Thought I would share what I have discovered after experimenting with ultra short cranks for a period of about just over 12 months, going down from 175mm to 120mm cranks.
I am 5ft 9", late forties, stocky build, 75kg, inseam measurement is 31", my flexibility is moderate to poor and I'm very lean for my height and weight. I probably should add I race at A grade level with my local club so I am an experienced moderately strong rider I guess!
My main reason for going shorter was,
1. Chronic low grade off and on issues with my hips, knees, back and dull aching buttock pain, nothing bad enough to stop me doing things, just annoying.
2. Being an open minded person I was keen to try shorter cranks after reading a lot of articles by Frank Day.
I first tried 155mm cranks which on the first ride (to be quite honest) felt terrible! At the time I guess my body and brain were so accustomed to using 175mm cranks for the last 40years I didn't really expect to feel so weird, literally to the point where I cut the ride short, road home to swap the cranks back again. Then I remembered Frank saying you need time to adapt, so I decided to persevere for a month to see if I could adapt. Best thing I ever did, withing about 2 weeks I felt completely normal on 155's, to my surprise I was not slower at all (despite people telling me I was just plain stupid) and interestingly some of the physical issues I had started to back off slightly, which gave me some real incentive to possible consider maybe going even shorter! So that's what I did over a period of about 12 months I slowly went shorter in length about 10mm jumps at a time until I reached 120mm. Interestingly each time I tried a shorter length I had that same "how could I ever get use to this feeling" but once adapted each length shorter I went the better I was riding, slightly faster but way more comfortable in the process! By the way all my physical issues completely disappeared by about the 135mm length, which was my initial incentive to try shorter.
Okay so after 12 months of experimenting down to my current length of 120mm, this is what I found. I can now ride on the flats at 50klm hr for short periods, where as before I could hardly handle more than 46klm which was a big one for me, that was probably where I was weakest. Hill climbing has changed in an interesting way I am climbing hills now seated almost the whole way where as before I use to stand up the whole time( I hated climbing sitting down) but now I love it, I would say my climbing is similar performance wise to before but I like having the option to stay seated rather than just stand all the time. The shorter hills I'm performing similar to before but over long mountain passes I'm definitely performing slightly better! Sprinting out of the saddle is an interesting one I find I don't quite have the initial jump I use to have on the 175mm cranks but I can hold the sprint for a lot longer without fatiguing and my top speed is always slightly higher. So I have to judge my sprints differently now, it took me a while but I'm starting to prefer that feeling!
Now my technique is were things have changed considerably, once I got down to about 140ish I started noticing pulling up a lot more on the up-stroke this not only helped me power wise but made the shorter cranks feel more natural to use and a lot smother while pedaling, once I got down to 120mm I now pull up almost as strong on the up-stroke as I do on the down-stroke. After reading a lot Frank Day's essay's on short cranks he made mention that once people get use to using Power Cranks they quite often start preferring shorter cranks because Power Cranks force you to pull up more on the up-stroke from what I gather, correct me if I'm wrong?
So generally I just feel strong now on the bike, much more comfortable in the aero position and all aches and pains have subsided!
Only real down side is finding cranks short enough, I use sinz square taper cranks they have a 110mm bcd spider and with a 103mm bb I have a Q-factor of 147mm and chainline almost identical to modern shimano cranks, they work perfectly. I have also drilled and tapped my own pedal threads in some Sram apex cranks they can easily be cut down to 125mm although I wouldn't cut them any shorter than that. Also I had to get a longer seat post as with the change in cranks length I had to lift my seat height a total of 55mm.
I actually ordered a set of sinz square taper cranks in size 115mm which I cant wait to try out!
Hopefully my findings inspire someone else to try going shorter, it's a fun eye opening experiment.