The crank arm is the component of the bicycle that transfers the force exerted on the pedals to the crankset. One crank arm is mounted to either side of the bottom bracket, usually they are mounted to the axle at a difference of 180 degrees. The left crank arm is always threaded counter-clockwise so that the crank will not loosen as the rider pedals. The length of the crank arms differ depending on the rider. Children's bikes have shorter crank arms than adult bikes. The crank arms can be made out of a variety of different materials including: titanium, aluminum, carbon fiber, and steel. Over the years, the size of the crank arm differs depending on the size of the wheels. Notice the short crank arms on this early bicycle.
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Types of Crank Arms
There are two main kinds of crank arms, a one piece crank arm and a three piece crank arm.
One Piece - A one piece crank arm resembles an S and this piece goes through the bottom bracket and the pedals attach on each side. This type of crank is typically seen on older bicycles and on children's bikes. This simple crank is the cheaper of the two types. The advantage to this crank arm is that there are no special tools needed to fix it. The disadvantage to this crank arm is that when the arms are tightened, the ball bearings in the bracket are also tightened and this can result in unwanted friction.
Three Piece - A three piece crank arm consists of three parts: a separate axle and two crank arms. The axle is inserted through the bottom bracket and the crank arms are attached on either side of the axle with bolts. This more expensive three piece mechanism is more difficult to service as compared to the simpler one piece crank. However, this assembly is stronger, smoother, and does not put pressure on the ball bearings in the bottom bracket, therefore less energy is compromised by friction.
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