Crank

Pedal cranks on a bicycle

Bicycle crank with screw points for a chainring and hole for connection to the bottom bracket shaft

A pedal crank is a lever attached to a shaft with a step for placing a foot on the free end. A drive torque is introduced into the shaft via the crank through a circular movement . Usually two pedal cranks offset by 180 ° are used so that both feet can be used.

The most common use case is the two pedal cranks attached to the ends of a short bottom bracket shaft on the bicycle . Pressing the bicycle pedals at the end of the cranks drives the shaft and chainring .

Pedal cranks are also used to drive pedal boats , mopeds , bicycle ergometers , pedal generators and washing machine drum drives in third world countries. In the past, pedal drives were also used to operate handicraft devices such as rotating whetstones for outpatient scissors and knife grinders .

The following explanations deal with pedal cranks on bicycles.

function

The person who operates the cranks usually sits on a saddle. The “bearing shaft” is driven by alternately stepping on the pedals. When driving standing still, the drive torque can also be obtained from the body weight. The drive force is usually transmitted to the rear wheel via a chain , gear , cardan or belt drive.

Material and dimensions

Pedal cranks used to be made of steel, while bicycles are now almost exclusively made of aluminum, some of them also made of carbon fiber reinforced plastic . The crank length is measured from the center of the pedal axle to the center of the bottom bracket axle. Most cranks are 170 to 175 mm long. There are also crank arms from 150 and up to 190 mm, which can offer an ergonomic advantage for short or long legs . Studies in track cycling show that shorter crank arms with a length between 155 and 160 mm not only have ergonomic advantages, but also lower air resistance.

The horizontal distance between the outer surface of the left and right crank is the so-called Q-factor .

inventor

Pedal crank by Heinrich Mylius (1845)

The first crank bicycle in Germany is documented by the German mechanic, dialect poet and designer Heinrich Mylius for around 1845 in Themar . Mylius is said to have built a two-wheeler that was equipped with cranks on the front wheel even before the Fischer crank wheel (1853). The date is now considered disputed. Compared to the Fischer bike, the peculiar rear fork is striking. This speaks for an older year of construction. It can be assumed that Mylius and Philipp Moritz Fischer (1812–1890) knew each other, because both lived only about 80 kilometers away from each other. Mylius fled to America in the course of the revolution of 1848/1849 .

As the inventor of the pedal crank on bicycles, the following are also associated:

Pierre Michaux (1813–1883), France - Pedal drive for bicycles in 1861, because he made two copies of his Velocipede public at the World Exhibition in 1867 , thus attracting great international attention and helping to achieve sales success.

Pierre Lallement (1843–1891), France - US patent for a pedal-powered bicycle 1866

technology

The connection point between the shaft and the crank is exposed to strong alternating loads. See: Bottom bracket # Connection of bottom bracket shaft and cranks .

Crank for cranks with square head to the axis of the bottom bracket .
A puller of better quality on the left. The other two pullers are available for well under € 10. Due to the rounded threads, they are already unusable. The middle puller has been disassembled into its two individual parts.
The pitch of the internal crank thread is low relative to the diameter and the tensile force acting. Simple quality pullers are often made of such soft material that their threads wear out after just a few uses. The internal thread of a stuck crank will then also be damaged. The tool slips through and the crank can only be removed with great effort.

Most bicycle cranks with a square connection to the bottom bracket shaft have an internal thread that is used to screw in a special screwing tool with which the cranks are removed from the shaft. The fine internal thread is easily damaged if the tool for pulling the crank is tilted or if its threads are worn after repeated use or through use in unclean internal threads. If the puller tool slips through the internal thread when tightening the inner screw, then another method of removing the crank must be used:

The crank is heated from outside by a flame or a hot air blower around the shaft mount. Rapid heating is an advantage. The crank can then be knocked off the shaft with a hammer or with a hammer and chisel, as long as the square socket of the crank has expanded due to the heat, while the shaft itself is still cold.

A two-armed or three-armed puller can be attached to the outside of the crank. As with the puller tool screwed in from the inside, a mandrel is then screwed against the shaft inside the tool in order to pull off the crank. In the case of cranks with rounded edges alongside, however, it is often hardly possible to hang the puller arms on the back of the crank without them slipping off. Extractors work more reliably with a cylinder slotted on one side, which is pushed over the head of the crank and embraces it on three sides (ball joint extractor or ejector).

If the crank is not to be used again, the head can be slit on both sides with a cutter until the crank can be broken off.

If the screw that attaches the crank to the shaft is loosened or removed, the crank will in many cases come loose on its own when the bicycle is ridden for a while. However, this method of pulling the crank is not recommended as it leads to material fatigue.

Shape of the cranks

Until the 1980s, the cranks on the bike were usually straight and parallel to the frame. Since then, "low profile" cranks have been predominantly used, which spread outwards and are therefore combined with a slightly shorter bottom bracket axle. This results in more freedom of movement for the heel and a certain amount of material saved.

Crank length

The length of the cranks should be adjusted to the length of the leg. The most important thing is not to choose the cranks too long.

The exact length of the cranks then has only a minor influence on the efficiency of the power transmission.

Even if the maximum length is not exceeded, the optimal crank length can seldom be predicted using formula values, because it depends on the geometry, design and area of use of the bike, personal preferences and other various factors. It is often only after a long period of getting used to it that you can determine how well your body can cope with a certain crank length.

With the same cadence, a shorter crank requires greater pedal force. However, this is offset by the fact that the legs are bent less with shorter cranks. Due to the more favorable lever arm with the leg extended, the increased pedal force can be easily applied without requiring greater muscle power. In addition, a higher cadence can be achieved with short cranks, which reduces the required pedal force again.

While cranks that are too long can lead to knee problems, short cranks do not result in any serious disadvantages. Even very tall people can usually get along well with the standard crank lengths of 170, 172.5 and 175 mm, and even for drivers up to 1.90 m in length, it can be worthwhile to try even shorter cranks.

It is important to ensure that bicycles for children, for small people and for people with relatively shorter legs are equipped with correspondingly shorter cranks.

Depending on the geometry and seating position, many recumbent cyclists also get along better with shorter cranks. A typical crank length for recumbent bikes is 155 mm. Occasionally it is stated that the crank length on recumbent bikes is 19–21% of the stride length and that the leg should not be bent beyond the right angle at “top” dead center in order to avoid knee problems.

Breakage due to material fatigue on the crank arm

Breakage due to material fatigue on the pedal arm. Fatigue fracture (dark, with snap lines ) on a cast aluminum crank arm ; light: force or brittle fracture

The transition from the crank to the shaft is subject to high loads and a high number of load cycle changes. Typical torques in road cycling are 100 Nm, in rail sports up to 500Nn when starting off, so damage and material fatigue are more common here. The fracture under load changes -Beanspruchung the crank arm is called fatigue fracture referred to as vibration breakage or colloquially as fatigue fracture.

literature

Fritz Winkler, Siegfried Rauch: Bicycle technology repair, construction, production. 10th edition, BVA Bielefelder Verlagsanstalt GmbH & Co. KG, Bielefeld, 1999, ISBN 3-87073-131-1