Bicycle wheel

From Wikipedia, the free encyclopedia

A bicycle wheel is a 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 fit into the frame and fork via dropouts.

Contents 1 Construction 1.1 Hub 1.1.1 Axle 1.1.2 Bearings 1.1.3 Hub shell 1.1.3.1 Hub Brakes 1.1.3.2 Gears 1.2 Rim 1.2.1 Clincher rims 1.2.2 Tubular or sew-up rims 1.3 Spokes 1.3.1 Adjustment 1.3.2 Radially or tangentially 1.3.3 Quantity 1.3.4 Form 1.3.5 Material 1.4 Alternatives 1.4.1 Disc wheels 2 Types 2.1 Road/racing bicycle wheels 2.2 Mountain bike wheels 2.2.1 26 inch wheel / 559 mm rim 2.2.2 29 inch wheel / 622 mm rim 2.3 Touring bicycle wheels 2.4 BMX wheels 3 Technical aspects 3.1 Sizes 3.2 Rolling resistance 3.3 Reaction to load 3.4 Rotating mass 3.5 Dish 4 See also 5 References 6 External links

[edit] Construction

The first bicycle wheels followed the traditions of carriage building, using a wooden hub, with a fixed steel axle (the plain bearings for which being 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 rim which locates a pneumatic tire.

[edit] Hub

A hub is the center part of a bicycle wheel. It consists of an axle, bearings and a hub shell. It supports (usually the head, but latterly the threaded ends of) the spokes; this is called the head shell or flange. Most hubs have 2 flanges and the hub assembly can be steel or aluminium (although composite and more exotic materials have been used). With steel hubs, the flanges are usually separate from the hub body and contain the bearings; on alloy (aluminium) hubs, the body and flanges form one unit and contain press-fitted steel bearing surfaces.

[edit] 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 special long axle that the fork/frame clamps onto. (found on some free ride and downhill mountain bikes)

[edit] Bearings

The bearings allow the hub shell to rotate freely about the axle. Most bicycle hubs use ball bearings. These can be further categorized into "cup and cone" and "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 very smooth to allow the bearings to roll with very 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 bearing is contained in a cartridge that are shaped like a hollow cylinder where the inner surface rotates with respect to the outer surface by the use of ball bearings. The cartridge is usually pressed into the hub shell and the axle rests against the inner surface of the cartridge. The cartridge bearing itself is generally not serviceable or adjustable; instead the cartridge is replaced in case of wear or failure.

[edit] Hub shell

The hub shell is the part of the hub that the spokes attach to. Usually the hub shell has a flange that extends outward from the axle. In a spoked wheel there are holes in the flange that the spokes pass through. Some wheels (like the Full Speed Ahead RD-800) have an additional flange in the center of the hub. Other (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.

[edit] 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.

For information on other types of bicycle brakes see the full article on bicycle brake systems.

[edit] 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.
• 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.
• 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 set of threads is for a track sprocket and the outer set is for a lock ring.
• Internal geared hub - the mechanism to provide multiple speeds is contained inside the shell of the hub.

[edit] Rim

The rim is an extrusion that is butted into itself to form a circle. Most rims are made of aluminum alloy, while some very high-end rims are made of carbon fiber, and some old or very low-end rims are made of steel. Rims have even been made of wood^[1] 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.

Rims can either have a single-wall or double-wall cross section. Single-wall rims are usually less expensive and weaker or heavier; double-wall rims tend to be stronger and more expensive. Double-wall rims may have a deep profile either to reduce aerodynamic drag or for additional strength or rigidity, especially for wheels with fewer spokes.

Aluminum rims are reinforced with either steel washers, single eyelets, or double eyelets. 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.

The number of spoke holes on the rim normally matches the number of spoke holes in the hub. For a double-walled rim there are usually holes for spokes in both walls, although some factory built wheels have rim designs without holes in the wall that contacts the tire. For example Campagnolo, Shimano and Mavic road rims/wheels and Velocity and Mavic mountain bike rims.

[edit] Clincher rims

Most bicycle rims are clincher rims for use with clincher tires (also known as wired-ons or wire-ons), which have a separate airtight inner tube enclosed by the rim and the tire. Modern clincher rims have hook-shaped edges to hold the bead of the tire in place, allowing high (8–10 bar, 120–150 psi) air pressure. If the inner part of the rim where the inner tube fits has spoke holes, they must be covered by a rim tape, 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.

[edit] Tubular or sew-up rims

Some rims are designed for tubular or sew-up tires (also known as singles in Australia), also known as tubulars or tubs, in which the tire is sewn into a tubular shape and then attached to the rim with an adhesive. A tubular tire may or may not have a separate innertube inside. Tubular tires requires more labour to repair a puncture than clincher tires. The tire must be removed from the rim, opened up, patched, sewn back up, then finally glued back to the rim. Clinchers have largely replaced tubulars for amateur racing, but tubulars are still commonly used for indoor track racing (where the closed track makes punctures from debris less commonplace), professional road racing, and road time trials.

Advantages of this system include a decreased chance of pinch flats, it can be made to operate in a wider range of tire pressures (from 25 to 200 psi). Furthermore, when a tubular goes flat at high speed there is a safety margin because it will not roll off the rim if it is properly glued on. Taken as a whole the total weight of a tubular rim and tire is usually lighter than its clincher equivalent. However, for all non-racing purposes the lightness advantage is somewhat offset by the need to carry at least one entire spare tubular tire (only a patch kit or inner tube are needed if using clincher tires)

Tubulars are most popular for bicycle road racing applications and it is argued that they provide a better road feel and are safer than clinchers in the case of a puncture while the bicycle is traveling at high speeds, also for track racing where punctures are rare.

[edit] Spokes

The rim is connected to the hub by several spokes under tension. The spokes on the vast majority of wheels are steel. They may be chrome- or zinc-plated to inhibit rust, or may be made of stainless steel. Plated spokes tend to rust, especially where the thread enters the nipple; stainless ones are much easier to true for maintenance or following damage. Butted spokes, with reduced thickness of the spokes over the centre section, are lighter, more elastic and more aerodynamic than spokes of uniform thickness. On high end wheelsets and custom builds lighter but expensive titanium, carbon fibre spokes are often used. Aluminium spokes, while lighter, are not often used as they fracture or snap more easily.

[edit] Adjustment

At the end of each spoke is a nut, called a nipple, which is used to adjust the tension. The nipple is usually at the rim end of the spoke, but some recent designs place it at the hub end to move its weight closer to the axis of the wheel, reducing the moment of inertia. The use of aluminium nipples at the rim also reduces the moment of inertia, but they are less durable. A nipple at the rim of a wheel usually protrudes from the rim towards the centre of the wheel, but in racing wheels may be internal to the rim, which is claimed to bring a slight aerodynamic advantage.

[edit] Radially or tangentially

With rim-brake wheels, the front spokes can be arranged radially, with 1 or more crossings or a mixture, such as with crows-foot pattern spokes. However, all disc brake-design wheels must have spokes attach to the hub tangentially, with a resulting crossing pattern, in order to transmit torque from the hub to the rim. Similarly, rear wheels almost always have some spokes tangentially attached to the hubs, with resulting crossing patterns, to transmit the torque applied to the rear hub by the drivechain out to the rim. In higher-end wheel designs, it is not uncommon for the spokes adjacent to the drivetrain to be attached tangentially while the spokes on the other side are configured radially. There are also often more spokes on the drivetrain side. This is because, due to the space occupied by the cassette, these spokes meet the hub closer to the center of the axle than spokes on the non-drive side, thus, they are under a greater amount of tension to keep the rim in the bike's plane of symmetry. It is possible for the rear wheel to be radially spoked on both sides, although this can lead to noticeable spoke "wind-up," where the hub has to twist a little before the rim is pulled along. This will happen upon both acceleration and braking, and the movement has an adverse effect on spoke life. However, the main criticism of radial wheels is that the spokes are prone to tearing out of the hub flange, as there is little metal between the spoke hole and the edge of the flange. Frequently, a number of spokes tear off a large piece of the flange, causing instant collapse of the wheel. Few hub manufacturers recommend or warranty their products for radial spoking, and hubs that are designed for such treatment need to be heavier and tougher, negating the (very small) weight saving from using this pattern. Nevertheless, some riders choose radial spoking for its striking appearance or due to the belief that there is an aerodynamic benefit, although the latter is negligible.^{[citation needed]}

Traditionally, most spokes are laced in a "three cross" fashion; this means that every spoke will directly cross three other spokes from hub flange to rim. In any but the cheapest wheels, the last spoke to be crossed will be threaded behind the first spoke to improve rigidity and strength. Certain lacings may employ soldering the last crossed spokes together with copper wire; this improves rigidity very well for velodrome riding, but has become obsolete by the use of more exotic wheel and spoke materials, notably carbon fibre.

[edit] Quantity

Wheels can have as few as a dozen or so spokes to well over 100 depending on the intended use, but traditionally most bike wheels on single rider bikes have had 28, 32 or 36 spokes and wheels on tandems have 40 or 48 spokes. Wheels with fewer spokes tend to have aerodynamic advantages for two reasons: firstly, the drag from the spokes themselves is reduced. Secondly, the reduced number of spokes means larger sections of rim are unsupported, necessitating stronger, deeper-section rims. Such rims tend to be aerodynamic, although there is often a weight penalty. More spokes usually means more strength and durability. However, equal and appropriate spoke tension is the key factor to strong bicycle wheels that remain true (circular and wobble-free).

[edit] Form

Spokes are usually circular in cross-section, but high-performance wheels may use spokes of flat or oval cross-section to reduce aerodynamic drag.

[edit] Material

Spokes are generally made of steel, but aluminium alloys, titanium, wood and exotic fiber and/or composite materials, such as kevlar and carbon fiber, have also been used.

Some modern wheels use titanium spokes which make the wheel lighter but also stronger compared with aluminium spokes^{[citation needed]}. Titanium tends to flex instead of break unlike other brittle metals but the heads of the spokes tend to snap, causing the wheel to buckle or become out-of-round, requiring the wheel to be re-trued^{[citation needed]}.

[edit] Alternatives

A wheel can be formed in one piece from a material such as thermoplastic or carbon fiber. The former is commonly used for inexpensive BMX wheels and have a maximum tire pressure of 45 psi. The latter may be used for high-end racing wheels.

[edit] Disc wheels

Disc wheels are designed to minimize aerodynamic drag. A full disc is usually heavier than traditional spoked wheels, and can be difficult to handle when ridden with a cross wind. For this reason, discs are commonly used only on the rear wheel.

A disc wheel may be simply be a fairing that clips onto a normal wheel, addressing the drag that the spokes account for by covering them. Or, the wheel itself can be an integral disc with no spokes inside. In the latter case carbon fibre is the material of choice. A spoked wheel with a disc cover may not be legal under UCI rules because it is a non-structural fairing.

A compromise that reduces weight and improves cross wind performance has three or four wide spokes that are integral to the rim – also typically carbon fiber.

[edit] Types

Bicycle wheels can be categorized by their primary use.

[edit] Road/racing bicycle wheels

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

• weight
• rotational inertia
• aerodynamics
• 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, because of the hub's proximity to the center of rotation reducing the hub's weight has less effect than reducing the rim's weight.

Semi-aerodynamic and aerodynamic wheelsets are characterized by greater rim depth, which is the 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 important, the rims must be heavier when there are fewer spokes, as the unsupported span between spokes is greater. While the increase in weight is somewhat important, it is the increased rotating inertia which is the greatest problem for "aero" wheels, as the rim, being farther from the axis of rotation, has the largest effect on rotational inertia, or in other words, moving 20 grams from the spokes (fewer spokes) to the rim will keep the weight the same, but will increase the rotational inertia. (But concerns about rotational inertia of bicycle wheels are vastly overstated--the inertia of all bicycle wheels is negligible compared to the mass of the rider.) "Aero" wheels are also reputedly more difficult to control in a "cross-wind" condition due to the larger projected lateral area. The tradeoff between rim depth, weight and spoke count is still under debate. However, a number of wheel manufacturers are now producing wheels with roughly half the spokes of a top of the line 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.

Almost all clincher carbon fiber wheelsets, such as those made by Zipp and Mavic, still use aluminum parts at the clinching part of the rim. Exceptions to this are the Campagnolo Hyperon Ultra Clincher, Bontrager Race XXX Lite Carbon Clincher, DT Swiss RRC1250, and Lightweight Standard C wheelsets, in which the rims are entirely made from carbon fiber.

[edit] Mountain bike wheels

[edit] 26 inch wheel / 559 mm rim

26-inch clincher tires (with inner tubes) are the most common wheel size for mountain bikes. The typically 26er rim has a diameter of 559 mm (22.0") and an outside tire diameter of about 26.2" (665 mm). Increasingly tubeless tires are becoming more popular. Tubeless tires are often called by the acronym UST (Universal System Tubeless). They allow the rider to run lower tire pressures for better traction and shock absorption without risking puncturing the tube in conventional bicycle tires.

[edit] 29 inch wheel / 622 mm rim

Main article: 29"er

“29-inch wheels” are becoming more popular for mountain bikes. Their rim diameter of 622 mm (~24.5 inch) is the same as that used on most road, hybrid and touring bicycles. The average 29" mountain bike tire has an outside diameter of about 28.5" (724 mm). There are advantages and disadvantages associated with this change discussed in detail in the main article.

[edit] Touring bicycle wheels

Touring bicycles may have wheels similar to road machines, or similar to mountain bikes, depending upon the terrain to be traveled, but they are built for strength, as heavy loads may be carried. Lightest possible weight and optimum aerodynamic performance is not required.

[edit] BMX wheels

Usually 20 inches in diameter, BMX wheels are designed to withstand the additional loads generated by BMX stunts.

[edit] Technical aspects

[edit] Sizes

Bicycle rims and tyres come in many different types and sizes. 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 tyres and rims in international standard ISO 5775. For example:

• For wired-edge tyres the ISO designation lists the width of the inflated tyre and the diameter with which the tyre sits on the rim, both in millimeters and separated by a hyphen: 37-622
• For beaded-edge tyres the ISO designation lists an overall diameter code (16, 18, 20, 22, 24, or 26) and a width code (1.25, 1.375, 1.75, or 2.125), defined by measurement tables given in the standard, separated by a cross: 20×1.375
• For rims the ISO designation lists the rim diameter (where the tyre sits) 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" = Crotchet-type): 622x19C

In practice, most tyres (and inner tubes) sold today carry apart from the modern ISO 5775-1 designation also some historic size markings, for which there exists no longer any officially maintained definition, but which are still widely used colloquially:

• an old French tyre designation that was based on the approximate, crudely rounded, outer diameter of the inflated tyre in millimeters: 700×35 C
• an old British inch-based designation: 28 × 1 5/8 × 1 3/8

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 use 622 mm rims. Many mountain bikes use “26 inch” wheels. Some mountain bikes use 24 inch or 29 inch wheels (due to evolved naming conventions, 29 inch wheels are identical in diameter with 700C road wheels, and 27 inch wheels are slightly larger in diameter than 700C road wheels). Some bicycles designed for triathlon or time trial purposes use 650c wheels. BMX bikes typically use 20 inch wheels, and some use 24 inch wheels.

The 650C triathlon size has the ISO diameter size of 571 mm. Size 650B is 584 mm and 650A is 590 mm.

Kids' bikes can have rim diameters ranging from 239 mm (12 × 1 3/8 × 1 1/4) to 400 mm (18 × 1 1/4). Older bikes may have, for example, 630 mm (27 × 1 1/4) or 597 mm (26 × 1 1/4) wheels that are incompatible with any of the sizes commonly used today.

Wheel rims also come in a variety of widths. High performance road racing rims are usually narrow, 18 mm or so, and less performance-oriented rims may be 24 mm wide or more.^[2]

[edit] Rolling resistance

Smaller wheels, all else being equal, have higher rolling resistance than larger wheels.^[3] "Rolling resistance increases in near proportion as wheel diameter is decreased for a given constant inflation pressure."^[4] An Oldenburg University's bicycle research group found that Schwalbe Standard GW HS 159 tires have a Crr of 455 for the ISO size 47-406 (20 in x 1.5 in) and, for the same model tire, a Crr of 336 for the ISO size 37-622 (700c): a size to resistance ratio of about -1.8.^[5]

Rolling resistance also is reduced with increasing tire pressure, although the reduction is slight above about 100 psi. While thinner bicycle tires are lighter and have less wind resistance, they actually have slightly higher rolling resistance than slightly larger tires at the same pressure.

[edit] Reaction to load

A load applied at the hub causes the wheel to flatten slightly near the ground contact area. The rest of the wheel remains approximately circular. The tension in all of the spokes is increased except for the few in the flat spot.^[4]

However according to^[6] and^[7] when a radial load is applied to a wheel at the hub eg by a rider sitting on the bicycle, the tension of all the spokes do not increase significantly, with only the spokes directly under the hub decreasing their tension.^[8]

[edit] 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. It is not the centre of gravity (mass) which matters, but the moment of inertia, which takes the rotation into account. Reducing the rotational inetria can be achieved by moving the spoke nipples to the hub or using lighter nipples such as aluminium. This can be pictured by imagining an ice skater spinning; when they pull in their arms, they rotate more quickly. A rule of thumb, in the case of acceleration only, is "the effect of a given mass on the wheels is almost twice that of the same mass on the frame."^[4] But again, the distinction between rotating and non-rotating mass is only felt during acceleration (and braking to a lesser extent), and mass at the hub matters a lot less than mass at the rim. See Bicycle weight and power for more detail.

[edit] Dish

Due to the need to mount multiple drive-train sprockets and disk brake rotors usually to only one side of a hub, and the need to have the rim be centered with respect to the frame or fork, spokes may be asymmetrical, with shorter and usually higher-tension spokes on the side with the additional components. This is referred to as dish. There are special tools, called dishing gauges, which indicate whether or not a rim is centrally positioned with the correct amount of dish.

Several different techniques have been tried to minimize dish. These include moving both hub flanges inboard the same amount, and placing spokes holes asymmetrically in the rim.

[edit] See also

• Cassette (bicycle part), the set of sprockets that attaches to the hub on the rear wheel
• ISO 5775
• Spoke wrench
• Wheelbuilding
• Wire wheels

[edit] References

[edit] External links

• Building Bicycle Wheels by Sheldon Brown
• Fix A Flat Tire
• How to True a Wheel by the Bicycle Tutor.