“The wheels just kind of fall apart,” said Chad Moore, the global brand manager for Mavic, a long-established wheel maker based in France. “It really just becomes an enormous safety issue for consumers.”

The rise of the knockoffs has been fueled by online retailing, the nearly universal adoption of carbon fiber for racing cycles and the general outsourcing of bicycle production to Asia, but the appeal is, quite simply, price. The use of carbon fiber in bicycle frames and wheels has raised the price of a high-end racing bike to the level of a modest used car. The frame and fork of the Specialized S-Works Tarmac bike that Contador has ridden on most stages at this year’s Tour have a list price of $4,000. Adding on the wheels and components similar to those that Contador uses bumps the final price to nearly $10,000.

Fake versions of Contador’s bike, or others now being ridden at the Tour, are about half that price on Chinese websites like DHGate and AliExpress, the consumer site of Alibaba, and they can be as little as $500.

Counterfeits of prestigious bike brands have a long history. But in the era of steel or aluminum frames, the deception was usually obvious even to an unskilled eye. The frames were usually substantially heavier than those they were imitating, the workmanship immediately obvious as inferior.

As their name might suggest, carbon fiber bicycles have almost as much to do with textiles as with traditional bike building. They are made of upward of 500 pieces of fabric woven from carbon fibers — a material pioneered by the aerospace industry — which are carefully built up in molds, impregnated with epoxy resins and then cured, usually with a combination of heat and pressure.