Crankset
From Wikipedia, the free encyclopedia
The crankset 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.
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[edit] Parts
[edit] 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.
[edit] Sizes
Crank arms can vary in length from 150 mm to 185 mm (usually varying in 5 mm increments) to accommodate different sized riders. 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. Common crank lengths for adult riders range from 170 mm to 177.5 mm.
Commonly available cranks are sized between 165 mm and 180 mm, and are most major manufacturers offer them in 2.5 mm increments in this range.
[edit] Materials
Crank arms are constructed of either an aluminum alloy, steel, or carbon fiber.
[edit] Attachments
[edit] To the bottom bracket
There are a variety of methods used to attach the crank arms to the bottom bracket axle.
- Older crank arms use a cotter pin for attachment to the bottom bracket.
- Newer arms slide onto
- a square tapered (with one of at least two non-interchangable dimensions, and with one of two orientations)
- splined (with one of at least two non-interchangable 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-TorqueTM, 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.[1]
- Finally, many kids 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.
[edit] 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 unthreading with use.
[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. Modern drive-side crank arms have an integrated spider.
Spiders have 4 or 5 arms, and the holes for attaching chain rings can have a variety of spacings.
[edit] Chainrings
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.[2]
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
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.
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.
[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.
There are tandem cranksets available, called independent pedaling system cranksets, which allow each cyclinst to pedal, or not, at their own pace.[3]
[edit] Chain guards
Some cranksets incorporate a chain guard that consists simply of a plastic or metal ring outboard of the largest chainring and a little larger in diameter to help prevent the chain from touching or catching a pant leg.
[edit] Freewheel
Some cranksets have been produced that incorporate a ratching mechanism to accommodate coasting. In this case, the chain continues to rotate with the rear wheel when the rider stops pedalling.
[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.
[edit] 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.
[edit] 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 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.
[edit] See also
[edit] References
- ^ Campagnolo Ultra-Torque Crankset (2006). Retrieved on 2006-12-14.
- ^ Bike Works NYC Chainwheel Archive (2006). Retrieved on 2006-12-14.
- ^ IPS Independent Pedaling System (2001). Retrieved on 2006-12-14.
- Barnett, John (2003). Barnett's Manual: Analysis and Procedures for Bicycle Mechanics, 5th edition, Boulder, CO: VeloPress. ISBN 1-931382-29-8.
