It's not even arbitrary pseudoscience - it's just wrong. Something like 90% of the pros use 170-175mm cranks and the few others use bigger ones to 180mm.

Longer cranks effectively give you higher gearing. If you consider that gearing is limited by the size of your outer cogs, then changing the crank might be something pros to gain a little extra torque (since they have stronger legs).

Or maybe 90% of pros just happen to have leg length that means they need something in the 172 range.

I can't imagine wanting to go much longer than 170 personally, since it decreases your clearance between your pedals and the ground during turns.

difference of perspective.
take this example then...

If you're a giraffe, you probably wouldn't know the difference in height between tom cruise and johnny depp.
If you're a donkey, you'll notice.

trying to pedal through tight turns is never a good idea. a good number the crit crashes i've seen are due to pedals clipping the ground. those are the crashes get the most broken bones

I agree with the others who feel that these "rules" are guidelines, they get you into the ballpark but you have to tweak the fitting for your style and preference.

Same thing with KOPS for cleat position.

BTW the formula gives me 180mm but I am very happy with my 175's....and my ss commuter is 170 which is also ok by me.

Doug

I have a 30" pant inseam, 70cm saddle height and I've always used 170's because that's what bikes came with. Two years ago I bought 165 SRAM Reds and it's been okay. I'm now switching back to 170's because the S975 doesn't come in 165.

Incorrect.

If you're good at judging lean angle and maximize your clearance (clipless pedals, short crank, etc), then you can power out of turns faster, which shaves seconds off laps... Obviously you're not going to be doing any Moto GP stuff while you're trying to pedal, but you can get a pretty good lean. The real limiting factor is road conditions and tire grip.

They give you a bit longer lever to turn the gear, but don't change gearing.

Thus effectively changing the gearing.

no, no. the gearing remains the same, but the gain ratio changes.
http://sheldonbrown.com/gain.html

I'm 6'4", and so the usual crank arm calculations always seem to point to some very long length that is made by no one, and would require a custom frame if it were (to avoid ground-clearance issues). So my current bike has 175's on it.

Ironically, my old 1980 Raleigh (25.5" frame) has 165mm crank arms. I don't know if shorter crank arms were just more typical 30 years ago, or what...

I have average length legs for my height, although I kind of think that I have relatively long calves, and somewhat short femurs. (But you should see my lemurs. They're huge!)

I just measured really rough. 28", about 71 cm. I'm 5'7" with short legs.

I ride 175s.

I pedal through turns, even relatively tight ones (but not really tight ones).

Although I have a custom frame, I requested no particular BB height - it's about what they all are.

I can ride 170s too - rode them for track, and rode them on the road for part of the late summer, until I crashed in Aug. I used to ride 167.5s, for a long time, maybe 1989-95. I used to be much, much faster. Like 20% faster in the sprints. I used to be lighter (15-20 lbs from now, and I'm 25-30 lbs lighter than last year). I haven't done a max speed test this year, but I've done multiple long (30+ seconds - those are long to me!) efforts at 35-36 mph without standing, i.e. leadout/chase type efforts, not sprints. I just put an 11T on after not having one for most of a year, so maybe I'll go out and do some sprints at some point :)

I thought 170s gave me speed (back in 2008), so I rode them the year I wasn't working. I trained a lot, rested a lot, BFed a lot, and I did worse in races than normal (at my elevated weight - about 180-200 lbs). I returned to 175s and promptly did better in crits and such, with 1/3-1/2 as much training.

Now, lighter, with 175s on still (I put them on back in the early winter of 2009), I've had a great season so far. I've been holding off on going to 170s, but I may try it to get more speed. If, that is, 170s give me more speed.

I notice 170s vs 175s. I raise/lower my saddle height to accommodate the crank difference (5 mm). I'm most concerned with leg extension. The leg "rise" at the top of the stroke, not so much.

If someone raised or lowered my seat arbitrarily in a 10 mm range (+/- 5 mm) I'd notice it. I think most riders would too. I definitely noticed a 10 mm difference in stem length, and I noticed 5 mm in saddle fore/aft. 5mm in crank length is pretty significant. 2.5 mm, not as much.

I am more aero on 170s. Although my saddle is higher, my torso is more flat and I can hold a low position more naturally. Since my leg extension remains the same, my frontal area is basically the same.

To go to a longer crank effectively takes months, to raise the cadence back up, to learn to use the leverage effectively. I went to 175s because on my 175 mtb (2" knobbies, suspension fork) I was faster than my 170 equipped Giant TCR road bike (campy 9s). I went on to do very well on the 175s, but I was 210-215 when I was on the mtb, dropping to about 190 for the first races on the 175. For me long cranks make up for a lack of fitness and a lack of FTP. Since I lack FTP even when I'm fit, I'm not sure when/if I move to 170s.

But this is all me. Keep an open mind. I refused to try 172.5s back when I was on 167.5s, although I "tried" them for about 3 days. It took me 10 years go try longer cranks, and when I did, it made a huge difference.

cdr

I have ridden 170, 175, 180, none of which have made any difference to me other than when getting fit on the bike, cops, seat set, back etc. Currently my Madone and litespeed have 175s and my orbea has 180. Makes no difference to me at all.
I have a 33.5 inseam and I am 6'-1".

Indeed. I used 172.5mm cranks for a while (5'8" 31.5 cycling inseam) and once I used 170mm cranks I preferred those right away because I felt spinning was easier on the shorter cranks.