The TRF421X D parts (suspension arms x2) can be used to replace a worn-out part for the TRF421X (Item 42391) or used as an upgrade to the standard TRF421 (item 42384). TRF421X D parts (suspension arms x2) Molded in carbon reinforced nylon to ensure suspension flexibility and to improve stability on hot temperature or high grip or carpet surfaces. Compatible with TRF421 (Item 42384) and TRF421X (Item 42391) chassis