There is a bit of a buzz these days for going to shorter cranks (crank arms) for both triathletes and cyclists. It's not for everyone, and like many things there are some misconceptions and misunderstandings of what's going on.
If you are in a great position on your bike and are having no issues, then going to shorter cranks is most likely not going to be helpful for you: Shorter Cranks does not automatically equal better/faster! If you are suffering some specific issues, relating to to your bike fit and in particular, where an opening of the hip angle will be helpful, then going to shorter cranks may be beneficial to you.
I had always ridden on 175mm cranks on every bike (road & tri bikes) that I have ridden since getting into cycling via triathlon in the early 80's. I had never given this a great deal of thought, because every, "58cm" bike I had ever owned, came with 175mm cranks on it! However, a couple of years ago, when I had just turned 50, I noticed a few things when riding the road bike:
1. I was getting increased lower back pain when riding
2. I could no longer ride in the drops of my road bike for extended periods of time
3. I had lost a bit of my jump
A swapping of kit on one of my wife Paolina Allan's bike made a set of Shimano Dura Ace 170mm cranks available to me. I looked into this a bit, and came across the findings of the 2001 Jim Martin study in several articles online (this one on the Cervelo web site), that came to the conclusion - basically, power generation is the same for a rider, using a wide range of crank lengths. I also compared notes with friend and Pro triathlete Jordan Rapp, who had just gone to shorter cranks on his set up, and noticed no difference in performance.
I started to put this together with what I knew was happening with my body - as I aged, I was becoming less flexible, and I was becoming weaker and less fit. Hey, it's inevitable! Shortening the crank arms, making the appropriate changes in the fit elsewhere (raising the saddle and bars slightly, to compensate), would open up my hip angle, and also make the circles that I pedal in slightly smaller.
The great news about the new crank and bottom-brackets, is that with the right tools, these are very easy to make changes - so, out with the 175mm cranks and in with the 170mm ones. I did this in the winter, so that the first few rides would be on the trainer. Just to check everything out.
I did notice a slight difference when I first started to ride with the 170's. However, by the end of a 45 minute ride on the trainer, I could not really tell any difference. Some informal bench-mark testing on the trainer over the next couple of weeks, backed up the research and observations previously mentioned - I noticed no difference in my "performance". Another difference I noted was that, at a given level of effort, I might need to drop down one cog to maintain the pedal RPM that I preferred. For me, not a big deal as I have always been a bit of a "spinner" with a naturally higher pedal RPM.
The real test came in the spring when I was outdoors again and going for longer rides:
A) Lower back pain was much reduced on longer rides
B) I could now ride in the drops like a used to
C) My jump in big accelerations on group rides was much better.
Conclusions: If you are an older triathlete or cyclist, (45+), and you are not feeling comfortable on your bike, and you have some other issues such as I noted, and all other things being equal, you might want to look at your crank length and consider trying shorter cranks. Even shorter than "recommended", or what just came on your bike. Note - at 6'2" (188cm) generally speaking, my recommended crank arm length would be 175 and as noted, on every bike I owned, it came with 175's. For me going shorter helped.
Hope this helps.
Are you an older cyclist/triathlete? Have you tried shorter cranks?
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