Bike Crank Arms Explained – What It Is and Important Things to Know

Mountain bike crank arms are an integral component in the drive assembly on your bicycle. However, there’s more to these machined components than just the transfer of power.

Bike crankarms transfer your pedaling power by connecting the pedal to the bottom bracket and forward gearing. Crank arms act as a rotating crank, part of the crankset, allowing the rider to apply force to the pedal transferred via the crank arm to the forward gears to drive the bicycle chain.

Let’s take a ride along the information highway to learn more about the design, lengths, types, and more information about crank arms. After all, if you’re going to learn about your bike, you might as well learn all you can get, so you get the right components to make your ride better.

Mountain Bike Crank Arm Design

The concept of the modern mountain bike crankset is that of simple yet effective design and engineering.

Mountain bikes can be found with one of three types of cranksets, and they are dependent upon the style of bottom bracket spindle on the bike. These styles are:

• Square Taper
• One-Piece Splined
• Spline (discontinued by Shimano)

The square taper spline is a shaft that runs through the bottom bracket, typically with sealed bearings, but some older or cheaper models still use cup and nut technology.

The shaft has two unique characteristics. First, the ends are squared off, so you can quickly see how the crank ark securely turns the spindle without slipping. Second, the shaft is drilled and tapped to accept a bolt threaded into the end. When you look at it from the end, it appears to have a circular hole in a square shaft or a circle in a square, in other words.

The one-piece splined crank arm concept is a take on the older and simpler one-piece cranksets that you still find on the smallest and cheapest bikes. However, the design principle is sound. These cranksets have the arm, spider (part of crankset that accepts bolts for chainring), chainring, and the spindle all in a single assembly. Many of these have spindles with splined ends. Cheaper bikes still use the square taper splines.

A splined bottom bracket is different from a squared type in that the outer shape is that of a spline and not square at all. This design was produced by Shimano but didn’t last and is not common anymore. That is to say, Shimano no longer produces the separate piece splined spindle in favor of the one-piece crankset style.

Physics And Bike Crank Arm Length

The whole concept, as you know, is that the crank arm rotates around the central point of attachment to the bottom bracket. Whether a new one-piece, a new or used tapered square type, or even one of Shimano’s older spline types, the concept remains the same.

However, have you considered what happens when you lengthen or shorten the arm? Not that the arms are adjustable, but you could purchase a replacement of a different length.

What would happen when you lengthen vs. shorten your crank arm? Well, to simplify the scenario, a shorter crank arm means you will apply more force per inch of foot travel along a smaller rotational circumference (the path your foot and the pedal travel through a rotation). (source)

On the other hand, a longer arm will mean you use less force per inch of foot travel, but require more speed to achieve the same effect as the shorter arm. However, the speed is the path of rotation, not necessarily the overall pedaling speed, which is faster on a smaller crank arm due to traveling a shorter distance at the same time.

Our friend physics dictates that the longer arm must now travel in a bigger circular path to achieve the same speed at the spindle as a shorter arm that travels a path of a smaller circle but requires more force to do so. Let’s summarize:

• Longer arm = Slower pedaling over a bigger circular path and more force per inch of travel to achieve speed.
• Shorter arm = Faster pedaling over a smaller circular path and less force per inch of travel to achieve speed.

How To Determine Which Length Crank Arm Is Best For You

There are three crank lengths on the market that most bikes will adhere to:

• 170mm
• 172.5mm
• 175mm

These sizes are the most common, however, there are cranksets available with lengths ranging from 150mm to 180mm. (source)

Shimano says that the ideal bike crank arm length is between 170mm and 175mm.

Keep in mind that it may not be suited to your frame when you move to a considerably longer crank arm. Very long crank arms work best with frames where the bottom bracket is raised.

Some studies have made some pretty outlandish claims about determining the crank arm length. One study, however, made more sense by examining the optimal length for maximum cycling power. The study found that the ideal crank arm length is about twenty percent of your leg’s length.

Measuring Mountain Bike Crank Arm Lengths

Most MTB crank arms worth their salt will have a stamped length on the arm. However, you may be wondering how the length is determined because when you put your crank arm to a tape measure, it doesn’t seem to match what’s stamped inside.

Bike crank arms are measured using a center point principle. The length of a bike crank arm is measured from the center point of the pedal threaded hole and the center point of the bottom bracket mount hole.

How To Remove A Bike Crank Arm

Tools:

• Small flat screwdriver – find on Amazon
• 14mm thin-wall socket – find on Amazon
• Ratcheting Wrench – find on Amazon
• 3/8 Drive Torque Wrench – find on Amazon
• Bike crank puller tool – find on Amazon
• Two adjustable crescent wrenches – find on Amazon
• WPL Grease – find on Amazon
• 15mm pedal wrench – find on Amazon

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Procedure:

1. Remove the pedals using your pedal wrench.
2. Remove dust covers from the spindle, exposing the crank bolt head inside the crank arm.
3. Undo and remove the crank bolt using your 14mm thin sidewall socket and an appropriate ratchet.
4. Carefully thread in your crank puller tool.  In my experience, it’s best to apply a touch of grease to the crank puller threads. It will make the extraction of the tool easier when you are done.
5. Tightening your crank puller’s secondary portion will force the tip of the crank puller to hit the bottom bracket spindle. Further tightening and the crank arm will break it free of the bottom bracket.

Replacing your crank arm is as easy as following these same steps in reverse order. The difference is on step 3, where, if you want to do things properly, you’ll install the spindle bolt using the torque wrench instead of the standard ratchet. Recommended torque settings are about 30-55 newton/meters force on the crank bolt. That’s about 3-5 hundred inch/pounds of force.

Similarly, when reinstalling your pedals, keep in mind that the optimal force to tighten them down is between 35 and 45-newton meters or about 300-inch pounds of force. (source)

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