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.
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 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 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 - 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 20 mm (110 mm width), 9 mm (100.33 mm 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 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. (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 100 mm wide fork spacing, road wheels generally have a 130 mm wide rear wheel hub. Off-road and "mountain" bikes have adopted a 135 mm 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.
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. Older designs used "cup and cone", whereas some modern wheels utilize pre-assembled "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
The hub shell is the part of the hub to which the spokes (or disc structure) attach. The hub shell of spoked wheels generally have 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 the 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.
Hub brakes
Some hubs have attachments for disc brakes or form an integral part of drum brakes.
- Disc brakes - a disc brake comprises 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 hubs have 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 and/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. This is an extremely robust design, although much heavier than more modern designs of multi-gear-ratio arrangements. Modern hubs are available from three-speed to 14 speeds or a continuously variable transmission hub , in the case of the NuVinci.
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 rim is designed for use with rod-actuated brakes, which press against the inside surface of the 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.
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.
Clincher rims
Most bicycle rims are "clincher" rims for use with clincher tires. These tires have a wire or aramid (Kevlar) 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
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