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  1. #1
    Senior Member himespau's Avatar
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    Fork measurements

    Considering buying a new fork and want to get the measurements right. The steerer tube length is from the top of the crown to the very top of the threads, right? And full length, you drop a straight line from the axle to the ground (assuming steerer is flat on the ground) and measure from that line to tip of steerer, right? And rake is distance from ground to actual axle? Am I missing anything? Pretty much all forks have 100 mm axle spacing, right? I'm trying to replace a rigid fork for an old mtb with 26" wheels.
    Punctuation is important. It's the difference between "I helped my uncle, Jack, off a horse" and "I helped my uncle Jack off a horse"


  2. #2
    Senior Member calstar's Avatar
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    Google "fork measurement", something like that, you'll get lots of into. On most rigid mtbs as long as the steerer is the same/right length and it has close to the same blade profile(straight or radiuse) you should be ok, with little if any noticeable ride difference. If you have a high end and/or custom frame best to be a little more particular in the selection.

    And you must be familiar with http://sheldonbrown.com/gloss_ra-e.html#rake lots of info on forks,

    Rake: The "rake" or "offset" of a fork is the distance between the wheel axle and the extension of the steering axis. This may be accomplished by bending the fork blades, or by attaching the fork ends to the front of the blades, or by tilting the blades where they attach to the crown.Rake is one of the three factors that affect the trail of the bicycle, which has a considerable influence on the handling qualities.
    A fork which is curved forward at the bottom also adds a small degree of suspension compared with one that is not, by acting as a leaf spring.


    Brian
    Last edited by calstar; 06-18-12 at 09:26 AM.

  3. #3
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    dimensions are as follows.

    Steerer length is from the crown race seat or base of the steerer to the top.

    There's no consistent way blade length is measured but many measure from brake hole to the axle (on centers), others measure wheel clearance from axle to the bottom of the crown. There's a difference of about 1/2" between these numbers so make sure you know how they're measured. Many sellers don't offer this specific dimension, and say only 700c, or 26". But this doesn't give you enough info to know the maximum size tire, the brake reach, or how much clearance there is for fenders. So you really want the distance to brake hole.

    Rake, is the distance the axle is forward of the steering axis. You can't measure this accurately by laying the fork on a table because the blade width and taper throws things off. Use a ruler along the blade to extend the centerline of the steerer, then measure forward to the axle.

    Yes most modern forks have 100mm axle width, but this isn't absolutely universal.

    There are a few other variations to consider, dropouts slot can vary either larger or smaller from the standard 9mm axle size, the crown seat may not be the ISO 26.4mm, and the thread may not be the ISO 1"x24tpi. These variations are rare, but are details you'd want to confirm before buying.

    Also, consider whether the fork has fender eyes, disc brake mounts, or any other features if any apply to your needs.
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  4. #4
    Senior Member himespau's Avatar
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    Ok, that's sounding a bit more complicated than I thought. Basically, I have an old Cro Mo mtb frame that came with a hi ten fork and I'm repainting the frame as part of converting it to a heavy duty drop bar commuter (it's been drop bar for a while, but I tore it apart to update the components/framesaver it anyway and figured I'd give painting a try as long as it's all apart). I had been thinking about repainting the fork to match, but then I thought, well it's a really ugly hi ten fork (unicrown, huge tubes that look out of place with fork ends almost tacked on to the ends), why waste the time painting it when it seems like there are relatively cheap cro mo forks out there (some chromed so they'll always match, some black which is less good of a match but still might work)? Maybe I should just go back to the original idea of painting the fork that I have.
    Punctuation is important. It's the difference between "I helped my uncle, Jack, off a horse" and "I helped my uncle Jack off a horse"


  5. #5
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    Quote Originally Posted by himespau View Post
    Ok, that's sounding a bit more complicated than I thought.
    .
    It's really not that complicated because forks fall into a narrow band of being very similar.

    99% odds you need an ISO threaded or threadless. If it's threaded, it's almost certainly an ISO thread, and most brands of your era came with ISO (26.4) crown seat, one common exception is Specialized which for some reason was slow to change from JIS.

    If you have a 26" wheel and canti brakes then any 26" canti fork will work there, and the issue of brake reach is moot. Likewise rake probably doesn't matter for your purposes.

    So it simply boils down to a 26" wheel fork for mtn bike, with canti boss (or without but then brake reach might become an issue) and a steerer of the right length. Note that threaded steerers usually have 2" of thread, and your headset will need one inch, so you have to be at least as long as yours, but not more than 1" longer. Threadless forks, only need to be long enough, and are easy to trim to your length.
    FB
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    An ounce of diagnosis is worth a pound of cure.

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    WARNING, I'm from New York. Thin skinned people should maintain safe distance.

  6. #6
    Randomhead
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    I see no reason to replace that fork. It does open up a can of worms, particularly if they happened to have their own ideas about fork geometry.

  7. #7
    Senior Member himespau's Avatar
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    Quote Originally Posted by FBinNY View Post
    It's really not that complicated because forks fall into a narrow band of being very similar.

    99% odds you need an ISO threaded or threadless. If it's threaded, it's almost certainly an ISO thread, and most brands of your era came with ISO (26.4) crown seat, one common exception is Specialized which for some reason was slow to change from JIS.

    If you have a 26" wheel and canti brakes then any 26" canti fork will work there, and the issue of brake reach is moot. Likewise rake probably doesn't matter for your purposes.

    So it simply boils down to a 26" wheel fork for mtn bike, with canti boss (or without but then brake reach might become an issue) and a steerer of the right length. Note that threaded steerers usually have 2" of thread, and your headset will need one inch, so you have to be at least as long as yours, but not more than 1" longer. Threadless forks, only need to be long enough, and are easy to trim to your length.

    Good points.

    There was one fork that looked like it'd work for everything until I looked at your response and realized that it lacked the cantilever bosses. Which is a shame because it was a 26" fork that was lugged as opposed to the ugly unicrown forks that appear to be my only options. If the threaded fork is less than 1" more than what I need, I just cut off the extra with a hacksaw (after measuring at least 3 times, right?

    Is there a reason that replacement fork manufacturers don't have most of the steerer threaded so that they only have to make one and then you cut it to the size you need rather than having to make multiple sizes? My bike is a 22.5" so I'm sure the fork will be on the larger end of the size range (for an mtb anyway that seems to be fairly large).

    Quote Originally Posted by unterhausen View Post
    I see no reason to replace that fork. It does open up a can of worms, particularly if they happened to have their own ideas about fork geometry.
    Yeah, at the end of the day you're probably right. Just because I can buy one that's slightly less ugly with tapered tubes as opposed to straight tubes (and is chrome so I don't have to paint it), at the end of the day it's still a fork and still a unicrown design (which I don't find to be particularly attractive).

    Might as well go with what works rather than spend money I don't need to spend to do something that should work.

    I'm not sure that a 4130 fork will feel any different than a hi ten fork and, since I'm putting my front rack back on there I can't go with a carbon fork to make it less buzzy.

    I don't know why I prefer tapered tubes with a lugged looking crown to straight tubing and unicrown forks, but I guess that just says I have the wrong kind of bike.
    Last edited by himespau; 06-18-12 at 01:38 PM. Reason: Put in quotes to clarify to what I was responding.
    Punctuation is important. It's the difference between "I helped my uncle, Jack, off a horse" and "I helped my uncle Jack off a horse"


  8. #8
    Senior Member Grand Bois's Avatar
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    Quote Originally Posted by himespau View Post
    Considering buying a new fork and want to get the measurements right. The steerer tube length is from the top of the crown to the very top of the threads, right? And full length, you drop a straight line from the axle to the ground (assuming steerer is flat on the ground) and measure from that line to tip of steerer, right? And rake is distance from ground to actual axle? Am I missing anything? Pretty much all forks have 100 mm axle spacing, right? I'm trying to replace a rigid fork for an old mtb with 26" wheels.
    The distance from the ground to the axle has nothing to do with the fork.

  9. #9
    Senior Member himespau's Avatar
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    Quote Originally Posted by Grand Bois View Post
    The distance from the ground to the axle has nothing to do with the fork.
    I meant with the fork still laying so the steerer was flat on the ground from my previous question, which someone above explained that this isn't the best approximation of a straight line through the steerer, but I didn't mean the height of the axle, which I'll agree is all about the size of the wheel (and of the tire) and is irrelevant to the rake.
    Punctuation is important. It's the difference between "I helped my uncle, Jack, off a horse" and "I helped my uncle Jack off a horse"


  10. #10
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    Quote Originally Posted by himespau View Post
    Good points.

    There was one fork that looked like it'd work for everything until I looked at your response and realized that it lacked the cantilever bosses. Which is a shame because it was a 26" fork that was lugged as opposed to the ugly unicrown forks that appear to be my only options. If the threaded fork is less than 1" more than what I need, I just cut off the extra with a hacksaw (after measuring at least 3 times, right?

    Is there a reason that replacement fork manufacturers don't have most of the steerer threaded so that they only have to make one and then you cut it to the size you need rather than having to make multiple sizes? My bike is a 22.5" so I'm sure the fork will be on the larger end of the size range (for an mtb anyway that seems to be fairly large).
    Too bad about the fork. That's why I listed all the considerations in my first response. It's too easy to forget a minor detail like brake bosses or fender yes, then find a fork can't do what you deed.

    Yes, you measure carefully, reconfirm the headset height, then cut. In some cases I prefer to fit the fork, attach the headset and any spacers, nut not the locknut, then cut 6-9 mm above the top of the stack. I cut to the long side (locknuts use 6mm) and add an extra spacer, because I want to leave room for a taller headset, or possibly some hear tube mounted accessory like a light bracket.

    There's a very good reason why forks are only threaded 2 or so inches. It relates to the minimum insertion height of the stems. The threaded portion of a fork isn't strong enough to take the stress flexing on the handlebars causes, and would break at th thread. However the stem extends below the last thread to the solid section carrying the stress below the threaded area. So you could safely have more than 2" of thread if you put the stem deeper, but since stems are marked at about 2.5" fork makers thread only about 2" leaving some fudge room.
    FB
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    An ounce of diagnosis is worth a pound of cure.

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    WARNING, I'm from New York. Thin skinned people should maintain safe distance.

  11. #11
    Senior Member calstar's Avatar
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    Quote Originally Posted by himespau View Post

    I'm not sure that a 4130 fork will feel any different than a hi ten fork....
    Not many, if any, people could feel any difference, everything else being equal.

    Brian

  12. #12
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    Quote Originally Posted by calstar View Post
    Not many, if any, people could feel any difference, everything else being equal.
    Actually many people can feel a difference. The problem is that it isn't in the fork, but in their head.

    There is some detectable difference in steel forks, but it isn't a function of the material but of the wall thickness, especially in the lower 6 inches or so.
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  13. #13
    Senior Member himespau's Avatar
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    Quote Originally Posted by FBinNY View Post
    Actually many people can feel a difference. The problem is that it isn't in the fork, but in their head.

    There is some detectable difference in steel forks, but it isn't a function of the material but of the wall thickness, especially in the lower 6 inches or so.
    Would a tapered tube (no matter the type of steel) provide a different feel from the cylinder of constant diameter that's on there now? Or is the taper just a thing to make it look more graceful? I suppose the preference for a tube that narrows as it reaches the fork ends could be just as much in my head and a desire for an older style look as the desire for a certain type of steel is in other people's heads.
    Punctuation is important. It's the difference between "I helped my uncle, Jack, off a horse" and "I helped my uncle Jack off a horse"


  14. #14
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    Quote Originally Posted by himespau View Post
    Would a tapered tube (no matter the type of steel) provide a different feel from the cylinder of constant diameter that's on there now? Or is the taper just a thing to make it look more graceful? I suppose the preference for a tube that narrows as it reaches the fork ends could be just as much in my head and a desire for an older style look as the desire for a certain type of steel is in other people's heads.
    There are two tapers involved.

    Tapering the outside reduces the OD making any tube more flexible. However there's a bit of a catch 22 here.

    When you taper a tube the wall thickness builds up which would increase the stiffness, partly undoing the benefit of tapering in the first place. That's why better fork blades are made of single butted tubes which are thicker-walled at what will become the top end, with the thickness tapering down toward what will become the lower end. When these blades are tapered down the wall thickness increases, but having started thinner, ends up thinner than if a non-butted tube were used.

    Using butted tubing allows people like Reynolds to precisely control how, where and how much a fork blade will flex.

    You see the results of this good engineering after a front end crash. Cheap fork blades predictably bend right at the crown, better blades bend lower down, and really good blades bend gracefully over much of their length, with no hard bends anyplace.
    Last edited by FBinNY; 06-18-12 at 09:01 PM.
    FB
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