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Wheel Cylinder Replacement

Wheel Cylinders for Drum Brake Systems

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 Shimano Dura-Ace freehub style hub
Freehub vs freewheel 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 2 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.
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 long axle, typically 20 mm (110 mm width), 9 mm (100.33 mm width) in diameter for durability, onto which the fork/frame clamps. (found on MOST free ride and downhill mountain bikes)
female axle – hollow center axle, typically 14, 17, or 20 mm in diameter made of chromoly and aluminum, which two bolts thread into on either side. This design can be much stronger than traditional axles. (found on higher end BMX hubs and some mountain bike hubs)
Modern bicycles have adopted standard axle spacing: the hubs of front wheels are generally 100 mm wide fork spacing, road wheels generally have a 130 mm wide rear wheel hub. Off-road and “mountain” bikes have adopted a 135 mm rear hub width, which allows clearance to mount a brake disc on the hub or to increase the wheel dish for a more durable wheel.
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. Older designs used “cup and cone”, whereas some modern wheels utilize pre-assembled “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
The hub shell is the part of the hub to which the spokes (or disc structure) attach. The hub shell of spoked wheels generally have 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 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 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. The mechanism is contained inside the bicycle wheel hub shell.
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. This is the case for most modern bicycles.
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. This style of hub was used before the freehub became practical.
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 (clockwise) set of threads is for a track sprocket and the outer (counter-clockwise) set is for a reverse threaded lock ring. The reverse threads on the lock ring keep the sprocket from unscrewing from the hub, which is otherwise possible when slowing down.
Flip-flop hub – Both sides of the hub are threaded, allowing the wheel to be removed and reversed in order to change which gear is used. Depending on the style of threads, may be used with either a single speed freewheel or a track sprocket.
Internal geared hub – the mechanism to provide multiple gear-ratios is contained inside the shell of the hub. Many bicycles with three-speed internally geared hubs were built in the last century. This is an extremely robust design, although much heavier than more modern designs of multi-gear-ratio arrangements. Modern hubs are available from three-speed to 14 speeds or a continuously variable transmission hub , in the case of the NuVinci.
Rim
Broken rim after a bicycle/car-door collision
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 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.
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.
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.
Clincher rims
Most bicycle rims are “clincher” rims for use with clincher tires. These tires have a wire or aramid (Kevlar) 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, 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
Straight-side (SS) rims
Crotchet-type (C) rims
Hooked-bead (HB) rims
Traditional clincher rims were straight-sided. Various “hook” (also called “crotchet”) designs emerged in the 1970s to hold the bead of the tire in place, allowing high (610 bar, 80150 psi) air pressure.
Tubular or sew-up rims
Main article: Tubular tires
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 air tight 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 for mountain bikes is the most common system of tubeless tires/rims for bicycles. 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.
Some cyclists have avoided the price premium for a tubeless system by sealing the spoke holes with a special rim strip and then sealing the valve stem and bead seat with a latex sealer. However, tires not designed for tubeless application do not have as robust a sidewall as those that are.
The drawbacks to tubeless tires are that they are notorious for being harder to mount on the rim than clincher tires, and that the cyclist must still carry a spare tube to insert in case of a flat tire due to a puncture.
In 2006, Shimano and Hutchinson introduced a tubeless system for road bikes.
Spokes
The rim is connected to the hub by several spokes under tension. Original bicycle wheels used wooden spokes that could be loaded only in compression, modern bicycle wheels almost exclusively use spokes than can only be loaded in tension. There are a few companies making wheels with spokes that are used in both compression and tension.
At the end of each spoke is a specialized nut, called a nipple, which is used to adjust the tension in the spoke. The nipple is usually located at the rim end of the spoke but on some wheels is 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 than brass. A third alternative is titanium nipples, which are extremely strong, but substantially lighter than brass. 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.
Butted spokes with reduced thickness of the spokes over the center section, are lighter, more elastic, and more aerodynamic than spokes of uniform thickness. In 2007, Mavic introduced their R-Sys, a new bicycle spoke technology that allows the spokes to be loaded in both tension and compression. This technology is promised to allow for fewer spokes, lower wheel weight and inertia, increased wheel stiffness, with no loss of durability.
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.
Material
The spokes on the vast majority of modern bicycle wheels are steel, stainless steel, titanium, aluminum, or carbon fiber. Stainless steel spokes are favored by most manufacturers and riders for their durability, stiffness, damage tolerance, and ease of maintenance. Titanium spokes are softer and more expensive than steel, and aluminum spokes are less durable.
Number of spokes
Conventional metallic bicycle wheels for single rider bikes commonly have 28, 32 or 36 spokes, while wheels on tandems have as many as 40 or 48 spokes. BMX bikes commonly have 36 or 48 spoke wheels. Wheels with fewer spokes have an aerodynamic advantage, as the aerodynamic drag from the spokes is reduced. On the other hand, the reduced number of spokes 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 refers to the pattern by which the spokes connect the hub to the rim. 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 vertically stiff wheel. 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). 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; 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, with 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, allows 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. 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 braking and drive forces are present. 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.
Wheelbuilders 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 wheel designs may be opted solely for 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. 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 leads to overstressed spokes which have a short 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 musical 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. It should be noted that 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