|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 crankarms, 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.|
Over the years, many different strategies have propelled crankset development, not the least of which have been the addition of new bottom bracket technologies, the desire for an increased amount of mechanical advantage, and weight savings.
When evaluating cranksets be mindful of weight differences, construction differences (better cranksets are stiffer, and have less play in them), and gearing differences.
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 most major manufacturers offer them in 2.5 mm increments in this range. Crank arms are constructed of either an aluminum alloy, steel, or carbon fiber.
Older crankarms use a cotter pin for attachment to the bottom bracket.
Newer arms attach to a square or splined bottom bracket axle
and are held in place by a bolt installed into the shaft of the bottom bracket.
Even newer designs have the bottom bracket
spindle (usually hollow and a 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.
Cheaper cranksets may have the chainrings welded or riveted directly to the crankarm. 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
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.
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.
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. Some freeride and downhill
bikes use a large metal ring called a bash guard to protect the largest cog from hitting rocks or logs.
A recent trend is a compromise between the standard road double crankset (with a 39 tooth and a 52 tooth chainring) 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.
Carbon Fiber Crank Arms
FSA makes a carbon fiber crank arm set for road bikes that absorbs a lot of the vibrations that are normally transferred up to the rider through the crank.
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.
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.