This is a replacement RPM Spline Drive Adapter Set, and is intended for use with RPM Spline Drive wheels. The RPM “Spline Drive” system is radically different than the standard hex drive. A hex is easy to machine and easy to manufacture, which is why it is so commonly used. However, splines are tougher and stronger, which is why they’re used in full-sized driveline components. All standard hex style adapters push out and up every time rotational force is applied to the adapter, which is a huge reason why so many hex-style adapters spin. RPM Spline Drives direct the rotational force of the axle in a near perpendicular direction, making the likelihood of a spun adapter virtually non-existent!