R31House Alum. GRK5 Rear A Arm Knuckle - RedR31House (#R31S326RD) Alum. GRK5 Rear A Arm Knuckle Red Feature: For: R31House GRK5; GRK4 Global 2 GRK M GRK GS2 RWD compatible with optional parts. The aluminum knuckle for the rear A arm, which was first adopted as standard on the GRK4, has been slightly changed to match the GRK5. In order to correspond to the geometry adopted in GRK5, SHIBATA have optimized the height of the toe control mounting part and reviewed the rigidity of the knuckle body.