BSL, how would you structure such at experiment, or model? Probably better to look at factoring strength rather than muscle mass? or try to estimate the efficiency of a large muscle rather than a small muscle?

I find a longer lever (recently switched up from 170s to 172.5s) feels more efficient, but after a while my knees don't like the extra pressure. I'm better off spinning.

For a thread that is a year old this is an interesting one. When I purchased my Burley, I did not check the size of cranks it had. I rode it for a few hundred miles and knew from the beginning that something was not right. I was ready to change the stem, the seat post to a zero set back, new bars, anything I could think of that would make the fit feel right. Then one day I measured the cranks and they were 175. All my other bikes have 170 cranks. I swapped the cranks and there it was. Spinning again and no knee problems.

Bill, I've recently noticed my knees feeling tired after a ride and on my stairway the day after. I've been using 172.5's on two bikes this year, and finally noticing it's hard to keep up a good spin. I have more force at lower cadences, but find it harder to maintain 85-90 like I was doing last year with 170s. My pedaling keeps dropping down to the 70-75 range. I think you hit on something, thanks!!

I think I'm going to change back to 170 on at least one of them and see what happens regarding cadence and knee pain. I do know last year I rarely had knee discomfort after long rides.

Road Fan

it's usually stamped on the back of one or both of the crankarms

It could be that "Father Time" as resulted in a slight loss of flexibility in your knee.
It's nice to have a different length arm available to try out to see IF it makes a difference.
Post back about any difference you did or didn't experience.
At least you have a good, established reference point to compare to.

I still find it a bit odd, never using the big ring anymore, while before that, was all I used.

I found the difference in cranks does affect how I spin. On my main road bike I felt like at speed I wasn't smooth, so I switched from 175 to 170 cranks and now I can spin better. But I did discover that those big hills are harder to climb. I have a cyclocross bike which I ride on the road and on trails with 175 cranks and it feels fine to me. My mountain definitely is better for me with longer cranks, have more power at slow speeds.

Just had to drop in. I`m not a `Fifty Plus` (I`m 38) but the title of this thread was too much to resist. I raced on 177.5`s as a Junior/Senior. Had no knee issues, problems with position, etc. Admittedly, I wasn`t often spinning along but that wasn`t ever my intention. I have a bit of a strange body. 6`2 with a 29 inch inseam...the lower leg (what`s that bone called???) being extremely short. Well suited to cycling, quite strange looking otherwise.

Starting off with that kind of leverage, I chose to maximize it. Seemed to work. A few medals from Nationals and a couple other results. I think the trick is to find what works for you. We all have our own riding styles. A fluid, super smooth rider is going to be lost on 177.5`s, while I would have been throwing away a bit of genetic luck with 170`s. I`m glad the OP found a crank to suit his riding style. No length is `best` for everyone. Your physiology, riding style, etc., have to be taken into account. Have fun, all.

Certainly you'll experience a loss of "leverage" with shorter cranks, but you simply use lower gears to regain that. Spin a lower gear faster and you end up with the same force to the rear wheel.
I'm basically using the same "cruising" cogs in back, but using 38T on front instead of 46.

Before, I was using a 13-28 and now use a 13-24. The 38/24 combo gives me an ideal start gear, and I now have more closely spaced cogs for the rest of my range. (13-14-15-16-17-19-21-24) I'm typically cruising on the 14 or 15 cog, so I have "room" to go up/down a bit as the wind changes and still keep my cadence in a rather narrow range that works best for me.
I don't do hills

I havent read all the thread cuz it's quite long now but if noone else has mentoned it already you can get crank shorteners. They bolt onto your cranks and lower the crank length by certain amounts. My crank shorteners lower my 170mm cranks by 30mm,60mm or 90mm.

been on the forums for a while because I lost my internet access.

I just recently got the 165MM cranks on 2 different bikes.
The verdict-
175MM 60 RPM
170MM 75""
165MM 85""
160MM 80""

BTW, I'm finally on an inhaler for my emphysema.
530 miles so far this month, with a couple days left.
Previous monthly high was 121 miles.
Previous yearly high was 515 miles.

Way to hang in there! Excellent.

I seem to have the opposite results. On my road bike I have 172.5 and on the tourer I have 170.0 I can spin the 172.5 much higher than I'm comfortable with on the tourer. Can't frame geometry have something to do with this?

Several bikes, 170 to 180, , it really doesn't matter..
though putting the 180's on a bike I felt had inadequate saddle setback,
gave me the illusion of more setback.