I'll try not to bore everyone by keeping this shorter than usual.
As I said, I'm 6ft with a 35.25" inseam. A few years ago I persisted with 180mm cranks for about 12 months (I even bought 3 sets of them for my 3 best bikes), but I ended up getting rid of them, and now use 172.5s. I ride no slower – and possibly faster -- with shorter cranks, and with no knee pain. I would've been happy to go back to 170s, but when I was 're-stocking' my bikes with shorter cranks, my local shop had more 172.5 DAs, and no 170s, so I took the 172.5s.
Firstly, long cranks impact more on position than they do on spin (unless the rider is a pure sprinter), which is something that I'm surprised gets overlooked. People think they can simply gain some extra leverage and power with long cranks, just as long as they can cope with a bit of a spin issue, but it's not that simple. For example, if someone gets 10mm longer cranks, then drops saddle the corresponding ~10mm for the extra reach at the bottom of the stroke, the pedals come up a whopping 20mm higher (relative to the rider) through the top of the circle. Being in a lower and weaker position, while trying to produce power through the downstrokes with larger hip and knee joint angles, makes long cranks harder to push. Relative to the top half of the downstroke -- which is where we get all our power -- It feels like you've simply dropped the saddle 20mm, and are hoping that the extra leverage will counter being in the weaker position.
That's the conundrum: there's more leverage, but they're harder to push because the rider is relative much lower, so it’s a bit of a wash. That’s why I gave them up (I also had some medial ligament pain which went away after I stopped using them).
My position became so screwed up with 180s that I was forced to try some 175s again, which felt much better (because I was higher over the pedals, so I could pound the crap out of them), so that was the beginning of the end for my 180s. I went back and forth between 180s and 175s for about a month, before finally getting rid of the long cranks.
I also found that my hamstrings were under a little more strain during 'red-line' efforts, perhaps due to the extra hip flexion, so kept lowering and lowering my seat, to the point where it felt as though my knees were in my chin at the start of each down-stroke.
So, there was the very frustrating feeling with my 180s of not being able to "get on top of" the pedals at the top of the downstroke to really pound the crap out of them. I'd sometimes have rides where I was constantly getting off the saddle to alleviate this annoyance.
There's also the issue of forward reach, which might not seem like much, but it's noticeable, and can be annoying. This may have contributed to my hamstring tightness and subsequent, repeated seat lowering, even though I moved the saddle forward
There's also a conundrum with torque and muscle force, which Arnie Baker explains much better than I can:
http://www.arniebakercycling.com/han...arm_length.htm
I still have some 177.5s on one bike with which I occasioanally experiment, but I much prefer the 172.5s. I don't mind the 177.5s for off-the-saddle, short hill efforts, but if I spend any amount of time grinding on the saddle with these cranks, they start annoy me all over again.
Secondly, the “21.6% of inseam” is totally bogus, and gets me a bit cranky!
It’s insane, especially for taller guys. It’s just one guy’s opinion (Kirby Palm), and he made it up to suit his own preferences, then Zinn jumped on it to suit his personal preferences, and to sell cranks.
Palm simply decided, just to suit his preferences, that, because there were a ‘certain number’ of riders with 31” inseams using 170mm cranks in the old days, this was the optimal ratio. That’s all it is! That’s where he got his oft-referenced, “magic”, “engineer-inspired”, dodgey ‘formula’. So what? There were also a zillion riders with 35” inseams using 170s, and a gazillion riders with 33” inseams using 172.5s. etc. What a crock! I could start a site saying that, because a large number of riders in the old days with 37” inseams used 170mm cranks, this then is the perfect ratio.
The 3 most prominent proponents of long cranks on the net -- Zinn, Kirby Palm and the ‘Thanks to My Cranks’ guy (his site has been down for ages) –- are all the same: they are all tall guys who were frustrated in the old days at the lack of cranks no longer than 175mm, so the sourced longer stuff, and went crazy with delite when they found what they were looking for. It snowballed from there.
The 21.6% of inseam thing would put me on 193mm cranks, which is so crazy I can’t even imagine it. I wish Palm would stop paying his server fee so that site would die.
I dunno how shorter guys ride on cranks longer than about 172.5. I have a theory that shorter guys are often weaker on flats, not because they are less powerful than bigger guys, but because they’re trying to push cranks which are way too long for them. Lots of shorter guys should be on 165s or shorter.
To rap up (I’m getting bored with my own thoughts
), no pro in the world, over about 5’9” is using cranks which are 21.6% of their inseam, yet they still all totally smash the crap out of us. Backstedt and Boonen use 177.5, but they’re both about 6’3”/6’4”. Ullrich also used 177.5s, but he’s 6ft. Indurain is 6’2”, and used 180s, and 190s for some time-trials. Considering Indurain is 2” taller than me, I’m gunna assume that his inseam is longer, or the same as mine (35.25”), which would still keep the 190s under 21.6%. His inseam would have to be 34.5” for 190s to be 21.6%, which is pretty short for someone 6’2”. And, ‘irregardless’, Indurain (and Ullrich) are two of the strongest ever riders! Sosenka reportedly used 192s for his hour record, but he’s 6’7”. Some claim that he used 200s, but it says on Bike Cult that he used 190s; and, anyway, 200 would still under 21.6% of his inseam. Pantani occasionally, reportedly, used 180s for mountain stages, but I remember him doing a lot of his climbing (especially attacking) off the saddle, which makes the position problem void. I don’t know of any current pro using 180s.
Thirdly, long cranks can hurt knees, because of force being applied with a sharper knee angle. I once hurt my knee in the late '90s when I jumped from 170s to 175s. This put me off long cranks for a long time. I also had some mild/moderate pain in both medials when I used the 180s.
'Fourthelelely', when I went back to shorter cranks, I had the strang sensation that I could ride at the same speeds with a lower heart rate, because the effort on shorter cranks was more of a 'short range stomp', rather than a longer range 'leg press', using muscles through a greater range of flexion.
To the original poster, I recommend getting some affordable 180s to try before spending big on anything longer.
More later….maybe.