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.
Still, looks pretty bad. Why on earth you’d design a bike frame with a crispy, nominally irreparable component right here in the line of fire is beyond me.
First, I taped off about a six-inch segment of the seatstay around the damage, and began sanding.
Here it is sanded down to the fiber level. You know when you get to this point because the sanding dust turns dark grey rather than white.
The epoxy and filler come from a local epoxy and filler shop. There is infinitely more of each than I'm ever going to possibly use but these are the minimum quantities available. Total cost was a little under $50.
What we're looking at is a large round bottle of marine-grade epoxy resin, a small round jar of "cab-o-sil" filler, and a smaller rectangularish bottle of epoxy hardener. Epoxy resin is a magic liquid that stays liquid indefinitely. Until you pour some of that hardener liquid into it. Which starts a crazy exothermic (ie, gives off a lot of heat) reaction that quickly results in a hard, solidified resinous mass. The speed of the reaction is determined by the types and amounts of epoxy resin and hardener used. Generally, the slower the hardening, the stronger the eventual bonds.
There are trade-offs to be made in the choice of epoxy resin. Some are stronger than others. Some dry clearer (as opposed to browner/yellower). For this job, I wanted the strongest I could find. But I also wanted as colorless an end-product as possible (because I was attempting not just a mechanical repair, but an aesthetic one as well). I ended up looking through the catalog (where these qualities are listed) and picking a resin that seemed to be good enough in each department.
And this "cab-o-sil", what is it? The label says "fumed silica". A white fibery powderous substance that is probably hell on a lung. Make sure you're wearing a dust mask. It's here to give the epoxy some body so that it can be stuffed into the hole and stay there. The other option would have been microspheres. Which are these tiny, bigger than dust but not much, hollow glass bubbles that do roughly the same thing. The difference seems to be that the bubbles produce a much lighter solid while the silica produces something a little stronger. Proper tradeoff? I don't know. The internet sez microspheres, the plastic shop guy sez cab-o-sil, and the price is the same. Since we're talking milligrams here, I go for the silica.
Is this the right approach? It seems like it but often these things have a way of being completely counterintuitive. It could be that by loading the tube at this point with a stiff epoxy will concentrate stresses at some other point, simply moving the failure to a different part of the tube. Or maybe not.
Plus it's hard or impossible to tell what stresses and strains this particular tube sees in action. I'll bet there's a mechanical engineer with a masters degree in exactly this but that mechanical engineer is not me.
Working out the physics of a static load on a bike frame isn't too difficult. But where's the fun in a static load? Once it starts moving everything gets awfully complicated. And all of the obvious simplifying assumptions (the chain pulls the center of the rear axle, the rider's weight is always on the pedals, the bottom bracket isn't flexing in three axes, the road is smooth, etcetcetera) all of them seem likely to also be wrongifying assumptions.
So I tape off everything around the hole.
This is hundreds of times more glop than I'm going to need. But it's fairly difficult to mix up small amounts of epoxy with any accuracy. And if you get the proportions wrong you end up with a sticky, non-hardening mess that's a huge bother to wipe off and start over.
It turns out that peeling off epoxied blue tape is not particularly easy and I've got to sand off a lot of the epoxy from the tape first. So the order of operations becomes sand, peel, sand.
Lesson: next time, be even less sloppy with the epoxy.
This stuff frays like mad at the edges and that's a bit worrying. I have no real idea how to cut it but scissors seem to work.
What's happening here is I'm going to cut out a piece of fabric that's the same complex shape as that white cardboard template. The fabric is going to be soaked in epoxy and then wrapped four times around the frame tube, covering the hole.
Time spent with plastic and tape at this point will pay off handsomely tomorrow. This paragraph has been brought to you by the voice of experience.
[Edit: note the comment below where it is suggested that this wide-to-narrow wrapping is precisely backwards and that I should have started with the narrow edge first. I believe that's probably the right way to do it.]
This will turn out to not be strictly necessary — it sands down pretty nicely — but I'd do it this way next time too.
At the time this didn't occur to me, so watch as I do it wrong.
(This is the point in the process where you go visit the bathroom, have a little snack, relax for a bit. When you come back the epoxy's going be prepared and the next hour is time- and attention- critical.)
To spread the epoxy over the fabric I needed a roller of some kind. The local horrible Michael's Craft Store seemed like a place to find this roller of some kind. I've been fooled that way before. What a disasterous place, it's 60% dried flower arranging, 35% scrapbooking (!) supplies, and then an assortment of random art supplies and other garbage. But today I was in luck. One of those random other things was a toy rolling pin for fifty cents.
The epoxy is glopped on and then spread out with the roller. You don't need to use a silly toy; a dowel or an empty beer bottle would also work. However, anything you use will never be able to be used for anything else ever again. So use something disposable.
This is also a good time to talk about ventilation. At this point in the process, the epoxy is giving off clouds of god-knows-what kind of volatile poisonous gasses. You should endeavour as much as possible to avoid breathing these vapours. Open windows, a gentle breeze, and a respirator rated for organic solvents are all good friends to have while working.
Once the fabric is in place, the next thing is to cover it with another tight wrapping while it cures. The material of choice for this seems to be black electrical tape wrapped so that the sticky side is out.
Something to be careful of here, which I was not, is to wrap the tape in the same direction as the fabric was wrapped. What you're seeing here, though you can't see it, is the tape being wrapped in the opposite direction. This has the misfeature of causing the fabric wrapping to loosen if you're not excessively careful. It eventually came out alright, so maybe not a major issue, but save yourself some worry and do everything in the same direction.
Once complete, I took a pin and poked a few dozen holes in the tape so that more could leak out. Lots did.
The loop of blue tape on the non-damaged tube is there at the corresponding location of the hole. This way, once it's all sealed up I'll have some idea where the damage was.
Still, there's now no longer any sign of the original injury.
This time we're putting on the top, outer layer which will be visible for the rest of the life of the bike. So I have to be careful to get the weave pattern in the fabric to match the direction of the weave pattern in the original fiber of the frame. Carbon fiber fabric seems to come in a large variety of weaves and matching the precise sizes of warp and weft and pattern and texture did not seem easily possible. But we can still come fairly close. And at least match the direction of the weave.
So look carefully at the non-damaged part of your bike. Is the weave squarely with the axis of the tube? Or is it at a diagonal? Try to cut this last piece of fabric out so that the weave will match when wrapped around the tube.
(If you don't get this exactly right, it's not a terribly big deal. From more than a couple of feet away, the patterns are invisible to the casual observer.)
The seam is on the inside, wheelside, where it will be less visible Then it's all wrapped with tape as before.
Five layers definitely adds some bulk. Though not as much as I was expecting. Five layers of dry carbon fiber is pretty thick but once it's soaked in epoxy and compressed, it's thinner than I thought it would be.
As an epilogue: I have ridden this bicycle almost 3000 miles since the frame repair. I haven't noticed any issues at all. I've left that little blue loop of tape on the left seatstay, directly opposite where the original hole was, so I can locate and carefully inspect the site of the repair. So far, it's holding up well.