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 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. In short, crank length alone affects bike fit for both power production and aerodynamics. The problem is that, until recently, this variable has been pretty much ignored by bike fitters and coaches because no one really knows what is best and cranks of various sizes really aren't readily available anyhow.
This is slowing you down and I will tell you why. First, lets look at what little science there is on the subject.
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. This paper looked at the effect of crank length on maximal power generation in a group of male cyclists of average height, 179cm. While this paper concluded there was little lost riding 170 mm cranks (about 1%) they actually found that maximum power was actually achieved using 145 mm cranks! The most relevant figure from that paper is reproduced below. . .
While the difference was small, I don't know of any serious cyclist who would willingly give up even a small difference in power if it were easily fixed. Yet, many argue that this paper "proves" that crank length makes no difference. 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.
Many have concluded from the Martin data that crank length really isn't 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, there are several problems with the Martin data. 1. Why would a serious rider want to give up any power? 2. The Martin study group averaged 179 cm tall with a 6 cm std deviation - what about the small rider or tall rider? Adult riders vary is size about 30%. Frames vary is size about 30%. Why does crank length that 99.9% of all riders use vary by about 6% (165-175). The Martin study did not analyze their data as it related to the different rider sizes they were testing. 3. Riders were presumably in the upright cycling position so the effects of crank length in the aero position was not tested.
Most experts agree that the lower one brings the front to get into a good aerodynamic position the closer the knee comes to the chest, the more difficult it is to get the pedal over the top, and the more the power is going to drop. We would expect that this "crank length does not affect power" finding would not be so clear if rider height (or leg length) and position on the bicycle were added to the mix. 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 possibly allowing one to lower the handlebars even more, reducing the frontal area and flattening the back, further improving aerodynamics without affecting power, and,
4. Making it easier to get the leg over the top causes less loss on the upstroke and also 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. High power requires high force on the pedals. It is easier to apply positive force to the pedal if it is moving slower rather than faster. So, in effect, most people are probably now riding at a pedal speed that is above their optimum so going to a shorter crank length helps slow pedal speed so should improve efficienc. 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.
Watch the video
Crank length formulas
A lot of people have come up with crank length formulas but our recommendation is if someone says they have a crank length formula that will tell you what your best crank length is do two things. Ask them on what science it is based on and then run away. I believe there are so many variables that there can not be a single formula. Variables include: 1. Rider height. 2. Ratio of leg length to height. 3. Thin or fat. 4. Flexibility. 5. Power output (higher power probably needs longer cranks). 6. Fitness level (beginners probably need shorter cranks. 7. Riding position (upright longer, aero shorter). In the past we have thought that this was so complicated that the only way to know for sure was to experiment, which is why almost all PowerCranks models allow crank length experimenting to some degree or another. But, we have learned that people really don't like that so we are now giving 4 easy charts that will tell you approximately where someone putting out average power for an endurance race (2hrs) should be based upon your height. The key principle is that crank length, just like frame size, should be proportional to the rider height and then modified to what fits the individual. There are 4 charts, two for the upright position and two for the aero position, depending upon how you race. The charts range from 4' (children race also) to 6'8". The reason there are two for each position is there is one chart for those who want to follow the science shown above where the starting point is the optimum crank length for the average man in the upright position is 145 and the other for those who think a better crank length for this man in this position is 170. Pick your poison.
The Modified Proportional crank length tables
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 different take on this issue go here.