Jim’s Tech Talk
By Jim Langley
Back in May, I wrote about my Cervelo S5 falling over and landing on a rock. The impact cracked the top tube. I said I might have the bike repaired by a professional carbon shop, such as Calfee Design since they’re just down the street from me and have been carbon gurus since the beginning (Greg LeMond raced on a Calfee!). In the comments to the article, several of you recommended Broken Carbon in Boulder, Colorado.
Here’s the story from May if you missed it and want to catch up: Do As I Say Not As I Did.
What jumped out at me in the comments, though, was one from reader Eric Brandt who said that he had learned how to fix carbon frames and that it was “fairly easy.” That got me Googling carbon repair and up came a link to Predator Cycling’s carbon repair kit, everything needed for just $99.95.
Before I bought it, I gave them a call to discuss my frame’s issue, the crack running more than halfway around my top tube. They assured me it was just the kind of problem I could fix myself with their kit. So, I purchased it.
The box arrived in just a couple of days and as promised, the carbon patch fabric, epoxy, brushes, alcohol cleaning pads, sandpaper, compression tape (heat shrink tape) and even rubber gloves were provided. Predator has 3 videos showing the full procedure, too.
The process for repairing my cracked top tube turned out to be just like Eric said, fairly simple.
Here are the steps:
- Mask and cover the frame to protect it.
- Wet sand the frame in the area of the crack to get all the paint and primer off and get down to the carbon. Make sure the crack/damage is smooth, no jagged edges.
- Cut 2 patches. I small one to go over the crack. I larger one to cover the small one and the full area around the patch.
- Mix the epoxy and hardener together.
- Clean the patch area with the alcohol wipes.
- Apply epoxy to the patch area on the frame.
- Apply epoxy to the patches being sure to saturate them.
- Apply the small patch over the crack and the larger patch on top of that.
- Wrap the repaired area with the compression tape.
- Using a hair dryer or heat gun, heat the compression tape to get it to shrink and compress the entire repair area.
- Wait 24 hours for the epoxy to cure.
- Remove the compression tape and all masking.
- Final sand and repaint the frame if desired.
This probably looks like a lot of steps and work, but it only took about 2 hours plus the 24 waiting for it to dry.
When you crash a carbon frame and can see damage, the “coin test,” is a good way to tell if it’s just a surface issue, such as a paint scuff or chip or real damage. To do it, you go down the carbon tube tapping on it with the edge of a coin. Undamaged carbon will be solid and you’ll hear the loud sound the coin strikes make. For a cracked frame, however, when you hit that bad spot with the coin, it will make a dead sound.
That’s what my Cervelo did when I tapped with the coin on the crack on the top tube. So I knew it was a failure, not just superficial paint damage. When I tested the frame after repairing it, the coin made the same loud sound all the way down the top tube and all around the formerly cracked area, too.
I also tested the bike on a couple of hilly rides, standing more than usual and feeling for any change. I’m happy to report that my bike rides as well as it ever did and I’m delighted that with only $100 and a couple of hours of labor, I could fix it myself.
In case you’ve got a carbon frame in need of repair, I made a video while I did the repair to help:
Follow-up on Tom’s Speed Wobble
I’m pleased to be able to report that my friend Tom with the speed wobble, who I wrote about a couple of weeks ago https://www.roadbikerider.com/avoiding-bicycle-speed-wobble/ is now wobble-free!
Thanks to a kind reader named Melissa who commented that she had experienced a wobble from her Roval wheels, Tom got the bike shop to give him a new front wheel. And voila, the wobble is now gone! Thanks Melissa!
Jim Langley is RBR’s Technical Editor. He has been a pro mechanic and cycling writer for more than 40 years. He’s the author of Your Home Bicycle Workshop in the RBR eBookstore. Check out his “cycling aficionado” website at http://www.jimlangley.net, his Q&A blog and updates at Twitter. Jim’s cycling streak ended in February 2022 with a total of 10,269 consecutive daily rides (28 years, 1 month and 11 days of never missing a ride). Click to read Jim’s full bio.