Recommended crank length to avoid cycling discomfort

Chris

4 years ago

The Problem

The length of cranks used on bicycles is important because:

Over two-thirds of cyclists are using cranks that are longer than recommended.
Using cranks that are too long can cause cycling discomfort and injuries, especially when riding longer distances, due to excessive articulation of the knee and hip at the top of the pedal stroke; using cranks that are too short is far less likely to cause problems.
For various reasons (which could be the topic of another entire post), the length of cranks that are fitted on new bikes varies very little.
Crank length should be proportional to leg length, just as saddle height and handlebar reach are proportional to other body dimensions.

Crank Length Recommendations

There are lots of recommendations and formulas for how to determine a cyclist’s “ideal” crank length, all of which are slightly different, but most lead to similar recommendations. I combined all of the information and further simplified things by only considering typical crank lengths and people’s heights to make 5 recommendations:

For people shorter than 1.65 m (5’5″), cranks no longer than 160 mm are generally recommended, but these are not so easy to find (such people’s bikes probably came with 165 or 170 mm cranks).
For people between 1.65 m & 1.72 m (5’5″ & 5’8″), 165 mm cranks are generally recommended (such people’s bikes probably came with 170 mm cranks).
For people between 1.72 m & 1.78 m (5’8″ & 5’10”), 170 mm cranks are generally recommended (such people’s bikes probably came with 172.5 mm cranks).
People between 1.78 m & 1.90 m (5’10” & 6’3″) are probably OK with the crank length that their bike came with (172.5 or 175 mm).
People taller than 1.90 m (6’3″) could consider cranks that are 180 mm or longer, but these are not so easy to find (such people’s bikes probably came with 175 mm cranks). Fortunately, riding a crank that is shorter than recommended is unlikely to cause major problems.

Summary

Everyone up to about 1.78 m tall (5’10”) is probably riding cranks that are longer than recommended and this may be the cause of discomfort or injury because it can be just as important to optimize the amount of articulation in the joints at the top of the pedal stroke as it is at the bottom. People taller than 1.78 m (5’10”) probably have a reasonable crank length for them, but they may still benefit from trying something else, especially if they are having bike fit issues or are particularly tall.

The shorter somebody is, the bigger the difference is between the recommended crank length and the length that the person’s bike came with, so the more likely it is that his/her crank length will cause problems, especially when Riding Far.

The above recommendations are very general and will not work for everyone. People pedal differently, have different leg length to body height ratios and react to changes in their bike fit differently. A professional bike fitter can hopefully give advice that is tailored to a specific individual. Unfortunately, many bike fitters don’t appreciate the importance of crank length because they often assume that the bike brands know what they’re doing when selecting crank lengths for each frame size.

For people shorter than 1.65 m (5’5″), the recommendation given above is deliberately vague. This is because the differences between the conclusions of the various methodologies are greatest for this height range. Hopefully those people can find a bike fitter who properly understands how to fit shorter people and they may well have to look outside of the major component brands to find a crank of the appropriate length (see options below).

Discussion

To maintain the same leg extension at the bottom of the pedal stroke when switching to a shorter crank, the saddle should be raised by the same amount as the difference in the two crank lengths. It is then the difference in pedal height between the top and bottom of the pedal stroke that is important, which is twice the crank length.

Therefore, a 2.5 or 5 mm difference in crank length corresponds to a 5 or 10 mm difference in pedal circle diameter and the pedal height at the top of the circle. Most experienced cyclists would consider a 5 or 10 mm difference in saddle height measured to the bottom of the pedal stroke to be a significant change, so they should not dismiss a 2.5 or 5 mm difference in crank length as being any less important because that makes a 5 or 10 mm difference in the saddle height when measured to the top of the pedal stroke.

Even so, I admit that a 2.5 mm change in crank length is hard to notice. I therefore normally recommend that if someone is going to experiment with different crank lengths that they should change it by at least 5 mm to make it noticeable and worthwhile. It would be better if manufactures only produced cranks in 5 mm increments and offered a wider range of sizes.

Many people wrongly assume that longer cranks = more torque = more power. However, torque doesn’t equal power if cadence isn’t kept constant and because longer cranks tend to reduce cadence, research has shown that power tends to stay relatively constant across a much wider range of crank lengths than has been discussed here. Crank length should therefore be chosen based on comfort and injury prevention, not on power optimization.

I’m not trying to invent anything new here, my recommendations are simply a summary of advice that has been offered by researchers and professional bike fitters.

Take-Home Message

I hope that this article makes people think more about their crank length and to question whether the narrow range of crank lengths selected by bike manufacturers has resulted in them actually having the most appropriate crank length for them.

Many ultra-distance cyclists (of all heights) have told me stories about how fit problems that they had struggled with disappeared when they tried using shorter cranks and so reduced the amount of articulation in their joints. Personally, I’m 1.72 m tall and am so much happier now that I’m using 165 mm cranks instead of the mix of 170 and 172.5 mm cranks that came stock on my bikes.

Crank Availability

The 4 major road crankset brands, Shimano, SRAM, FSA/Vision and Campagnolo offer road cranks from 165 mm to 175 mm, with certain models going to 177.5 or 180 mm. Some smaller manufacturers offer a wider range of sizes, including Rotor, TA Spécialités and Lightning. For extra-short cranks, the Stonglight Impact Kid is offered in sizes from 130 to 155 mm, but unfortunately the square-taper bottom bracket won’t fit in many modern frames. Lennard Zinn is a specialist in extra-long cranks plus bikes with geometry adapted to them and he also makes extra-short cranks. Please let me know in the comments if I’m missing other good options.