Mongoose 24 in. Mountain Bicycle, 21 Speed, Teal Mongoose 24" Flatrock Mountain Bike, Hardtail, 21 Speeds. Explore rad new trails with the Flatrock mountain bike by Mongoose. The Flatrock comes equipped with a lightweight aluminum hardtail frame and Element suspension fork that are ready to tear down the trail. 21-speed twist shifters and rear derailleur provide effortless gear changes, while alloy v-brakes deliver reliable stopping power. Plus, double-wall alloy rims offer lightweight durability. Get out there and explore with the Mongoose Flatrock.