Ha! 🙂 I also have marks on my stays, but that's coz I ride with my heels in (toes out). I ride 43s, and I don't think that's big. I had to get rid of a Cinelli frame because of the shape of the stays.
I'm 182cm with long legs (89.5cm inseam). I tried 180s for about a year when I was obsessed with this issue, but didn't really have any extra problems hitting the stays, however, I ended up feeling a bit uncomfortable with them for other reasons. Like I hinted at above, I found it harder to push through most of the down-stroke because my feet were coming up too high at the top of the stroke, which is obviously compounded by having to drop the seat for the extra length. I had some rides where I kept getting off the saddle, due to my frustration at not being able to 'get on top of' the pedals to pound the [censored] out them. I also felt like I kinda had to reach forward. The day I went back to 175s I felt like Superman, so I went a step further and bought 172.5s. I'm definitely no slower with shorter cranks. The only time I miss my long cranks is when I'm off the saddle stomping over short hills, so I bought some 177.5s, but I hardly use them 🙂
Without getting into knee height vs saddle height (or, hip angle, I guess), I reckon, even though the 'percentage of inseam' method may not be the be-all and end-all of determing crank length, it's ridiculous that some shorter guys are using cranks which are 23%+ of their inside leg! I can't imagine it. That's like me using 205mm cranks! :p
Anyway, on the whole long crank issue (which I reckon is way overrated -- sometimes by me 😄), despite the increased availability of extra lengths, no pro, over about 5ft9", is riding cranks which match Zinn's recommended method, yet they're still smashing amazing times. Some guys are using 177.5s (Ulrich, Backstedt, and Boonen, I think), and we all know about Indurain using 180s and 190s for some time-trials, but they're all tall guys, with the exception of Ullrich, but I'd be amazed if 177.5 was anywhere near 21.6% on his inseam.
One of the 'infamous' long crank advocates says the main reason pros don't ride very long cranks is because of stubborness and tradition, but I don't buy it.
People usually only talk about spin/cadence and leverage when they speak about long cranks, but, in my opinion, the biggest impact long cranks have is on position.
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