Cracked carbon bars- Mtbr.com
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  1. #1
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    Cracked carbon bars

    Had a pretty big crash on my 2019 Canyon Neuron. Bent the bars pretty good but otherwise bike seems ok. Bought some Box carbon bars to replace the bent Iridium bars and they cracked in the stem when tightened. They were installed correctly with carbon paste and using torque wrench with a stepped sequence. 5 Nm is all that was applied (Box said max is 7 Nm). Was not overtightened or tightened on one side.

    What gives?

  2. #2
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    I'd be calling Box.....

  3. #3
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    Got any pics? A few years ago I had some bars that had a decal under clearcoat and the stem clamp smooshed the decal/clearcoat making it look like the carbon cracked.
    Do the math.

  4. #4
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    Quote Originally Posted by sharkynrk View Post
    Had a pretty big crash on my 2019 Canyon Neuron. Bent the bars pretty good but otherwise bike seems ok. Bought some Box carbon bars to replace the bent Iridium bars and they cracked in the stem when tightened. They were installed correctly with carbon paste and using torque wrench with a stepped sequence. 5 Nm is all that was applied (Box said max is 7 Nm). Was not overtightened or tightened on one side.

    What gives?
    How was the torque wrench calibrated?
    "It's only when you stand over it, you know, when you physically stand over the bike, that then you say 'hey, I don't have much stand over height', you know"-T. Ellsworth

    You're turning black metallic.

  5. #5
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    The clicker torque wrench was calibrated by the manufacturer but I verified it with a beam style torque wrench as well. I submitted a claim to Box. Havenít heard back. Iíll post pics tomorrow.

  6. #6
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    The hole in the stem might not be round. Take an old al handlebar and spot the stem with some valve grinding compound. Betcha the hole is imperfect.
    lean forward

  7. #7
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    Cracked carbon bars-img_1371.jpg

  8. #8
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    ^^ I slid the bar out to the right to take the picture, but the bar was centered when installed.

  9. #9
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    Was the stem, hardware, or faceplate damaged from the last crash? Any minor bend could easily create a hot spot and wreck the carbon bar.

    I would never torque a carbon bar to 7nm. That's way too high and completely unnecessary. 4nm is probably sufficient, 5 max.
    Rigid SS 29er
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    Stop asking how much it weighs and just go ride it.

  10. #10
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    yeah 7nm is what Box said, 5 is all the higher I went. I didn't see any damage or issue with the stem - the original bars bent out toward the end. I also checked the mating surfaces prior to installation and noticed nothing out of the ordinary.

  11. #11
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    haven't heard from box yet

  12. #12
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    I never torque carbon bars, one should only go a hair past where you cannot twist it by hand in neck.

    As for your case, bar sure does look thin where the crack is, and it also looks like a hot spot pushed it in

  13. #13
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    Go to the auto parts store and buy the smallest container of the finest grit valve grinding compound they sell. Spot the stem and see whatcha got. It will tell ya how round the hole is and any high spots that might be. I have spotted all the stems I have ever bought and none were very good out of the box. If ya work it enough ya can remove the high spots and even make the hole round.
    lean forward

  14. #14
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    I will definitely try that with lapping compound and the old bent bar before putting any new bars back in the stem. I am quite surprised that a hotspot that is small enough to be removed with valve lapping compound could do that to a bar under so little clamping force, but obviously it can be an issue as I am learning.

  15. #15
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    For anyone that was reading this thread. Box is making good with a replacement. Standing behind their product.

  16. #16
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    I'd replace that stem too. Clearly something is up with it and I wouldn't want my bar failing out on the trail. Not worth it.
    Rigid SS 29er
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    Full Sus 29er (Yuck)

    Stop asking how much it weighs and just go ride it.

  17. #17
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    Quote Originally Posted by *OneSpeed* View Post
    I'd replace that stem too. Clearly something is up with it and I wouldn't want my bar failing out on the trail. Not worth it.
    I grabbed an aluminum replacement while I am waiting for the replacement bar, I guess I will by a new stem to go with the carbon bar and keep the other set as a backup.

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