Haven't seen them, but they might be like the fluo triangles on truck wheels so they can tell if a bolt becomes loose.
@Emyr started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
Haven't seen them, but they might be like the fluo triangles on truck wheels so they can tell if a bolt becomes loose.