Technical FAQ: Tour de France timing transponders
Question: While watching the Tour de France this year I noticed a piece of hardware on the riders’ bikes that I did not recognize attached to the non-drive-side rear chainstay. It was a red circular tab that hung down, usually two-thirds the way back toward the dropout. Several times a motorcycle cameraman would zoom in on the little device, but I never heard any explanation on what it was.
It’s a timing transponder