DIY carbon repair

Backstory

In 2016 I crashed my 2013 Sworks with 2km to go at Masters 50+ Nationals when someone slid up between me and another rider and took my rear derailleur off. The major problem from that incident was tremendous sciatica, which turned me in to "The Falls Church Night Weirdo" for the entire summer. But in addition to my piriformis muscle , the seat stay was also damaged. The drive-side seatstay crack had been growing for years, while I  monitored it. By Spring 2021 it was large enough to affect the shifting, and I retired the bike. I had already had it repaired once before in 2015, after an Artemis guy chopped me at the Tour of Page Valley, and either my heel or the heel of the guy I fell on cracked the chainstay. 

The crack in the seat stay extended about 2/3 of the way around.

Since the frame was a writeoff at this point, a repair attempt seemed like an entertaining retirement project. As everyone does, I watched about twenty youtube videos on DIY carbon-fiber repair. I ended up buying a kit from Predator Carbon: https://predatorcycling.com/products/carbon-repair-kit , mostly because their videos had seemed useful. I actually bought the individual components instead of their pre-packaged kit, because it was only 3/4 the cost. 

All the repair kits I have seen from various sources are similar. Mine had three components:

1. a strip of 2x2 twill-weave carbon fiber 150 mm x 1300 mm,
2. 75 ml of resin with catalyst, and 
3. 2 m of industrial heat-shrink tape.

The third component is the "secret sauce" that makes the whole process work for the neophyte end user. The heat-shrink tape provides the compression over the fiber layup to squeeze  out the excess resin and compact the layers.

The kits represent a big markup over the marginal cost of materials, but most carbon fiber suppliers only sell in much larger, and ultimately wasteful, volumes. My experience was that the 75 ml of resin is sufficient for four or five repairs. And it has a very finite shelf life, so buying a 4 liters just means that 3.9 liters will go to the solid waste transfer station.

I practiced once using Marc Klein's Cannondale that Pete Lindeman had given me. I smashed the top tube on the concrete steps and then patched it.  After the resin had cured, I sawed a section of the tube out from the repair and verified that the patched area didn't contain unconsolidated fiber/resin. 

For the Sworks Chainstay repair, I opted for a simple three-layer overwrap, instead of attempting to create a scarf joint. The resulting repair will be a bulge like a recently healed broken leg, but more layers seem safer than a more attractive repair. 

The Repair

First I removed the clearcoat by hand sanding 5 cm on either side of the crack. 

The crack with the clear coat removed, and prepped for an overwrap. 

Mixing the resin and catalyst is straightforward by weight: 3.5:1 . I used the triple-beam balance that I rescued off the loading dock of Bard Hall 35 years ago. 

The twill-weave carbon fiber cut to size, and the components ready to weigh out. I mixed them with a recycled Trader Joe's popsicle stick in a used a carry-out sauce container . 

I masked the area of the seat stay with painters tape and electrical tape, and I wrapped the entire frame in newspaper and garbage bags, which is important, because the resin runs out and drips everywhere.  One of the videos suggested tacking the end of the carbon fiber sheet in place using partially cured (40 minutes) resin, so I did that.  

Before wrapping, I staged the the correct length of heat shrink tape by taping it to one size of the repair area, so it would be quick to just wrap it around. 

With the resin and catalyst mixed, I painted the whole exposed section of the seat stay with resin using a plumber's acid brush and started wrapping the carbon fiber sheet, while simultaneously painting more resin onto the new layer. The entire repair was three layers of sheet, which makes a repair about 1.0 mm thick.

With everything in place, I spiral wrapped the heat-shrink tape like a handlebar, overlapping about 50 % of the tape on each wrap. Then I taped the end with electrical tape and hit the whole area with the heat gun for about 90 s. As the tape shrunk, it squeezed out the excess resin, which I (mostly) wiped up with a shop towel. Some dripped on the floor, unfortunately. 

The repair after the tape had shrunk

I let the resin cure for 48 hours before stripping the heat-shrink tape and then lightly sanding it with 320 grit paper. I plan to just clear coat the repair with automotive clear coat, because the resin is known to be UV sensitive. 

The completed repair before clear coating. 

Postscript

Of course, the proof will be whether it survives being ridden..