From the Manufacturer

This is a set of optional ST Racing Concepts Aluminum Oversized Rear Hub Carriers with Bearings. Using high quality CNC machined aluminum, these oversized Aluminum rear hub carriers greatly increase the strength of your rear suspension, while adding great looks to your Traxxas electric Stampede, Rustler, Bandit or Slash. Because of the increase in size of bearing for durability, ST Racing has included a full set of 5x11mm ball bearings (replacing the stock 5x9mm bushing) for improved efficiency and durability of the rotating axle.