Changing gearing when changing crank length

[JWTS]

I’m not talking about the aerobic effect or the “overall” feel. I’m talking about the force vector that goes into turning the pedals. That MUST increase at the same cadence and gear ratio with smaller cranks, so skeletal and neuromuscular strain he feels at any particular gear ratio and cadence will be higher. So of course you basically must compensate with higher cadence and lower gearing, and in some cases that really throws things off.

I went to short cranks long before it was cool--both TT bike and road bike. What I noticed was that my cadence increased without any real effort. It didn't feel any harder, and I wasn't consciously trying to pedal faster. This confused me so I went down the rabbit hole of trying to figure out why.

What I found in the publish research I could find at the time suggesting that "cadence" isn't really the self-selecting variable here--it's "foot speed". And when the cranks get shorter, foot speed decreases as cadence stays the same. Which is to say, you end up pedaling faster without really trying, because foot speed is what will remain relatively constant. This ends up reducing the peak forces. Peak forces (as measured with an SRM) stayed pretty much the same across crank sizes. So, everything corrected itself without any real effort. It just got rid of the pain in my hip from being folded over and my knee hitting my chest...

I also went to a 50/34 after the swap on my road bike, because like the OP I felt a bit overgeared on hard climbs. However, the 50/11 was actually fine, for the reason stated above. The only time it got uncomfortable was when I got a wheel change in a race and got a wheel with a 12T small cog, and a crazy tail wind. Not good times.

[SixThirteen]

Across nearly 3500 rides, my average cadence is

170mm 80rpm
172.5mm 76rpm
175mm 72rpm

[TobinHatesYou]

I’m not talking about the aerobic effect or the “overall” feel. I’m talking about the force vector that goes into turning the pedals. That MUST increase at the same cadence and gear ratio with smaller cranks, so skeletal and neuromuscular strain he feels at any particular gear ratio and cadence will be higher. So of course you basically must compensate with higher cadence and lower gearing, and in some cases that really throws things off.

I went to short cranks long before it was cool--both TT bike and road bike. What I noticed was that my cadence increased without any real effort. It didn't feel any harder, and I wasn't consciously trying to pedal faster. This confused me so I went down the rabbit hole of trying to figure out why.

What I found in the publish research I could find at the time suggesting that "cadence" isn't really the self-selecting variable here--it's "foot speed". And when the cranks get shorter, foot speed decreases as cadence stays the same. Which is to say, you end up pedaling faster without really trying, because foot speed is what will remain relatively constant. This ends up reducing the peak forces. Peak forces (as measured with an SRM) stayed pretty much the same across crank sizes. So, everything corrected itself without any real effort. It just got rid of the pain in my hip from being folded over and my knee hitting my chest...

I also went to a 50/34 after the swap on my road bike, because like the OP I felt a bit overgeared on hard climbs. However, the 50/11 was actually fine, for the reason stated above. The only time it got uncomfortable was when I got a wheel change in a race and got a wheel with a 12T small cog, and a crazy tail wind. Not good times.

I went to 140mm cranks as an experiment. Compared to a cadence of 90rpm on 165mm cranks, I would have to gear down and pedal at 107rpm with the 140s. And for sprints where I was typically at 110rpm on 165s, well yeah that felt chaotic on 140s. All to feel the same force feedback through the pedals (at that higher leg speed.)

https://imgur.com/a/3Cexos4

[DaveS]

It's not that complicated. You need about 2t less up front or 2t more in back to get a 4-5% lower gear ratio. A 50/34 should work. Either that or do nothing and accept that you lost 4.5% in mechanical advantage at the low end.

I'm a Strava user and use Sauce for post ride analysis. Buckhorn/Stove Prairie/Rist wall/Glen Haven are my favorite places for my climbing science experiments. Come ride with me and we can chat about science. I'll send you my contact info. via PM.

I ride the same areas, but I'm a 70 year old retired mechanical engineer who's not racing and just wants to complete 55 miles with 5000 feet of climbing at a reasonable pace. I couldn't care less about ultra-short crank lengths. I've ridden 170-175mm cranks since I came back to cycling in 2018 with two new knees. 170 works fine with my 73cm saddle height. My bikes are set up with 46/30 cranks and 10-44 cassette that produces a 675% range. If necessary, I can spin 80 rpm at 4.3 mph. I haven't needed that low gear yet, but I like the 30/38. There are places where a 30/36 became difficult last year. I rode 6400 miles with 400,000 feet of climbing last year.