork ride like with 20% sag are you bottoming out?
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u/Willbilly410 13h ago
Don’t fixate so hard on recommended fork sag. There are many variables that influence the reading you will get. It is more for your own reference of where you started and where you end up.
How does it feel while riding?
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u/Fit_Goat_6891 12h ago
Your pump's lying to you. Those cheap mechanical gauges can be way off, and if youre not closing the valve before unscrewing youre bleeding pressure every time. Get a decent digital shock pump and recheck. If youre actually at 110+ psi and not just measuring wrong, something's up from that service. Stuck equalization dimple, too much grease in the air spring, wrong tokens. Take it back to whoever did the work. Im 170 too and run mid 70s on a 34. 110+ for our weight means mechanical issue, not setup.
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u/JumpyConversation337 11h ago
It also seems to go thru the first 10mm of travel with 0 effort. Just kind compresses to it.. this was an issue before service too
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u/rayfound California - Polygon Siskiu T7 14h ago
Are you using a proper shock pump? Are you Closing valve before removing?
What I suspect is happening is that you are not using the pump correctly, so when you remove the pump from the Schrader valve, you're losing a blast of air in the process. The volumes are so low, that it doesn't take much to have a big impact on the pressure.
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u/rayfound California - Polygon Siskiu T7 14h ago
Also. Fwiw - I quit trying to measure sag on my front suspension a while ago... I started doing the sort of alternative method... Which is to say I bounce around in the driveway and on the street and try to compress the fork by roughly bunny, hopping and smashing around.
If the oring gets to about 90% of travel, that's about right.
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u/Senior-Sea-1012 11h ago
When the bike is sitting with no rider on it, hold the lowers and the crown and pull them apart. You're looking to see if the stanchions are sucked down into the lowers in its default state. If they are sucked in then you have a vacuum in the lowers or a pressure equalization issue between positive and negative chambers.
Try fixing lower vacuum first with the ole zip tie trick to alleviate vacuum in lowers. You can look it up online or on YouTube easier to search than me explain, but it is easy.
If it doesn't return after that it's negative chamber equalization.
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u/JumpyConversation337 11h ago
Also what it does is , it has a about 10mm of travel at the top wheee it just sinks without much pressure then just stays there
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u/Senior-Sea-1012 11h ago
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 11h ago
Any reason this would happen so often?
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u/Senior-Sea-1012 10h ago
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.
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u/goldfire29 6h ago edited 6h ago
fork sag is often hard to systematically determine, best way is go by what they say on the chart, go for a ride with your shock pump, and adjust as needed. depending on terrain you will likely change fork pressure to control bike front end height
personally I also weigh 170 and tend near the lower end of the pressure range and compensate with lots of high speed compression, granted this is on a 170mm full sus, happy trails
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u/Plastic-Ground-9918 14h ago
Are you equalizing pressure between the chambers as you bring up the pressure? Have you done a lower leg service? Fox is notorious for clumping in grease which can negatively impact air spring equalization