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 2 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 as separate hub shell.
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 without any tools (found on most modern road and mountain bikes).
* nut - the axle is threaded and protrudes past the edges 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 20mm (110mm width), 9mm (100.33mm width) in diameter for durability, onto which the fork/frame clamps. (found on MOST free ride and downhill mountain bikes)
* female axle - hollow center axle, typically 14, 17, or 20mm in diameter made of chromoly and aluminum, which two bolts thread into on either side. This design can be much stronger than traditional axles. (found on higher end BMX hubs and some mountain bike hubs)
Modern bicycles have adopted standard axle spacing: the hubs of front wheels are generally 100mm wide fork spacing, road wheels generally have a 130mm wide rear wheel hub. Off-road and "mountain" bikes have adopted a 135mm rear hub width, which allows clearance to mount a brake disc on the hub or to increase the wheel dish for a more durable wheel.
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 without any tools (found on most modern road and mountain bikes).
* nut - the axle is threaded and protrudes past the edges 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 20mm (110mm width), 9mm (100.33mm width) in diameter for durability, onto which the fork/frame clamps. (found on MOST free ride and downhill mountain bikes)
* female axle - hollow center axle, typically 14, 17, or 20mm in diameter made of chromoly and aluminum, which two bolts thread into on either side. This design can be much stronger than traditional axles. (found on higher end BMX hubs and some mountain bike hubs)
Modern bicycles have adopted standard axle spacing: the hubs of front wheels are generally 100mm wide fork spacing, road wheels generally have a 130mm wide rear wheel hub. Off-road and "mountain" bikes have adopted a 135mm rear hub width, which allows clearance to mount a brake disc on the hub or to increase the wheel dish for a more durable wheel.
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