Every now and then I come across an article that challenges the status quo. This was a case a few weeks ago when I read a section concerning crank length from Bicycle Design by Mike Burrows.
Whatever bicycle you buy, there seems to be little option but to purchase the standard crank lengths of between 165mm and 175mm. There seems to be little evidence that these sizes are the best for everyone. It’s just the way things are. But there might be some cases when a shorter crank would benefit you.
The Crank Length Debate
A cursory search on the Internet reveals that crank length is a frequently debated topic on various forums. Although there are many individual claims that shorter cranks have been helpful, there seems to be little conclusive research in this area.
The first argument in regards to crank length is that shorter cranks allow the leg to work over a more powerful range of knee extension. If you have been squatting in the gym, then you know that you can lift more with a partial squat than a full squat. One set of test results done by NASA shows the maximum force that can be created in a leg press for varying angle of hip flexion and knee flexion. (NASA has done extensive work on measuring human performance, and the research makes for fascinating reading, by the way.)
Since cycling uses varying degrees of knee and hip flexion, this information is highly relevant. The above graph demonstrates the results for 75-79 degrees of hip flexion from the trunk (shown as 15-19 degrees from the seat in the reference) and knee extension of 80-170 degrees. As expected, increasing knee extension up to 160 degrees allows more force to be generated.
On bicycle, this force is used to turn a circular crank, so although the downward force might increase, the ability to turn the crank is related to the force that can be created around the circumference (tangential force). At the extremes, when the crank is near the top and the bottom, there is little useful torque being generated. A larger torque can be generated when the cranks are near horizontal.
A smaller crank would enable the knee to be extended slightly more and thus create a little more torque. This is shown in the diagram below. The grey lines represent the bicycle frame and the blue and red lines represent the positions of the knee joint with longer (red) and shorter (blue) cranks. The shorter (blue) crank has also enabled the seat to be slightly raised.
Reducing Fatigue Through Form
The second argument for smaller cranks is improved cycle fit. A smaller crank enables the seat to be raised slightly while maintaining the maximum knee extension at the bottom of the stroke. This change in seat height means the body can be tilted more toward the horizontal. This may help your aerodynamic profile if you are a racer. It also means the hip flexion at the top of the stroke can be slightly reduced.
I test a lot of cyclists and I often find that a lack of hip flexion at the extreme top of the pedal stroke means the hips tilt from side to side, the knees splay out, or the ride is just uncomfortable on a long distance. The hip flexors are not usually strong muscles and requiring them to lift the leg to full flexion several hundred times during a cycle ride can be a source of fatigue. Eventually the opposite leg is required to push harder on the crank to lift the leg over the top. This is a waste of energy.
Those with lower back problems may also find a slightly reduced hip flexion more comfortable as the lumbar spine is not repeatedly flexed to accommodate the lack of hip flexion movement. A more upright position would also reduce the hip flexion with possibly greater aerodynamic drag. Regular exercise aimed at helping improve hip ranges would also help.
Consider Tweaking Your Position
Everyone has a different body shape and you may be quite happy with your long cranks. But the range of standard cranks is small, and while any improvement you get from changing your cranks may also be small, it may be helpful. If you wish to tweak your position on the bike, it may be worth exploring shorter cranks with a bike fitting specialist.
Check out these related articles:
- Why Heavy Lifting is the Best Winter Activity for Cyclists
- How to Reduce Lower Back Pain From Cycling
- Why Serious Cyclists Should Consider Strength Training
- What’s New On Breaking Muscle Today
References:
1. Mike Burrows, Bicycle Design (Alpenbooks, 2000).
2. NASA, “Man-System Integration Standards,” accessed February 23, 2015.
