SFM135 Forever Mountain Bike is a bike designed for off-road cycling. Mountain bikes have some similarities to other bicycles, but incorporate features designed to increase durability and performance in rough terrain, which makes them heavier, more complex and less efficient on slippery surfaces. These typically include suspension forks, knobby tires, more durable wheels, stronger brakes, straight, extra-wide handlebars for improved balance and comfort on rough terrain, and wide teeth optimized for terrain ratio gearing and application (e.g., steep climbs or fast descents). Rear suspension is ubiquitous in heavy bikes, and now even common in light bikes. A drop bar can be installed to allow the rider to quickly adjust the seat height (a raised seat position is more efficient for pedaling, but can be dangerous in aggressive maneuvers).

SFM135 Forever Mountain Bike is usually dedicated to mountain roads, single track, fire roads and other unpaved roads. Mountain biking terrain often has rocks, tree roots, loose dirt, and steep grades. Many trails have additional Technical Trail Features (TTFs) such as log posts, log rides, rock gardens, leans, gap jumps and wall rides. Mountain bikes are designed to handle these types of terrain and features. The heavy-duty construction combined with stronger rims and wider tires also made this style of bike popular with urban riders and couriers who had to negotiate potholes and curbs.

Since the development of mountain biking in the 1970s, many new subtypes of the SFM135 Forever Mountain Bike have evolved, such as cross-country (XC), enduro, all-mountain, freeride, downhill, and various track and slalom types. Each places different demands on the bike and requires a different design for optimum performance. The evolution of MTB has led to increases in suspension travel, now typically up to 8 inches (200 mm), and can go up to 13 speeds to facilitate climbing and fast descents. Advances in gearing have also led to the ubiquity of "1x" drivetrains (pronounced "one-by"), which reduce the gearing to just one sprocket in front and various cassettes in the rear, usually nine to 12 sprockets. 1x gearing reduces overall bike weight, increases ground clearance, and simplifies the gear selection process, but 2 or 3 ring drivetrains are still common on entry-level bikes.