The Pulsar pulley set is our fastest set of replacement wheels, running the same full ceramic bearings as the Kolossos Pulsar cage: ceramic balls turning in ceramic races, not steel. No metal means nothing to corrode and no lubrication at all, so a 30 second free spin on the lower wheel becomes standard. Aluminum cogs and full ceramic bearings, finished in black. Please note: compatible ONLY with Kolossos Oversized Derailleur Cages, and available with road seals. Speed Kolossos takes the 20T lower wheel, Aero Kolossos the 19T. Shimano runs a 12T upper, SRAM AXS a 14T. We do not recommend Pulsar for off road use.