A bicycle wheel is a wheel, most commonly a wire wheel, designed for a 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. A typical modern wheel has a metal hub, wire tension spokes and a metal or carbon fiber rim. The hub consists of an axle, bearings and a hub shell.
Invention The first wheel to use the tension in metal spokes was invented by Sir George Cayley to achieve lightness in his 1853 glider.In his notebook, dated March 19, 1808, Cayley proposed that in order to produce "the lightest possible wheel for aerial navigation cars," one should "do away with wooden spokes altogether and refer the whole firmness of the wheel to the strength of the rim only, by the intervention of tight strong cording … " See: J.A.D. Ackroyd (2011) "Sir George Cayley: The invention of the aeroplane near Scarborough at the time of Trafalgar," Journal of Aeronautical History [Internet publication], paper no. 6, pages 130–181. Cayley's tension-spoke wheel appears on page 152, "3.7 The Tension Wheel, 1808".
Construction 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.
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.
Axle The axle is attached to dropouts on the fork or the frame. The axle can attach using a: Quick release skewer - 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 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 removable axle with a threaded end that is inserted through a hole in one fork leg, through the hub, and then screwed into the other fork leg. Some axles have integrated cam levers that compress axle elements against the fork leg to lock it in place, while others rely on pinch bolts on the fork leg to secure it. Diameters for front thru axles include 20 mm, 15 mm, 12 mm, and 9 mm. Rear axles typically have diameters of 10 or 12 mm. Most thru axles are found on mountain bikes, although increasingly disc-braked cyclocross and road bikes are using them. Thru axles repeatably locate the wheel in the fork or frame, which is important to prevent misalignment of brake rotors when using disc brakes. Unlike other axle systems (except Lefty), the thru axle is specific to the fork or frame, not the hub. Hubs/wheels do not include axles, and the axle is generally supplied with the fork or frame. Adapters are usually available to convert wheels suitable for a larger thru axle to a smaller diameter, and to standard 9mm quick releases. This allows a degree of re-use of wheels between frames with different axle specifications. Female axle - hollow center axle, typically 14, 15, 17, or 20 mm in diameter made of chromoly and aluminum, with two bolts thread into on either side. This design can be much stronger than traditional axles, which are commonly only 8 mm, 9 mm, 9.5 mm, or 10 mm in diameter. (found on higher end BMX hubs and some mountain bike hubs)
Since the 1980s, bicycles have adopted standard axle spacing: the hubs of front wheels are generally 100 mm wide fork spacing, road wheels with freehubs generally have a 130 mm wide rear wheel hub. Mountain bikes have adopted a 135 mm rear hub width, which allows clearance to mount a brake disc on the hub or to decrease the wheel dish for a more durable wheel. Freeride and downhill are available with both 142 and 150 mm spacing.
Bearings The bearings allow the hub shell (and the rest of the wheel parts) to rotate freely about the axle. Most bicycle hubs use steel or ceramic ball bearings. Some hubs use serviceable "cup and cone" bearings, whereas some use pre-assembled replaceable "cartridge" bearings.
A "cup and cone" hub contains loose balls that contact an adjustable 'cone' that is screwed onto the axle and a 'race' that is pressed permanently into the hub shell. Both surfaces are smooth to allow the bearings to roll with little friction. This type of hub can be easily disassembled for lubrication, but it must be adjusted correctly; incorrect adjustment can lead to premature wear or failure.
In a "cartridge bearing" hub, the bearings are contained in a cartridge that is shaped like a hollow cylinder where the inner surface rotates with respect to the outer surface by the use of ball bearings. The manufacturing tolerances, as well as seal quality, can be significantly superior to loose ball bearings. The cartridge is pressed into the hub shell and the axle rests against the inner race of the cartridge. The cartridge bearing itself is generally not serviceable or adjustable; instead the entire cartridge bearing is replaced in case of wear or failure.
Hub shell and flanges The hub shell is the part of the hub to which the spokes (or disc structure) attach. The hub shell of a spoked wheel generally has two flanges extending radially outward from the axle. Each flange has holes or slots to which spokes are affixed. Some wheels (like the Full Speed Ahead RD-800) have an additional flange in the center of the hub. Others (like some from Bontrager and Zipp) do not have a noticeable flange. The spokes still attach to the edge of the hub but not through visible holes. Other wheels (like those from Velomax/Easton) have a threaded hub shell that the spokes thread into.
On traditionally spoked wheels, flange spacing affects the lateral stiffness of the wheel, with wider being stiffer, and flange diameter affects the torsional stiffness of the wheel and the number of spoke holes that the hub can accept, with larger diameter being stiffer and accepting more holes. Asymmetrical flange diameters, tried to mitigate the adverse effects of asymmetrical spacing and dish necessary on rear wheels with many sprockets, have also been used with modest benefits.
Hub brakes Some hubs have attachments for disc brakes or form an integral part of drum brakes. Disc brakes – a disc brake comprises a circular plate or disc attached to the hub which is squeezed between brake pads mounted within a caliper that is fixed to one side of the wheel forks. The brake disc can be attached in a variety of ways using bolts or a central locking ring. Drum brakes – a drum brake has two brake shoes that expand out into the inside of the hub shell. Rear-mounted drum brakes are often used on tandems to supplement the rear rim brake and give additional stopping power. Coaster brake – coaster brakes are a particular type of drum brake which is actuated by a backward pressure applied to the pedals. The mechanism is contained inside the bicycle wheel hub shell. For information on other types of bicycle brakes see the full article on bicycle brake systems.
Gears The rear hub has one or more methods for attaching a gear to it. Freehub – The mechanism that allows the rider to coast is built into the hub. Splines on the freehub body allow a single sprocket or, more commonly, a cassette containing several sprockets to be slid on. A lock ring then holds the cog(s) in place. This is the case for most modern bicycles. Freewheel – The mechanism that allows the rider to coast is not part of the hub, it is contained in a separate freewheel body. The hub has threads that allow the freewheel body to be screwed on, and the freewheel body has threads or splines for fitting sprockets, or in the case of most single speed freewheels an integral sprocket. This style of hub was used before the freehub became practical. Track sprocket – There is no mechanism that allows the rider to coast. There are two sets of threads on the hub shell. The threads are in opposite directions. The inner (clockwise) set of threads is for a track sprocket and the outer (counter-clockwise) set is for a reverse threaded lock ring. The reverse threads on the lock ring keep the sprocket from unscrewing from the hub, which is otherwise possible when slowing down. Flip-flop hub – Both sides of the hub are threaded, allowing the wheel to be removed and reversed in order to change which gear is used. Depending on the style of threads, may be used with either a single-speed freewheel or a track sprocket. Internal geared hub – the mechanism to provide multiple gear-ratios is contained inside the shell of the hub. Many bicycles with three-speed internally geared hubs were built in the last century.
Rim
The rim is commonly a metal extrusion that is butted into itself to form a hoop, though may also be a structure of carbon fiber composite, and was historically made of wood. Some wheels use both an aerodynamic carbon hoop bonded to an aluminum rim on which to mount conventional bicycle tires.
Metallic bicycle rims are now normally made of aluminium alloy, although until the 1980s most bicycle rims - with the exception of those used on racing bicycles - were made of steel and thermoplastic.
Rims designed for use with rim brakes provide a smooth parallel braking surface, while rims meant for use with disc brakes or hub brakes sometimes lack this surface.
The Westwood pattern rim was one of the first rim designs, and rod-actuated brakes, which press against the inside surface of the rim were designed for this rim. These rims cannot be used with caliper rim brakes.
The cross-section of a rim can have a wide range of geometry, each optimized for particular performance goals. Aerodynamics, mass and inertia, stiffness, durability, tubeless tire compatibility, brake compatibility, and cost are all considerations. If the part of the cross-section of the rim is hollow where the spokes attached, as in the Sprint rim pictured, it is described as box-section or double-wall to distinguish it from single-wall rims such as the Westwood rim pictured. The double wall can make the rim stiffer. Triple-wall rims have additional reinforcement inside the box-section.
Aluminum rims are often reinforced with either single eyelets or double eyelets to distribute the stress of the spoke. A single eyelet reinforces the spoke hole much like a hollow rivet. A double eyelet is a cup that is riveted into both walls of a double-walled rim.
Bicycle rims utilizing inner tubes also require rim tapes or strips, a flexible but tough liner (usually rubber or woven nylon or similar material) attached to the inner circumference of the wheel to cover the ends of the nipples. Otherwise, the nipple ends wear a hole in the tube causing a flat tire.
Clincher rims
Most bicycle rims are "clincher" rims for use with clincher tires. These tires have a wire or aramid (Kevlar or Twaron) fiber bead that interlocks with flanges in the rim. A separate airtight inner tube enclosed by the rim supports the tire carcass and maintains the bead lock. If the inner part of the rim where the inner tube fits has spoke holes, they must be covered by a rim tape or strip, usually rubber, cloth, or tough plastic, to protect the inner tube.
An advantage of this system is that the inner tube can be easily accessed in the case of a leak to be patched or replaced.