Wednesday, November 07, 2012

Crank Length And Single Speeding: More Opinions

Two years ago I sent this missive off to the digital realms concerning my thoughts on crank arm length and single speeding. Well, I was asked just yesterday about my thoughts concerning this subject again, and in specific, to what I thought about these.

Crank length? Here we go again....
If you do not clink the link, what it shows you are some new cranks made for IRD that are available up to 225mm in length.

On the surface of it, this isn't all that alarming, nor does it seem bad or good. Weird? Yes....perhaps. But let's consider what the longer crank set is doing when you use it.

I think modern power meter gadgets have helped us understand what is going on here in a better way. So, as most anyone even vaguely familiar with power meters knows, your pedaling circle has a sector in its arc that produces power, then tapers off, then there is a "dead sector", then things ramp up back into the power making sector, and so on, and on as you pedal. (I am simplifying this for my example.) Let's assume that you produce power for a certain amount of degrees of crank arm rotation. That distance you move the arm changes as your crank arm gets longer. Longer arms mean you are in the power sector longer. Is this "more leverage", or is it "spreading the butter thinner"? You only make "X" amount of power, so my thinking is that the longer the arm, the longer you are in the power sector, and unless you increase the effort in that part of the arc, you aren't making more power, just spreading it over a longer arc. Maybe I'm screwed up there, but that's my take on that, all else being equal.

Longer crank arms = lower saddle height.
But as we all know- all else isn't equal. One of the things a person can do is to use a longer crank arm to adjust your weight on the bike, and relationship to the bars. Since your legs are a fixed length, (well, I assume that most of you are not related to Gumby), when you go with a longer crank length, you should adjust your saddle height downward an appropriate amount.

I used this trick to make the Singular Gryphon I have fit me better. The stem I used is a "standard" Thomson issue, not a high rise one, which I would have had to have gone to if the cranks on this bike were 170mm instead of 180mm. Not only that, but my weight is set differently on the bike as well. More "in the bike", than "on the bike", as it were.

But getting back to those "freakishly long" cranks- I actually rode a bike with 225mm arms. My friend, Ben Witt, had a bike set up with some of these long armed deals and I tell ya- it was a very odd feeling.  It was as if the amount you could push down was never ending. It promoted a really low cadence, (which was abetted by the high gear inch this fixed gear bike was set up with), and I felt as if I was on an exercise machine, not a bicycle. Which is telling, actually, because I figure my muscles would have been fried long before my lungs were on that rig.

All that said- it makes sense for a longer legged individual to get the longer armed cranks. What you are attuned to will help determine this length, but in general, taller, longer limbed folks may want to look at "alternative length" crank arms like the IRD ones. For the rest of us? I think the 180's are not offered enough, but the lengths available serve most of us rather well, and that's why the industry settled where it did on crank lengths.


mw said...

two bikes with 180mm, the rest with 175mm. i like the difference and i like the 180mm on my flat bar SS as well as my flat bar geared bike.

Adam Lorenz said...

Maybe I am not attune to my set-up as I should be but over the years I've run 172.5, to now 175 and haven't been able to feel a difference (even on the rouge 180 on my polo rig).

I was intrigued by the points Zinn brings up in his justification for longer crank arms... granted he is also trying to sell the cranks he produces as one with a naturally slower cadence by his math I should be 200+ mm crank length to match my inseam. Too bad there wasn't any easier way to demo these.