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 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.^[2] 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.^[3] (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,^[4] which allows clearance to mount a brake disc on the hub or to decrease the wheel dish for a more durable wheel.^[4] Freeride and downhill are available with both 142 and 150 mm spacing.^[5]

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.^[6] 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.^[6]

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.

The ISO 5775-2 standard defines designations for bicycle rims. It distinguishes between

1. Straight-side (SS) rims
2. Crochet-type (C) rims
3. Hooked-bead (HB) rims

Traditional clincher rims were straight-sided. Various "hook" (also called "crochet") designs emerged in the 1970s to hold the bead of the tire in place,^[9][10] allowing high (6–10 bar, 80–150 psi) air pressure.

Tubular or sew-up rims

Some rims are designed for tubular tires which are torus shaped and attached to the rim with adhesive. The rim provides a shallow circular outer cross section in which the tire lies instead of flanges on which tire beads seat.

Tubeless

A tubeless tire system requires an airtight rim — capable of being sealed at the valve stem, spoke holes (if they go all the way through the rim) and the tire bead seat — and a compatible tire. Universal System Tubeless (UST), originally developed by Mavic, Michelin and Hutchinson^[11] for mountain bikes is the most common system of tubeless tires/rims for bicycles.^[12] The main benefit of tubeless tires is the ability to use low air pressure for better traction without getting pinch flats because there is no tube to pinch between the rim and an obstacle.^[11]

Some cyclists have avoided the price premium for a tubeless system by sealing the spoke holes with a special rim strip or tape and then sealing the valve stem and bead seat with a latex sealer.^[11] However, tires not designed for the tubeless application do not have as robust a sidewall as those that are.^[11]

The drawbacks to tubeless tires are that they are notorious for being harder to mount on the rim than clincher tires,^[11] and that the cyclist must still carry a spare tube to insert in case of a flat tire due to a puncture.^[11]

French tire manufacturer Hutchinson has introduced a tubeless wheel system, Road Tubeless, that shares many similarities to the UST (Universal System Tubeless) that was developed in conjunction with Mavic and Michelin. Road Tubeless rims, like UST rims, have no spoke holes protruding to the air chamber of the rim. The flange of the Road Tubeless rim is similar to the hook bead of a standard clincher rim but is contoured to very close tolerances to interlock with a Road Tubeless tire, creating an airtight seal between tire and rim. This system eliminates the need for a rim strip and inner tube.

In 2006, Shimano and Hutchinson introduced a tubeless system for road bikes.^[13]

Cross section

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.^[15]

Material

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.^[17] Spokes are also available in titanium,^[17] aluminum,^[18] carbon fiber,^[17] and non-rigid materials such as polyethylene composites.^[19]

Number of spokes

Conventional metallic bicycle wheels have 24, 28, 32 or 36 spokes. Wheels on tandems and BMX often have 40 or 48 spokes to support additional stresses and weight. Lowrider bicycles may have as many as 144 spokes per wheel.^[20][21][22]

Wheels with fewer spokes have an aerodynamic advantage, as the drag is reduced. The reduced number of spokes also results in a larger section of the rim being unsupported, necessitating stronger and often heavier rims. Some wheel designs also locate the spokes unequally into the rim, which requires a stiff rim hoop and correct tension of the spokes. Conventional wheels with spokes distributed evenly across the circumference of the rim are considered more durable and forgiving to poor maintenance. The more general trend in wheel design suggests technological advancement in rim materials may result in further reduction in the number of spokes per wheel.

Lacing

Lacing is the process of threading spokes through holes in the hub and rim^[23] so that they form a spoke pattern.^[24] While most manufacturers use the same lacing pattern on both left and right sides of a wheel, it is becoming increasingly common to find specialty wheels with different lacing patterns on each side. A spoke can connect the hub to the rim in a radial fashion, which creates the lightest and most aerodynamic wheel.^[24] However, to efficiently transfer torque from the hub to the rim, as with driven wheels or wheels with drum or disc brakes, durability dictates that spokes be mounted at an angle to the hub flange up to a "tangential lacing pattern" to achieve maximum torque capability (but minimum vertical wheel stiffness).^[24] Names for various lacing patterns are commonly referenced to the number of spokes that any one spoke crosses.

Conventionally laced 36- or 32-spoke wheels are most commonly built as a cross-3 or a cross-2, however other cross-numbers are also possible. The angle at which the spoke interfaces the hub is not solely determined by the cross-number; as spoke count and hub diameter will lead to significantly different spoke angles. For all common tension-spoke wheels with crossed spokes, a torque applied to the hub will result in one half of the spokes - called "leading spokes" tensioned to drive the rim, while other half - "trailing spokes" are tensioned only to counteract the leading spokes. When forward torque is applied (i.e., during acceleration ), the trailing spokes experience a higher tension, while leading spokes are relieved, thus forcing the rim to rotate. While braking, leading spokes tighten and trailing spokes are relieved. The wheel can thus transfer the hub torque in either direction with the least amount of change in spoke tension, allowing the wheel to stay true while torque is applied.

Wheels that are not required to transfer any significant amount of torque from the hub to the rim are often laced radially.^[24] Here, the spokes leave the hub at perpendicular to the axle and go straight to the rim, without crossing any other spokes - e.g., "cross-0". This lacing pattern can not transfer torque as efficiently as tangential lacing. Thus it is generally preferred to build a crossed-spoke wheel where torque forces, whether driving or braking, issue from the hub. Where braking is concerned, the older-style caliper devices that contact the rims to apply braking force are not affected by lacing patterns in this way because braking forces are transferred from the calipers directly to the rim, then to the tires and then to the roadway. Disc brakes, however, transfer their force to the roadway via the spokes from the disc's mounting point on the hub and are therefore affected by the lacing pattern in a manner similar to that of the drive system.

Hubs that have previously been laced in any other pattern should not be used for radial lacing, as the pits and dents created by the spokes can be the weak points along which the hub flange may break. This is not always the case: for example if the hub used has harder, steel flanges like those on a vintage bicycle.

Wheel builders also employ other exotic spoke lacing patterns (such as "crow's foot", which is essentially a mix of radial and tangential lacing) as well as innovative hub geometries. Most of these designs take advantage of new high-strength materials or manufacturing methods to improve wheel performance. As with any structure, however, practical usefulness is not always agreed, and often nonstandard wheel designs may be opted for solely aesthetic reasons.

Adjustment ("truing")

There are three aspects of wheel geometry which must be brought into adjustment in order to true a wheel. "Lateral truing" refers to elimination of local deviations of the rim to the left or right of center. "Vertical truing" refers to adjustments of local deviations (known as hop) of the radius, the distance from the rim to the center of the hub. "Dish" refers to the left-right centering of the plane of the rim between the lock nuts on the outside ends of the axle. This plane is itself determined as an average of local deviations in the lateral truing.^[25] For most rim-brake bicycles, the dish will be symmetrical on the front wheel. However, on the rear wheel, because most bicycles accommodate a rear sprocket (or group of them), the dishing will often be asymmetrical: it will be dished at a deeper angle on the non-drive side than on the drive side.

In addition to the three geometrical aspects of truing, the overall tension of the spokes is significant to the wheel's fatigue durability, stiffness, and ability to absorb shock. Too little tension leads to a rim that is easily deformed by impact with rough terrain. Too much tension can deform the rim, making it impossible to true, and can decrease spoke life. Spoke tensiometers are tools which measure the tension in a spoke. Another common method for making rough estimates of spoke tension involves plucking the spokes and listening to the audible tone of the vibrating spoke. The optimum tension depends on the spoke length and spoke gauge (diameter). Tables are available online which list tensions for each spoke length, either in terms of absolute physical tension, or notes on the musical scale which coincide with the approximate tension to which the spoke should be tuned. In the real world, a properly trued wheel will not, in general, have a uniform tension across all spokes, due to variation among the parts from which the wheel is made.

Finally, for best, long-lasting results, spoke wind-up should be minimized. When a nipple turns, it twists the spoke at first, until there is enough torsional stress in the spoke to overcome the friction in the threads between the spoke and the nipple. This is easiest to see with bladed or ovalized spokes, but occurs in round spokes as well. If a wheel is ridden with this torsional stress left in the spokes, they may untwist and cause the wheel to become out of true. Bladed and ovalized spokes may be held straight with an appropriate tool as the nipple is turned. The common practice for minimizing wind-up in round spokes is to turn the nipple past the desired orientation by about a quarter turn, and then turn it back that quarter turn.^[26]

In wheel truing, all these factors must be incrementally brought into balance against each other. A commonly recommended practice is to find the worst spot on the wheel, and bring it slightly more into true before moving on to the next worst spot on the wheel.

"Truing stands" are mechanical devices for mounting wheels and truing them. It is also possible to true a wheel while it is mounted on the bike: brake pads or some other fixed point may be used as a reference mark, however this is less accurate.

700C road bicycle wheels / ISO 622 mm

Touring, race, and cyclo-cross bicycles may have vastly different design goals for their wheels. Aerodynamic performance and low weight are beneficial for road bicycles, while for cyclo-cross strength gains importance, and for touring bicycles, strength becomes more important again. However, this diameter of rim, identical in diameter to the "29er" rim, is by far the most common on these styles of bicycles. Road wheels may be designed for tubular or clincher tires, commonly referred to as "700C" tires.

650C triathlon bicycle wheels / ISO 571 mm

These wheels experienced a brief popularity in the 1990s on triathlon bikes.^[30]

650B gravel bicycle wheels / ISO 584 mm

In the late 2010s, 650B wheels began appearing on gravel bikes.^[30]

24 inch / ISO 507 mm

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 / ISO 559 mm

26-inch clincher tires (with inner tubes) were the most common wheel size for new mountain bikes until the early 2010s.^[31] 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 / ISO 584 mm

27.5-inch mountain bike wheels^{[32][33][34][35][36]} (which some also refer to as 650B^[37][38] 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 / ISO 622 mm

"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. 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.^[39]

36 inch / ISO 787

36-inch wheels have been developed to create bikes more proportional for taller riders.^[40]

20 inch / ISO 406 mm

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.

20 inch / ISO 451 mm

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.

26 inch

The common "26-inch" wheel used on mountain bikes and beach cruisers is an American size using a 559 mm rim, traditionally with hooked edges.

28 inch

Traditionally, there were four different sizes of 28-inch diameter wheels, from the narrow tires to the widest, they all measured the same outside diameter, which coincide with four different families of 700 tire sizes, these are 700, 700A, 700B and 700C. The largest of these rims (ISO 647mm/642mm) with the narrower tires are no longer available.^[37][47][48]