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  1. #1
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    Breaking spokes at the bend?

    My radial laced front wheel, (20hole, Sapin CXray spokes, Kinlin XR270 rim) has been popping spokes lately. The spokes are breaking at the hub.

    I built the wheel myself with Bitex hubs. Is this a symptom of too much or too little tension? Or does it "depend"?

    I don't have a tensionometer. The rear wheel has been bombproof.

  2. #2
    A wheelist
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    Spokes break due to metal fatigue almost always caused from low tension. That wheel doesn't have many spokes so 20 spokes will fatigue faster than, say 32, or 28 even in a correctly tensioned wheel. Maybe you're too heavy for 20 spokes. If you are, then even correctly tensioned spokes will go through deeper load/unload tension cycles as the wheel rotates.

    So spokes break from Work Done and more work will be done by -

    • Fewer spokes.
    • Low tension spokes.
    • Spokes carrying more weight.
    • Spokes in use for a long time. (a very relative term in the life of a spoke and it's based on the above three criteria)


    So how does your wheel stack up against that list? Too much or very high tension is actually better for spoke metal fatigue as the load/unload strain (fatigue) cycle is lower than in a low tensioned wheel. Highly tensioned spokes tend to crack rim holes. This happens long before spokes' ultimate tensile strength is reached.
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  3. #3
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    Every time the wheel revolves,each spoke is "loaded" and "unloaded".
    It's like bending a paper clip. All "J bend" spokes will break, given enough time.
    Low tension, uneven tension, and low spoke count speeds things up.
    Usually a rear wheel will start breaking spokes sooner.
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  4. #4
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    Thanks, all. I weigh 138lbs, and I built the wheel a couple of years ago. Sounds like I might need to try a higher tension when I rebuild, although the first spoke broke while riding the GAP and C&O on a cx bike with a seatpack. Since then, two more have broken on the road while on my road bike, JRA.

  5. #5
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    Quote Originally Posted by Dream Plus View Post
    My radial laced front wheel, (20hole, Sapin CXray spokes, Kinlin XR270 rim) has been popping spokes lately. The spokes are breaking at the hub.
    How old is the wheel and how many miles?

    A lack of stress relieving is the usual culprit or insufficient tension if it's the NDS rear, but it is very odd that you are having issues with the front and not the rear.

  6. #6
    changingleaf
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    I agree with previous posters. The tension was/is too low and the spokes are all significantly fatigued. You will need to replace all the spokes.

  7. #7
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    "How old is the wheel and how many miles?"

    Old and high mileage doesn't always kill a wheel.

    I do all of my training with a 10 year old, 20 spoke Mavic Aluminum Cosmic front wheel. I've had some pretty bad crashes on that wheel, also.
    I believe that the key to it's longevity is that it uses straight pull, nail head spokes, and that it has pretty high tension.
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  8. #8
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    Breaking spokes at the bend?

    Quote Originally Posted by Dream Plus View Post
    Thanks, all. I weigh 138lbs, and I built the wheel a couple of years ago. Sounds like I might need to try a higher tension when I rebuild, although the first spoke broke while riding the GAP and C&O on a cx bike with a seatpack. Since then, two more have broken on the road while on my road bike, JRA.
    My suspicion is that your front spokes break because they are not properly supported at the elbows by the hub flange and fatigue has caught up with you. Bitex hubs have typically thinner flanges and I believe the spokes have not been properly set to account for that. You may have to bend the elbows a bit so they are supported by the flanges.
    If you happen to have a copy of Brandt's "The Bicycle Wheel" look at page 80 for a picture of what I'm talking about.

  9. #9
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    It could indeed be the hubs not fully supporting the spokes, or straining the angle of the bend.

    Often the the 90 degree bend doesn't bend far enough. The spoke should be bent to perfectly match the path to the rim before it is tensioned. Don't rely on spoke tension to straighten it out, or you'll have residual stresses that can't be relieved through the usual techniques.

    Thin flanges or overly large holes cause the opposite problem, "unbending" the bend. I'm not sure you can deal with this other than to get better hubs, or use those tiny washers, which is iffy.

    It's simply not true that all J-bend spokes will eventually break. If there are no residual stresses and they're tensioned properly, and not overloaded (too-low spoke count, uneven tension due to bent rim), they should last practically forever.

    Jobst Brandt put hundreds of thousands of miles on his spokes, replacing his rims when they wore out. And he's a big, strong guy (6'6"?) whose typical weekend ride is stomping over 10,000' mountain passes in a 42-21.
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  10. #10
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    Quote Originally Posted by mattotoole View Post
    The spoke should be bent to perfectly match the path to the rim before it is tensioned. Don't rely on spoke tension to straighten it out, or you'll have residual stresses that can't be relieved through the usual techniques.
    You don't have to do it *before* tensioning, but the spokes need to be yielded so they end up take straight paths when you are finished.

  11. #11
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    Quote Originally Posted by rruff View Post
    You don't have to do it *before* tensioning, but the spokes need to be yielded so they end up take straight paths when you are finished.
    But to get it to yield, you have to bend it past where you want it to be...
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  12. #12
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    Quote Originally Posted by mattotoole View Post
    But to get it to yield, you have to bend it past where you want it to be...
    I tap 'em down with my plastic hammer. It works a tweet. The thumb works too but not as good as my hammer.
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  13. #13
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    I think it's time to purchase a tensiometer. They're a fantastic tool, because proper spoke tension becomes more important with low spoke counts and thin spokes. Any baboon can build a rideable 32 spoke 14-15 wheel, but wen building wheels with smaller margins of error, you need all of the help you can get , especially if you're not too experienced in wheel building.

  14. #14
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    Quote Originally Posted by Mike T. View Post
    I tap 'em down with my plastic hammer.
    Yup...

    And then stress-relieve... done.

  15. #15
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    This discussion of elbows reminds me of a quesition...

    As for the elbow height or length, DT, Pillar and CN's spokes are more or less identical in this dimension(6.2mm elbow for 14g spokes),I've never used Sapim spokes and wonder if they're any different?

    Is there such a scenario that certain hubs' flanges favor a specifc brand of spokes?
    Or which brand has the best elbow geometry?

  16. #16
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    Quote Originally Posted by wrcompositi View Post
    This discussion of elbows reminds me of a quesition...

    As for the elbow height or length, DT, Pillar and CN's spokes are more or less identical in this dimension(6.2mm elbow for 14g spokes),I've never used Sapim spokes and wonder if they're any different?

    Is there such a scenario that certain hubs' flanges favor a specifc brand of spokes? Or which brand has the best elbow geometry?
    Bump'd, 'cuz it's an interesting question.
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  17. #17
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    I don't know. I usually use Sapims which seem about the same as Pillar and DT, and Wheelsmith seems to be a little tighter on the J.

  18. #18
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    I'd say that even tire pressure can add to the workload done by the spokes.

    Riding with too low tire pressure can add to the metal fatigue, most noticeable in lower spoke count and radial lacing, ime.
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