THOUGH STEEL, aluminum, wood, bamboo and magnesium alloy have all been used to craft stylish bike frames, carbon fiber is today the go-to for serious cyclists with lots of cash. Glued and reinforced by epoxy resin, it can now be mass-produced and shaped to achieve an optimum balance between weight, comfort, aerodynamic performance and a mystic quality known in cycling jargon as “stiffness” that efficiently transfers power from your legs to the wheels.

Lately, however, a carbon-fiber alternative—first used to shape posh frames…