No, Zinn is all wrong about cranks. He suggests crank arms that are much too long. In my opinion most of us use cranks that are far too long, but he advocates much longer ones.
There are a few factors that matter, and leg length or femur length is really not among them. What's important is:
When riding a fixed gear, make sure your pedal can't hit the ground. If it does, you are leaving the ground and coming down sideways. When in doubt get shorter cranks.
Short cranks are advantageous at high cadences. Short cranks put more strain on the cardiovascular system and less on the legs and in particular less on the knees, except at very low cadence, when you might not have enough leverage to turn them over.
Long cranks are advantageous at low cadences. That's especially relevant on rough trails where you may have to power over rocks and things at walking speed. Long cranks put your knees through a wider range of motion which can be painful or even injurious, especially if your not used to it.
I don't recommend cranks over 170 mm for anyone except Zinn himself. I have 165s on most of my bikes because a vintage version of anything shorter than that is hard to find.
I'm amazed that we can feel a difference of 5mm or less. I was doubtful, but I found that I can.
I think rhm might be onto something. I have a pair of TA 155mm cranks I'm going to try soon. They were made for junior racers.
rhm, my question is, I kinda understand how short cranks help us spin, but given their lower leverage, wouldn't they also increase the amount of force going through he knee? Well, yes, of course they would, but the question is, what's the important factor? They change the angle that the knee is bent at when applying maximum force, and maybe that is more important than the
amount of force.
I strongly suspect this has not been studied extensively enough. The trend of increasing crank lengths might prove to be a bad one. I've been happy with 167.5mm and 170mm cranks. I recently borrowed my sister in law's bike, a 2003 Specialized Sequoia. She and I are the same size, a 56cm frame. After riding taking a long ride on very big hills, I wondered if it had long cranks. They felt funny. Sure enough, they were 172.5. 2.5 seems tiny. That's a tenth of an inch. Yet I felt it. It wasn't uncomfortable, but is that really appropriate for someone with legs as short as mine?
I want cranks with adjustable lengths so I can do experiments. Rudi, any ideas for building those?
I never got comfortable on my mountain bike (which is now gone, stolen). I realized later that it might be because of the 175mm cranks, standard on MTB's. I was just riding it on roads, fwiw.