Crankset
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 or belt, which in turn drives the rear wheel. It consists of one or more sprockets, also called chainrings
or chainwheels, attached to the cranks, also called arms or crankarms, 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.
Parts
Cranks
The two cranks, one on each side and usually mounted 180° apart, connect the bottom bracket axle to the pedals.Lengths
Bicycle cranks can vary in length to accommodate different sized riders and different types of cycling. Crank length is measured from the center of the pedal spindle to the center of the bottom bracket spindle or axle. The larger bicycle component manufacturers typically offer crank lengths for adult riders from long in increments, with cranks being the most common size. A few small specialty manufacturers make bicycle cranks in a number of sizes smaller than and longer than. Some manufacturers also make bicycle cranks that can be adjusted to different lengths.While logic would suggest that, all other things being equal, riders with shorter legs should use proportionally shorter cranks and those with longer legs should use proportionally longer cranks, this is not universally accepted. However, very few scientific studies have definitively examined the effect of crank length on sustained cycling performance and the studies' results have been mixed. Bicycle crank length has not been easy to study scientifically for a number of reasons, chief among them being that cyclists are able to physiologically adapt to different crank lengths. Cyclists are typically more efficient pedalling cranks with which they have had an adaptation period. Several different formulas exist to calculate appropriate crank length for various riders. In addition to the rider's size, another factor affecting the selection of crank length is the rider's cycling specialty and the type of cycling event.
Historically, bicycle riders have typically chosen proportionally shorter cranks for higher cadence cycling such as criterium and track racing, while riders have chosen proportionally longer cranks for lower cadence cycling such as time trial racing and mountain biking. However, the evolution of very low rider torso positions to reduce aerodynamic drag for time trial racing and triathlon cycling can also affect crank selection for such events. Some have suggested that proportionally shorter cranks may have a slight advantage for a rider with a very low torso position and an acute hip angle, especially as the rider pedals near the top-dead-center position of the pedal stroke. Cranks can be shortened for medical reasons using shorteners such as Ortho Pedal.
Unicycle cranks vary in length to accommodate different unicycle wheel sizes, and different unicycling disciplines. As almost all unicycles are ungeared, crank length is a major factor in determining how much force is transmitted to the wheel. Larger wheel diameters, such as, require longer cranks, as do disciplines such as Mountain Unicycling, Trials, Street, and Flatland. These unicycles and disciplines commonly use cranks lengths greater than. For indoor unicycling such as freestyle or hockey, shorter cranks give a smoother pedaling motion and enable tighter turns without the pedal hitting the floor. Crank lengths of are common, although some riders use cranks as short as. As there is no chainwheel on a unicycle, right and left cranks are identical, except for the pedal attachment thread in the left-hand crank, which is reverse threaded.
Materials
Cranks are constructed of either an aluminum alloy, titanium, carbon fiber, chromoly steel, or some less expensive steel. Tubular steel cranks can be light and very strong, are usually found on BMX bikes, and are slowly finding their way to mountain bikes. Aluminum cranks may be cast, hot forged or cold forged. 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.Attachments
To the bottom bracket
There are a variety of methods used to attach the cranks to the bottom bracket spindle.- Older cranks use a wedge-shaped pin, called a cotter, for attachment to the bottom bracket spindle.
- Newer cranks slide onto
- * a square tapered spindle. The taper is 2 degrees with respect to the centerline. There are at least two non-interchangeable dimensions, and two orientations: diamond and horizontal square. The hole in the crank in which one screws a crank puller almost always has a diameter of, though some old French cranks require a different puller.
- * a hexagonal tapered spindle
- * a splined bottom bracket spindle with two prominent specifications, and numerous uncommon ones. The ISIS spline may be the most common splined standard as it was decided on and supported by several companies. Shimano's Octalink is a common proprietary standard that comes in two forms: version one for XTR, 105, Ultegra and Dura Ace; and version two for every other groupset such as XT, LX and Deore. Truvativ and DMR also have their own proprietary spline interface standards. A 48 splined spindle, such as from Profile Racing, Demolition, and Eighth Inch, is mostly used for BMX, unicycles, and fixed-gear Freestyle.
- Even newer designs have the bottom bracket spindle, usually hollow and larger diameter than is possible for bottom brackets with bearings held inside the bottom bracket shell of a bike frame, for reduced weight and increased stiffness, permanently attached to the right crank or the left crank. The left crank slides onto a spline and is tightened with one or more pinch bolts or is pressed onto a spline by a bolt on the bottom bracket spindle.
- The latest from Campagnolo, called Ultra-Torque, has each crank permanently attached to one half of the spindle which then join in the middle of the bottom bracket with a Hirth joint and a bolt.
- Certain companies such as Cannondale have made their own unique bottom bracket standards requiring changes in the bottom bracket shell of the bicycle frame in order to accommodate the bottom bracket/crank design. The open BB30 standard is gaining popularity on high end bicycle and component manufacturers.
- 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.
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. One-piece or children's cranks use a 1/2 inch hole. Some cranks on children's bikes have more than one pedal hole so that the pedal can be moved to accommodate growth.The right-side hole is right-hand threaded, and the left-side hole is left-hand 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. However, this would require manufacturers to change a well-established standard which currently allows most pedals to be fitted to most cranks.
The solution to the issue of fretting is to use a metal based anti-seize lubricant; being composed of assorted mixtures of aluminium, copper, graphite and nickel powders in a grease base – that allows repeated assembly and disassembly without wear and the elimination of fretting corrosion during use.
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.
Bolt circle diameter (BCD)
Many modern bicycles have removable chainrings, to allow for replacement when worn, or to change the gear ratio provided.The holes on the spider arms used for attaching a chainring can have a variety of spacings, referred to as the bolt circle diameter, commonly abbreviated as BCD. This measurement is sometimes referred to as pitch-circle diameter. 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 bigger, outer rings and the smaller to mount the smaller, inner ring. Most modern two-chainring cranks use either a bolt circle diameter.
Bolt circle diameters of common cranks:
; vintage single : 151 BCD
; vintage double : 144 BCD
; Single : 130, 135, or 144 BCD
; Road double : 130 BCD, 135, or 122
; Road triple : 130/74 BCD, or 135/74 BCD
; Cyclocross/compact/touring double : 110 BCD or
; Mountain bike /compact/touring triple : 110/74 BCD
; Mountain bike : 104/64 BCD
; Mountain bike : 94/58 BCD
; Single, double or triple : 70 BCD