A bicycle wheel is a wheel, most commonly a wire wheel, designed for bicycle. A pair is often called a wheelset, especially in the context of ready built "off the shelf" performance-oriented wheels. Bicycle wheels are typically designed to fit into the frame and fork via dropouts, and hold bicycle tires. The first bicycle wheels followed the traditions of carriage building: a wooden hub, a fixed steel axle (the bearings were located in the fork ends), wooden spokes and a shrink fitted iron tire. A typical modern wheel has a metal hub, wire tension spokes and a metal or carbon fiber rim which holds a pneumatic rubber tire. [edit]Hub A hub is the center part of a bicycle wheel. It consists of an axle, bearings and a hub shell. The hub shell typically has two machined metal flanges to which spokes can be attached. Hub shells can be one-piece with press-in cartridge or free bearings or, in the case of older designs, the flanges may be affixed to a separate hub shell. [edit]Axle The axle is attached to dropouts on the fork or the frame. The axle can attach using a Quick release - a lever and skewer that pass through a hollow axle designed to allow for installation and removal of the wheel wit out any tools (found on most modern road bikes and some mountain bikes). Nut - the axle is threaded and protrudes past the sides of the fork/frame. (often found on track, fixed gear, single speed, BMX and inexpensive bikes) bolt - the axle has a hole with threads cut into it and a bolt can be screwed into those threads. (found on some single speed hubs, Cannondale Lefty hubs) Thru axle - a long axle, typically 20 mm (110 mm width), 9 mm (100.33 mm width) in diameter for durability, onto which the fork/frame clamps (found on most mountain bike forks). Female axle - hollow center axle, typically 14, 17, or 20 mm in diameter made of chromoly and aluminum, which two bolts thread into on either side. This design can be much stronger than traditional axles[citation needed]. (found on higher end BMX hubs and some mountain bike hubs) Modern[when?] bicycles have adopted standard axle spacing: the hubs of front wheels are generally 100 mm wide fork spacing, road wheels generally have a 130 mm wide rear wheel hub.[where?] Mountain bikes have adopted a 135 mm rear hub width[citation needed], which allows clearance to mount a brake disc on the hub or to decrease the wheel dish for a more durable wheel.