bicycle wheel

Invention

The first wheel to use the tension in metal spokes was invented by Sir George Cayley to achieve lightness in his 1853 glider.

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.

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)

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.

Spokes

The rim is connected to the hub by a number of spokes, which are rods. While early bicycle wheels used wooden spokes that could be loaded only in compression, modern bicycle wheels almost exclusively use spokes that can only be loaded in tension.

The rear wheel is subjected to greater stress because more weight is carried on the rear wheel. The rear wheel spokes on the right are more likely to fail. The rear wheels are asymmetrical to make room for multisprocket gear clusters. This asymmetry means that the spokes on the right are twice as tight as those on the left. The spokes break due to fatigue and not excessive force.

One end of each spoke is threaded for a specialized nut, called a nipple, which is used to connect the spoke to the rim and adjust the tension in the spoke. This is normally at the rim end. The hub end normally has a 90-degree bend to pass through the spoke hole in the hub, and a head so it does not slip through the hole. This is the J-bend type. Another type is straight pull spokes, which have no bend at the hub end, just a head. The major materials for spoke nipples are aluminum and brass.

Double-butted spokes have reduced thickness over the center section and are lighter, more elastic, and more aerodynamic than spokes of uniform thickness. Single-butted spokes are thicker at the hub and then taper to a thinner section all the way to the threads at the rim. Triple-butted spokes also exist and are thickest at the hub, thinner at the threaded end, and thinnest in the middle.

Spokes are usually circular in cross-section, but high-performance wheels may use spokes of flat or oval cross-section, also known as bladed, to reduce aerodynamic drag. Some spokes are hollow tubes.

The spokes on the vast majority of modern bicycle wheels are steel or stainless steel. Stainless steel spokes are favored by most manufacturers and riders for their durability, stiffness, damage tolerance, and ease of maintenance. Non-stainless steel spokes on older or cheaper bikes are sometimes surface-treated by galvanization, paint, or, more rarely, chrome plating, and can rust eventually. Spokes are also available in titanium, aluminum, carbon fiber, and non-rigid materials such as polyethylene composites.

Nipples

At one end of each spoke is a specialized nut, called a nipple, which is used to connect the spoke to the rim and adjust the tension in the spoke. The nipple is usually located at the rim end of the spoke but on some wheels it is at the hub end in order to move its weight closer to the axis of the wheel, reducing the moment of inertia. A variant of this is integrating nipples into the hub, its flange containing the threads for usually bladed spokes.

The most common materials used for bicycle nipples are brass and aluminum (often referred to as "alloy"). Brass nipples are heavier than aluminum, but they are more durable. Aluminum nipples save weight, but they are less durable than brass and more likely to corrode.

A nipple at the rim of a wheel usually protrudes from the rim towards the center of the wheel, but in racing wheels may be internal to the rim, offering a slight aerodynamic advantage.

Alternatives

A wheel can be formed in one piece from a material such as thermoplastic (glass-filled nylon in this case), carbon fiber or aluminium alloy. Thermoplastic is commonly used for inexpensive BMX wheels. They have a low maximum tire pressure of 45 psi (3bars or atmospheres). Carbon fiber is typically used for high-end aerodynamic racing wheels.

Disc wheels

Disc wheels are designed to minimize aerodynamic drag. A full disc is usually heavier than traditional spoke wheels, and can be difficult to handle when ridden with a cross wind. For this reason, international cycling organizations often ban disc wheels or limit their use to the rear wheel of a bicycle. However, international triathlon federations were (and are still) less restrictive and this is what led to the wheels' initial usage growth in popularity in the 1980s.

A disc wheel may simply be a fairing that clips onto a traditional, spoke wheel, addressing the drag that the spokes generate by covering them; or the disc can be integral to the wheel with no spokes inside. In the latter case, carbon fiber is the material of choice. A spoke wheel with a disc cover may not be legal under UCI (Union Cycliste Internationale) rules because it is a non-structural fairing while it may be legal under ITU (International Triathlon Union) rules.

A compromise that reduces weight and improves cross wind performance has a small number (three or four) tension-compression spokes molded integral to the rim – also typically carbon fiber.

Types

Bicycle wheels can be categorized by their primary use.

Road/racing bicycle wheels

For road bicycle racing performance there are several factors which are generally considered the most important:

aerodynamics

weight

rotational inertia

hub/bearing smoothness

stiffness

Semi-aerodynamic and aerodynamic wheelsets are now commonplace for road bicycles. Aluminum rims are still the most common, but carbon fiber is also becoming popular. Carbon fiber is also finding use in hub shells to reduce weight; however, because of the hub's proximity to the center of rotation reducing the hub's weight has less effect to the rotational inertia than reducing the rim's weight.

Semi-aerodynamic and aerodynamic wheelsets are characterized by greater rim depth, which is the radial distance between the outermost and the innermost surfaces of the rim; a triangular or pyramidal cross-section; and by fewer numbers of spokes, or no spokes at all—with blades molded of composite material supporting the rim. The spokes are also often flattened in the rotational direction to reduce wind drag. These are called bladed spokes. However, semi-aerodynamic and aerodynamic wheelsets tend to be heavier than more traditional spoked wheelsets due to the extra shapings of the rims and spokes. More importantly, the rims must be heavier when there are fewer spokes, as the unsupported span between spokes is greater. A number of wheel manufacturers are now producing wheels with roughly half the spokes of the highest performance traditional wheel from the 1980s, with approximately the same rotational inertia and less total weight. These improvements have been made possible primarily through improved aluminum alloys for the rims.

Most clincher carbon fiber wheelsets, such as those made by Zipp and Mavic, still use aluminum parts at the clinching part of the rim. An increased number of all-carbon rims, such as Campagnolo Hyperon Ultra Clincher, Viva v8 wheels, Bontrager's Carbon Clincher wheels, DT Swiss RRC1250, Corima Winium and Aero (also tubeless, see below) and Lightweight Standard C wheelsets are now available.

Mountain bike wheels

Mountain bike wheels are described by the approximate outer diameter of the rim plus a wide, ~2+ inch tire.

24-inch clincher tires (with inner tubes) are the most common wheel size for junior mountain bikes. The typical 24-inch rim has a diameter of 507 millimetres (20.0 in) and an outside tire diameter of about 24 inches (610 mm).

26-inch clincher tires (with inner tubes) were the most common wheel size for new mountain bikes until the early 2010s. This tradition was started initially because the early mountain bike pioneers procured the wheels for their early bikes from American-made bicycles rather than the larger European standards in use. The typical 26-inch rim has a diameter of 559 millimetres (22.0 in) and an outside tire diameter of about 26.2 inches (670 mm).

27.5-inch mountain bike wheels (which some also refer to as 650B use a rim that has a diameter of 584 mm (23.0″) with wide, knobby tires (≈27.5 ⋅ 2.3 / ISO 58-584) are approximately the midway point between the 26-inch (ISO-559mm) and the 29-inch (ISO-622mm) standards. They carry some of the advantages of both formats, with a smoother ride than a 26-inch wheel and more stiffness and durability than a 29″ wheel.

"29-inch wheels", which also conform to the popular 700C (622 mm diameter clincher) wheel standard are becoming more popular for not only cyclocross bikes but also cross-country mountain bikes. Their rim diameter of 622 millimetres (24+1⁄2 in) is identical to most road, hybrid, and touring bicycle wheels, but they are typically reinforced for greater durability in off-road riding and wider to accommodate significantly wider tires. The average 29-inch mountain bike tire is ISO 59-622 - corresponding to an outside diameter of about 29.15 inches (740 mm).

32 inch / ISO 686

32-inch wheels have been in use on unicycles and have been appropriated for bicycles to create bikes more proportional for taller riders.

36 inch / ISO 787

BMX wheels

There are two distinct wheel sizes that get described as 20 in., and both get used in the BMX sport.

Usually 20 inches in diameter (rim diameter of 406 mm), BMX wheels are small for several reasons: they are suitable for young and small riders; their lower cost is compatible with inexpensive bicycles; the size makes them stronger to withstand the additional loads generated by BMX jumps and stunts; and to reduce rotational inertia for easier wheel acceleration.

Nominally 20 x 1-1/8″ or 20 x 1-3/8″, with rim diameter 451 mm. These are intended for racing by lightweight BMX riders, and sometimes referred to as "skinnies". The size is also used on classic British folding or shopping bikes.

Technical aspects

Sizes

Bicycle rims and tires came in many different types and sizes before efforts were made to standardize and improve wheel/tire compatibility. The International Organization for Standardization (ISO) and the European Tyre and Rim Technical Organisation (ETRTO) define a modern, unambiguous system of sizing designations and measurement procedures for different types of tires and rims in international standard ISO 5775. For example:

For wired-edge tires the ISO designation lists the width of the inflated tire and the "bead-seat diameter", both in millimeters and separated by a hyphen: 37-622. The bead seat diameter (BSD) is the diameter of the surface of the rim upon which the tire bead sits.

For rims the ISO designation lists the rim's bead seat diameter and the rim's inner width, both in millimeters and separated by a cross, along with a letter code for the rim type (e.g., "C" = Crochet-type): 622x19C

In practice, most tires (and inner tubes) sold today carry, in addition to the modern ISO 5775-1 designation, some historic size markings, which are still widely used:

an old French tire designation that was based on the approximate outer diameter of the inflated tire in millimeters. The Japanese Industrial Standard JIS D 9112 continues to provide an official definition for the French tire designations. For example: 700×35 C.

an old British inch-based designation: 597 mm (26 × 1+1⁄4), 590 mm (26 × 1+3⁄8), 630 mm (27 × 1+1⁄4), and 635 mm (28 × 1+1⁄2)

Which designation is most popular varies with region and type of bicycle. For a comprehensive equivalence table between old and new markings, see the ISO 5775 article, the table in Annex A of the ISO 5772 standard, as well as Tire Sizing by Sheldon Brown.

Most road and racing bicycles today use 622 mm diameter (700C) rims, though 650C rims are popular with smaller riders and triathletes. The 650C size has the ISO diameter size of 571 mm. Size 650B is 584 mm and 650A is 590 mm. 650B is being promoted as a 'best of both worlds' size for mountain biking. Most adult mountain bikes use 26 inch wheels. Smaller youth mountain bikes use 24 inch wheels. The larger 700C (29 inch) wheels have enjoyed some recent popularity among off-road bicycle manufacturers. These rims are the same bead seat diameter as 700C wheels and are generally compatible with bicycle frames and tires designed for the 700C standard, however, rims designated as 29 inch are designed for wider tires than rims designated 700C, so frame clearance may be an issue. The formerly popular 27 inch (630 mm) wheel size is now rare.

Rolling resistance

There are a number of variables that determine rolling resistance: tire tread, width, diameter, tire construction, tube type (if applicable), and pressure are all important.

Smaller diameter wheels, all else being equal, have higher rolling resistance than larger wheels. "Rolling resistance increases in near proportion as wheel diameter is decreased for a given constant inflation pressure."

Previously mountain bikes were based on 26 inch wheels but modern mountain bikes are often with 29 inch (622mm) wheels resulting in lower rolling resistance but changes to manoeuverability.

Rotating mass

Due to the fact that wheels rotate as well as translate (move in a straight line) when a bicycle moves, more force is required to accelerate a unit of mass on the wheel than on the frame. In wheel design, reducing the rotational inertia has the benefit of more responsive, faster-accelerating wheels. To accomplish this, wheel designs are employing lighter rim materials, moving the spoke nipples to the hub or using lighter nipples such as aluminum. Note however that rotational inertia is a factor only during acceleration (and deceleration/braking). At constant speed, aerodynamics are a significant factor. For climbing, total mass remains important. See Bicycle performance for more detail.

Dish

The hub flanges of modern tension-spoked bicycle wheels are always spaced wider than where the spokes attach to the rim. When viewed in cross section, the spokes and hub form a triangle, a structure that is stiff both vertically and laterally. In three dimensions, if the spokes were covered (visualize paper covering the spokes on each side), they would form two cones or "dishes." The greater the separation between the hub flanges, the deeper the dishes, and the stiffer and stronger the wheel can be laterally. The more vertical the spokes, the shallower the dish, and the less stiff the wheel will be laterally.

The dishes on each side of a wheel are not always equal. The cogset (freewheel or cassette) of a rear wheel and disc brake rotors, if installed, takes up width on the hub, and so the flanges may not be located symmetrically about the center plane of the hub or the bike. Since the rim must be centered, but the hub flanges are not, there is a difference in dish between the two sides. Such an asymmetrical wheel is called a "dished" wheel. The side of the wheel with less dish has slightly shorter but significantly higher-tensioned spokes than the side with more dish. Several different techniques have been tried to minimize this spoke asymmetry. In addition to modified hub geometry, some rims have off-center spoke holes, and the mounting of common J-bend spokes at the hub flange can be altered "inboard" or "outboard".

A truing stand or a dishing gauge, can be used to measure the position of the rim relative to the hub. Thus "dishing" is also used to describe the process of centering the rim on the hub, even in the case of symmetrical wheels.

Stiffness

The stiffness of a bicycle wheel can be measured in three primary directions: radial, lateral, and torsional. The radial stiffness is primarily a measure of how well the wheel absorbs bumps from the surface on which it rolls. Lateral stiffness, especially of the front wheel influences the handling of the bicycle. Torsional, or tangential stiffness is a measure of how well the wheel transmits propulsive and braking forces, if applied at the hub, as in the case of hub or disc brakes.

Several factors affect these stiffnesses to varying degrees. These include wheel radius, rim bending and torsional stiffness, number of spokes, spoke gauge, lacing pattern, hub stiffness, hub flange spacing, hub radius. In general lateral and radial stiffness decreases with the number of spoke crossings and torsional stiffness increases with the number of spoke crossings. One factor that has little influence on these stiffnesses is spoke tension.

Too much spoke tension, however, can lead to catastrophic failure in the form of buckling. The "most significant factor affecting the lateral spoke system stiffness" is the angle between the spokes and the wheel midplane. Thus any change that increases this angle, such as increasing the width of the hub, while keeping all other parameters constant, increases the resistance to buckling.