Posts
31
Joined
2/28/2023
Location
Madison, WI, USA
Edited Date/Time
5/26/2025 6:11pm
This past weekend I helped a buddy do his lower leg service on his Fox 36 for the first time. It has been through some pretty rough wet rides and the dust wipers were very dirty but to our surprise there was a good amount of oil that poured out when we dropped the lowers. Also recently, I have heard quite a few stories of bikepacking racers who have had their forks lock up partway through a race (anywhere from 300 to 3,000 miles). Obviously they surpassed manufacturer recommend intervals, but could using more bath oil in their lowers have helped prevent their forks from failing? For riders who don’t service their suspension themselves or as often, could extra lubrication help their fork feel better longer?
The oil in the lowers is just for bushing lubrication, adding more than recommended might reduce the volume and cause the fork to ramp up more than wanted.
On long rides, flipping the bike upside down so the oil gets into the bushings and foam rings, might do more for performance than running more oil. Or maybe there is a better, higher end oil that could last longer? But it might be dirt and dust that is contaminating the oil.
Did the oil mostly come from one leg or both leg? If there was significantly more oil it could be from the damper leaking oil into the legs. Yes too much oil will cause it perform differently from more ramp up to completely locked out. I can notice a difference if I put much grease on the spring side or if the damper leaks oil.
Short answer: yes it does matter and there are some dramatic downsides to overfilling your bath oil. There’s not a lot of room for air when a single crown fork chassis bottoms out. Don’t need to make that space even smaller.
While lower leg lubrication volume is not huge, it definitely isn't going to cause fork locking up even if you run out if it. And running out of it is unlikely unless you have very leaky seals. Modern low friction seals do leak some oil on purpose, end even then after few days riding in dusty bikepark conditions there is still good amount of oil left. What can cause lockout is actually running more oil in the lowers that then migrates to the damping cartridge. Some cartridges are designed to purge the excess oil (fox grip, Manitou), but some don't and will actually hydrolock after some time (Suntour r2c2 for example). That hydrolock happens gradually though, so it manifests as hard bottom out at less travel than normally.
Thanks for the responses so far! These all seem to make sense to me. I guess I should clarify that the “good amount of oil” in my buddy’s fork was what I would expect to come out of a freshly serviced fork on the damper bath side, so I don’t think anything was leaking.
Is the potential for oil from the bath to migrate into the lowers the reason Fox uses two different weights of oil between the air and damper sides?
Yep they use the same oil in the lowers as in the dampers on the non sealed cartridge dampers which I think is most of them now
Bike packing forks usually seize up due to having copious amounts of rain on them and pushing past the seals, so even with relatively low k's or hours they will be in a rough state. Also if there is no visible leaking, there is usually a similar amoutn of oil in the lowers than when it was serviced, it will just be very degraded.
In general, more oil = better and will last longer but in a lot of forks the air spring side has very little space in the lower leg and any more than the specified amount will cause excessive ramp up, leaking or blowing out the seal. The damper side of sealed cartridge forks is an issue too - a certain percentage of the bath oil gets ingested in to the damper so even if there is plenty of air volume in there, adding more oil will just work its way in to the damper and cause damage (blown bladder).
Self-bleeding forks like Grip and Charger 3 don't have that concern which is why they can have 30-50cc of bath oil quite happily.
Heavier oil lubricates better and last longer, but if its too thick it creates drag and can also bypass seals easier, getting past the dust wipers, damper seals and even the air spring negative chamber. Oils like the Maxima Plush dynamic "light" is a good compromise
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