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Fox F120 RL - low psi = low travel?!

individual6891individual6891 Posts: 120
edited December 2010 in MTB workshop & tech
I'm having some issues with my fork and I'm not sure whether its normal or not..

Just got a F120 second hand, changed the oil in damper side (10w), cleaned seals & wipers and relubed with 80w etc. I.e. a semi-full service/rebuild.

I'm 5 foot, 6 inches and weigh a measly 50kg. Now, setting my ideal pressures for 20-25% sag (40-50psi ish), I find that the fork doesn't quite fully extend to 120mm. It's lost about 5-10mm. Higher pressures (70psi +) and the travel maxes out to 120mm.

Now I swear my other F120 on a different bike has a constant 120mm even at 40psi? The only thing I can think of to justify the difference is that I might have put a tad too much 80w in the top air-side chamber (I'm talking 8ml instead of the recommended 5ml!).

Is there something wrong? I'd rather not take it for a £100 service, re-building it wasn't all that difficult. :? :? :?

Posts

  • gongagonga Posts: 225
    So the fork doesn't extend fully when your off the bike?
  • Yeah, about 5-10mm when I'm off it with an air pressure of 35-40. I know fox recommend 45psi minimum, but even at 45psi, there's still 5mm or so in there!
  • thel33terthel33ter Posts: 2,684
    Sounds like the same problem as I'm having with my Wotan, too much oil somewhere, so at low pressures it doesn't extend fully.
    And now you know, and knowing is half the battle
    05 Spesh Enduro Expert
    05 Trek 1000 Custom build
    Speedily Singular Thingy
  • bennett_346bennett_346 Posts: 5,092
    Yeah, about 5-10mm when I'm off it with an air pressure of 35-40. I know fox recommend 45psi minimum, but even at 45psi, there's still 5mm or so in there!
    How much do you weigh?
  • thel33terthel33ter Posts: 2,684
    In the first post 5'6" and 50kg :shock:

    I'm around 5'4", skinny, and still weight 55kg or so.
    And now you know, and knowing is half the battle
    05 Spesh Enduro Expert
    05 Trek 1000 Custom build
    Speedily Singular Thingy
  • Oddly enough, I'm going to be doing the same (service, oil change) with my just arrived, second hand f100 rl's, and I too weigh a paltry 60kg. I'll be using 10w rock oil damper side, which I'm guessing is what you've used? I'll let you know how I get on.

    Now, two things. I'm not convinced a few mls too much on the spring top side would have an affect, but I'm no expert; surely quickly whipping off the top cap and emptyin the oil would let you test this. If it works only put 5 mls back.

    Second, I went with 10w rock oil as it happens to have a similar centi-stokes rating as the overpriced fox stuff. I assume you have too. Now, I've been concerned that light spring pressures might necessitate a lighter damper oil, and my inexpert theory is that your spring pressure is too low to overcome the rebound damping. Might be worth considering lighter damper oil.

    Last thing is it's prob worth giving the fork a bed in period post-service to see if it settles down once the oil has found it's way aroun the fork.

    Phil
  • bennett_346bennett_346 Posts: 5,092
    thel33ter wrote:
    In the first post 5'6" and 50kg :shock:
    Must of been because it took me ages to read to the bottom, forgive me :lol:

    Change the volume of oil, check for improvement, if not then up the pressure to the minimum as that cant be helping either. If you dont like them that firm then those forks aren't for you.
  • fuelexfuelex Posts: 165
    Eat some pies, then up the psi ;)
  • goldfish2 wrote:
    Oddly enough, I'm going to be doing the same (service, oil change) with my just arrived, second hand f100 rl's, and I too weigh a paltry 60kg. I'll be using 10w rock oil damper side, which I'm guessing is what you've used? I'll let you know how I get on.

    Now, two things. I'm not convinced a few mls too much on the spring top side would have an affect, but I'm no expert; surely quickly whipping off the top cap and emptyin the oil would let you test this. If it works only put 5 mls back.

    Second, I went with 10w rock oil as it happens to have a similar centi-stokes rating as the overpriced fox stuff. I assume you have too. Now, I've been concerned that light spring pressures might necessitate a lighter damper oil, and my inexpert theory is that your spring pressure is too low to overcome the rebound damping. Might be worth considering lighter damper oil.

    Last thing is it's prob worth giving the fork a bed in period post-service to see if it settles down once the oil has found it's way aroun the fork.

    Phil

    I used Silkolene Pro Fork Oil 10w for damper side. Pretty sure I got the oil volume as I used syringes to measure. Same colour as fox and apparently its just rebranded and had the classic Fox-tax. For Fox Fluid I used Silkolene 80w Gear Oil (which cost about 25% of Fox Fluid, again rebranded stuff. Heck if you used Redline Gear Oil it's even blue to match).

    I don't think it's rebound from the slightly weightier oil (old oil must be 7w) cos the damper side shouldn't affect travel, just the rate of rebound & damping. Plus, if I open the rebound damper fully, it still suffers the same problem. With the rebound being adjustable, you can counter the heavier oil. I can't see how the oil volume in the damper side can affect the travel according to psi.
    thel33ter wrote:
    In the first post 5'6" and 50kg :shock:
    must have been because it took me ages to read to the bottom, forgive me :lol:

    Change the volume of oil, check for improvement, if not then up the pressure to the minimum as that cant be helping either. If you dont like them that firm then those forks aren't for you.

    I have another pair of F120's, same year (2009) on my full-sus Trance and it works perfectly.
    fuelex wrote:
    Eat some pies, then up the psi ;)

    :twisted:
    thel33ter wrote:
    Sounds like the same problem as I'm having with my Wotan, too much oil somewhere, so at low pressures it doesn't extend fully.

    Did you manage to figure it out? I reckon my original guess of too much oil in the top air chamber as that's the only chamber where air is under pressure. I'll give it a good seeing to later in the week and report back.
  • thel33terthel33ter Posts: 2,684
    The Wotan has a positive and negative air chamber. So it is a bit more complex than the Fox fork.
    Sounds like too much lubrication oil from the positive air chamber has leaked into the negative air chamber. A service will get this sorted

    That's what they said to me. I'm just waiting to call the service centre to see if that will be covered under warranty.
    And now you know, and knowing is half the battle
    05 Spesh Enduro Expert
    05 Trek 1000 Custom build
    Speedily Singular Thingy
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