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  1. #1
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    Fox air forks - Modifying volume to get full travel?

    Hi there,

    I own a 2008 Fox F120 fork that i've been riding for the past 6 months for primarily XC and trail riding. I weigh around 67kg and when i first used the fork, could only get about 100mm of travel out of it regardless of sag setup.

    After reading magazines and forums about others having similar issues, i decided to modify the push rod length in the air spring side of the fork in order to increase the volume and reduce the forks inherent progressiveness.

    I cut about 20mm off the top of the push rod, with the DIY article suggesting up to 30mm could be severed.
    The fork has been a lot more plush since the modification, though there's still another 10mm of travel to be had.
    I'm thinking of taking the push rod out and cutting some more to get a bit more volume.

    Im just interested to see if anyone else has performed this sort of modification and if so what sort of results they had.
    Im pretty sure that i can get full travel out of this fork, but id like to hear others experiences before i go hacking into its internals again

    Cheers,
    Wil

    FYI, the article about the modification was published in an Australian MTB magazine.

  2. #2
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    A guy in Finland modified a -07 Fox float 140 mm, see pictures:

    http://suksiseppo.pinkbike.com/album/Huoltoa/

  3. #3
    TNC
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    Another possibility...

    Where is the schrader valve for your fork? What about adding a remote volume device of some kind...maybe even something adjustable. Risse makes or made this little doodad. It can be used for shocks or forks. It is just a chamber that has its own valve to add or release air. Its volume can be adjusted much like the piggyback on many modern rear shocks. I post this just to provide another angle on the issue. One could even make their own simple device with a fixed volume to suit their needs. Any attached or hosed remote device could creatively be fastened to some part of the fork or frame as long as there is a decently accessible air valve. Dirt motorcycles did something like this a long time ago. If added volume is desired, something along this vein might be better than hacking into the internals of your fork. This or something similar could be removed if you sell the fork later without any difficulties.
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  4. #4
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    Thanks for posting this info.
    I have a 2006 f100 RLT that I can only get 80mm of travel out of. I think I'll be trying this modification soon.

  5. #5
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    Make sure your dampener side is not overfull. I have a 2007 F100 RLC only getting 80mm and that was the problem. To correct it I unthreaded the the top of the damper assembly, released all the air from the air spring side, compressed the fork all the way, extracted oil until under the threads, reassembled. Works perfect & has full travel now.

  6. #6
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    If there is too much oil in the damper side, then if you let out all the air from the spring side, the fork will still not compress all the way, even with without air in the fork.

    However, if the damping side has the correct oil levels, then you should be able to compress the fork all the way without any air in the fork.

    If this is the case but you are still finding the fork isn't using all the travel, then its likely to do with the forks volume in relation to your bodyweight.

    Correct me if im wrong.

    cheers

  7. #7
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    Thanks guys.
    I'll be trying the oil level thing first and if that doesn't do it I'll try rod trimming mod.
    The fork works great except for the limited travel.

  8. #8
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    Quote Originally Posted by Little Willy
    If there is too much oil in the damper side, then if you let out all the air from the spring side, the fork will still not compress all the way, even with without air in the fork.

    However, if the damping side has the correct oil levels, then you should be able to compress the fork all the way without any air in the fork.

    If this is the case but you are still finding the fork isn't using all the travel, then its likely to do with the forks volume in relation to your bodyweight.

    Correct me if im wrong.

    cheers

    You are correct.

    I've never owned a fork that got 100% of its advertised travel at recommended sag.
    Usable travel and advertised travel are differant animals for us light riders.

  9. #9
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    Yeah I agree about the never getting full tarvel out of a fork before but missing one out of 4 inches seems excessive.
    That said this Fox is soo much better than the SID's I've had in the past its ridiculous.

  10. #10
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    Quote Originally Posted by Little Willy
    If there is too much oil in the damper side, then if you let out all the air from the spring side, the fork will still not compress all the way, even with without air in the fork.

    However, if the damping side has the correct oil levels, then you should be able to compress the fork all the way without any air in the fork.

    If this is the case but you are still finding the fork isn't using all the travel, then its likely to do with the forks volume in relation to your bodyweight.

    Correct me if im wrong.

    cheers
    Correct, that is how I determined the dampener was over full. The Fox web site documents F100/F90 RLC/RL/R all have an oil volume of 155ml. This is incorrect because the RLC dampener cartridge has more components, than the RL, that take up more volume thus the necessity for less oil. I contacted Fox after repairing my fork and the technician agreed with my conclusion.
    It is also very common to over fill the damper when servicing the fork. The damper cartridge retains oil, if not evacuated, and the fork will be way over full when the recommended oil volume is added.

    Prost

  11. #11
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    also, removing some of the lubrication oil from the air chamber should free up enough volume for u to get u full travel, it will also make the air spring a bit less progressive...be sure to leave about 1cc (5cc is stock) in though to keep everything running smoothly

    i would check the oil levels and remove some lubricating oil out of the air chamber before i would go ahead with the mod
    V-10c (custom)
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  12. #12
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    Well Push actually has a high volume kit for the float forks which I imagine they are doing similar to what you are doing because obviously they cant add the volume through diameter.

    Also to the poster above the RL and RLC dampers are very similar to each other it is just the top pison or valving (the compression piston inside the top cap) that is different. The RLS had an adjustable orifice (port) for the low speed compression where as the RL all the orifices (ports) are a fixed size.

  13. #13
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    Quote Originally Posted by Jesse Hill
    Well Push actually has a high volume kit for the float forks which I imagine they are doing similar to what you are doing because obviously they cant add the volume through diameter.

    Also to the poster above the RL and RLC dampers are very similar to each other it is just the top pison or valving (the compression piston inside the top cap) that is different. The RLS had an adjustable orifice (port) for the low speed compression where as the RL all the orifices (ports) are a fixed size.
    The RLC has low-speed compression & lockout force adjustment. The lockout force adjustment internal workings is what displaces more oil in the RLC fork.

  14. #14
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    I've just checked my fork under full compression without air in the spring and it compresses all the way, so i can safely assume i need to increase the volume rather than modify the damping oil levels - though i will refill the forks after i do the mod and check them anyway.

    As for taking out some of the lubricating oil from on top of the piston, i only have about 3cc in there at the moment, and i think i will need substantially more volume to achieve full travel.
    If i end up cutting off too much, then i can always add float fluid on top of the piston to decrease the volume.

    I'll report back once i've done the mod (V.2)
    thanks for the help.

    Wil

  15. #15
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    Quote Originally Posted by Jacobus
    The RLC has low-speed compression & lockout force adjustment. The lockout force adjustment internal workings is what displaces more oil in the RLC fork.
    Ah forgot about that. Not sure if it really takes up too much more though as it is just an adjustable preload spring on the base valve and I cannot remember of the top of my head but I thought the RL still had the blow off it was just set tight and not adjustable but there was still relief so the cart wouldnt blow. I cant picture it though its been a few months since I had one apart.

  16. #16
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    Just an update:

    i've just put the fork back together this evening after cutting an additional 10mm off the piston rod to increase the air volume.
    Will get out tomorrow to see if the fork has improved and will write back.

    cheers

  17. #17
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    Fairly brief ride today but i can already tell the difference; fork is much more linear in action now.
    Im getting pretty much all the travel now and the fork feels smoother.

    FWIW im putting 70psi in the fork, whereas i had 65psi in before and 60 psi when i had the fork new.

    Highly recommend this mod if you arent reaching full travel of your fork!
    cheers,
    wil

  18. #18
    Doctor
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    Kudos to you for having the moxie to take a hacksaw to such an expensive component!

    I felt brave just servicing the thing...

    jeff

  19. #19
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    Did this mod to 36 float

    A few months ago I cut 20mm and redrilled my Float 36 air piston control rod. It's very simple and easy modification that provides immediate results. On normal rides my average full travel went from 5" to 5 3/4". The most travel I have used post mod is 6" so there still is 1/3" for the OH Sh!T I went off that wrong move. Besides warrenty I can think of no down side to this mod for my useage...
    HoJo

  20. #20
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    howsabout on a 100mm?

    i have a float r 100mm; thinking i can get 120mm ? can i do this mod on this fork?

    I'm thinking fox did not make a 100mm and a 120mmm float?
    who would know...push, fox, ?

  21. #21
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    Will not help you 100mm getmore than 100mm

    ecoast, in a word NO. This helps you get full travel or close to the full speced travel of the fork

  22. #22
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    Quote Originally Posted by Little Willy
    Just an update:

    i've just put the fork back together this evening after cutting an additional 10mm off the piston rod to increase the air volume.
    Will get out tomorrow to see if the fork has improved and will write back.

    cheers
    Are you noticing brake dive and lack of mid stroke support symptom since the mod?
    sth

  23. #23
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    Quote Originally Posted by HoJo
    ecoast, in a word NO. This helps you get full travel or close to the full speced travel of the fork

    So you think fox made float in 100mm, 120mm, 130mm an, as well as 140mm?
    That's alot of separate parts for essentially the same thing...

    ...personally find it hard to believe my 100mm is not a longer travel model w/ spacers in it...was hoping someone would know, but looks like I'll have to pull it apart to find out.

  24. #24
    TLL
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    Quote Originally Posted by HoJo
    A few months ago I cut 20mm and redrilled my Float 36 air piston control rod. It's very simple and easy modification that provides immediate results. On normal rides my average full travel went from 5" to 5 3/4". The most travel I have used post mod is 6" so there still is 1/3" for the OH Sh!T I went off that wrong move. Besides warrenty I can think of no down side to this mod for my useage...
    HoJo
    Interesting mod. It is my understanding the part of the reason the 36's don't get full travel is that the bottom-out resistance is set firm at the factory. Changing it to med or light gives additional travel. How much I don't know.
    Hadley rear hub service here and here.

  25. #25
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    Quote Originally Posted by ecoast
    So you think fox made float in 100mm, 120mm, 130mm an, as well as 140mm?
    That's alot of separate parts for essentially the same thing...

    ...personally find it hard to believe my 100mm is not a longer travel model w/ spacers in it...was hoping someone would know, but looks like I'll have to pull it apart to find out.
    You're correct that Fox makes one air piston rod for all model forks. Your fork probably doesn't have spacers in it, but the rod will have holes in it so that you could push the pin out of the negative spring stop and move the stop, thus changing the travel. Moving the stop to the holes closer to the piston head will increase the travel. Moving the stop the other way reduces the travel.

    One thing to make note of: Depending on the model year of your fork, your damper (right leg stuff) might not be long enough for you to increase the travel to 120mm. I have a '03 Float that can only go to 100mm because the damper isn't long enough.

    A by-product of increasing the travel is the air chamber size is increased as well. A Float at 140mm will act more linearly in it's travel than one set at 80mm, which makes sense.

    The mod described in this thread is a way of increasing the air chamber size without increasing the travel of the fork, making it more linear at that given travel.

    op
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