At r=172.5, the circumference of the circle your pedal makes is 1084mm (2*pi*r), for a 175 it's 1099. So in 96 revolutions of the 172.5 your pedal has covered 104,064mm (341.4 ft), 94.7 revolutions of the 175 is about the same distance.

Remember we are only talking about a 0.1" difference in crank length. If you did the same rpm comparison for 175 to 165 its a bit less trivial at 94.7 rpm = 100.5 rpm.

It's a static change independent of distance equal to (Old Crank Length/New Crank Length)^-1

I'm 5' 10" and went from 175 to 170. The 170's are right for me.

But if you can maintain that "pedaling in circles" and don't create dead spots (lose power and efficiency) in the stroke with 175's, go for it.

What I noticed when I went from 175's to 170's is that my knee and quad soreness went way down. And that 170's allowed me to finally learn how to pedal in circles rather than mashing it out using primarily my quads. Now my hamstrings and glutes are more engaged. It's a better pedaling technique now because the "circle" is slightly smaller.

Consider the changes that will take place in your leg angles at the top and bottom of the stroke if you use 175's. To get the same efficiency of 170's, it seems (at least, to my thinking) that the seat must then be set higher. Which might cause your hips to bounce. Hence, more possibility of chafing. And maybe more soreness in muscles and joints.

But you can always try it and change it if you don't like it.

One of the things I like about the 175s is that I seem to recruit more muscles. I get sore going to the 175s, not so much to the 170s. Specifically my glutes get more sore on 175s. This is when I make the change from one crank to another, not in general.

Since I drop my saddle when I go to the longer cranks (I tried keeping it the same, dropping 1/2 the difference, but my knees couldn't take either of those approaches), I keep the same extension. Therefore I bring my knee up further, stretching out the glutes more. Quads too but I have pretty flexible quads so it doesn't bother me at all.

Although the distance covered by longer cranks is marginally longer, I think coordination, fitness, and gear selection play a role in cadence too. On 170s I'd hold 90 rpm all day, and my efforts (TT, climbs, etc) were at 110 rpm. When I first went to 175s, I felt like my legs were flying at 80 rpm. After 3-4 months I could turn 175s over at 85-90 rpm comfortably (my average cadence five years later is 87-89 almost all the time). My max cadence on 175s is about 240; on 170s it was closer to 280+.

With 170s I can't turn the same big gears I can turn on 175s. I can't prove this scientifically, at least not now. I want to go over my power data from 2008/170s and 2009/175s which were on the same bike, just different actual arms (I swapped out the arms on the Cannondale SI cranks). I figure there'll be a relationship between length/cadence and cadence/HR

cdr

cdr, would be interesting to see some power data on this. From purely an engineering perspective this differences 5mm gives in leverage, and in angles/range of motion (any measureable) are very small. I would venture to guess they are within the range of what you could easily reach with a small variation in bike fit.

I'm curious about the power/cadence/HR data too. As you point out, the differences in measurements are pretty small. At the same time, I also know that 5 mm in saddle height can make a pretty big difference in feel. For example, say anyone could randomly change your saddle height 5 mm. That wouldn't be very comforting.

My riding style changes with different crank lengths, for sure. I don't think it's just a conscious thing either. There's one bit of road, maybe 200 meters long, where I get a good idea of how I feel. It's a false flat into a short rise followed by a false flat and a bit of downward sloping road. On 175s I fly on this section, on 170s not so much.

My position changes too, so that may be part of it. With 170s my saddle is higher, I have a flatter back, I feel more hunkered down, more aero. I'm sure I use different muscles.

So anyway I'll go look at some data.

cdr