Thread: TR: repairing a carbon frame...

So my buddy snagged a warranty replacement bike last year. It has a slight crack on the seat stay, and was likely destined to be junked. It is a Cannondale supersix Himod, so its a pretty bitchin frame. The only damage is this rear stay, and the rest of the bike is in solid shape.

I was under the impression it was a 56cm when I got it, but unfortunately I found its a 58 after measuring. Thankfully it was a good deal, and this will be good practice working with composites. I found a matching fork for $50, and the going rate for this frame used with a fork, is in the neighborhood of $800 (for a not-broken one). All said and done, I will have just a bit over $100 into the frame and fork (missing one dropout, no headset or BB) once repaired. It will be a few grams heavier than the original, but I like to think it will be worth more than what I paid

Not sure what I am going to do with the frame after repair, probably send it to my uncle who is a big time roadie. Unless someone here wants to trade me something in the 54cm zone, or roadie components...

I am doing all of this work with a friend who has worked in composites for years, primarily in aerospace. This is his first time working on a bike, but it should be interesting to see what happens.

Here it is...



So I located the crack, and sanded the paint off with 150 grit. Its amazing how little paint is on these frames. I gave about two inches on either side of the crack, and went all the way around the tubing. (The crack probably runs 1/3rd of circumference) Had to be pretty anal about not sanding too hard, so I didnt compromise the existing carbon. Checked for more cracking visually and didn't see anything. Could go to campus and set up some sort of ultrasonic testing rig, but that seems like too much work. On summer break.



Finished sanding.



Next step is to do a few wraps of carbon around the sanded region, then some finishing work.

Very interesting. Looking forward to seeing how this progresses.

I would taper your wraps so as to not create additional stress risers but your buddy probably already knows this.

One method is to inject a small amount of expanding foam inside the tube then grind out the damaged area entirely. The area can then be built back up laying in CF strands in multiple directions as dictated by the damage and needs of the structure.

You guys using impregnated CF or a contact layup with Epoxy?

Good luck and post back with status/pics.

Rad project Single, interested to see how this all goes.

Originally Posted by scottyb

I would taper your wraps so as to not create additional stress risers but your buddy probably already knows this.

One method is to inject a small amount of expanding foam inside the tube then grind out the damaged area entirely. The area can then be built back up laying in CF strands in multiple directions as dictated by the damage and needs of the structure.

Thats a really cool idea. I haven't done much research on this stuff, but that makes perfect sense. Luckily the crack is in the middle of the tube and will not affect tire clearance. I imagine that could be a problem, and you would probably have to take the foam approach to remedy that.

And definitely going to taper it, hard edged stress concentrations are not ideal. I dont know what it looks like in a composite, but in general metal shaft design the concentration factors for a hard edge are in the range of 2+, which I would assume is over the safety factor for a frame like this.

You guys using impregnated CF or a contact layup with Epoxy?

Good luck and post back with status/pics.

Doing a contact layup. Thanks, hopefully this little project will be done in the next week or two. Will definitely post pictures, and will try to weigh everything that goes into the repair.

So you're just repairing it with carbon warps and epoxy? Seems surprisingly simple.