Common sense. The longest crank you can turn will maximize your leverage, or more accurately in this situation "torque" over a given gear ratio. This thinking has lead to the common 165-175mm crank length. Leverage and torque are easy to comprehend. What is more complex and challenging to fully wrap your head around is power, or, the amount of work done as a function of time. The utmost concern of the dedicated cyclist is to optimize power output at a given fitness level, while maintaining an efficient position. Torque, I've found, is irrelevant. Old fashioned thinking lead me to believe that riding the longest crank that didn't hurt my knees was going to be fastest, and certainly best when the road turns upward. I now know that at least for me, this is not the case.
Five years of cycling culminated in the ability to average 19.5 mph. Nothing to write home about. In 4 months 172.5mm powercranks built me up to average over 21 mph, but more importantly allowed me to train knee pain free for the first time ever. As a bike messenger, an overuse injury would put me out of work. After hearing about Courtney Ogden winning an ironman on 145mm cranks and subsequently shortening to 115mm, I couldn't resist the urge to try out shorter cranks.
A quick google search revealed that my initial experiment would cost me 30$ for a set of 145mm sinz juniors bmx racing cranks. I was a little worried that changing my position on the bike and pedaling a drastically smaller circle (intended for 10 year old kids on bmx bikes) in the middle of my buildup for the 204 mile seattle to portland classic would spell disaster for my knees. I was ready to scrap the idea at the first sign of knee pain, and maybe try again when my training volume is reduced.
The first trip around the block was interesting, pedal stroke felt buttery smooth, difficult to "spin out" and it didn't feel like I lost any power, but they did feel strange. The next day, I went out on my first ride. My standard fitness testing route, 25 miles of rollers with a 2 mile hill that maxes at 10%. What I noticed most was how much smoother the short cranks are, and how comfortable I was in the drops. for the first time, I felt like I could ride in the drops all day long. My best ride on 172.5's put me at 21.06 miles in one hour, and the hill in 8min 14 sec. Rough wattage estimate for the hill 260. First time out with the 145's put me at 22.5 miles in an hour and the hill in 7min 34sec. Rough wattage estimate 292! Knees felt fine, I ordered a pair for my work bike. The next ride was 120 miles, I was amazed by how fresh I still felt after the ride.
The Seattle to Portland is not a race, but more akin to a marathon where the majority of the 10,000 participants are just there to finish. I was hoping to be in the top 100. I ended up finishing 4th, 6 min behind a recumbent rider and two guys who were working together the whole way even though I was riding solo for about 90 of the last 100 miles! The guy behind me was 20 min back.
It has been about 10 weeks since my first ride on 145's and I am continuing to see gains that I mostly atribute to the adaptation to shorter cranks. My new best ride on my standard training route put me at 23.02 miles in an hour and the hill in 6min 44sec. wattage estimate for the hill is 349. The way I see it, the advantages of short cranks are least apparent in cx where aerodynamics plays less of a role than being able to repeatedly pull hard out of corners. The first cyclocross preseason race of the year was this week, and I won on my 145mm sinz. This was my first race in 3 years, with no off road training, and the first cross race I've ever even finished.
I belive strongly enough in my experience with shorter cranks and their superiority at least for my genetic stew, that I sent my powercranks in to be irreversibly shortened to 145mm. I plan on experimenting even shorter to find my perfect length after I hit a plateau on 145's.