What Size Cranks do I Need?


Optimise Your Crank Length – Podcast Interview


Here’s a summary of the podcast I did with Mark Florence on the cycling time trial podcast.

General Info


I worked with a former guest Coach Joe Beer, beat all my TT PB’s 10-100 miles, won 3 British National Titles. Learned a huge amount from him and was really inspired by the coaching process, which led me to start on some coaching courses.


Since we last spoke, I have completed my bike fitting certification and I’m in the final year of a Masters degree in Clinical Biomechanics. Prior to the pandemic I started working through the coaching levels. The pandemic left me with more time, so I speeded up the process and am now a level 3 ABCC cycling coach, now into my second winter with my athletes and really enjoying the process.

The history of crank arm length is shrouded in mystery. It seems no one really knows why a medium bike came or comes with 172.5 cranks. Do you have a better understanding of crank length history?

I have no idea why cranks are the length they are…it’s probably to do with the happy medium of usability by the rider, the mechanical constraints of the bike (front-wheel clearance and ground clearance).

There are studies that have come to the conclusion that crank arm length doesn’t have much if any effect on power production. What is your experience?

I’m probably like most people who start out cycling…their first bike has cranks of a particular length, didn’t think about it, just started riding. For many, this will be just fine. It’s only when you are forced to think about it through pain/injury or trying to make performance gains that you start looking around for different options.


There’s a general misconception that longer cranks will give a performance advantage, this just isn’t true. It’s like seat height, people think higher is better! Actually, the correct height is best…the correct crank length is best…Hopefully, the messages are starting to filter through.

All the research I’ve seen says that crank length doesn’t affect the power/metabolic cost of cycling. In fact for novice cyclists there is an economy benefit to shorter cranks…potentially due to smaller radius requiring smaller joint ranges, slower muscle contraction speeds and lower neuromuscular demand.

Mark’s experience, 172.5-165-150.

I’ve changed from 172.5mm to 165mm about 1 year ago.

Where does evaluation of crank length take place?


Before the practical/on bike part of a bike fit two things need to happen:

  1. evaluation of the bike

Measure up the key elements, seat height, saddle fore/aft, reach, stem length, stem angle, and of course includes crank length.

You need a baseline of where you started. It also gives you a picture of why certain things may be happening…injuries, aero-fit issues, etc

  1. evaluation of the person

A little bit like a physio evaluation, it’s important to get a subjective and objective history. You need to understand them as an individual before diving into the bike fit.

On the subjective side I want to know their:

  • injury history
  • training history (how many years, frequency, volume etc)
  • bike fit goals
  • cycling goals

Objectively, it’s a physical evaluation:

  • joint ranges, muscle ranges
  • assess any injuries
  • movement quality
  • postural tendencies on and off the bike

As a Physiotherapist/coach/fitter, You would initially question crank length when observing someone on the bike, what do you see that would make you consider suggesting a different crank length?

It’s a  tricky one because it’s not the easiest thing to change. As I fit people to their own bikes, I can’t make a change on the day, so it would only be a recommendation. Rarely if ever are people so far out that it stops the process in its tracks.

The primary goal when I’m fitting someone is to establish stability on the bike. This for me starts with stability around the pelvis. If that’s not stable, nothing really will be…it’s a great anchor point that you then build out from. Crank length can play a big part in that, but on the day you have to fit them on the bike in front of you.

Are your recommendations more based on physical needs and/or injury prevention?

Because you are trying to fit a huge amount of information together, it’s like a giant puzzle and if things start coming together, that’s great. But often alarm bells start ringing if someone on the shorter side has particularly long cranks…that’s probably the biggest issue. For someone 5’ 8” to 6’, cranks that are between 170-175 will be no real problem unless there are other issues in the mix:

  • short legs for your height
  • desire a really aggressive position
  • particularly inflexible
  • injury history to hips, back, knees

If you are shorter than that you are likely to need shorter cranks. Or if you have particular anatomical variations, movement or injury history that makes longer cranks a bit of a disadvantage.

For time trial it makes more sense to be on the shorter side of crank lengths. Shorter cranks give you more options in terms of fit. So particularly when you are trying to get very aerodynamic, where other margins are being heavily constrained (hip angle for instance) the shorter cranks make life a lot easier.

I think as a general rule shorter cranks give a greater window to fit people in. The closer you get to their limit of crank length the more precise the fit has to be to achieve all the goals of comfort, injury prevention and performance.

For example, if you have anterior knee pain, 175mm cranks might mean your fit window to successfully treat the knee issue may be really small…and sometimes non-existent. Whereas a shorter crank will be more forgiving and offer more flexibility in your fit. Also, you aren’t the same athlete every day…or even on the same ride!! If the fit window is so small any further complication (stiffness from a previous ride, stiffness from a bad nights sleep, fatigue late on in a long ride) might mean your fit no longer works for you, even if it was perfect on the day.

What type of riders might benefit from shorter cranks?


  • Short legs?

Definitely related somewhat to anthropometry…though shorter riders will proportionally ride longer cranks there’s no need to go much shorter than the 160-165mm that are more commonly available.


  • Time trialists?

Opens up the hip angle and gives greater scope for adjusting the fit for optimum aerodynamics.


  • Shorter riders?

As above

  • Pain/injury…particularly anterior knee pain

The longer cranks will tend to increase shear forces at the knee, so shorter ones may help those with anterior knee pain.

  • Stiff hips

It’s hard/impossible to get someone stable on the saddle if they don’t have enough range of hip flexion to get the pedal over the top of the pedal stroke…they have to compensate by shifting on the saddle or sticking their knee out to the side…short cranks are a must here


There are many complicating factors when considering changing crank arm length.

  1. Availability of really short (shorter than 165) cranks is challenging (Rotor/Cobb).
  2. Cost of changing bikes.
  3. Integrating power.

These are big issues, I changed to pedal-based power meter when I changed to 165mm cranks because the change of cranks was an experiment and if I wanted to change again it was less painful as the power meter didn’t have to change again!!



  1. Closing thoughts?

It’s like many other things in cycling…the downside is much greater on one side than the other…slightly too short cranks are no drama, slightly too long can cause a lot of problems… like tyre pressure, the losses from slightly too low are much smaller than the losses from slightly too high…like training with inflated FTP, train with zones from an FTP that is slightly too low, not a big problem, slightly too high is a big problem


  1. Thanks for joining us!

 Contact info





Scroll to Top