Fork Length

Discussion in 'Mountain Bikes' started by sjsmithjr, Sep 19, 2003.

  1. sjsmithjr

    sjsmithjr New Member

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    Can anyone confirm that the following is true for most mountain suspension forks. The dimension I am referencing is from the top of the crown to the centerline of the axle.


    450 mm = 80mm travel forks
    471mm = 100mm travle forks
    501 mm = 130mm travel forks

    and so on. I am assuming the dimensions are fairly standardized so that manufacturers can determine frame geometery for a given travel.

    Thanks
    Sam
     
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  2. mooseknuckle

    mooseknuckle New Member

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    This is a great question because I am trying to figure how much travel I have with my fork. I have a 1999 judy c wich comes w/ either 63 or 80 mm of travel. Is there anywhere on the fork that I could look for this number or is it something I am going to have to measure. I am pretty sure that it is 80mm, but I want to be positive because I am thinking about upgrading.
     
  3. sjsmithjr

    sjsmithjr New Member

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    As a followup to my original post, measuring the distance between the axle and the crown is the only reliable means by which to determine whether a fork is a 63mm, 80mm, 100mm model as this distance is common across fork makes. Measured travel isn't. By example, I have a Z5 air 80mm that with nearly 90mm of measureable travel. The distance between the axle and the crown is 451mm, which definately makes it a 80mm model.


     
  4. jeminhi

    jeminhi New Member

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    Measuring axle center to crown race is not a reliable way of measuring actual fork travel, if you compare 100mm travel forks from 2 different companies the axle center to crown race will vary as much as 10mm. A more accurate way of measuring actual travel is to install a zip-tie (cable tie to electricians) push it down against the wiper seal, remove coil spring or release all air from fork, then push down. Reinstall coil spring or reinflate to your pressure then measure distance between wiper seal and zip tie. When pushing down on air sprung forks it helps to release air as you push down on fork as pressure builds as you push down. An important thing to consider when extending the travel on your stock fork (or upgrading to a longer travel fork) is that a 2" change in axle center to crown race will slacken your head angle approx. 1 degree. Also when you install a longer travel fork it will sag further into it's travel, effectively maintaining your original geometry so long as the change is less than 1". I swapped the 80mm fork on my stumpy fsr for a 130mm fork and found the slackness of the head angle appropriate for the riding style that a fork that long was built for anyway.
     
  5. sjsmithjr

    sjsmithjr New Member

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    We're actually talking about two different things here. To clarify, if you want to determine fork model, you need to check the crown to axle dimension. To determine actual fork travel, use a zip tie. When I made my original post I was trying to determine if a used fork I had was a 80mm model or a 100mm model. Actual fork travel was 85mm. By measuring the crown to axle dimension, which is fairly consistent between manufacturers, I was able to determine that it was a 100mm model. It's fork length, not travel, affects the "static" geometry of the bike.

    -Sam

     
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