An electric bicycle (e-bike, eBike, etc.) is a motorized bicycle with an integrated electric motor used to assist propulsion. Many kinds of e-bikes are available worldwide, but they generally fall into two broad categories: bikes that assist the rider's pedal-power (i.e. pedelecs) and bikes that add a throttle, integrating moped-style functionality. Both retain the ability to be pedaled by the rider and are therefore not electric motorcycles. E-bikes use rechargeable batteries and typically travel up to 25 to 32 km/h (16 to 20 mph). High-powered varieties can often travel more than 45 km/h (28 mph). In some markets, such as Germany as of 2013, they are gaining in popularity and taking some market share away from conventional bicycles, while in others, such as China as of 2010, they are replacing fossil fuel-powered mopeds and small motorcycles. Depending on local laws, many e-bikes (e.g., pedelecs) are legally classified as bicycles rather than mopeds or motorcycles. This exempts them from the more stringent laws regarding the certification and operation of more powerful two-wheelers which are often classed as electric motorcycles. E-bikes can also be defined separately and treated under distinct electric bicycle laws. In UK legislation the vehicles are called EAPC or Electrically Assisted Pedal Cycle, in EU legislation EPAC or Electrically Power Assisted Cycle. E-bikes are the electric motor-powered versions of motorized bicycles, which have been in use since the late 19th century. Some bicycle-sharing systems use them. E-bikes are zero-emissions vehicles, as they emit no combustion by-products, but the environmental effects of electricity generation and power distribution and of manufacturing and recycling batteries must be accounted for. Even with these issues considered, e-bikes have a significantly lower environmental impact than cars, and are generally seen as environmentally desirable in an urban environment.
A mountain bike (MTB) or mountain bicycle is a bicycle designed for off-road cycling. Mountain bikes share some similarities with other bicycles, but incorporate features designed to enhance durability and performance in rough terrain, which makes them heavier, more complex and less efficient on smooth surfaces. These typically include a suspension fork, large knobby tires, more durable wheels, more powerful brakes, straight, extra wide handlebars to improve balance and comfort over rough terrain, and wide-ratio gearing optimised for topography and application (e.g., steep climbing or fast descending). Rear suspension is ubiquitous in heavier-duty bikes and now common even in lighter bikes. Dropper posts can be installed to allow the rider to quickly adjust the seat height (an elevated seat position is more effective for pedaling, but poses a hazard in aggressive maneuvers). Mountain bikes are generally specialized for use on mountain trails, single track, fire roads, and other unpaved surfaces. Mountain biking terrain commonly has rocks, roots, loose dirt, and steep grades. Many trails have additional technical trail features (TTF) 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. Typical features of a mountain bike are very wide tyres. The original 26 inch wheel diameter with ~2.125" width (ISO 559 mm rim diameter) is increasingly being displaced by 29 inch wheels with ~2.35" width (ISO 622 mm rim diameter), as well as the 27.5 inch wheel diameter with ~2.25 widths (ISO 584 mm rim diameter). Mountain bikes with 24 inch wheels are also available, sometimes for dirt jumping, or as a junior bike. Bicycle wheel sizes are not precise measurements: a 29-inch mountain bike wheel with a 622 millimetres (24.5 in) bead seat diameter (the term, bead seat diameter (BSD), is used in the ETRTO tire and rim sizing system), and the average 29" mountain bike tire is (in ISO notation) 59-622 corresponds to an outside diameter of about 29.15 inches (740 mm). It can also be made into an electric mountain bike, which can better enjoy the thrill of speed.