Picture of Repairing a Carbon Fiber Bicycle Frame
One fine morning I was riding up the mountain and I guess I was pedaling too hard or something.

Somehow — and we're not precisely sure of the precise sequence of events — it seems that I managed to break the bike chain. The broken chain then got itself entangled in the rear derailleur. Since I was still pedaling, fairly hard, this caused the rear derailleur to be ripped from the frame.

All of this happened within the space of about a second. The next second would be even more exciting.

Since I was still pedaling, and the chain was still on the front chainring, the derailleur was then dragged through the narrow space between the rear wheel and the right-side seatstay. Since this part of the frame is a crispy brittle carbon fiber tube, the derailleur managed to take a nice chunk of tube with it as it went sailing past.

This is an interesting bit of damage. I'd never seen it before and although the bike, after a bit of chain tool activity, was still seemingly ridable, it was difficult to know exactly how dire a problem it was. Unlike steel which will bend considerably, carbon fiber tends to fail all at once and catastrophically. So it's a safe conclusion that this is a big problem.

The internet wasn't terribly helpful. Apparently, this is not an uncommon thing. There's a company outside of Santa Cruz called Calfee Design which seems to do almost all the repair work for everybody and everybody seems to recommend their work. Sadly, their work starts at a $300 minimum, requires the entire frame to be stripped down and shipped, and they don't make any particular guarantees.

The first bikeshop people I talked to were anxious to sell me a new bike. A friend who knows a thing or two about frames said to chuck it and buy a new frame. Trek, the company who made the frame with the brittle part right there in the derailleur flight path, would happily sell me a new frame through their "crash replacement" program which borders on a complete scam and briefly made me hate them.

Things weren't looking good.

Then I ran into James at the Missing Link bike shop in Berkeley. While everyone else had been pessimistic and dour, James was brim full of gung ho. And while he had no actual personal experience in this department, he knew someone who had heard of someone who had done it himself.

Good enough! This was precisely the encouragement I needed. So I rolled the bike home and set about figuring out what I'd need.

The entire project took three days, almost all of that spent waiting for stuff to dry. Materials mostly came from TAP Plastics, a local chain selling all kinds of fascinating and deadly polymers. There's a large selection of cheaper alternatives on the internet but I wanted to get this done in a hurry. You probably have a similar place in your nearest large urbanization.

What follows is a set of photos and notes on the process. I hope this is helpful. As far as whether this is a safe or effective way to repair a frame, you're on your own. Seek the advice of a qualified professional. I am neither.
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rnorman32 years ago
the frame you have is a TREK frame, and if you take your frame into a TREK authorized dealer, they will warranty you a new frame. It may cost you a little, like if you need a new headset or BB. a new frame would be safer
art medlar (author)  rnorman32 years ago
I can't believe you are telling me this! This is _instructables_ not "purchase-ables". As I very clearly state in the opening paragraphs here, Trek does NOT "warranty" anything at all. They do have a "crash replacement" program which means that they will sell me a replacement frame for MORE THAN THE COST OF AN ENTIRE REPLACEMENT BICYCLE! It's basically a scam that they use in their advertising literature in an attempt to fool people who are not good at math. If you'll actually read what I spent the time to write in the introduction, you'll see that I already took that into consideration.

And I'm afraid I need to take issue with your unsupported claim that somehow a new frame would be "safer". Do you have any evidence that this repair is not "safe"? Do you have any metrics for quantifying this supposed "safety"? Because without them your claim just seems to be coming out of fear: the fear of experimenting, learning new processes, making something on your own. And retreats into passive consumerism.
I concur....same thing happened to me...very small minute ding in the seat stay area and TREK "crash replacement" option was BS!!! They offered me a brand new frame at market value -5%. I bought a kit off EBay and conducted my own repair. It's a 2013 Domane 4.0 CF. The repair came out awesome and thank you for your write-up!!! Ignore the "haters" I'm not worried about transferring any type of load because one seat stay is stronger than the other. This frame has ISO-flex in the seat post area, so that load gets transferred up stream.
SteveB45 months ago

Great job Art!

I stumbled across your instructable since I just bought a CF bike and wanted to do a little reconnaissance in preparation for some "likely" future issue. Your open-mindedness and willingness to tackle this project (even through necessity) is inspiring as well as very helpful to others who follow in your footsteps. I enjoyed going through it and appreciate the time and humor that you invested here. I will definitely refer to it when the need arises. Thank-you.

a-dog5016 months ago

Hi, I have a similar problem on a carbon frame on the right hand side chain stay, due to its position in is next to the cable stop for the rear derailleur and possibly underneath it as it is epoxied in place. I am temped to try this method but speaking from experience would you wrap several layers around and either side of the stop or cut small holes in the fibre to allow the stop to poke through when going over the top of the stop. Thanks and well done on your well written instructable!

rick.stout.946 months ago

Good job Al. A little suprised about your comment about why Trek would put a "crispy, nominally repairable part right in the line of fire". Never seen a "carbon fiber" bike with steel, ti or aluminum chain stays. Of course they make mixed material frames with stays and other tubes of various materials but this is a carbon fiber bike.

PatrickF27 months ago

I cannot tell you how much of a Godsend finding this article was! I have discovered an area on my top tube that looks completely like only the clearcoat has sustained some type of crack. I examined the crack under high intensity light while wearing my surgical magnification loupes and it does not look like any of the CF is affected.

This coming Tuesday, I'm taking the bike to the surgery center where I do my surgeries, and I'm going to use the fluoroscope (portable x ray machine) as well as a diagnostic ultrasound machine to image this area of the crack to determine if any blowout of the internal side of the tube has taken place.

My question is this- If I find that there is no full-thickness crack, nor any blowout of the internal side of the top tube has occured, do I simply have to sand down the clear coat and re-spray it with the enamel as pictured in your instructable?

всмок7 months ago

I had my carbon fork cracked when I was hit by a car. In Ukraine got it peraired for $40 (at Garbaruk company). I am shocked by the $300 starting price in US, but I understand labor cost is much higher there.

alinezam8 months ago

Exactly the same thing happened to my Norco bike. However, while I was getting ready to follow these instructions, to my surprise, the company agreed to give me a new frame (I only paid for moving parts from the old frame to the new one). I just wanted to share some lessons I learned:

- keep the derailleur adjusted all the time

- keep track of the gear you are at (i.e., don't try to change gear in uphill if you are already at the lowest gear)

- regardless of how careful you are, carbon bikes don't last as long as aluminum or steel bikes

Finally, thanks to the author for this useful article and thanks to Norco for their great customer service!

Devinchi421 year ago

On my carbon frames, I topcoat with a final coat of the UV stable epoxy that I used for the carbon layup.

CobyUnger2 years ago
I did this to a bike months ago. Worked great and it's holding up really well..
toyowner2 years ago
Very nice repair.
Gelcoat is the ususal finish for carbon fiber and fiberglass as well.
Gelcoat is Fiberglass resin thinned with acetone so it can be sprayed. Not as easy as it sounds and is very hard on spray equipment.
toyowner2 years ago
use cellophane wrap and "squeegee" the resin out and seal it from air then you can wrap it in cellophane or tape to hold it. unless you could somehow vacuum it closed.
chuck1902 years ago
I commend you for your courage, but there's a saying that "fools step in where angels fear to tread". A failed stay could lead to serious injury, even death in heavy traffic. I was biting my nails as I read each step.

I have done some modification work with carbon on my own bike, and I try to guarantee against failure by prayer. I also prepared myself by doing a lot of research. Here's some info that might help you and other readers:

1. I worry about your using 3 layers of carbon (of unspecified specs) mainly because the stays should be equal in strength and flexibility. If the repaired one is stronger, shock absorbancy from bumps will be unbalanced and could put the weaker side under loads that could eventually lead to failure. Frankly, if I were you, I would sand off the layers and get 24K carbon toe at This video demonstrates how to apply it I would apply it on BOTH stays. Give the damaged one an extra two layers.

Note that how the excess epoxy is constantly squeezed from the application. The problem with using the cloth like you did is that it's almost impossible to eliminate air bubbles in between layers, and this will cause delamination with time. If you want to use the excess cloth you have, cut it into ribbons about 1-1/2 to 1-3/4" wide and apply the way the demo shows how to do it with toe. But it could get quite messy and difficult to control because squeezing the epoxy will screw up the weave. The toe isn't that expensive. I think you can get the minimum for about $15.

2. The silicone balls don't strengthen a layup in your application. It's principle purpose is to keep the epoxy from running on vertical surfaces AND filling in spaces. I don't think you needed it, but it's not worth removing.

As I remember, you used West System epoxy - which is a good choice. You can also get advice from their technical staff. They are very very good and helpful.

I hope this helps. Like I say, I wouldn't assume the repaired stay won't give trouble - and injury with time because it violates some basic principles of carbon layups.
art medlar (author)  chuck1902 years ago
Thanks! I was not aware of 'carbon tow' and it looks like it might be useful for other types of repairs. To clear up some things that might not have been clear:

1. There are five layers of carbon fiber, not three. I'm not sure how to make that any more obvious in the instructions.
2. The silica (not silicone balls) was only used in the fill step, for exactly the purpose you suggest, and was not used for wetting the fiber prior to wrapping. I'll try to make that more clear.
3. The process of wrapping with plastic tape squeezed out a considerable amount of epoxy and air. From watching that video, I'm not convinced at all that the 'carbon tow' method is any less likely to have voids.
4. I think you're completely mis-calculating the danger from over-strengthening this section of tube. First, the tubes are extremely stiff to begin with, so even if there were any measurable extra stiffening of this section of the tube, it would likely be no more than a small percentage. And even if this section were over-stiffened, this section is only about 10% of the entire tube. So we're talking about *maybe* at most making the repaired tube 1% stiffer than the undamaged one. If this "unbalanced" condition is enough to cause a failure, I'd say the problem wasn't with the repair.

Finally, as I noted, I've put a considerable amount of mileage on the bike post-repair and I keep a careful watch on the repair site. No problems whatsoever.

I'm concerned that you claim this "violates some basic principles of carbon layups". Because you don't specify what those principles are. Aside from my mis-communicating that there actually were five layers and that the silica was not used while wetting the carbon fiber, did I do anything else wrong?
jheiss2 years ago
By "about 1/4 of the tube inside" do you mean you stuffed in enough to fill the tube a few inches in either direction (i.e. 1/4 of the length of the tube), or something more localized like 1/4 of the cross section of the tube right near the hole?
art medlar (author)  jheiss2 years ago
You're right. What I'd written made no sense at all. I wonder what I was thinking? Anyway, I've edited the text of this step to, I hope, be more clear about exactly what I did. Thanks for pointing this out!
actimm2 years ago
I haven't ever done a CF patch, but when I did fiberglass work what I found works best is actually to do the layers opposite what you did. start small and work to big. This way you only have one layer of "frayed" edging to smooth out rather than having a candycane effect of frayed material if you make a mistake with the epoxy.
art medlar (author)  actimm2 years ago
Oh wow. That never occurred to me! And it is, in retrospect, so obviously the right way to have done it. Thanks!
Great job and so well explained. I like your style
voron882 years ago
Really nice job. I have done a lot of fiberglass work and you handled this like a
camp6ell2 years ago
This is the best instructable I've ever read. Exactly the right amount information - and disinformation - laced with human experience. Nice.
I'll be coming back here when I need to do this.
nmcrae12 years ago
I'm an aircraft mechanic and have experience with Carbon Fibre and repairs - and you did an admirable job. Your repair is probably stronger than the original frame is.
I was also impressed that you used tape to compress it and let the excess epoxy leak away - most people wouldn't think of that, but it is important for the strength of the repair. Too much epoxy weakens the matrix. Well done!

Yes, working with epoxy like this IS messy, and if you use liquid hardener (Methyl Ethyl Ketone Peroxide or MEKP) - be EXTREMELY careful with it - just getting the fumes of MEKP near your eyes can cause INSTANT blindness. Nasty stuff - be VERY careful with it, and wash your hands before you touch your eyes - even if you have gloves on! Cream hardener is less dangerous, but also much weaker, and not as easy to use in this kind of situation. Great work!
Slay. nmcrae12 years ago
Not to be sarcastic or anything, but you really are an aircraft mechanic, aren't you? Totally wish I had that job...
graydog1112 years ago
Really nice job. I have done a lot of fiberglass work and you handled this like a pro. At least carbon fiber sanding dust should not make you itch like fiberglass does. Good instructable....very thorough.
wilgubeast2 years ago
Great documentation, and you certainly have a way with words.
maintann2 years ago
We'll done. An easy way to clean off sunset epoxy is with either alcohol or white vinegar. One of the tricks with sanding epoxy is judging how long to leave it to set. There is a period of an hour or so to a day or so depending on the epoxy and the temperature when it sands fairly easily, then it goes as hard as! Best if possible to do the sanding then.
lapsmith2 years ago
Excellent job and very well written. Thanks!
tqwinter2 years ago
By far the best Instructable I've ever seen! Thanks for taking all the time to document each step carefully and with outstanding photos.
Wow that repair looks professional!
oneline1802 years ago
You seem to have done things with care, therefore you should not worry too much about the repair you made.

Well done. Now go ride your bike more often.
alames2 years ago
Don't know if it would have helped, but methyl alcohol is a thinner for epoxy. With a very small amount you can make the epoxy more like a thin paint, and you can also clean up your brushes if you get to them before it cures. You can buy at the hardware store, or some hobby shops (model airplane and car types with gas engines) might sell you some if you bring your own bottle.
Best instructable ever, well done keep up the good work, very brave of you
DreamHazard2 years ago
Excellent job! Especially considering having had no guidance you've done a brilliant piece of work :)
Workstead2 years ago
This is a great instructable. It is written by someone who has been there. In my wotking life I was a technical writer and this is one well written article. If I ever need this type of repair I now have the knowledge to do it properly, without making mistakes that I otherwise would like have, because of your cautions about the wrong way to do it.Thanks,
DreamHazard2 years ago
A commonly used technique here is to pre-pierce the tape beforehand so the excess resin can bleed through the tape, resulting in a tighter wrap and (theoretically) less mess to sand off afterwards.
WVvan2 years ago
A most excellent instructable. Double plus good!
mikeyc.20092 years ago
Good job! Like maintann said alcohol does a great job cleaning off uncured epoxy. I get the 99% stuff from a local beauty supply shop. A good place to get small amounts of carbon fiber fabrics and tapes is hobby shops that specializes in radio control airplanes. They will also have a epoxy resins there too.
phburns2 years ago
Very nice explanation, thank you.
Nico3ck2 years ago
Carbon fiber itself is very strong, you don't really need to put a blob of epoxy inside your tube (but it can help while wrapping).

On simple way to use epoxy & carbon (or glass) fiber is to cut the fiber in a long strip, "paint" it with epoxy (fiberglass will be clear when enough epoxy is inside, carbon is a bit more tricky) and then wrap it tight around the sanded part.
You dont need to wrap it with plastic after, but you can add a special cloth with will absob the excess resin and limit the finishing sanding.

To work with epoxy, temperature is important (> 15 C°), the air must also not be moist. You will need to paint or varnish it, uv rays will tarnish your resin.
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