What is a Crank Arm?

What many people believe are the pedals on a bike are actually a number of important parts that drive the bike forward. The pedal itself is the platform on which a cyclist’s feet rest. The pedal is then attached to a crank arm connecting the pedal to the bottom bracket, which is a series of bearings within the frame that allow the crank arms to rotate. Although this sounds a bit complicated, the system itself is quite simple and provides a smooth pedal pull that propels the rider forward.

The crank arm is usually made of steel, aluminum or higher end materials such as carbon. Affordable bikes will have a simple steel crane arm that is very strong and much heavier than other materials, and more expensive bikes will use lighter materials to save weight. It is important for this section to be very rigid so that the traction force is not lost for flexing the cranks; instead, power should transfer directly to the chain, which drives the bike forward using a series of gears.

A crank arm set contains two arms. The non-drive side crank arm, or crank arm on the opposite side of the chain, is a simple arm connecting pedal and crank. The drive side of the crank, but is a bit more complicated: it holds the pedal at one end in the same way as the non-driven crank arm, but at the other end, the arm must be attached to the bottom bracket and it must be attached to the chain rings, or the front “gears” “of a bicycle. These crank drives are part of the driveline, which is the system that drives a bike forward. It includes the wreaths, chain, and rear cassette – or cluster of gears.

In recent years, the design of the crane arms has changed considerably. The method that the pedals attach to the crank arms has been largely the same, but the way that the crank arms connect to the crank has gone through many changes in design meant to make the system more efficient and less prone to power failure due to flex. The crank arms have historically attached to the bottom bracket by a square hole which is attached to a square conical arm on the crank; The boom arms are then secured with a threaded boom arm bolt. Many types of attachment systems exist, from wedge crank systems to lighter and stiffer shaft systems.

by Abdullah Sam
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