One of the oft debated topics for MTB set up is crank length. Typical lengths range from 170mm long to 180mm long, but the huge majority of crank arm lengths on MTBs is an industry standard 175mm. Why? No idea, really. I have no clue on how this length was picked out, but it is ubiquitous regardless or frame size (most of the time) and as a result, rider height/leg length.
The thought here is to match the length of the two levers...or maybe three levers...or maybe one lever and a fulcrum or two...heck, I am just as clueless to mechanical engineering and how it relates to the human body as the next guy, but often a longer leg length points to a longer bicycle crank arm, at least for MTB use where high RPMs are not the norm over long distances (unless you are Fixie Dave...but NO ONE else is Fixie Dave but Fixie Dave so that is silly).
I ran 180s for YEARS and years, maybe from the early 90s on. We all were on geared bikes then, no SS, etc. I even ran 177.5s on one road bike. This came from a suggestion from Doug Curtiss of Curtlo Cycles, he being very tall (at least 6'4"). He ran 190s if he could find them. I was having some knee pain and he looked at me and said, "run a longer crank". Now I am not sure where that idea came from...too much welding fumes, perhaps, but he had way more experience on a bike then I did so I jumped up to 180mm XT cranks and never looked back. No knee pain either, so there ya' go.
I found that it worked well with my 'diesel' approach to climbing where I would get in a taller gear, slide back on the saddle, and 'dig in'. Chug chug chug. I really never found an issue with spinning as we all had triples and we rarely spun out the gearing in those days unless you were under gravity's hold on you. If that was the case, we were just holding on for dear life (pre-suspension, you know).
So when I got back into riding after taking some time off from Martial Arts, I was right back on 180s. Then, the bike testing gig comes along and guess what? No one specs 180mm cranks on a stock bike. So I was spinning these 'tiny' cranks and I noticed the loss of torque right away. I ended up riding in lower gears and at a higher RPM but that became the norm and I got used to it. After a while, only the Lenzsport (hardly ridden) and my SS had 180s.
Then I got the first SS test bike in with 175s on it. I was concerned about losing all that leverage. So then one day I led a group ride that was a few hours of loops around our local trails and at the end I found the results to be intriguing. I did notice more effort on steep climbs, but I also felt like I could get the crank 'over the top' easier and I even felt like I was less tired afterwards. Huh. How about that?
Another few years of 175 cranks and now I am on NO bike with 180s, even my SS. I begin to wonder if I am missing something, so on a recent build up of the Blackbuck to test some wheels, I went back to an old, trusty set of XT Hollowtech II 180 cranks. I also geared a bit lower by 2 teeth in the front to get this bike more set up for difficult trail rides, thinking that the combo of long crank and low gear should make steep climbs or slow trails a breeze. Normally I run 34/21 on an SS and this was now 32/21 and the 180s.
Last night I got out on the bike for the '20 Mile Loop'. Ed The Tall was testing the carbon Stumpy SS for me, so I was on the SS Blackbuck with the180s and low gears. Pretty much sucked, that set-up. Part of this were the long paved road climbs to get to the trail head (and this bike was not built for that stuff). But when I hit the dirt, I would have my revenge. It begins with an ugly grunt up a rutted road that is a grind on a single geared bike. I figured I would be pedaling in tall cotton here with all those wise choices I made in setting up this bike. Watch out Ed The Tall...here comes the wind. No wind. Well maybe a headwind. Slooooowww and well, slow. Hard, too. Whaaaahhh???
I think a couple of things went wrong. One being the longer crank gave me the overall result of a lower effective gear AND, two, I geared down at the same time. I should have just run a longer crank with the 'normal' gearing OR the shorter crank with the lower gearing...not both. Both in combo created a dead spot in the crank rotation that was so big and long lasting that I could check email and text someone while I pedaled through it. 'BAAA-LUUMP...BAAA-LUUMP' etc. My legs were WORKED and I was going slower.
The last ride on the Super Stumpy 'S-S' for me was on that same route. My legs were slightly fresher but I was just getting over being sick so I was hardly 'strong', yet I was moving faster EVERYWHERE on this loop. Now the 'S-S' is a fast bike in its own right but I also had begun to experiment with and refine my standing pedal stroke to where I was making more of a 'circle', less of a 'piston' action and I was surprised to find how my climbing speed increased. I need to get better at this as it was bringing in different muscle groups but I think once I get this figured out it will be a big step ahead for me.
I could not do that last night on the 180mm cranks. Too big a circle...too much dead stroke. Well, I COULD do it but, it felt like I was walking though an ankle deep, muddy field..pick your feet waaaay up and step, etc. Now another note of interest is the comments from my buddy, mentor, and editor Guitar Ted, who states that the 'old timers' of single speeding said that 170mm cranks were better for SS use. Really? I do not know ANYONE that even HAS a set of 170s. And that wisdom comes with no information behind it, only that it 'works better'. But I can see a point where it might allow a quicker, smoother rotation getting over and past TDC on the crank position. Interesting. However, I would think that gearing would need to be adjusted down a bit too. Dunno if tiny cranks are in my future but it is something to try.
So guess what I am going to do for now? I am going back to 175s on this SS build and I am not looking back at all. The 180s will be dust binned till a longer legged person comes along....so ends an era, in some ways.