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.
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.
Hub
A hub is the center part of a bicycle wheel. It consists of an
axle,
bearings and a 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)
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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)
Bearings
The
bearings allow the hub shell to move freely and independently of the axle.
Bearings can be loose balls that contact a 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 move with very little friction. Alternatively bearings can be contained in a cartridge that are shaped like a hollow cylinder where the inner surface moves independently of the outer surface by the use of ball bearings. The cartridge bearings are usually pressed into the hub shell and the axle rests against the inner surface of the cartridge.
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.
Hub Brakes
Some hubs have attachments for disc brakes or form an itegral part of drum brakes and some hubs.
★
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 avariety of ways using bolts or a central locking ring.
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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 the pedals.
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.
★
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 intregal 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.
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 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
tyre. For example
Campagnolo road rims and the
Velocity Zvino MTB rims.
Clincher rims
Most bicycle rims are clincher rims for use with 'clincher' tires, 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 (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.
Tubular or sew-up rims
Some rims are designed for 'tubular' or 'sew-up' tires, 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.
Spokes
The rim is connected to the hub by several
spokes under tension.
Adjustment
At the end of each spoke is a nut, called a
nipple that can be used to adjust the tension in that spoke. The nipple has traditionally been located at the rim, but some recent designs place it at the hub to reduce the
moment of inertia. 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.
Radially or tangentially
With rim brake-design 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 centre 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.
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 have had 32 or 36 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. Fewer spokes increase the danger of a stick or animal become lodged in the rim, leading to a catastrophic accident. 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).
Form
Spokes are usually round in cross-section, but high-performance wheels may use bladed (flattened or ovalized) spokes to reduce aerodynamic drag.
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. 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.
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: either tri-spoke or disk.
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.
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:
★ 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. A couple exceptions to this are the
Campagnolo Hyperon Ultra clincher and
Bontrager Race XXX-Lite Carbon Clincher wheelsets, in which the rims are entirely made from carbon fiber.
Mountain bike wheels
A 29" and 26" Mountain Bike wheel
26-inch/ISO 559
26-inch clincher tires (with inner tubes) are the most common wheel size for mountain bikes. The typically 26er rim has a diameter of 22.0" (559mm) and an outside tire diameter of about 26.2" (665mm) Increasingly
tubeless tires are becoming more popular. Tubeless tires are often called by the acronym UST. (Universal System Tubeless) UST tires allow the rider to run lower tire pressures for better traction and shock absorption without risking puncturing the tube in conventional bicycle tires.
29-inch/ISO 622
Main articles: 29"er
29-inch wheels are becoming more popular for mountain bikes. The rims have a diameter of approximately 24.5" (622mm), the same as 700c rims used on most road, hybrid and touring bicycles. The average 29" mountain bike tire has an outside diameter of about 28.5" (724mm). There are advantages and disadvantages associated with this change discussed in detail in the main article.
BMX wheels
Usually 20 inches in diameter,
BMX wheels are designed to withstand the additional loads generated by BMX stunts.
Technical aspects
Sizes
Bicycle wheels come in many different sizes. Most road and
racing bicycles use 700C wheels (approximately 700 mm diameter with a tire mounted) due to regulation, while many
mountain bikes use 26 inch wheels. Some mountain bikes use 24 inch or
29 inch wheels (29 inch wheels are in fact identical in diameter to 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.
Kids' bikes can have 12 inch, 14 inch, 16 inch, and 18 inch wheels. Older bikes may have 27 inch or 26 inch wheels that are incompatible with any of the sizes commonly used today.
Wheel rims also come in a variety of widths. High performance rims are usually narrow, 18 mm or so, and less performance-oriented rims may be 24 mm wide or more.
ISO, the
International Organization for Standardization has defined a tire sizing standard that assignes a rim diameter in millimeters for each of the sizes listed above. For a complete table, see the
ISO 5775 article or
Tire Sizing by Sheldon Brown.
Rolling resistance
Smaller wheels, all else being equal, have higher
rolling resistance than larger wheels.
[1]
"Rolling resistance increases in near proportion as wheel diameter is decreased for a given constant inflation pressure."
[2]
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.
[3]
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.
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.
[2]
However according to
[5]
and
[6]
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.
Rotating mass
Due to the fact that wheels rotate as well as translate when a bicycle moves, more force is required to
accelerate a unit of mass on the wheel than on the frame. 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."
2 But again, the distinction between rotating and non-rotating mass is only felt during acceleration. See
Bicycle weight and power for more detail.
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.
See also
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Cassette (bicycle part), the set of sprockets that attaches to the hub on the rear wheel
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ISO 5775
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Spoke wrench
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Wheelbuilding
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Wire wheels
References
1. VREDESTEIN Bicycle Tires
2. Bicycling Science, , Frank R., Whitt, Massachusetts Institute of Technology, 1982, ISBN 0-262-23111-5
3. Rolling resistance of bicycle tires
4. Bicycling Science, , Frank R., Whitt, Massachusetts Institute of Technology, 1982, ISBN 0-262-23111-5
5. The Bicycle Wheel, , Jobst, Brandt, Avocet, 1981, ISBN 0-9607236-2-5
6. Bicycling Science, , David Gordon, Wilson, Massachusetts Institute of Technology, 2004, ISBN 0-262-73154-1
External links
★
Building Bicycle Wheels by Sheldon Brown
★
Fix A Flat Tire
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