Short Cranks

As Mike says, bodies are complex.

In case it is a useful consideration, some generalisations:-

Muscle produces most power in mid range. (Inner range being close to as short as it gets, outer being, close to as long as it gets.)

Maxiamal compression between knee cap, (patella,) and thigh bone, ,(femural condyles,) mechanically will occur when the quads are contracting through 90 degrees of knee bend (flexion.) Which is why patello-femoral problems manifest, walking/running up and down hills/stairs; grinding on the pedals with longer cranks, and presumably why shorter cranks help those with such a problem.

If there is a theoretical down side to shorter cranks; perhaps one draws on a larger collection of muscle fibers , when contracting through the full range of a muscle, and fewer when the using a shorter range. Superior blood supply to great number of fibers, perhaps.

It would be a surprise, if like many other things in life, there was not a balance to be found!

Edited by Bill B-J - 29/January/2012 at 14:41

I personally find old school cranks have more meat on (so to speak) for allowing shortening.
There are ways to do hollow cranks, by moulding in inserts.

For standard cranks I generally seek blackwidows, BMX. I think they go down to 140mm but on 110mm BCD spiders. So you may need to make carries for your 130 shams, or 135 cantis.

Not sure the are generally available new either, as heard a rumor the where not being sold anymore. But i have seen them for sale since.

Cost around 60quid.

if you don't take a fishing rod with you, you wan't catch any fish! or if you don't take a bike racing you can't race

Bill B-J - Apologies for mis-naming you. Embarrassed

Some unicycles use chains or very very long legs ;)

if you don't take a fishing rod with you, you wan't catch any fish! or if you don't take a bike racing you can't race

Bill B-J wrote:

Does one reduce the gearing, in relative to the percentage shortening of the crank?

You certainly should expect to need to reduce your gearing. Simplistic mechanical thinking would indeed suggest that it should be by the same proportion as the reduction in length of the crank, so keeping the overall gearing (in terms of distance moved by the pedal to distance moved on the road) the same.

This is pretty much what I've done to my road recumbent, going from 170mm to 145mm cranks and from a 50T chainring to a 42T. My limited experience so far is that I'm spinning out at a speed about 10% lower than I did do, so either I shouldn't have gone for quite such a small chainring, or I just haven't learned to spin those short cranks properly yet.

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