This is an optional Traxxas HD Cold Weather Rear Suspension Arm Set, a heavy duty set of rear suspension arms for 2WD Slash, Rustler and F-150 Raptor feature an updated design which incorporates the latest proprietary composite material, available in multiple colors. Traxxas designed this composite specifically for high loads and hard landings with just the right amount of flex to resist breaking—even in cold weather. Whether you’re sending it off a curb, or preparing for the next speed run, these arms were built for extreme wear.