Turning a push into a spin
A rider pushes a pedal down, but a wheel must turn round. The crank is the part that makes that conversion. It is a lever arm fixed at one end to a rotating axle, with the pedal at the other end. Pushing the end of the arm downward turns the axle. A pair of cranks set on opposite sides, half a turn apart, lets the legs take turns, so one is always pushing while the other recovers.
The crank is an old mechanical idea. It was familiar from hand-turned grindstones and from the steam engines and water mills of earlier centuries, which used cranks to turn reciprocating motion into rotation. It was also used in treadle sewing machines, which turned a foot’s up-and-down motion into rotation of a wheel. Bicycle makers applied the same principle on a smaller scale.
The front-wheel crank
On the velocipedes of the 1860s, the cranks were attached to the axle of the front wheel. That put the pedals directly on the wheel, which had two consequences. First, the crank length mattered in a new way: a longer arm gave more leverage but also required a larger circle for the foot to travel. Second, the crank could not be separated from the wheel’s size and the rider’s leg length, which limited the design.
On high-wheelers, makers sometimes provided adjustable cranks with a series of holes or slots for the pedal, so that the rider could change the effective crank length. Such adjustment was a way of fitting the machine to the person at a time when the size of the front wheel was already the main fitting choice.
The crank moves to the center
With the chain-driven safety bicycle, the crank moved from the front axle to a separate shaft inside the frame, called the bottom bracket axle. The crank no longer turned the wheel directly. It turned a chainring, and a chain carried the motion to the rear wheel. This freed the crank from the constraints of the front wheel and let makers choose crank length to suit the rider.
It also meant that the crank joint was a critical point. The pedal force was concentrated at the junction between the crank arm and the axle, and a loose joint could wear quickly.
The cotter pin
For many years, the usual way to fix a crank to its axle was the cotter pin. The axle had a flat cut on it, and the crank had a hole around it. A wedge-shaped steel pin, the cotter, was driven through a hole in the crank and pressed against the flat. When tightened with a nut, the cotter held the crank firmly.
Cottered cranks were simple, cheap and easy for a local workshop to make. They were common on bicycles through much of the twentieth century. They had weaknesses, though. The cotter could work loose and needed to be re-tightened, and repeated hammering could deform the parts. The design also required careful fitting to ensure that the two cranks were exactly opposite one another.
One-piece cranks
Another solution was to avoid the problem of attaching a crank to an axle by forging the cranks and the axle as a single piece. This is a one-piece crank. It is cheap and sturdy, and it is still found on many children’s bicycles, cruisers and low-cost models. It is heavy and the bottom bracket is large, but there is little to loosen. One style used in the United States is known as the Ashtabula crank, a name that refers to an Ohio town, though details of its history vary between sources.
Cotterless cranks
In the early to mid twentieth century, cotterless designs spread, first on better bicycles. In this arrangement the end of the axle is shaped as a square taper, and the crank has a matching socket. A bolt or nut draws the crank onto the taper, and the force of the fit holds it in place. This eliminated the cotter pin and made the cranks easier to remove. It also let manufacturers use lighter forged or cast aluminum cranks, developed with the help of advances in aluminum smelting.
Later versions used splined shafts, with ridges that engage the crank, or an axle that is part of one crank and fits into the other. In recent decades, many cranks have been made with hollow arms or a bolt-together outer shaft, in the interest of stiffness and low weight. Carbon cranks appeared in the later twentieth century and have found a place on some racing bicycles.
Crank length
Crank length is the distance from the center of the axle to the center of the pedal. Typical adult bicycles use lengths around 165 to 175 millimeters, with children’s bicycles using shorter ones. Longer cranks can give more leverage but need a larger circle for the foot to travel. Choices between lengths are matters of preference and body proportions, and this article does not give fit advice.
Motors and new demands
On electric bicycles with a motor at the crank, the crank axle may contain a sensor and gears, and the crank itself must be strong enough to bear the added force. The motor joins the story as another source of power on the same axle, much as steam engines once shared cranks with human-powered machines.
Looking back
The crank is a reminder that the bicycle was assembled from older ideas. The invention was in applying the crank to a rider’s legs and making it light, adjustable and durable. Its development from the front-wheel crank to cotterless axles reflects improvements in metalworking and in standardization.