GMC Topkick Dual Suspension Mountain Bike 21-speed mountain bike features a full-suspension, strong aluminum frame Floating beam suspension design is mated to a Zoom suspension fork with elastomer/spring technology Micro shift push shifters and Shimano TZ-30GS derailleur Strong aluminum wheelset featuring V-shape rim profile and high-flange alloy hubs Front and rear Promax disk brakes work great, even in adverse conditions