The cranks ensure that the force on the pedals is converted into a moment on the crankshaft (left crank) or directly in a moment on the chain wheel (right-hand crank).
The crank is a simple lever with internal screw thread at the end for the attachment of the pedal. Previously, the crank was confirmed by means of a spline to the bottom bracket.
In this so-called spline connection, the crankshaft had a flat recess at right angles to the longitudinal direction of the shaft.
The cranks are provided with a round hole. This is diametrically opposed to the longitudinal direction of a crank.
The disadvantage of this way of connecting, is that there is rapid slack comes on the link, and that the disassembly and reassembly is a difficult task. Therefore all bikes nowadays, have a sline free crankset.
The chainrings are mounted to the right crank. The number of chainrings can vary from one to a bike with or without gear hub to three to a bike with a derailleur system.
A chain wheel can be attached in two ways with the crank.
- By middle of a seam.
This is used in bicycles with a chainring with or without a hub.
- The chainring is screwed to the crank by means of a spiderarm . This construction is used in bicycles with a derailleur system.
By default, bicycle cranks are fitted with a length of 170 mm. The crank length should actually be adjusted to the leg length. At the expensive market segment there are loose cranks available in lengths of 130, 145, 160 (children’s) 165, 167.5, 170, 172.5, 175, 177.5 and 180 mm.
In the cheaper price range in 170 mm length and 175 mm. Cranks with just clinched chainring for bikes without gears or hub gears are usually only available in the length of 170 mm. The length is measured from the center of the bottom bracket to the center of the pedal.
Simple cranks are made of steel, luxurious cranks of aluminum . The best cranks are cold forged aluminum.
Cranks that are from cold forged aluminum are the lightest and have a narrow ‘low profile’ form.
Most luxury cranks are made hollow to save weight without weakening them. Steel cranks are chromed. Aluminum cranks are painted, polished or anodized.
Cranks can bend by a crash. Steel cranks can, when they are not deformed too severe, be straightened. Aluminium cranks need to be replaced. The straightening of aluminum cranks can lead to sudden rupture of the crank with serious falls as a result.