Mongoose Impasse HD 29" Wheel Mountain Bicycle, 18" Frame Size Aluminum hardtail frame with front suspension fork and rear disc mounts is durable and responsive for strong trail riding 21-speed SRAM twist shifters with Shimano rear derailleur for smooth shifting everywhere you go Front and rear mechanical disc brakes ensure quick stops and precise speed control down the mountain Light and durable alloy rims support knobby mountain tires 3-piece alloy crank ensures optimal gearing and less pesky maintenance on your end