Development of Molding and Repair Technology for CFRP Chain Sprockets for Bicycles

Abstract

A 54-tooth CFRP chain sprocket for bicycles was molded using short 3K carbon fiber bundles impregnated with epoxy resin under a pressure of several kPa in a silicone rubber mold, with the carbon fibers oriented in a mountain shape toward the tooth tips. The weight is 50% lighter than the 7075 super duralumin chain sprocket. The developed CFRP chain sprocket was used to travel 3,500 km over a four-year period. Compared to the 7075 super duralumin chain sprocket, the CFRP chain sprocket required approximately 10% less watts when traveling at 20 km/h on flat ground, and approximately 25% less watts when climbing a 10% gradient. Experimental analysis showed that the CFRP tooth tips deform in the tooth width direction due to the chain tension, which makes it easier to climb hills by releasing the stress accumulated at the bottom dead center of the crank. All 54 teeth of the CFRP chain sprocket were worn, with 26% of the tooth tips chipped and 27% of the tooth tips having holes with a diameter of 0.3 to 0.5 mm. In addition, 22% of the tooth bases were chipped and 15% of the tooth bases had chips. There were holes. The tooth wear was greatest at ±45° to the crank axis. The CFRP chain sprocket also had low lateral rigidity, about 1/5 that of a 7075 super duralumin chain sprocket, and when shifting gears at the front, the chain sprocket deformed and the chain could not move, requiring three and a half turns to change gears. For comparison, a front single chain sprocket with 56 teeth machined from a CFRP plate was 280% heavier than a silicone rubber molded CFRP chain sprocket. A 1000km running test was conducted over a four-year period. All teeth were worn, but no chips or holes were observed. A repair technique was developed to reinforce the tooth base of a worn CFRP chain sprocket using a commercially available chain as a mold, with 3K carbon fiber bundles and epoxy adhesive.