Safe to reshape carbon fiber frame?
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  1. #1
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    Safe to reshape carbon fiber frame?

    I sanded down a small part of my carbon bike's chainstay just behind the bottom bracket. This was to clear the new super-compact chainring that I installed. I sanded down about 1mm or a maybe a bit more. The frame had very little clearance even with the regular size chainrings, but with the new chainrings that I got, the rings are offset 2.5mm closer to the frame. The inner ring actually contacts the frame so I thought I'd sand the paint off. Turns out I had to sand down the carbon too to clear the rings and so the cranks would fully set into the bottom bracket. I didn't sand down through the carbon completely but I sanded down at least two layers from the bands that I can see. The sanded area still seems strong; there's no give when I push hard on it with my finger.

    Is this going to be safe to ride?

  2. #2
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    I would not have done what you did, but it's your bike. The benefit of carbon is that it can be laid-up to be strong where it needs to be and thin where it doesn't. The tube walls aren't that thick to begin with, so shaving off 1mm just to fit a chain ring sounds like a bad idea to me. I guess you'll find out if it's safe to ride or not.

  3. #3
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    So now that it's done, your having second thoughts? That's not how it's done.
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  4. #4
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    Yeah probably shouldn't have done it. But too late now. I do have a carbon repair kit, so I can rebuild the chainstay again if needed.

    I'm thinking I could also go all the way and shave some more. Right now the chainring is still almost touching. I could shave some more for better clearance, and rebuild the carbon with a slight bend.

  5. #5
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    That's not going to work. The material you take off is going to be way stronger than anything you could replace it with and still maintain clearance.

    Just go ahead and make more room and try it. It may work! ... if it don't send it to someone.

    Why didn't you just use a spacer on the crank to clear the CS? Maybe that wouldn't work, but obviously you should reconsider all your wrenching.
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  6. #6
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    If you shaved the chainstay down enough to clear 2.5mm inwards on the chain line, you have almost certainly compromised the integrity to the point where it's unsafe to ride.

    Ask yourself some questions.


    • If someone were selling this bike, knowing about this 'work' having been done, would you buy it? Even at a significant discount (or even 'free')?
    • Would you let someone you care about ride this bike? S/O, child, sibling, or your mother?
    • Do you honestly trust the opinion of random, anonymous people on the internet to decide if your bike is safe to ride, without so much as seeing a photograph (not that it would help much)?
    • Do you have good health and dental insurance? (ok, I admit, that was me being snarky, but seriously, don't ride this bike).


    I am sure others reading this thread have similar questions going through their minds as I have. Namely, 'WHY?' But the answer to that doesn't really matter.

    On the bright side, you now have sufficient justification to go out and purchase yourself a shiny new bike (or frameset)...

  7. #7
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    I agree shaving 2.5mm would compromise the carbon. In fact, it would not be a question at that point because I don't think carbon walls would be 2.5mm thick, so you'd really end up with a hole in the carbon tube.

    I sanded around 1mm deep about an inch diameter area. The chainstay is really beefy though. That's part of the reason why there's so little clearance because they made the chainstay tubes so large. It's actually more rectangular cross section rather than circular cross-section (tube).

    I'm thinking the design was for extra stiffness around the bottom bracket. So thinning out a small section probably would not be catastrophic. There's still the other chainstay intact. I would not try to sand down a fork or a downtube. But if a chainstay breaks, that shouldn't be any worse than getting a rear spoke break or a rear tire flat. I mean it's not going to throw you over the bars I don't think.

  8. #8
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    Quote Originally Posted by duriel View Post
    Why didn't you just use a spacer on the crank to clear the CS?
    There's no room for a spacer on the bottom bracket.

  9. #9
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    Quote Originally Posted by roaden View Post
    I sanded around 1mm deep about an inch diameter area. The chainstay is really beefy though. That's part of the reason why there's so little clearance because they made the chainstay tubes so large. It's actually more rectangular cross section rather than circular cross-section (tube).
    Did you ever think that the BB area is really beefy because that's where the frame has the most torsional forces acting against it? While the cross section of the tube is huge, the wall thicknesses aren't necessarily huge.

    Quote Originally Posted by roaden View Post
    I'm thinking the design was for extra stiffness around the bottom bracket. So thinning out a small section probably would not be catastrophic. There's still the other chainstay intact. I would not try to sand down a fork or a downtube. But if a chainstay breaks, that shouldn't be any worse than getting a rear spoke break or a rear tire flat. I mean it's not going to throw you over the bars I don't think.
    See what I said above. Yeah, the other chainstay is intact, but no one has made a 'Lefty' version of a rear triangle. I agree, you may not get thrown over the bars when failure occurs, but please post vids. I sure wouldn't want to experience a chainstay snapping from the BB shell at 25 mph and sending my rear tire into the chainstay.

    To steal a quote, 'Dibs on your stuff'

  10. #10
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    The only real answer is 'maybe'.

    I can tell you I wore a gouge maybe that deep in a chain stay from breaking a spoke and riding home with the tire rubbing and subsequently rode about 30K miles on the frame. But that's a far cry from meaning sanding off 2.5mm from any bike would be no problem.

  11. #11
    What the what???
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    If it were me, that bike would be relegated to the trainer... permanently.
    Given his penchant for nicknames, and his aversion to reading, I've decided to shorten Donald J. Trump to it's essence: Dump*

    I was "social distancing" before it was cool.

  12. #12
    Huge in Japan
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    Might as well have stuck some grip-tape to the chainring and let it sand it's own clearance. I think you're into 'test of faith' territory at this point. Ride at your own peril, just don't sell it.
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  13. #13
    'brifter' is f'ing stupid
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    OP have you heard of the 'Darwin Awards'?

    I know I shouldn't be surprised at what people think is ok to do but then things like this happen.
    #promechaniclife

  14. #14
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    I ain't afraid...

    I ain't smart enough to be afraid.
    Too old to ride plastic

  15. #15
    Schuylkill Trail Bum
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    Pics, please.

  16. #16
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    I really can't imagine how anyone would think this was a good idea.
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  17. #17
    Russian Troll Farmer
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    After reading this, I wasn't sure if you were either a genius or incredibly stupid. I'm tending towards the latter, though......
    "L'enfer, c'est les autres"

  18. #18
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    How lucky do you feel? Just go hop off the curbs few times and see if it holds up. If it lets go, at least it won't catch you by surprise.

    There's no repair that will restore the original structural integrity, just make sure the area is properly protected from moisture. I believe you provided the depth, but not the length of your handiwork, which I believe is the more critical value.

  19. #19
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    Quote Originally Posted by cxwrench View Post
    OP have you heard of the 'Darwin Awards'?

    I know I shouldn't be surprised at what people think is ok to do but then things like this happen.
    You can always fix it with some EEEEpoxy Raysun!

  20. #20
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    Quote Originally Posted by Jay Strongbow View Post
    The only real answer is 'maybe'.

    I can tell you I wore a gouge maybe that deep in a chain stay from breaking a spoke and riding home with the tire rubbing and subsequently rode about 30K miles on the frame. But that's a far cry from meaning sanding off 2.5mm from any bike would be no problem.
    You're the 2nd person who thought I sanded off 2.5mm. I sanded off only 1mm (approx.; i didn't measure it). The 2.5mm is how much the new super-compact chainring offsets the rings towards the frame compared to normal chainrings. Regular chainrings don't rub on the chainstay, but there's so little clearance, that moving the chainrings 2.5mm inward contacts the frame. I was able to make the new chainrings clear the frame by sanding off 1mm from the chainstay.

  21. #21
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    Quote Originally Posted by SPlKE View Post
    Pics, please.
    Will post a pic later. I realized I can't leave the bare carbon unsealed, so I'll have to take off the cranks again, and spray paint the carbon. Will take pics.

  22. #22
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    Quote Originally Posted by craiger_ny View Post
    Might as well have stuck some grip-tape to the chainring and let it sand it's own clearance. I think you're into 'test of faith' territory at this point. Ride at your own peril, just don't sell it.
    I actually thought I'd just let the chainring gouge it's own clearance. But maybe a better idea would have been to sand the chainring instead. That probably would have been less risky because you would be putting stress on the small chainring only when you're climbing during which you would be going at slower and therefore safer speeds.

  23. #23
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    Quote Originally Posted by nova_rider View Post
    How lucky do you feel? Just go hop off the curbs few times and see if it holds up. If it lets go, at least it won't catch you by surprise.

    There's no repair that will restore the original structural integrity, just make sure the area is properly protected from moisture. I believe you provided the depth, but not the length of your handiwork, which I believe is the more critical value.
    The sanded area is about 1.5-2 inches diameter. I originally estimated 1 inch diameter but that was too low estimate. I'll measure it when i take off the crank again and take a pic.

  24. #24
    'brifter' is f'ing stupid
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    Quote Originally Posted by roaden View Post
    I actually thought I'd just let the chainring gouge it's own clearance. But maybe a better idea would have been to sand the chainring instead. That probably would have been less risky because you would be putting stress on the small chainring only when you're climbing during which you would be going at slower and therefore safer speeds.
    You should give up cycling and find something very, very safe to occupy your time. Possibly meditation, you can think about not doing stupid things.
    #promechaniclife

  25. #25
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    As a mechanic I would NEVER ride that frame again and recommend you dont either. Carbon is strong or flexy or weak depending on how it is layered. The fame could last and never fail BUT it also could have a catastrophic failure at any time.

    Question is how lucky do you feel when cruising downhill at speed and that is the point the frame brakes in half where you did your work????? Hope it works out but I would not ride it!

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