Introduction: Carbon Fiber Repair: Damaged Bicycle Dropout

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About: Trying to keep the things I own from owning me.

A short muddy section of trail stopped my 3 month old Canyon Lux mountain bike dead in its tracks. It didn't take much but some caught between the low pulley and the chain causing the derailleur to lever backwards, up, and over the cogs. The set screw boss broke from the hanger, but not before the derailleur sheared through part of the carbon fiber dropout. A subsequent rocky ride on the weakened part resulted in completely severing the drop out.

New style mountain bike derailleurs are so often damaged by rocks, sticks, and other stuff that they are often designed to fail, or break, with minimal cost or convenience. In nearly all cases, field replacement of a $25 hanger or a 40 cent allen screw is all it takes to get back on the trail. However, because of a manufacturing defect in either the frame or a $25 derailleur hanger, a $3,500 mountain bike was totaled while moving at less than 3 mph.

Step 1: Parts Damage Inspection

It's worth noting how flimsy this dropout is. It's neither enclosed, nor support by an external load-bearing component. I've seen beefier key rings!

Between Canyon's customer service and the pandemic's effects on bike demand, a new frame was at least 6 months to a year away. So, I decided on repairing my useless frame. After hours spent researching technical documents and the ever growing number of online write-ups on carbon fiber repair, I felt like the process was well within the skill set of a typical "do-it-yourselfer" like myself. The cost of a DIY repair was pretty compelling too!

Carbon Fiber Mtb Frame Replacement = $2,500 to $4,000+
Dedicated CF Frame Repair Service = $300 to $600
DIY = $50 (plus incidentals)

Step 2: Prepare Your Work Space

Despite having a vacuum pump and bagging materials, I avoided any technique not amenable to a quick trip to a typical hardware store. This entire process could easily be modified for repairing pivot points and bearing enclosures.

  • Warm and dry, enclosed space, free from breezes that would disturb carbon fiber bits on the floor or work surfaces. Note that mixed epoxy takes much longer to set when below room temperature
  • Bright steerable lighting; something like a 150W white LED flood light clamped on a tripod that can be easily moved around. You want to clearly see your work and steering the light lets you look for uneven finishes.
  • Floor, work surface, and bike protection.
  • A $10 infrared light bulb may be used speed epoxy curing.
  • Cured epoxy is rock hard and difficult to remove from delicate surfaces. Saran wrap for cutting boards and bike parts, a plastic tarp for the floor.

Step 3: Gather Your Tools

  • Adjustable speed Dremel with high-speed steel cylindrical burr bit. It's not critical, but makes paint removal much easier.
  • Barber scissors for cutting hair-like carbon fibers and sheet. They really make a difference when cutting complex shapes and can be found at any Walgreens.
  • Fine tweezers for pulling and manipulating carbon fiber strands.
  • Smartphone camera for documenting your project to impress your friends.

Step 4: Consumables

    Time. I did about 5 separate lay-up steps that each required 24hrs of cure time in between. So, despite very little time actually working on the repair, I spent at least 5 days waiting for stuff to cure. Other than the carbon fiber cloth and resin epoxy, you can easily find everything else at regular hardware and drug stores.

    • Two-part epoxy and some appropriate carbon fiber cloth
    • Methanol and isopropyl alcohol for degreasing and cleaning without leaving residues
    • Super Glue, for rigidly setting loose parts into place.
    • Paper Towels
    • Fine Tweezers for handling carbon fibers (and removing tiny bits from your skin)
    • Canned Air for blowing alcohol puddles out of the recesses of your work.
    • 2 Graduated Syringes, dedicated to each part of the 2-part epoxy.
    • 6, or so, cough syrup cups for epoxy mixing, for working with small batches of epoxy. Get them at Walgreens
    • Small popsicle sticks for mixing resin epoxy and maneuvering wet carbon fiber.
    • Clean, grease-free teethbrush
    • 220 grit, wet sandpaper
    • Ziplock bags for fiber and epoxy mixing
    • Masking tape
    • Surgical gloves for manhandling wet carbon fiber onto and over parts.
    • If repairing bearing enclosures, you'll want spray-on mold release which will provide a very thin, non-stick surface from which a bearing surface can easily slip from once resin epoxy is cured.

    Step 5: Paint Removal and Surface Preparation

    Clamp your bike to your work stand and begin removing the paint and lacquer from the carbon fiber. This will give the new carbon fiber a good surface to stick to. Set the Dremel to low speed and carefully work the edges of the paint chipping it off a little at a time. Using a low speed keeps you from melting the paint or accidentally grabbing your work and gouging a hole through the carbon fiber. Be patient and do not completely grind down to the carbon fiber itself.

    Step 6: Parts Alignment, Initial Fixing, and Reinforcement

    It's obvious why your repaired parts must be properly aligned. In this case, aligning the broken dropout piece was relatively easy as I had the flat surfaces of the derailleur hanger, the axle itself, and part of the dropout surface on the chainstay. I just fixed the axle in place, built everything around it, then superglued the broken parts together. Lightly coating the axle and metal parts prior to assembly with a very thin film of silicone grease made removal of these parts easy.

    Now that the broken part is aligned and (sort of) fixed, I had to make it more robust before I could really work on it. So, a few strands of carbon fiber over the top and under and 24 hours later I was in business; I could fill, sand, and wrap without fear of accidentally breaking the part off.

    Step 7: Gap Filling With Carbon Fiber Filler

    The void space around the derailleur set hole needed to be filled in before I could reinforce the exterior faces of the dropout. After lightly greasing up a cable ferrule, I placed it in the future set-screw hole then made my own carbon fiber "hairball" filler using epoxy and many individual carbon fiber strands and carefully filled the void space, eliminating air bubbles along the way.

    When cured, the ferrule was easily pulled out and the excess cured material was carefully removed. The two sides of the dropout were then reinforced with fitted pieces of epoxy impregnated carbon fiber cloth. The entire assembly was wrapped lightly in Saran Wrap, sandwhiched bewteen thick cardboard, and held in place with a pony clamp for the duration of the curing.

    Step 8: Form Preparation

    The main carbon fiber reinforcement had to be held in place during curing and it was clear that without some sort of "mold" or "form", both the fibrous mass and the liquid epoxy resin would just slough off the sides, reducing the final thickness (and strength) of the repair.

    Any pair of stiff smooth plastic discs would work to help keep the fiber/epoxy mass in place. I cut a couple of beer can holders to fit and clamped it together using a simple screw and butterfly nut. Obviously a small hole was hand drilled through the axle hole to accomodate the assembly.

    Step 9: Final Reinforcement

    For such a unidirectionally loaded component, it was not useful to use the burlap-like carbon fiber weave that I had, so I pulled about 40 long individual carbon fiber strips out of the weave, and placed them in a Ziploc bag with about 60cc of pre-mixed resin epoxy. The plan was to gently knead the contents and thoroughly saturate the fibers within the strands, then pull out the strands and lay them up individually....

    ...but here's what happened:

    As I kneaded the fiber/epoxy mixture, all of the air bubbles were worked out of the mixture and the individual strands began separating. Removing the air bubbles became easier, which was great, but even better, my collection of carbon fiber strands became a thick, homogeneous cable of many thousands of individual fibers.

    After 10 minutes of this gentle kneading, I was satisfied that all of the fibers were were coated and there were no bubbles in the mixture. Now wearing surgical gloves, I pulled the entire mass down to the bottom of the ziploc, cut part of the edge of the Ziploc and pulled the entire mass out of the bag. Working quickly with my free hand, I poured some epoxy on the dropout, and used the small brush to paint the epoxy onto every surface and into whatever nook and cranny there might have been present. Then I laid my carbon fiber cable on the chainstay, wrapped it through the guides and on to the seatstay in one simple move. Lightly pinching opposing ends, I carefully pulled my cable down snug against the dropout, and carefully splayed the ends smooth against the stays.

    As the assembly cured, I wiped off dripping epoxy and used leftover epoxy to repair an exposed section of the chainstay near the tire where my muddy had already worn away some paint.

    Step 10: Assembly, Testing, Cosmetic Layup

    While waiting for a complete cure, I straightened my derailleur hanger and plotted an improvement to the set screw design, determined to make sure I'd never suffer this kind of failure again. After drilling out the nut plate where the threaded boss had been sheared off, I put in a flathead screw that I had substantially weakened around the head, with a hacksaw. Now, if the derailleur were to ever get caught in the cassette, it's rotation would easily sever the weakened screw, preventing the screw from shearing through the dropout. I may desroy the derailleur, but, at least, save the frame.

    After an impatient re-assembly, I tested my repair on my home trails, moving slowly over rocks, dips, and ruts, carefully looking for cracks, changes in surface texture, and coloration, upping the ante after every inspection. With no visible damage to my repair, I trimmed and rounded some edges, added a thin fiber wrap on the chainstay, and smoothed the entire assembly with epoxy, making future damage easier to see. I have no plan to repaint the repair.

    After about 100 miles that has included high speed, deep washboard roads, some rock gardens, and 18" drops, I see no hint of cracking. Stay tuned for updates as I pile on the miles!