Front/Rear Active Differential set for XB4. The unique XRAY Active Differential is a high-performance, adjustable front gear differential for XB4 ... vastly improving the buggy\'s speed and handling characteristics. The special design of the internal components using different angled segments allows the diff gears to engage on-power to increase the forward traction, making it faster and easier to drive. While the XRAY Active Differential is a performance option for serious racers who want to gain an edge on superior speed and handling, it is also of great benefit to drivers of all levels that want a more predictable and easy-to-drive car in all racing conditions. Ball-bearings not included Fits XB4\'20-21 Notice: When buidling the Active Diff always use all 4 internal satellite gears. Never use only 2 internal satelitte gears. (click to enlarge) (click to enlarge)