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Old 11-03-21, 07:29 PM
  #79  
Carbonfiberboy 
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Originally Posted by smashndash
I'd just like to point out a few things:

- once you adjust your crank length, you should no longer merely be discussing cadence. You should also be discussing foot speed. If you went from a 170mm crank to a 160mm crank and your cadence only increased by ~6%, your effective foot speed is the same and you have seen no gain. In fact, you're now at a detriment because you have to fire your legs at a higher rpm, which requires more focus, more overhead etc. That being said, I think people who move to shorter cranks find that their rpms increase more than the "break even" amount due to the limited knee flexion.

- One reason why you may see less knee pain when switching to a shorter crank is that, if your knee has any wobble (even a couple mm), a shorter vertical ROM will also reduce lateral displacement. Preventing your knee from bending or straightening too much will allow you to more easily compensate for poor cleat alignment. This does not mean you "need" shorter cranks.

- shorter cranks allow you to sit farther back relative to the pedals while still achieving the same minimum hip angle. This can improve muscle engagement and reduce the amount of weight on your hands.

- All bike fit is a compromise. More specifically, it involves optimizing for the intended use. If you're going to be climbing steep hills out of the saddle frequently, you should probably go for longer cranks. Most people have a natural rhythm (60-80rpm) that they like when swaying the bike and it's going to be hard to increase that, as is necessary if you reduce crank length.

- If you're doing flatter rides, shorter cranks are almost always faster because they will almost always allow you to get more aero and rarely will it compromise your power.

- crank length optimization follows a hockey stick curve for most people. Too short is slower, as is too long. But going *slightly* too long can be disastrous whereas going significantly too short will only be a slight detriment. When in doubt, go shorter.
Good points. But why move back with shorter cranks? When my stoker went from 170 to 151mm cranks, I tried doing a KOPS but her power dropped off. I moved her saddle back to where it was and that seems to work better for her.

We have several bikes with crank lengths all over the place. They all work fine - one simply has to do what one has to do to move the bike, i.e. put down the same power. It doesn't take long to figure out what one has to do, even without a PM. I just now took it into my head to go around and see where the aft pedal spindle is w/r to sit bone location on the saddle on our bikes. It's all over the place too. The bikes have differing seat tube angles, different seat post setbacks, different saddles, and the idea of having the seatpost clamp not out at either end of the rails is also in play. And like I say, they all ride fine, i.e. exact BB placement doesn't seem a huge deal unless it's way off as in the case of moving her saddle all the way to the end of the rails with a CoblGoblr post which has a 30mm setback.

We know that KOPS is not the way to go, and I'm thinking that the rider CG w/r to BB might be a better way to think about it. Balance while pedaling seems most important to me. That'll vary somewhat with crank length, but in general, we don't push down all that hard, so it shouldn't vary by much. Another argument for going by BB location is that we push forward quite a bit at the top of the pedal stroke and pull back at the bottom, so the physical mechanics of that are also in play.

I'm thinking I need to develop some intelligent way to determine where both pedals and the BB should be w/r to sit bone location and set all our bikes up the same way. We have about the same hip angle on all of them.
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