The Relationship Between Crank Length and Gear Selection

In the last installment of this column, I discussed the general principles of how your bike and crank length influence power production in cycling. Even the fundamentals can be a little confusing, so let's review the basic points of the argument between shorter and longer cranks:

Long Cranks

  • Pro: Give you a bigger lever arm to push harder during the power generation phase.
  • Con: Can pull you out of position by requiring you to reduce saddle height.

Short Cranks

  • Pro: Take the "inside track" on longer cranks and complete a full revolution faster. Higher cadence can be more efficient for your quads and overall position.
  • Con: Shorter crank reduces the amount of torque you can achieve.

Just as a reminder, remember the relationship for torque:

Torque = (Force x Moment Arm) (where "moment arm" is just a fancy way of saying "crank length")

Climbing Gears

With that taken care of, we need to put things in context. The fastest way to do that is to imagine yourself pedaling up a very steep hill two different ways. The first way, we'd use a typical, common-sense climbing gear (small chain ring and large cog on the rear wheel). Pretty easy, right?

Now, switch to your largest, all-out racing gear ratio. It's almost impossible to get rolling before you fall over. What happened? The hill obviously didn't get steeper, but people very rarely consider the cause of these circumstances.

This is related to my introductory discussion on gear ratios, but here's the important thing to know: the ratio of your chain ring and cog size is also the ratio of the torque you generate at the pedal to the torque that happens at the wheel.

More: An Introduction to Gear Ratios

In other words:

(Torque at Pedal)/(Torque at Wheel) = (Size of chain ring )/(Size of cog)

Here's the part where we need to think a little. Given this relationship, we see that we get the most torque at the wheel for the least effort against the pedal in the climbing gear. That makes sense based on our experience, and it's why that gear is best for overcoming the force of gravity.

Think about it from gravity's perspective. In its view of things, the goal is to pull backward on your wheel hard enough that you come to a complete stop and fall off. But what is a climbing gear for you is a speed gear for gravity. It's pulling on a big gear that's attached to a smaller one.

In this way we see that changing gears literally turns the tables in our favor against the forces working against us. That leads to a new question, though. If we get more torque when we're in a climbing gear than in a "racing" gear, how is it that the racing gear makes us so much faster? The key to that is speed. We're almost there.

More: 9 Tips for Better Hill Climbing

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About the Author

Jim Gourley

Jim Gourley is a four-time Ironman finisher and part of a four-man division that finished the Race Across America. He earned a degree in astronautical engineering from the United States Air Force Academy and has written on science and technology in triathlon for four years. He is author of the book Faster: Demystifying the Science of Triathlon Speed.

Jim Gourley is a four-time Ironman finisher and part of a four-man division that finished the Race Across America. He earned a degree in astronautical engineering from the United States Air Force Academy and has written on science and technology in triathlon for four years. He is author of the book Faster: Demystifying the Science of Triathlon Speed.

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