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 connect to the frame and fork via dropouts.
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[edit] Construction
A wheel consists of a rim connected to a hub by spokes. The art of assembling these components is known as wheelbuilding.
[edit] Hub
The hub uses bearings, usually ball bearings (loose or in a cartridge), to reduce friction with the axle. Except in fixed gear bicycles, the rear hub is connected to the freewheel or freehub and the rear sprockets. Some bicycle wheels are attached to the bicycle frame using a quick release for ease of removal. A quick release skewer requires a hollow axle, otherwise a solid axle may be used. Hubs have a flange at both ends with holes that the heads of the spokes engage. The flange may be either wide or narrow. Wide flanges may have additional lightening holes in them.
![Bicycle with rims made of wood [dubious — see talk page] (1896)](../../../upload/shared/thumb/1/18/Bamboobike.jpg/180px-Bamboobike.jpg)
[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 and thermoplastic.
The number of spoke holes on the rim normally matches the number of spoke holes in the hub. Some unusual rim designs have no holes for spokes, for example Campagnolo road rims and the Velocity Zvino MTB rims.
Rims meant for use with rim brakes provide a smooth 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, and 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; 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.
[edit] Clincher rims
Most bicycle rims are for 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 rim has spoke holes, they must be covered by a rim 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.
[edit] Tubular or sew-up rims
Some rims are 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), and it is usually lighter. 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.
[edit] Spokes
The rim is connected to the hub by several spokes under tension.
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 rotating mass.
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, all rear wheels must have some spokes (typically it is all) tangential attached to the hubs, with resulting crossing patterns, to transmit the torque applied to the rear hub by the drivechain out to the rim.
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. Fewer spokes usually means less weight and aerodynamic drag; 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 straight).
Spokes are usually round in cross-section, but high-performance wheels may use bladed (flattened or ovalized) spokes to reduce aerodynamic drag.
[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: either tri-spoke or disk.
[edit] Disk wheels
Disk 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. The discs can be simply clipped onto existing wire spoked wheels, so addressing the considerable drag that the spokes account for, or can be an integral part of the wheel itself. In the latter case carbon fiber is the material of choice. 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, although some argue that its proximity to the center of rotation means that it is less useful than reducing rim 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 1980's, 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. One exception to this is the Campagnolo Hyperon Ultra clincher wheelset, in which the rims are entirely made from carbon fiber.
[edit] Mountain bike wheels
[edit] 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.
[edit] 29-inch/ISO 622
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.
[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 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.
Coker Tire makes a 36 inch tire and wheel that is available on their Monster Cruiser™.
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.
[edit] Brakes
Bicycle wheels are usually an integral part of a bicycle's braking system. In the case of rim brakes, brake pads press on the side of the rim; or, in the case of disk brakes, a disk is mounted to the hub. In either case, the wheel transfers the braking force from the brake to the ground surface.
[edit] 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.
[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.[2]
However according to [4] and [5] 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.
[edit] 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.
[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
- ^ VREDESTEIN Bicycle Tires. Retrieved on August 14, 2006.
- ^ a b c Whitt, Frank R.; David G. Wilson (1982). Bicycling Science, Second edition, Massachusetts Institute of Technology, 119. ISBN 0-262-23111-5.
- ^ Rolling resistance of bicycle tires. Retrieved on August 22, 2006.
- ^ Brandt, Jobst (1981). The Bicycle Wheel. Avocet, 12-20. ISBN 0-9607236-2-5.
- ^ Wilson, David Gordon (2004). Bicycling Science, Third edition, Massachusetts Institute of Technology, 389-390. ISBN 0-262-73154-1.
