Mountain bikes are generally specialized for use on mountain trails, single track, fire roads, and other unpaved surfaces, although perhaps the majority of them are never used off pavement, and it is common to find hybrid road bikes based on “mountain bike” frames for sale. This type of terrain commonly has rocks, roots, loose dirt, and steep grades. Many trails have additional TTFs (Technical Trail Features) such as log piles, log rides, rock gardens, skinnies, gap jumps, and wall-rides. Mountain bikes are built to handle these types of terrain and features. The heavy-duty construction combined with stronger rims and wider tires has also made this style of bicycle popular with urban riders and couriers who must navigate through potholes and over curbs.