Ortlieb Thru Axle M6-Connector – 1mm Black. An M6 thru axle is a robust and secure fastening system used to attach bicycle racks to bicycles. It provides enhanced stability and strength compared to traditional quick-release skewers ensuring that the rack remains firmly in place even under heavy loads. The M6 designation refers to the 6mm diameter of the axle making it compatible with a wide range of bike frames and racks.