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i have suspected for some time now that the amount of air in the lowers of a fork affect the overall spring rate in an air fork.

today i got an idea of how much it can.

usually, i have been in the habit of compressing the lowers down about half-way before closing the up the bottom of the fork during an oil bath change. i started doing this because it appeared that excess air in the lowers was preventing me from getting full travel on one of my forks. and on that same fork i was having trouble with oil going past the seals and down the lowers. so, i figured if i create a slight vacuum in the lowers during full extension, oil won't be as likely to leak out. so, i've been doing this on all my forks since then just to prevent potential problems.

well, today i got a bit lazy while installing new enduro seals. i figured these seals won't let the oil out the top, so i'll pump up the fork with normal pressure, at full extension, and close it up.

get the fork on the bike and give it good shove, and......stiff as a tree trunk! so, i took a break, thought about it for a while.......went back and compressed the fork halfway down, let out the pressure from the lowers, and closed it up and pumped the air piston back to normal.....wow, what a difference, fork feels great and sags exactly where it's supposed to with the recommended air pressure.

i've never seen anyone post about this peculiarity, and i haven't seen anything about this in the sram repair manuals.

often i read articles on here about people having trouble tuning their rock shox forks, and i am wondering if air pressure ramping-up in the lowers might be contributing to their troubles.

has anyone had these same experiences? and how did you cope with it if you did?
 

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Agree 100% although the effect is relative to how much the trapped air volume changes. Some forks will have more spring effect than others. Also, keep in mind that MANY air forks over time or due to lack of maintainance can leak air into the lowers. Even if it is just a PSI or two at full extension that gets out of the air chamber...this will make a big difference when the fork is compressed.

FYI although I have never heard this brought up on this forum (seems like most posts are about people having issues with year+ old forks that have seen zero service..and trying to convince users that all new forks need very regular service, how to do the work, what parts/tool/lubes are needed etc) it has been discussed a fair bit on other forums (RM) mostly in reference to Boxxer WC and lighter riders that would be more prone to notice the spring ramp.
 
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