Bike Friday crank length
I know one advantage of a BIke Friday is that it can be set up with the same geometry as a full-sized bike. Like many people I have 170 mm cranks on my road bike and 175 mm cranks on my mountain bike (which I now think are causing some knee pain). Is there any reason to go to a different crank length on a folder? I was actually thinking about going to 165.
If you think that the bottom bracket is lower to the ground, then there is an obvious reason to go with shorter cranks. Since the gearing should still be approximately the same, there is no reason to think that you will have to spin more.
Generally, I don't think that the crank length effects folded size ... maybe on something like a Brompton it would make the fold longer.
Originally Posted by invisiblehand
I was thinking of it for both those reasons: more compact for packing, although it is probably a small effect; and in case it is lower to the ground, but haven't seen the specs on bottom bracket height.
Also I think that it might be better ergonomically. Certainly the 175s on my mountain bike "feel" too long and I think my knee started to feel a little sore when I switched to that bike for winter commuting last December, and the discomfort is felt as I push through the top of the stroke.
I also have this vague idea (which may be totally fanciful) that smaller cranks will somehow be better suited to the twitchy feel of a 20" wheel, since there won't be as big a shift in weight and effort from side to side while pedalling.
Car free since 1995
That twitchy feel will disappear after the first ride, and you will be addicted to it from then on.
I put 165s on my bike because I have knee pain. The pain mostly went away, but I'm not sure whether it was that or not. Remember, we are talking about 1/2 of a centimeter here, so this isn't going to make much difference one way or the other. It also doesn't affect the gearing much (check Sheldon's gear calculator, and you will see hardly any difference between 170mm and 165mm cranks).
I'm a physician with almost zero physics, and thus can't claim any expertise here, but crank length seems to me to be a bit more of a religious argument than a scientific one. If you do some web searches, you'll see all sorts of ideas about inseam length relative to crank length.
As for me, I'm 5'10", and 165s subjectively seem about right. If you are shorter or have short legs, you might consider going lower. But then, you'll be stuck in a vicious downward cycle with your other bikes!