Did you have an unfortunate accident on your dream bike that has left your rear wheel bent and unfixable. What is a Flip Flop Hub?. A flip flop hub has threads on both sides. Flip Flop 3/8. Wheel 20x175/2.125Rear Weinmann DW - W/M MX-4000 Flip Flop 3/8 axleto fit 3/8 Dropouts Black New.