Bicycle crank length
Crank length is a, pretty much, ignored variable that can have a big impact on cycling performance. Crank length affects pretty much everything we do on a bicycle since we only touch the bicycle at three points, the seat, the handlbars, and the pedals and crank length is the only thing that affects where the pedals are in space when we touch them so they affect the orientation of the legs when they are trying to apply power. Crank length has an impact on the power one can generate AND the aerodyanamic position one can achieve while still generating good power.
There are two major studies here. First is Determinants of maximal cycling power: crank length, pedaling rate and pedal speed by Martin and Spirduso of the University of Utah. While the paper concluded there was little lost riding 170 mm cranks they actually found that maximum power was actually achieved using 145 mm cranks, the difference being only abut 1%. The most relevant figure from that paper is reproduced below. . .
The second paper by McDaniel, et. al. entitled Determinants of metabolic cost during submaximal cycling looked at the effects of crank length and pedaling rate (cadence) on the metabolic cost of cycling. Again, crank length had almost no effect.
Now, it would appear at first glance of the Martin data that crank length really isn't really important. Maybe a little bit but not a lot. Then, why bother about crank length, why not just ride what came with your bike? Well, one problem with the Martin study is he didn't look at this relationship when the rider was in the aerodynamic position. Most experts agree that the lower one brings the front in trying to get into a good aerodynamic position the more the power is going to drop because of the difficulty in getting the thigh close to the chest. We would expect that this "crank length does not affect power" finding would not be so clear if position on the bicycle were added to the mix. And, if you are really serious about improving or maximizing your output why would you voluntarily give up even 1% power just because the difference is small? Martin couldn't possibly have understood the typical cyclist when he concluded that a 1% change wasn't important. But, even beyond this consideration is the effect of crank length on aerodynamics. Simply shortening the crank and moving the seat up the same amount (and doing nothing else to the bike) does four things in this regard.
1. It moves the butt up in relationship to the handlebars and shoulders so it flattens the back and chest, generally regarded to be a better aerodynamic shape.
2. A shorter crank reduces the frontal area, the most important element to good aerodynamics, and,
3. It opens the distance between the knee and the chest at TDC, making it easier to get the crank up and making it possible to, possibly, lower the handlebars even more, again reducing the frontal area and flattening the back, improving aerodynamics without affecting power, and,
4. Making it easier to get the leg over the top makes it easier for the rider to apply power over the top, generally the weakest part of almost everyones pedal stroke, which is the part of the stroke most affected by PowerCranks training accounting for most of the big power improvements seen by new users.
The McDaniel study says pedal speed is the real important determinant regarding efficiency (not crank length) but, in our experience, when people go to shorter crank lengths they tend to stay at or near the same cadence (their natural cadence) as they use with longer cranks which automatically lowers pedal speed. So, in effect, going to a shorter crank length slows pedal speed so should improve efficiency if their current pedal speed is too high. Does this work out in the real world? You bet it does, at least for most who experiment with this.
Three anecdotal reports to illustrate the effects of this.
1. One fitter reported to us he was able to lower the front of a cyclist 7 cm by changing the crank length from 175 to 145. Despite this lowering he still was seeing a drop in his HR while riding at the same power. Improved aerodynamics and improved metabolic efficiency (and probably improved power) with one simple change.
2. Here is a screen shot of a test done by a customer testing this hypothesis. He has a bike that allows him to race previous efforts on a screen in front of him. What he did instead of racing was to ride the same course at exactly the same effort in a single gear (so cadence is identical) following a previous ghost. In this effort he saw the average HR drop 10 bpm with the shorter cranks compared to the longer cranks. Here is a screen shot of his HR comparison.
3. And, an article in Triathlete Magazine in 2008 on this subject reported that John Cobb reported a 30% reduction in drag, determined in a wind tunnel, from simply shortening the crank length which allowed a lowering the front. According to analyticcycling.com a 30% reduction in drag should result in about a 7% speed improvement for almost everyone. This change alone would take about 21 minutes off a 5 hour Ironman bike split.
At PowerCranks we believe this will be very important to most riders, but most especially triathletes who do all of their racing in the aero position, in the future. Therefore, all PowerCranks now come with the ability to adjust crank length to almost any length you might want. Our basic cranks will now adjust from 90 mm to 175 mm. Now, with PowerCranks, not only can you learn how to pedal in a more efficient powerful pattern we make it easy to work on other important issues related to power production, efficiency, and aerodynamics. One product, lots of benefits. For a more detailed and "scientific" discussion of this issue go here. Watch the video