Hello from Predator Cycling. This Instructables shows you how to repair a small crack in a carbon fiber bicycle frame. We also encourage our readers to visit our supplemental Youtube video at predatorcycling.com/go/carbon-repair-kit where we also offer a complete kit to help provide the tools to assist with this repair.
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Step 1: Inspect Your Frame
Inspect the damaged area of your bike frame. Check for soft spots, chips, bubbles or areas where the paint and carbon are raised. The tap test is a helpful way to determine unseen cracks. This video shows you how to complete a tap test with a coin.
Step 2: Mask Off Your Frame
Using masking tape or painters tape, mask off two inches around the damaged area of your frame. Next, use plastic wrap, newspaper, or an equivalent material to tape and wrap off the rest of the undamaged frame. The idea is to protect the frame from water, epoxy, carbon dust, and other spills, leaks, or messes.
Step 3: Begin Wet Sand of Damaged Material
Put on gloves and safety glasses / mask and begin to wet sand away all clear coat and/or paint around the crack/damage within the masked-off area in need of repair. We sanding utilizes water, so spray our pour water on both the carbon repair area and sandpaper throughout the sanding process. You will begin to notice the water turning black indicating you have reached the carbon tubing.
Step 4: Continue Wet Sand and Filing
Remove all damaged composite material using 120 wet/dry sandpaper OR a course rat tail file for thicker areas.
Step 5: Sanitize the Area
Continue to wipe the damaged area with rags, sterile cleaning pads, or acetone. After the area is clean, be sure to re-wrap the edges of the repair area where it meets the plastic wrap with electricians tape so that epoxy does not seep through to the rest of the bike going forward.
Step 6: Lay Carbon Weave
You are now ready to lay your carbon weave. Analyze your current carbon frame layup so that you match the direction of the carbon weave to the weave of the loose new carbon. Plan out your repair pieces by measuring how many carbon plies/pieces you will need to cut out from the fabric. Cut out carbon pieces with scissors that will cover the cracked/damaged area with an excess of about .5 inches to 1 inches around. Use as many carbon pieces as needed to build up the area over the damage to match the original thickness of the existing frame. Cut ONE extra layer of carbon sheet to lay around the repaid to give you extra thickness for wet sanding later.
Step 7: Mix the Epoxy
Next, mix the epoxy. Read the mixing instructions for your epoxy to hardener ratios. Follow all instructions. Stir the contents in a mixing cup with an acid brush for approximately one minute.
Step 8: Saturate the Repair Area With Epoxy
Using an acid brush, apply the epoxy/hardener mix to the repair area on the frame. Brush on a light thin coat of the mix. Lay just enough so that the area has a nice gloss shine. Do no over-epoxy the area.
Step 9: Saturate Carbon Fabric With Epoxy
Using the carbon pieces that you cut out earlier, begin brushing epoxy/hardener mix on each square, saturating the weave. Place the saturated pieces on the repair area, lining the first sheet to match the carbon weave of your existing frame. If the first sheet is at a 90 degree angle, every sheet after should be oriented at 45 degrees. Taper the sizes starting with the smallest piece to create evenness throughout the repair. Later when dry, wet sanding will even and blend all of the areas.
Step 10: Apply Compression
Using compression tape, wrap tightly around the repair area. Clamp the end of the tape using a utility or spring clamp to the frame to hold the tape secure. Make sure you have a rag or towel on hand for messes. Using a heat gun or hair dryer, heat the tape all the way around to create compression around the repair area. Beware of overheating and burning through the tape.
Once the tape is compressed, let dry/cure in a hot spot between 60/90 degrees F. We recommend placing the repaired frame in front of a small heater. Let dry for 24 hours, but please ensure the heater is operated safely.
Step 11: Unwrap and Inspect Repair
After the repair has dried properly, unwrap and inspect the repaired area. Using 120 sandpaper, lightly wet sand off excess epoxy. Blend the repair area to match the original frame shape by wet sanding with 120, then 220, then 320 sandpapers. Be sure to sand just enough so that the repair is smooth. Do not sand down too far.
Step 12: Complete Repair
After the repair as been wet sanded smooth, your repair is now done. Repairs can be finished by either paint match finish, clear coat, or by using vinyl sticker decals to cover up areas. Please visit your local bike shop or paint supply store to create your own finishing technique.