Crankset

A Shimano Deore crankset, drive-side showing crank arm, spider, three chainrings and chainring guard

A left crank arm attached with a cotter.

A one-piece crank as seen from the drive side.

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 attached to the crank arms, which attach the pedals to the crank. 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.

Parts

Crank arms

The two crank arms, one on each side and usually mounted 180° out of phase, connect the bottom bracket axle to the pedals.

Sizes

Crank arms can vary in length from 150 mm to 185 mm to accommodate different sized riders,lengths for adult riders range from 170 mm to 177.5 mm. People with a shorter inseam (inside leg measurement) usually use shorter cranks, while those with longer inseams use longer crank arms. 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.

Most major manufacturers offer crank arms from 165mm to 180mm in 2.5 mm increments.

Materials

Crank arms are constructed of either an aluminum alloy, titanium, carbon fiber, chromoly steel, or some less expensive steel.

Attachments

To the bottom bracket

There are a variety of methods used to attach the crank arms to the bottom bracket axle or spindle.

• Older crank arms use a wedge-shaped pin, called a cotter, for attachment to the bottom bracket.
• Newer arms slide onto
  • a square tapered (with one of at least two non-interchangeable dimensions, and with one of two orientations)
  • splined (with one of at least two non-interchangeable specifications) 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 for reduced weight and increased stiffness, permanently attached to the drive-side crank arm. The non-drive-side crank arm slides onto a spline and is tightened with one or more pinch bolts.
• The latest from Campagnolo, called Ultra-Torque^{TM, has each crank arm permanently attached to halves of the axle (called semi-axles) which then join in the middle of the bottom bracket with a Hirth joint and a bolt.}
• Finally, many kids bikes and older, or less-expensive bikes have one-piece cranks where the two arms and bottom bracket spindle are forged as one piece of steel.

See the bottom bracket article for more details.

To the pedals

Crank arms have a threaded hole 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 kids cranks use a 1/2 inch hole. Some crank arms on kids bikes have more than one pedal hole so that the pedal can be moved to accommodate growth.

The right-side (usually the drive-side) hole is right-hand threaded, and the left-side (usually the non-drive-side) hole is left-hand (reverse) threaded to help prevent it from becoming unthreaded by an effect called precession.

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.

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.

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 cranksets use either a 110mm or 130mm bolt circle diameter.

Bolt circle diameters of common "named" crankset configurations:

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

Compact/touring triple

110/74 BCD

For an extensive list of bolt circle diameters and their applications, see Sheldon Brown's Bolt Circle Diameter Crib Sheet.

Chainrings

A Shimano chainring, detached from crankset

Chainrings 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.

Sizes

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

By convention, the largest chainring is outboard and the smallest is inboard. Chainrings vary in size from as few as 22 teeth to as many as 55 or more.

Materials

Chainrings are constructed of either an aluminum alloy, steel, or carbon fiber.

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.

Variations

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 arms. Both left arms have spiders and chainrings to be connected by a timing chain, and only one of the right arms 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.

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 a pant leg. 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.

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.

Left-side-drive

Introduced by Specialized in the late 1990s for BMX riders who grind on their right side, 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.

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.

Ovoid chainrings

Several manufacturers have tried non-round chainrings, such as Shimano's Biopace. 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.

Compact crankset

A recent trend is a compromise between the standard road double crankset (with a 39 tooth and a 52 tooth chainrings) and the road triple (with 52, 42, and 30 tooth chainrings). The compact crankset has just two chainrings and a different spider, usually with a 110mm BCD, that allows the smaller chainring to have as few as 33 teeth. This provides nearly the same low-end as a triple but without the added weight of a third chainring.

In the context of road cycling, compact drivetrain typically refers to cranksets with gearing of 50/36 or 50/34 and a 110 mm bolt circle diameter. 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.

Compact gearing is not necessarily lower than standard gearing given 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.

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". Possible differences from comparable straight crank arms include more weight and more flex.

See also

• Bicycle gearing
• Bicycle pedal
• Biopace
• Bottom bracket

References

• Barnett, John (2003). Barnett's Manual: Analysis and Procedures for Bicycle Mechanics (5th edition ed.). Boulder, CO: VeloPress. ISBN 1-931382-29-8. {{cite book}}: |edition= has extra text (help); Cite has empty unknown parameters: |accessyear=, |origmonth=, |accessmonth=, |month=, |chapterurl=, |origdate=, and |coauthors= (help)