Bike Crank Arm

A bike crank arm is the part of the bike that transfers power to the chains. A pedal will almost always be mounted to a bike crank arm to help generate its energy. The chain ring is also bolted to the crank arm, and is the primary lever that you use to drive the chain with pedal action. This particular bike crank arm is 170 millimeters long, and they use a square taper-type bottom bracket which is made out of aluminum.

Why might you need a new bike crank arm?

It's pretty common if you're peddling hard to pull the threads out where the pedal threads in. Some bike shops will offer a Helicoil repair service, where they bore the old threads out, put an insert in, and then thread your pedal in. But crank arms nowadays are so inexpensive. The labor to do a Helicoil repair is usually more than just to buy a new crank arm. The other thing that can happen is if the bottom bracket bolts that hold the crank arms to the bottom bracket get loose, and this frequently happens on the left-hand side of the bike. The bolt sort of backs out and allows the square taper bottom bracket to sort of start rounding out the square hole where the crank arm meets the bottom bracket spindle. And then, it doesn't spin true. It's blocking around inside the rotation. Those are two main reasons people replace their bike crank arm. Thread failure, and then rounding out of the square taper hole.

What differentiates bike crank arms?

Bike crank arms are a fairly standard, universal part. Most bicycle manufacturers will create a standard, replaceable bike crank arm so that you could potentially source a replacement part elsewhere. You wouldn’t necessarily have to buy your crank arm from the same manufacturer of your bike. A measurement that is necessary to keep in mind is the crank arms pedal thread width. Most bike crank arms come stock at 9/16 inch, but other more built custom bikes may have a different measurement.

Bike crank arm installation

If you stripped out the pedal threads or something was stuck, you may find yourself needing a new bike crank arm. Luckily they are relatively easy to install. You will need to remove your bike chain and then remove the bolts bolting the crank arm to the frame of the bike. Most bikes are right-hand drive. Meaning you've got your crank arm, your crank arm bolts, and then you've got your sprocket attached with the bolts all on the right side.