Crankset
From Wikipedia, the free encyclopedia
The crankset, or chainset, is the component of a bicycle drivetrain that converts the reciprocating motion of the rider's legs into rotational motion used to drive the chain, which in turn drives the rear wheel. It consists of one or more chainrings[1][2][3] or chainwheels[3] attached to the cranks or arms[4] to which the pedals attach. It is connected to the rider by the pedals, to the bicycle frame by the bottom bracket, and to the rear sprocket, cassette or freewheel via the chain.
Contents |
[edit] Parts
[edit] Cranks
The two cranks, one on each side and usually mounted 180° out of phase, connect the bottom bracket axle to the pedals.
[edit] Sizes
Cranks can vary in length from 150 mm to 185 mm to accommodate different sized riders,lengths for adult riders range from 165 mm to 177.5 mm. People with a shorter inseam (inside leg measurement) usually use shorter cranks, while those with longer inseams use longer cranks. Various formulae exist to calculate appropriate crank length for various riders, however, the length an individual cyclist feels most comfortable with may vary from this. Generally the same rider will prefer shorter cranks for fast cadence activities such as time trialling and track racing, while longer cranks are preferred for lower cadence use such as mountain biking.
Most major manufacturers offer crank lengths from 165mm to 180mm in 2.5 mm increments.
[edit] Materials
Cranks are constructed of either an aluminum alloy, titanium, carbon fiber, chromoly steel, or some less expensive steel. Tubular steel cranks (such as Tioga's Revolver) can be light and very strong, are usually found on bmx bikes, and are slowly finding their way to mountain bikes (Dirt jumping and Urban assault) . Aluminum cranks may be cast, hot forged or cold forged ("cold" in this context means the billet from which the crank is to be made is heated to a specified temperature well below the melting point, not room temperature). Cold forging gives the metal additional strength, and the cranks can therefore be made lighter without increasing the risk of breakage. Shimano "Hollowtech" aluminum cranks are made by forging the main arms around a hard steel insert which is then withdrawn, leaving an internal void to save weight. They are then welded up before final machining.
[edit] Attachments
[edit] To the bottom bracket
There are a variety of methods used to attach the crank to the bottom bracket axle or spindle.
- Older crank use a wedge-shaped pin, called a cotter pin, for attachment to the bottom bracket.
- Newer cranks slide onto
- a square tapered axle (with one of at least two non-interchangeable dimensions (Shimano and Compagnolo have competing standards in square taper, though most parts made by other manufacturers are to the Shimano standard), and with one of two orientations)
- a hexagonal tapered axle (Tune components cranks are an example)
- splined (with one of at least two non-interchangeable specifications, the ISIS spline being the most common splined standard as this standard was decided on and supported by several companies. Companies with their own standard for splines include Shimano (Octalink - 2 standards, one for XTR, one for everything else), Truvative and DMR) bottom bracket axle
- and are held in place by a bolt installed into the axle of the bottom bracket.
- Even newer designs have the bottom bracket axle, usually hollow and larger diameter than is possible for BB's with bearings held inside the BB shell of a bike frame, for reduced weight and increased stiffness, permanently attached to the right crank ([Shimano] and others) or the left crank (Race Face). The left crank slides onto a spline and is tightened with one or more pinch bolts (Shimano) or is pressed onto a spline by a bolt on the BB axle axis (Race Face).
- The latest from Campagnolo, called Ultra-TorqueTM, has each crank permanently attached to one half of the axle (called semi-axles) which then join in the middle of the bottom bracket with a Hirth joint and a bolt.[5]
- Certain companies such as Cannondale have made their own completely proprietary BB standards requiring changes in the BB shell of the bicycle frame in order to accommodate the bottom bracket/crank design.
- Finally, many children's bikes and older, or less-expensive bikes have one-piece cranks where the two cranks and bottom bracket spindle are forged as one piece of steel.
See the bottom bracket article for more details.
[edit] To the pedals
Crank arms have a threaded hole (or "eye") at their outboard end to accommodate the pedal spindle. Adult or multi-piece cranks have a 9/16 inch hole with 20 TPI (a combination that appears to be unique to this application). One-piece or children's cranks use a 1/2 inch hole. Some crank on children's bikes have more than one pedal hole so that the pedal can be moved to accommodate growth.
The right-side (usually the chain side) hole is right-hand threaded, and the left-side hole is left-hand (reverse) threaded to help prevent it from becoming unthreaded by an effect called precession.
Pedal spindles are hard steel, and gradually fret and erode the crankarm where the two meet. This can eventually be a cause of crank breakage, which commonly occurs at the pedal eye. Some manufacturers advise the use of a thin steel washer between the pedal and crank, but this is ineffective because the hard washer frets against the crank instead. A solution, suggested by Jobst Brandt, is to use a 45 degree taper at the surface where crank and pedal meet, as this would eliminate precession-induced fretting and loosening (it is already done for most automobile lug nuts for the latter reason). However, this would require manufacturers to change a well-established standard which currently allows most pedals to be fitted to most cranks.
[edit] Spider
On older styles, the spider—the multi-armed piece that connects the chainring to the bottom bracket axle—was a separate piece from the crank arm. The most common modern cranks have an integrated spider on the drive-side crank arm. However, Middleburn, TA, and Surly currently produce cranks with separate detachable spiders, enabling a wide variety of chainring patterns to be used with the same cranks.
Spiders usually have 4 or 5 arms, although some models have had as few as 3 and many as 10 arms with 6 having been popular in the past.[6]
[edit] Bolt circle diameter (BCD)
The holes on the spider arms used for attaching chainrings can have a variety of dimensions, referred to as the bolt circle diameter, commonly abbreviated as BCD. Cranks designed to mount one or two chainrings will almost always use a single bolt circle diameter. Cranks designed to mount three chainrings will almost always use two different bolt circle diameters; the larger to mount the two outer rings and the smaller to mount the inner ring. Most modern two-chainring cranks use either a 110mm or 130mm bolt circle diameter.
Bolt circle diameters of common "named" cranks:
- Track
- 144 BCD
- Road double
- 130 BCD (Shimano and others), or 135 (Campagnolo)
- Road triple
- 130/74 BCD (Shimano and others), or 135/74 BCD (Campagnolo)
- Compact/touring double
- 110 BCD or (Campagnolo carbon 4x110/1x113 BCD)
- Compact/touring triple
- 110/74 BCD
- Mountain bike (4 arm)
- 104/64 BCD
- Mountain bike (5 arm standard)
- 110/74 BCD
- Mountain bike (5 arm compact)
- 94/58 BCD
For an extensive list of bolt circle diameters and their applications, see Sheldon Brown's Bolt Circle Diameter Crib Sheet.
[edit] Chainrings
Chainrings (also called "chainwheels" or "sprockets", although sprocket is used this way mostly in the BMX community[3]) engage the chain to transfer power to the (usually rear) wheel. They usually have teeth spaced to engage every link of the chain as it passes over; however, in the past, some designs (called skip-tooth or inch-pitch) have had one tooth for every other link of the chain.[7]
[edit] Sizes
By convention, the largest chainring is outboard and the smallest is inboard. Chainrings vary in size from as few as 20 teeth to as many as 55 or more.
Chainrings also come in several widths:
- 3/16" for old-time bikes (especially skip-tooth or inch-pitch), heavy duty BMX, Worksman, and exercise bikes
- 1/8" for track, BMX, cruiser bikes, one-speed, three-speeds, and the rare derailleur bike.
- 3/32" for road, hybrid, mtb bikes, single-speed and 5-, 6-, 7-speed freewheels.
- 5/64" for any bike with 9- or 10-speed cassettes
[edit] Materials
Chainrings are constructed of either an aluminum alloy, titanium, steel, or carbon fiber.
[edit] Construction
Cheaper cranksets may have the chainrings welded or riveted directly to the crank arm or spider. More expensive sets have the chainrings bolted on so that they can be replaced if worn or damaged or to provide different gearing.
Replacement chainrings must be chosen with a bolt-hole count and spacing that matches the spider.
Chainrings designed for use with multi-chainring crank arms may have ramps or pins to aid in shifting. The middle chain ring, in the case of a triple crankset, usually has the most shaping to aid in shifting up and down. The smallest chainring usually has the least, if any shaping.
[edit] Variations
[edit] Tandem cranksets
On tandem bicycles the pedalling contribution of both riders is often combined and coordinated by the crank arms. There may be a second set of chain rings, often on the opposite side from the regular drive train, one on each crank set and connected by a separate chain. The most common implementation has both cyclists pedaling at exactly the same pace and usually in phase, although it is possible to configure the system for out-of-phase pedaling.
The most common tandem crankset is a set of four cranks. Both left cranks have spiders and chainrings to be connected by a timing chain, and only one of the right cranks has a spider for the drive chain.
There are tandem cranksets available called independent pedaling system cranksets, which allow each cyclist to pedal, or not, at their own pace.[8]
[edit] Chain guards
Some cranksets incorporate a chain guard that consists simply of a plastic or metal ring outboard of the largest chainring and slightly larger in diameter to help prevent the chain from touching or catching clothing. Bicycles that are going to be used in abusive applications, such as freeride and BMX, will often incorporate a very heavy-duty chain guard that is designed to protect the chainrings from physical damage caused by impact with fixed objects; also called 'bashguards', these commonly replace a third (large) chainring.
[edit] Freewheeling cranksets
Some cranksets have been produced that incorporate a ratcheting mechanism to accommodate coasting. In this case, the chain continues to rotate with the rear wheel when the rider stops pedaling. The ultimate goal of freewheeling cranks is to allow the rider to shift the chain while coasting. Shimano's discontinued FF system (Front Freewheeling) is one of the more commonly seen examples, although it was one case of a solution in search of a problem.
[edit] Left-side-drive
This configuration consists of a left crank arm with a spider and chainring, and a right crank arm without a spider. This is exactly the opposite of a normal configuration. The special crankset must also be paired with a rear hub that can be driven from the left side. These hubs typically have left hand threads for a special freewheel, which is also threaded left hand and ratchets the opposite direction of a normal freewheel. If the freewheel and hub were threaded with right hand threads, the torque applied by pedaling would loosen and unthread the freewheel from the hub.
Left-side-drive is sometimes done with a fixed gear drivetrain. Because a correctly installed track cog can not be loosened from the hub no matter which way torque is applied, it can be used for left-side drive without requiring special left hand threaded parts.
Note that a normal right-side-drive crankset can not be installed backwards to create a left-side-drive bicycle because the threaded pedal holes at the end of the crank arms would be backwards of normal. Even though some kinds of pedals could simply be installed on the wrong sides to get around this issue, precession would tend to loosen them over time, causing the pedals to become detached and/or damaging the pedal threading in the crank arms.
[edit] Independent crank arms
At least one manufacturer offers a crankset in which the crank arms may rotate independently. This is supposed to aid in training by requiring each leg to move its own pedal in a full circle. See PowerCranks.
A different system mounts the chainrings on a bearing mounted eccentric to the BB with linkages connecting the right and left cranks (Pivoted around the BB axis) to the chainrings. The cranks can turn independently to each other on the BB axle. The purpose of this is to pull the upward crank past the top dead centre point before the lower crank reaches bottom dead centre, eliminating the dead centre in the pedalling action of normal cranks. See Rotor Cranks.
[edit] Ovoid chainrings
Several manufacturers have tried non-round chainrings, such as Shimano's Biopace and Rotor's Q-Rings[1]. These are designed to provide varying leverage at different points in the pedal stroke, effectively changing the gear ratio at different angles of rotation to provide some type of advantage.
[edit] Compact crankset
In the context of mountain biking the term compact crankset refers to smaller triple cranksets, giving a small benefit in weight at the expense of increased wear and also giving the bike better clearance over obstacles. Typical ratios would be 22/32/44 teeth as opposed to 28/38/48 or 24/36/46 teeth. These would be used with smaller cassettes (Generally cassettes are available with 11 tooth minimum gear sizes for compact chainsets while standard chainsets were designed for cassettes with a 13 or 14 tooth top gear), giving the same overall ratio. Compact chainrings are the dominant standard for mountain bike cranks for the past decade or so.
In the context of road cycling, compact drivetrain typically refers to double cranksets with a smaller (usually 110mm) bolt cycle diameter than the standard 130mm or Campagnolo's 135mm. As of 2006, all of the major component manufacturers such as Shimano and Campagnolo offer compact cranks in their midrange and high-end product lines. The compact crankset provides a compromise between the standard road double crankset (with 39/52 or 39/53 tooth chainrings) and the road triple (with 30/42/52 or 30/39/53 tooth chainrings). The compact crankset has two chainrings and typical ratios are 34/48, 34/50 and 36/50. This provides nearly the same lower gear ratios as a triple but without the need for a third chainring, a triple front derailleur and a long cage rear derailleur. Note that both Shimano and Campagnolo recommend and sell front derailleurs specifically designed for compact cranksets, claiming better shifting.
Compact gearing is not necessarily lower than standard gearing if cassettes with smaller sprockets (such as 11-23) are used. A high gear of 50x11 on a compact drivechain is actually slightly higher than the 53x12 of a standard set.
Compact gearing usually has a large percentage jump between the two chainrings. In balance, it may also allow small jumps in the rear by allowing a closer ratio cassette to be used, except for the 9% jump at the high end between the 11 and 12 tooth sprockets.
[edit] Bent crankarms
Marketed and known by a variety of names (Z-cranks, P.M.P cranks, etc.) non-straight crank arms have been introduced several times. However, "as long as the distance from crank axle to pedal is fixed, there is no pedaling advantage in using bent crank arms".[9] Possible differences from comparable straight crank arms include more weight and more flex.
[edit] See also
[edit] References
- ^ Bike Works NYC Chainring Archive. Retrieved on 2007-10-21.
- ^ Shimano Technical SG-X Chainrings. Retrieved on 2007-10-21.
- ^ a b c Brown, Sheldon. Sheldon Brown's Bicycle Glossary: Chainring. Sheldon Brown. Retrieved on 2007-10-21.
- ^ Brown, Sheldon. Sheldon Brown's Bicycle Glossary: Crank. Sheldon Brown. Retrieved on 2007-10-21.
- ^ Campagnolo Ultra-Torque Crankset (2006). Retrieved on 2006-12-14.
- ^ Bike Works NYC Chainring Archive. Retrieved on 2007-01-16.
- ^ Bike Works NYC Chainwheel Archive (2006). Retrieved on 2006-12-14.
- ^ IPS Independent Pedaling System (2001). Retrieved on 2006-12-14.
- ^ Bike Works NYC Chainring Archive: Bent Crank Arms. Retrieved on 2007-01-16.
- Barnett, John (2003). Barnett's Manual: Analysis and Procedures for Bicycle Mechanics, 5th edition, Boulder, CO: VeloPress. ISBN 1-931382-29-8.
