The 50/50 Prime Frames are great for flat and even better for anti-rockered setups. They use 50/50's Core System 2 which has extruded aluminum in the core of the frames with injection molded glass-filled nylon walls. This makes them very responsive but also easy to grind on the outside. The system is also designed to be used with an anti-rockered setup as all flat. They have single-sided 8mm axles with spacers included and the frames come in two different lengths.