Originally Posted by
Polaris OBark
Why is the idea of using a shorter crank arm for the shorter leg frowned upon (vs a shim)?
The problem I had when I tried a shim was that the stack height really made it hard to tolerate, whereas the shorter leg only bothers me if the saddle is at the high/optimum for the longer leg. (This difference is small enough that I didn't notice it existed until my bike fitter/framebuilder mentioned it to me, but I can feel one leg can be over-extended when the other is comfortable.)
The gist of it is work=force x distance. This means that the leg attached to shorter crank arm will be doing a different amount of work than a leg attached to a longer crank arm.
Ideally, you'd want both legs to be doing the same amount of work. Generally the way to do this, is to align the work to where the rider happens to be. I.e., a shim.
Though there is a bunch of different ways one limb can be different than the other. So, there are undoubtedly cases where a different crank arm length
could be appropriate. But, that would between that person & their licensed doctor professional based on that persons particulars.