Curious about crank lengths

I occasionly use my bike instead of the uni (because its faster) and at the weekends I work as a rickshaw rider which is a great way of getting fitter and stronger and getting paid for it!!

I am curious about changing crank lengths on my bike and rickshaw so they are the same length as the cranks I use on the uni (normally 125ish) so my legs and muscles become more used to spinning in a smaller circle.

Would this help my uni riding?

Probably not.

It would however make your rickshaw harder to ride.

The gearing of a bike/trike/rickshaw is not simply about the size of the cogs relative to each other. It also takes into account the length of the cranks and the size of the driven wheel.

What is really happening is that the entire system, from pedal - crank - chainring - chain - rear sprocket - tyre, is taking the movement of your feet and multiplying it before it reachest the tyre.

So if you just put shorter cranks on, but everything else remains the same, your feet will be moving in smaller circles, and therefore less distance, but the multiplication from chainring - chain - rear sprocket - tyre will remain the same.

Or to put it simply, you will have less leverage and your rickshaw will be harder to pedal. In more technical terms, you will have less torque.

On a standard unicycle, you do not have the chainring-chain-rear sprocket combination. There are no gears to multiply the revolutions. Therefore there is just the direct relationship between the size of the circle made by the cranks, and the size of the wheel.

If you were to ride a 24" unicycle on 170mm cranks, the gearing would be very similar to a mountian bike in “granny gear” which means you would be putting hardly any torque. Your muscles would be working hard to move your feet round in big circles, but you would not have to be pushing hard to move the uni.

If you put 125s on the 24, you would still need to put hardly any torque in, but your feet would be moving in a smaller circle, which is less unwieldy, and can be done more smoothly and quickly.

So the effect of 125s on a direct drive uni is that you [B]still[B] have to put very little torque in, but you can spin faster.

The effect of 125s on a bike/trike/rickshaw would be that you would have to put more torque in (to cope with the gearing and the additional weight of the machine and passengers/cargo) so you would probably end up pedalling at a lower cadence - the opposite of what you want to achieve.

Another aspect of short cranks that is often overlooked is that they use a smaller part of the available muscle movement. With long cranks you can use all or most of the movement available from the leg muscles. With very short cranks, you are focussing all that work on a much smaller part of your muscle, which can be more tiring if you are pedalling under a heavy load.

To visualise this, consider riding a uni with absurdly short cranks - say 5 mm - and imagine what a tiny little twitch of the muscles it would require, and how tiring it would eventually become if you were going up a hill.

The reason we often use short cranks on unis is because we do not have the option of selecting the gears. We choose the wheel size and that dictates the gearing. (Even with a schlumph hub, there are only 2 gearing options.) We can however get away with short cranks because the torque required is so low.

The reason that bikes/trikes and rickshaws have cranks of around 165 - 170 mm is that this is the most efficient size for using the leg muscles. Think about the Tour de France, or any other hard core cycling event: they could chose whatever size cranks they wanted. They must have thought about it because they’ve thought about everything else to optimise their machines. Do they have very short cranks? No. They have optimal length cranks and then choose their gear ratios appropriately.

Thanks for your comprehensive answer.

I was thinking that I could stay in a lower gear for longer and spin faster to simulate spinning on the uni.

Most of the time I ride the rickshaw pretty close to the highest gear to make the loading on my legs heavier to build up strength, I only switch down if I have a full load on (total weight 400-500kg) so I think I could manage the extra loading of short cranks :slight_smile:

Mike Burrows of the BHPC has stuff to say about shorter cranks:
I use 150mm on my velomobile trike which weighs in at 70lbs unladen. I do have 72 gears to play with.

Thanks he seems to suggest that the shorter better, certainly I can see it shraining knees less :slight_smile: