I found this to be a good discussion about crank length https://blog.bikefit.com/bf-podcast-ep-9-does-crank-length-matter-with-dr-jim-martin/
It’s been over a month since I listened to it and I want to be careful about generalizing but there was a lot of discussion towards going shorter for many people.
Hey Carytb, I’m 5’11” and been riding 172.5 for years.Ive been recommended switching to 165s like you but am wondering how you found climbing after the switch?
Cheers.
At 91 kgs I never was much of a climber and the cranks unfortunately haven’t turned me into one! Seriously I haven’t noticed any difference in climbing
I just know I can spin easier and long rides are more comfortable. The biggest help to me climbing was putting a 46/30 crankset on coupled with an 11-32 cassette.
Thanks replying so quickly, fascinating I’m setting up 165 on bike dnd hing yo test. 
This. Switching to shorter cranks does not mean your cadence will magically “improve”. You can always switch to a lower gear with your existing crank length to achieve a higher cadence.
Shorter cranks can improve bike fit issues for some, esepcially on a TT bike. They are not a panacea for improved cadence or higher power.
Been using 165s with the TT bike for a couple years now with good success. I recently switched to 165s on my road bike and I agree there is no free lunch on power or cadence for a geared bike. Fixed might be different but, I don’t know.
The older I get it’s all about economy and I think this is where shorter cranks might help certain riders with shorter torso to leg dimensions. Economy meaning lower or slower HR rise for the same power. Best I can tell there are at least two reasons for this. 1. Hip angle which has been well covered. 2. Foot/cleat position relative to the BB comes back which helps me get on top of the pedal/gear easier.
Extremely difficult to write anything definitive as we all seem to want to see what we want to see. Im no different. What seems to suffer is supra threshold power at high cadence. Think 5-10 minute power efforts. It sure seems like I want a longer arm in these moments. But, then when the pace settles back to threshold or just under the shorter cranks feel better than the longer cranks.
Peak sprint power appears to be the same however…
Going to 165 mm cranks has helped me the most in both comfort and pedaling efficiency however it does come at a cost, the torque required. Power (W) = Torque (N*m) x angular velocity (rad/s) so one needs to increase cadence if applying the same force (N) to the pedals compared to longer crank arms. To me it seems easier to keep the cadence higher on the shorter cranks but one needs to to keep the power the same.
Colby Pearce talked a little about this is his recent Fast Talk podcast about pedaling. He stated the GC guys like Porte use 175 mm cranks for when they run out of gears on the climbs. It was an interesting podcast to listen to. Not to say crank length isn’t important but I found the entire “this is how to pedal a bike” discussion a bit more interesting, even noticed some issues I feel on the bike being discussed.
I have awful knee problems and used to ride 175mm, for the past week I’ve gone to 172.5mm and have seen both an increase in cadence and a decrease in pain
I think it’s the other way. To replicate 34:12 I. 172.5 you’d be closer to 34 sprocket. with 165’s
Really good calculator on Sheldon Hall - work out with gain ratio and it takes account of crank length 