AtomicRC's Optional/Upgrade UPPER ARM MOUNT, aluminum, 0, 2 & 4 degrees of additional caster, GOLD. Caster is known to add steering traction, "0" is standard and works well on small, slow tracks..."2" offers slightly more steering traction and is used on slightly longer and faster tracks as well as faster, longer tracks. "4"is used on all tracks with assorted infields and straight a ways...the trick is to have all three for: specific track conditions. Easy to change out. EACH SOLD SEPERATLY