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The Right Crank Length?

mike burrows

writes Mike Burrows

When the HPV movement got underway in the early 80's, it was not long before some people realised that the usual constraints of bottom bracket height were no longer a factor, and we saw a few 200-250mm cranks. But none of them won anything and it probably hurt a lot and so we got on with more important things like aerodynamics. And it was an attempt to improve the aerodynamics that saw Frank Reinhard from Germany fit a pair of 110mm cranks. This would allow him to reduce the size of his fairing, the idea being that the reduced drag would more than compensate for any power loss. Although Frank did not win anything (the fairing was small but not very good) his legs did not hurt, and he learned a couple of things. The first thing is that if you want to race on short cranks, you need to train on them, and after you have done this for a little while your performance is just as good as it was on the 170mm cranks, even with no fairing.

Over to me

I first adopted the idea for my son who had to have a knee joint replaced, the operation was successful but it rather messed up the tendons in the upper leg and he could no longer bend his leg far enough to cycle, at least not on 170mm cranks. So I cut them down to 125mm (he is 183cm tall) I also put a set on one of my own bikes (I am 177cm tall). They felt very strange, especially out of the saddle, and as I was using several different bikes at the time I could not get used to them. My son on the other hand, had no choice, and in a matter of weeks was totally at home with them. He has now been riding with them for about 15 years.


It was a while before I got back to the idea, by which time it was becoming popular with many in the HPV movement, some even going down to 100mm. I chose 150mm for although I no longer ride quite so many bikes as before, there is the occasional 8 Freight to deliver. I changed cranks on all three bikes in the same week to get off to a clean start. The first to be ridden was my Ratcatcher touring recumbent which I rode for about 70 miles at an 18mph average and the most remarkable thing was the complete lack of any sensation at all. It was just like it always was, possibly a bit nicer. That could have been placebo effect, I had to by the way reduce the chainring size by the same percentage so as to use more or less the same gears as before. Not quite the same on the uprights, an 8 Freight carrier cycle and a 2D city bike, both used for short things on a daily basis. It felt a bit odd for a week or more, neither is ridden hard so I cannot assess performance for them, but having got used to them, using 170mm is no longer very nice, especially using them in anger. And on my racing recumbent I am down to 145mm and going well for an old man, but the recumbent world is very variable, both machines and riders, so no real comparisons can be made other than they feel nice, are lighter, and the doctors say they are good for us.


There are some similarities between the human body and the internal combustion engine, and being a bike rather than body centred person, and having raced noisy toys I know a bit more about the latter, and the direction that engine development is going is towards shorter strokes and higher RPM. This will allow more power strokes and the short stroke will reduce the inertia loads and losses, for even if our feet are restrained, reversing the direction of our upper leg will require an amount of energy and it can only come from one place.

The Question

One of the ways engines have changed to allow the shorter cranks to work is in the design of valves and cams. This is an area that seems to have been ignored in the studies of the human body. A lot has been written about how our bodies convert carbohydrates and how this is then converted into motion by the muscles, but not about the best way of getting it there, because once a muscle has tired it needs the blood to flow through it to remove waste product (exhaust gas?) and provide fresh fuel. But how long does this process take? For example, take two bikes, one with 200mm cranks and a 100 tooth ring, the other with 100mm cranks and a 50 tooth ring, in engineering terms, the same thing. The short cranks need to be turned twice for the bike to cover the same distance but the foot will not have travelled further or pushed any harder than with the 200mm cranks. So they are the same except for the fuel supply to the muscles which mainly occurs when the muscles are relaxed i.e. the upstride or in fact some 4/5ths of a revolution if you are pedalling correctly. It is then quite possible that when pedaling this gear at say 50 RPM there is so much time to refuel the muscles that they end up waiting to be used, so that by changing to 100mm cranks and 100 RPM you could get say 3/4 of the fuel per revolution, so 1 1/2 times as much usable energy per minute. Or it could be the other side of the equation, with long cranks and low revs you are pushing and so the the muscle fibres are very much firing in sequence, as you shorten the cranks and increase the revs you get closer to a punch with the fibres simultaneously. To really find out for certain will take a lot of very serious testing of many athletes (not my department) but in the meantime I can tell you that a lot of recumbent riders are going very well on short cranks, and if someone like me can cut 1" off his cranks and not notice it, then changing from 170 to 172.5 is pure placebo!

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