Mismatched crank lengths (probably very stupid)

But that doesn't affect the length of a crank, your foot is 2.5mm higher at the bottom of the stroke and 2.5mm higher at the top of the stroke too so the crank length is exactly the same with or without the shim.

Hang on, if I have a 175 crank, and place a 2.5mm shim under my cleat, then while the crank is higher/lower, my foot is surely in the same place as a 172.5 crank, no? This is the whole thinking behind bike fitters shimming cleats - to compensate for different leg lengths. And it's much cheaper than changing cranks. 

In this case, though we have someone who has the same length legs, but different length cranks, the logic is the same. 

Shimming makes your leg longer. It doesn't make the crank shorter. Try thinking about it as wearing a built up shoe instead.

Recoveryride wrote:

if I have a 175 crank, and place a 2.5mm shim under my cleat, then while the crank is higher/lower, my foot is surely in the same place as a 172.5 crank, no?

Only at the bottom of the pedalstroke. At the top it's 5mm higher - 2.5mm extra crank length plus 2.5mm shim.

You say that, but I definitely noticed  the difference in feel when I switched from 172.5 to 170mm cranks (needed a new crankset, same size was out of stock). 1.5% difference in lever length, it's not much but it's something. 

I wonder how much of that is in your head?

That's not a dig at you - we're all liable to noticing things that aren't really significant but we anticipate that they might be. I suppose it's a sort of placebo effect.

Alternatively, there may have been other minor - but unstated - differences which might have contributed singly or in combination to a different feel (e.g. the Q factor)?

But you knew that had happened so it was always a 'chimp' in your brain. 

I couldn't source a same length crankset for my son's bike and fitted one 2.5 mm longer - he's never noticed and I haven't told him. 

Whiii wrote:

No excellent brains here but I'd say you're correct. If my maths is right that 2.5mm translates to around 16mm of extra travel relative to the other leg for every pedal stroke, or 1.6m for every minute you spend at 100rpm. I suspect what you've saved on the power meter, you'll be spending on a physio several times over! And I've never tried it, but sprinting in a straight line might be tricky?

Your overall change in length of pedal stroke would be 5mm, as the force is largely applied to the vertical vector, the horizontal "float" is incidental I would have thought.

The difference in length of path that your foot travels would be 16mm as you say, however only half of that would be applying pressure, the other half relaxed on the return stroke.

If you are wearing cleats it might also be worth considering the precision to which you have adjusted the cleat position - eg is it within a tolerence of 2.5mm?

Considering the variance that people generally have in their bodies I might hazard that 2.5mm is negligible and unlikely to make any noticable difference. discrepencies in limb length 10 times that aren't considered to be a clinical condition.