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Short and Sweet - A Discussion on Crank Length by Mike Burrows

Wednesday, November 10, 2010 3:08:00 PM Categories: Chairman
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Much has been written by myself and others on the matter of crank length, but not as I have realised lately in this, our own magazine. So for those who do not get to read the bigger and shinier magazines here is what (we think) we know to date.

It started as an attempt to improve the aerodynamics of fairings; feet on pedals for a laid-back rider being a limiting factor for size and optimum shape and it was hoped that the probable small power loss would be a worthwhile trade off. In practice it was discovered that these short (down to 100mm) cranks, which had to be used on the unfaired training machines to get adapted, seemed to work just as well as the usual 170mm once the riders had got used to them.

There was a time when all cranks were ‘short’ and also related to wheel diameter – i.e. on ‘pennys’. The shorter the crank was, the larger the wheel (and thus the gearing) could be. But ever since we added chains and sprockets all our gearing has been done with them. How 170mm became the normal length I do not know, but it was not mentioned in any stone tablets and is almost certainly wrong. Length should obviously be related to rider height but what the starting point should be cannot at present be stated with any certainty. I am currently using 140mm which seem quite normal and could possibly run even shorter, but that would need all my other machines altered as well, so that is on hold just for now.

So why would short ‘levers’ give you more power? Well there is the obvious fact – or at least what should be obvious to the readership of this publication – that torque, which is what is decreased as length is reduced is only half of the equation. ‘Power’ which is what we are interested in is torque x speed, i.e. how often you push on the levers. But this is still only part or the equation, which gets a lot more complicated when we start to consider what it is that pushes the levers.

Our bodies are very complex, messy and not fully understood by anyone, let alone engineers who are used to working with things that have instruction books (not that real engineers read the instructions of course – ed), but we are happy to take an occasional guess.

The force and rate of any pushing that our limbs do will be governed by the ability of heart and lungs to process the blood and to deliver it to the muscles and some sort of balance between supply and demand needs to be reached. Also there is the other side of the equation, how the muscles deliver the force.

A muscle is a bundle of fine fibres, each of which can do only one thing. That is to contract and then relax. As individual fibres they do not do hard or fast or gentle, they just contract. Variation in your effort is determined by how many contract at any given time. So for a gentle sustained push a small number fire in sequence, then relax, refuel and fire again. A punch on the other hand means all firing at once, and then all relaxing at once, which of course uses up all the energy so you can’t ‘punch’ continuously. But it does seem to me that the more the fibres fire in unison the less internal friction will be generated. To test any of this in a scientific way would be very difficult, bearing in mind the unreliability of the power source.

But this is a very holistic thing to do as shorter cranks are of course lighter/stiffer, more aerodynamic, even without a fairing, run at higher revs, and thus less torque to be resisted by the frame, which is thus stiffer/lighter. And it is better for our bodies, pushing with a very bent leg is apparently not good for us. But best of all you get to send me money to have me shorten your shiny Shimano!

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