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  1. #1
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    Nexus hub SG-8R36: dimensions?

    Hi guys:

    Can you clarify these issues?
    Shimano lists the following dimensions for the above hub.

    O.L.D. = 132 mm
    Flange Diameter = 92.6 mm
    Center to Flange Left = 30.25 mm
    Center to Flange Right = 24.85 mm

    1. Where is this center located?
    Obviously not mid-way between the two flanges.

    2. The "center to flange" measurements = are they to the inside or outside of the flange?

    3. With the asymmetric measurements, the spoking is dished on the drive-side. Did anyone try non-standard dishing to maintain chainline?

    thanks

  2. #2
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    1. Center is located midway between the outside of the lock nuts; the center of the hub axle.
    2. They are the center of the flange. The spokes will be laced one outside, the next inside. In any case, it isn't likely that this would change the required spoke lengths.
    3. Dishing is always set to have the rim run through the centerline of the bike. It really has no impact on chainline. No matter how much you dish the wheel away from the centerline (in either direction), the cog will be in the same relative position to the chainring.

  3. #3
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    Quote Originally Posted by FlintPaper View Post
    1. Center is located midway between the outside of the lock nuts; the center of the hub axle.
    2. They are the center of the flange. The spokes will be laced one outside, the next inside. In any case, it isn't likely that this would change the required spoke lengths.
    3. Dishing is always set to have the rim run through the centerline of the bike. It really has no impact on chainline. No matter how much you dish the wheel away from the centerline (in either direction), the cog will be in the same relative position to the chainring.
    With reference to the third point, keeping the center of the rim aligned with the center-line of the bike, I am under the impression that dishing moves the flanges relative to the center-line of the bike.

    For example, if the spokes are laced without any dishing, then the flanges move to the right by (30.25 - 24.85)/2 = 2.7mm, keeping the rim aligned with the center-line of the bike. In turn, this moves the rear cog to the right by 2.7mm.

    I cannot see the flaw in the above reasoning. If you do, please explain.

  4. #4
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    You've got your reference points backwards.

    The hub is always kept centered on the axle (depending on the model, there is no way for it not to be). You wouldn't want the hub to be anything BUT centered properly on the axle. If you did move the hub away from center on the axle, the hub and/or axle wouldn't fit properly in the dropouts.

    When you dish a wheel, you are changing the position of the rim relative to the hub. The hub (and the flanges) is always in the same position relative to the frame.

  5. #5
    All That is Man
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    The rim can move side to side, but the hub is locked in by the frame no matter what.
    John

  6. #6
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    The OLD is 132 mm but the standard spacing between rear dropouts is 135mm. (In my case, it is 140mm) So there is 3mm (7mm) to play with.

    It is not clear to me why the hub is fixed with respect to the frame and not the rim?

    When one dishes a wheel, this changes the relative position of the rim and the hub. At this point, is it desirable (setting aside feasibility) to align the centerline of the bike with the center of the rim or the center of the hub axle?

    I would go with the rim and hence the reasoning in my post.

    With default dishing, the the centerline of the bike is aligned with the center of the rim and the center of the hub axle.

    With zero dishing, the centerline of the bike is still aligned with the center of the rim and the center of the hub between the two flanges. Of course, one would need spacers to ensure this alignment.
    Last edited by anga; 10-20-2011 at 10:30 AM.

  7. #7
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    Quote Originally Posted by anga View Post
    The OLD is 132 mm but the standard spacing between rear dropouts is 135mm. (In my case, it is 140mm) So there is 3mm (7mm) to play with.

    It is not clear to me why the hub is fixed with respect to the frame and not the rim?
    With 140mm dropouts, you won't have enough axle length to install a hub meant for 130/135mm dropouts.

    With some hubs, like the old XT's, you could convert to a longer axle relatively easily. In this case, you could use spacers to mount the hub off-center in the frame. I don't know of anyone who has actually done this.

    With a nexus, the axle is not a simple solid threaded axle. You can sort of see it in this picture (multiple diameter axle with flat sections). There is a very narrow range of shifting the axle on the hub and a very narrow range of the hub fitting in a bike (too much axle on one side will give problems with the non-turn washers, on the other with installing the acorn nuts or shiftbox).

    The ~2.5mm of adjustability that a nexus has is in which side of the dropout you install the non-turn washers in. There is no room to do more than that or to move the hub on the axle.

    Please purchase one and prove me wrong.

  8. #8
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    FlintPaper:

    No selective quoting please. This is not a debate.
    While the 140mm dropout spacing is not what this thread is about, I do have the Nexus hub and the axle length is sufficient, it fits in the rear dropouts and needs some spacers. Proved you wrong. Does that make you happy? Any case, this is a non-issue.

    The real issue is getting the chainline correct and for this need to play with dishing. Appreciate responses on that.

  9. #9
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    As FP inducated, the centerline of the rim and the chainline are not related in any way. The chainline is fixed by the hub's dimensions and the frame's alignment. The centerline of the wheel is set by spoke length/tension. You really need to think about this before you make another post on this subject.

  10. #10
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    pursuiter: You ought to follow your advice on thinking before posting.

    Let me summarize my understanding of the issue.
    1. The centerline of the bike is determined by the bike's frame.
    2. The ideal chainline is parallel to the centerline of the bike.
    3. The BB, crank, hub, wheel, dishing etc determine the locations of the chainring and the rear cog and hence the chainline.
    4. With standard dishing, the centerlines of the rim, wheel and bike are aligned with the geometric center of the axle rear hub.
    4. With non-standard dishing, the centerlines of the rim, wheel and bike are still aligned but the geometric center of the axle rear hub shifts from this centerline. The position of the rear cog shifts too.
    5. To ensure the alignment in 4, one must use spacers between the rear dropouts.
    6. With zero dishing, the rear cog shifts to the right by 2.7mm.

    Given the limited flexibility on the location of the rear cog for a Nexus hub, I propose using non-standard dishing to get close to the ideal chainline.

    If you think any of the above points is not correct, please respond. I am open to logic and geometry.

  11. #11
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    Quote Originally Posted by anga View Post
    I am open to logic and geometry.
    Apparently not!

    Anga, go move the axle 2.7mm (or whatever) towards the drive side or space the non-drive side out with washers then lace the wheel up off center and show us the light.

    Pictures please, I've never seen someone do this before.

  12. #12
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    Flintpaper: You are a moving target---always throwing up irrelevant challenges. Such a fragile ego. Since you have nothing useful to contribute, why don't you move somewhere else?

  13. #13
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    Just do it the way you want and when the tire rubs the stays, come back and tell us it didn't. LOL

  14. #14
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    wow..so many clones!!

  15. #15
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    Is it just me, or does this feel like a Davidcopperfield thread?

    Anga, maybe you should restate your outstanding questions for other users to comment on. I'm not clear on what's preventing you from lacing up the wheel.

    I can't wait to see the wheel built up and in the frame.

  16. #16
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    BTW, here is the exploded diagram of your hub:

    http://techdocs.shimano.com/media/te...9830689520.pdf

    Notice the VERY unique axle. It won't be possible to relocate the axle in the hub because of that. I think your only choice will be to add 8mm worth of spacers to the hub as needed to achieve your chainline. Don't forget that you're going to need enough hub exposed on the drive side to be able to mount the shift box, and enough space on the non-drive for both the non-turn washer and the acorn nut.

    You should be able to do your own center to flange measurements.

    C2Do = Space between the dropouts with the hub in place / 2 (~70mm?)
    C2Left = C2Do - (dropout to left flange)
    C2Right = C2Do - (dropout to right flange)

    Then go here: wheelpro spoke calc and find out which spokes you need.

    Then build up your wheel.

    Keep in mind that the closer the spokes come to vertical, the less strong the wheel will be. If the spokes go 'inverted', I wouldn't ride the wheel at all. If either your C2Left or C2Right number is unusually small (or negative), I would bail on the project altogether.

    This picture (from this page) sort of paints the picture:



    Build up that wheel and show us something new!

  17. #17
    All That is Man
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    That picture is really pointy.
    John

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