GKTech - REAR SUBFRAME SLIP-IN COLLARSThe GKTECH rear subframe collars are used to eliminate axle tramp and to also tighten up the rear end giving more precise control of the vehicle under harsh driving conditions. CNC Machined 8 Piece Set FAQS Whats the advantage of the slip in collars? The slip in collars sit above and below your standard subframe bushes and essentially compact them. This results in less movement of the subframe and eliminates axle tramp and increases stiffness in the