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  1. #1
    Merendon Junkie
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    Cracked carbon Rim, long term performance

    About two years ago I cracked a rear carbon fan, dh layup rim when I cased a jump onto a rock. This is on a bike used for dh duty only. I discovered the crack about two weeks after the event. I was going to replace the rim but never got around to it. I marked where the crack ended and watched for growth. Its a 36 hole rim, I removed almost all of the tension from the cracked spoke hole and rebalanced it with the others around it. The wheel is very true.

    After a couple of runs, no growth. Couple of months later, no growth. Couple of years, still no growth. I am not a super fast dh rider, I race from time to time and do some runs every couple of weeks. Like to get some air and ride "fast" through rock gardens. I have also managed to case some jumps after it cracked.

    I now feel comfortable running this rim like this and feel like a cracked carbon rim is not the immediate death sentence we make it out to be. I run this bike with tubes, if it was tubeless it would likely compromise the seal, that would probably warrant a replacement, but with tubes its been fine. I think a catastrophic failure is very unlikely. Maybe a crack starting from the outside part of the rim would seem to be more prone to failure and/or growth.

    Any others with similar experiences using a cracked rim? Do you guys think this is reckless and should immediately give up on it?

    Cracked carbon Rim, long term performance-img_3904.jpg

    Cheers
    Abel

  2. #2
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    I haven't messed with carbon rims but I do know that when carbon fails, its usually an all out failure. Obviously yours has been pretty damn durable but I'd be looking to lace up a new rim. Its a lot cheaper than a trip to the hospital (unless you live in Canada).

  3. #3
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    It's fine until it catastrophically fails and leaves you stranded. It may have been ok for the last 2 years, but all it takes is just one perfect impact and it's done. I'd at least put some resin or epoxy on it. I mean, what have you got to lose? You've been running a cracked rim for all this time already.

  4. #4
    Oaktown Honkey on Strava
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    I personally value my health over money. Especially for a few hundred bucks. If there is a catastrophic failure on an exposed section and you fall 50-100 ft that would not be awesome. If it fails and sends you head first into a tree, bad. Maybe the rim folds on an easy section and you skid to a halt, laughing. If you take cracked rim off now, you can do fun and informative destructive tests. This thread is interesting and informative, thanks, Good luck, and I hope you replace that rim.

  5. #5
    Ride Fast Take Chances :)
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    Rim is toast. You can superglue carbon and press it back together, but your break looks ready to catastrophically fail. I would not rid that rim.

  6. #6
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    I ran a cracked Giant branded rim for over a year. It finally wouldn't hold tubeless anymore so I replaced it when given the opportunity. It stayed true and evenly tensioned the whole time.

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  7. #7
    Elitest thrill junkie
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    The one rim I cracked, from running pressure too low and failing to check it pre-ride, held together for 10 miles of rough including 7 miles of rough downhill, I didn't pamper it. If it had been an alloy rim, I would have gotten a nasty flat spot and the results would have been the same (lost tubeless capability and had to retire rim after ride). It did go off with a good "bang" and I couldn't see what was wrong at first, but continuing to ride and I started loosing pressure the problem became obvious. Put in a tube and was good to go. One local shop showed me someone who has been riding all season on a cracked rim. I wouldn't, but there are some people that will evidently. It helps to have wheelbuilding skills so you can whip the replacement together.
    "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

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  8. #8
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    That rim could be repaired, probably without even having to unlace it. If it is strong enough to survive heavy use with the crack then even if a scarf patch didn't get it back to full strength it should be close enough to remove the rest of your worries, and allow you to properly tension that spoke.

  9. #9
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    Do we know of "catastrophic" mtb carbon rim failures that have immediately sent the rider to hell?

    I reckon (most) carbon rims are so damn strong you could cut out a 3" inch section and still ride it to safety before death and brimstone. Especially a 36 spoked 26" rim. With that said another solid shot in the same area would probably finish it off.
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  10. #10
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    Quote Originally Posted by WHALENARD View Post
    Do we know of "catastrophic" mtb carbon rim failures that have immediately sent the rider to hell?

    I reckon (most) carbon rims are so damn strong you could cut out a 3" inch section and still ride it to safety before death and brimstone. Especially a 36 spoked 26" rim. With that said another solid shot in the same area would probably finish it off.
    I was just about to say the same. I've never read, or heard of any wheel destroying itself [other than the Mavic R-Sys spoke failure that will and did cause a catastrophic failure].

    The rim in a built wheel is under massive compressive force. Take a pin jointed alu rim, at the joint it has a crack going 360 degrees around the rim. Those little alignment pins are not there to prevent catastrophic failure. Does carbon react differently?

  11. #11
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    Quote Originally Posted by roger-m View Post
    Take a pin jointed alu rim, at the joint it has a crack going 360 degrees around the rim. Those little alignment pins are not there to prevent catastrophic failure. Does carbon react differently?
    I would say that those pins are primarily there to prevent a catastrophic failure, what happens when the joint is knocked out of alignment? The reason that the cracked carbon rim has held up is because it is not cracked all the way through and there is a great deal of strength left in the uncracked portion, which is helped by the compressive forces holding it together. And carbon cracks behave differently than aluminum and are less likely to propagate.

  12. #12
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    I say just run it..... I mean, what's the worst that could happen?

  13. #13
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    Everything is absolutely fine until the instant it isn't.

    It MAY only be a surface layer crack, which is why it's not a big deal.
    Have you looked to see if it's cracked inside as well?
    "Go soothingly in the grease mud, as there lurks the skid demon"

  14. #14
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    Might not be cracked, no way to tell fromthat...cracked carbon can sometimes make a ticking, pop/ high pitch sounds...even, while not riding the bike. Does it give when you apply pressure? That wall is probably cracked thru if so
    ...

  15. #15
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    I would NEVER ride on that. I certainly would not be moving very fast through any chunk on it, assuming I had the balls to ride on it at all.

    If I had enough money, I would buy you a new rim myself.

  16. #16
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    I'd ride it until a new rim arrived to replace it, but not plan on continue riding on it any longer than necessary.

    If it were on the front wheel then no way in the world would I ride that, but for the most part a rear failure won't normally end in disaster.

  17. #17
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    I'd ride it, or not, depending on the type of ride. Mellow trail riding, yeah. DH, no.
    Do the math.

  18. #18
    Merendon Junkie
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    We the people ...

    I decided to keep on riding this rim more as an experiment than to skimp off on changing it after some time of not getting around to order a burly alu rim to replace. If it were to fail or crack thoroughly, what would be chance of it hurting me seriously or causing a major crash? I would be way more concerned about this if it was my front wheel. My odds of getting hurt are way higher from my inherent clumsiness in the general type of riding I do, as you can see in this video (crash is at the end). This was with the rear rim cracked btw. The way I landed with the front wheel would have tacoed many a wheel, this wheel did not flinch.

    https://m.youtube.com/watch?v=6J5UK8UY__g

    These rims are very burly, the crack was not caused by the impact against the rim bead itself but by the spoke wanting to pull through. Maybe we can say that the spoke bed is underbuilt when compared to the rest of the rim.
    Quote Originally Posted by mtnbkrmike View Post

    If I had enough money, I would buy you a new rim myself.
    Thats is very kind of your sir.

    Quote Originally Posted by DethWshBkr View Post
    Everything is absolutely fine until the instant it isn't.

    It MAY only be a surface layer crack, which is why it's not a big deal.
    Have you looked to see if it's cracked inside as well?
    The inner wall (where you put the tape on) is not cracked, I cant actually see if the crack went all the way through in the inside of the rim but I would guess it did.

    Cheers

  19. #19
    transmitter~receiver
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    Quote Originally Posted by abelfonseca View Post
    Any others with similar experiences using a cracked rim?
    I ran a carbon rim that developed stress cracks at the nipples during the initial build. I sent pictures and got a warranty replacement but decided to ride it while I waited. Rode it hard for two years with no problems. Ended up selling the wheelset and giving the buyer the unused rim with the wheelset.

    Quote Originally Posted by abelfonseca View Post
    Do you guys think this is reckless and should immediately give up on it?
    You asked for it... and got it.
    Quote Originally Posted by pvd
    Time to stop believing the hype and start doing some science.
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