threaded steerer tube/head tube length, and axle to crown distance?
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  1. #1
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    threaded steerer tube/head tube length, and axle to crown distance?

    i'm looking to replace the fork on my bicycle to a 1 inch steel fork. the vast majority are threaded.
    the head tube looks to be 118mm
    how do i determine the minimum amount of steerer tube needed?
    also, if i find one long enough, can a threaded steerer tube be used with a threadless headset and stem combo?

    also, the current fork's axle to crown measurement is 370mm
    i try to get the measurement from the sellers, but their responses are sporadic at best. some don't know what i'm talking about.
    would i be ok with just any road bike specific steel fork, and not worrying if its 370, or 375mm? or what ever other lengths they come in?

  2. #2
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    It's all simple math.

    Start with the head tube length. Add the stack height of the headset, making sure that you take into account the top lock nut. I usually add 10mm or so (taken up with a spacer) for good luck and to allow you some flexibility to re-use the fork on other bikes.

    Can you use a threaded fork with a threadless headset and stem? You shouldn't. This is mostly because you really don't want to clamp a threadless stem onto the threaded portion of a threaded fork. You run the risk of snapping the steering tube. If the fork has a monster steering tube and you are able to get it to fit and clamp your threadless stem on the unthreaded portion of the tube, Yatzee!

    Should you worry if the axle to crown measurement matches your old fork? Only if you like the way that you bike handled with the old fork. If you jack up the front of the bike, or lower it, you alter the geometry. Simple as that. One problem is that the number that you are given - from the axle to the bottom of the crown, i.e. a measurement that is aimed at whether a tire size fits or not - sometimes isn't terribly useful. To keep the handling the same, you want to measure from the axle to the bottom of the crown race on the fork crown. Different fork crowns have different heights/specs, and the only way to get it to all to match up is to measure from the axle to the crown race.
    It's Been Fun...See You Down The Road.

  3. #3
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    Quote Originally Posted by Gregory Taylor View Post
    It's all simple math.

    Start with the head tube length. Add the stack height of the headset, making sure that you take into account the top lock nut. I usually add 10mm or so (taken up with a spacer) for good luck and to allow you some flexibility to re-use the fork on other bikes.

    Can you use a threaded fork with a threadless headset and stem? You shouldn't. This is mostly because you really don't want to clamp a threadless stem onto the threaded portion of a threaded fork. You run the risk of snapping the steering tube. If the fork has a monster steering tube and you are able to get it to fit and clamp your threadless stem on the unthreaded portion of the tube, Yatzee!

    Should you worry if the axle to crown measurement matches your old fork? Only if you like the way that you bike handled with the old fork. If you jack up the front of the bike, or lower it, you alter the geometry. Simple as that. One problem is that the number that you are given - from the axle to the bottom of the crown, i.e. a measurement that is aimed at whether a tire size fits or not - sometimes isn't terribly useful. To keep the handling the same, you want to measure from the axle to the bottom of the crown race on the fork crown. Different fork crowns have different heights/specs, and the only way to get it to all to match up is to measure from the axle to the crown race.
    thanks for the response. and that was what i meant by axle to crown measurement

  4. #4
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    are there standard lengths for this measurement? the one i have now is 370. do forks come in like standard 5mm increments?

  5. #5
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    No, they don't, at least on in my experience.

    This is an older list that is up on the Sheldon Brown website.

    Fork Lengths, by Damon Rinard. Carbon forks and steering geometry changes.

    A quick search of the internet should confirm that the situation is not that different today, even though the differences may have gotten smaller.

    (The article is also pretty good - it tells you how to figure out what the change in geometry will be if you use a different length fork. He also suggests that changes of half a degree or less may not be too noticeable. Or, stated another way, a change within that range could actually be beneficial, if you want to change the handling and plan it right.)

    And don't forget that you also will have to figure in offset...
    It's Been Fun...See You Down The Road.

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