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Thread: 120mm fork

  1. #1
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    120mm fork

    on a maybe XC frame... i'm not even sure... i have a 1999 Schwinn Moab 2 that I use for trails. I don't race or do exclusively XC since it's my only mtb and will be my all-around ride. i've upgraded to a RS Reba dual air fork, currently set at 100mm. from factory, the bike had a 80mm fork. i understand that it's alright with the +/- 20mm from factory for safety measures. but i also see that geometry has something to do with optimal or max suspension travel as well.

    the frame is a 17" and i weigh not more than 120# (if that matters) and this is the geometry (with the 80mm fork)



    the current 100mm setting has more likely changed the HTA already so now i'm kind of curious with the 120mm if it could improve the ride. the local trail i go to is pretty technical (roots and rocks) and i haven't seen any other hardtails aside from dirt jump ones.

    sorry if i'm asking in the wrong section but i figured trail category would belong in the all-mountain section.

  2. #2
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    First i would get the axle-to-crown measurements of each fork, then go from there.


  3. #3
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    thanks for the help! i'm still using the same rs reba fork. just going to remove the spacer so it becomes 120mm from 100mm so i'm guessing the fork length will increase by 20mm from what it is now.

    original fork (80mm RS judy C) was 17.5 inches fork length. the RS Reba at 100mm is 18.5 inches. so over 1 inch difference then from the stock fork if i set it at 120mm... too much stress on the frame?

  4. #4
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    If i understand you correctly, you are jacking the front of the bike 40mm higher than intended (by 50%). This will slacken the head angle and slow steering dramatically vs stock. You are creating a chopper.

    If you do this, you should prolly go with some wider bars and shorter stem.
    '95 M2 StumpJumper FS
    '11 Cannondale RZ 120-two

  5. #5
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    I did some quick trigonometry: @ 100mm, HA is slacked to under 70.2 deg, BB is raised by 0.38", WB is 0.33" longer, with respect to the stock fork; @ 120mm, HA is slacked to under 69.2 deg, BB raised by 0.68", WB is 0.60" longer, again with respect to stock fork.

    You probably already have some intuition as to how these numbers can affect handling.

    We can approximate a trail measurement increase of 9% and 17%, respectively, for 100mm and 120mm travel forks. (Note that this only takes into account a slacked HA.)

    Personally, i wouldn't go above 100mm. I would try 80mm, perhaps 90mm. I don't know if this is a matter of stress on the frame as much as it is a matter of stability and handling.

  6. #6
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    wow, thanks voodoo! it's looking like it would be too much for the frame looking at the huge difference if i do go 120mm. i guess i'll keep it at 100mm for now but might try 120mm when i perform a full service on it to get a feel and put it back to 100mm.

    i posted this thread with regards to stress on the frame and not much on handling/stability as i am yet learning. haven't tuned into it to a point where i'm able to say i felt a difference in handling and that it sucks now compared to before. i'm just finding out that i'm enjoying going down the rough and technical parts of the trail and thought maybe the increase in travel would help (but risk frame integrity) bought this as an entry-level bike and just trying to make do by making changes to it. can't afford a nice all-mountain full squish rig yet that i would really want...

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