There is no single general rule, but there are many rules of thumb.
You have five variables: the diameter of the wheel, the length of the cranks, the section of the tyre, the skill of the rider, and the preferred style of riding.
Of these, it is easy to pontificate about the first three, rather than working on the fourth and developing the fifth!
The benefit of short cranks is that you can spin them faster. Your feet (and therefore your ankles, shins, knees and thighs) have less distance to move to complete a revolution of the wheel. That means you can ride faster, and also more smoothly.
The benefit of longer cranks is that you can apply more toque (turning force, or “leverage”) to the wheel. This is useful on hills - especially on descents where the extra confidence you get from long cranks is a real benefit.
However, it is not just a simple matter of choosing a perfect crank length. The ratio of the length of the crank to the size of the wheel is also important. Very crudely, a 20 inch wheel with 5 inch cranks will behave very similarly to a 24 inch wheel with 6 inch cranks.
However, this is only true in “ideal” circumstances on a smooth level surface such as a gym or hockey court. The big wheel will always be better on uneven ground and long fast journeys. The small wheel will always be easier to control in tight manoeuvres.
There is a secondary effect that a large wheel (especially a heavy one) has a flywheel effect, and will tend to smooth out your pedalling, meaning you can get away with longer cranks without getting into that horrible bouncy style that can come with trying to ride too fast on a smaller wheel.
On the other hand, on a bigger wheel, you “tend” to do faster straighter riding, so for a lot of the time you can get away with a shorter crank than you might think. See - conflicting advice already!
Until you can confidently freemount and idle, altering your cranks is not a brilliant idea. You would be modifying your unicycle to suit an undeveloped riding style. In turn, this would hamper the development of your riding style.
As a near beginner, struggling to cope with my 26, I fitted extra-long cranks. Hey ho, it made it easier for me to freemount, but I later found I was using a very very poor freemounting technique. Once I corrected this, I found that not ony could I go back to shorter cranks, but I preferred them - and they made freemounting easier!
There is a safety element too: a uni at high speed can be difficult to stop under control with short cranks. A Coker or 28 can take 5 or 10 wheel revolutions or more to stop. Shorten the cranks and this effect is magnified. It is generally considered a bad thing to fall off the back of your unicycle at a junction, cracking the back of your head and firing your unicycle torpedo-like at another vehicle.
Many people fit short cranks to achieve speed, but what is speed? It could be:
- Top speed on the flat.
- Average speed over a ride with obstacles and changes of terrain.
- Cruising speed on average terrain.
Fit super short cranks and you could find yourself with a high top speed that you never use, and you have to slow down well in advance of obstacles, and walk some of the hills, so your average speed over a journey falls dramatically.
As a very rough rule of thumb: if you want to ride far and fast, choose the shortest cranks that you can comfortably idle with. Any shorter would be counter-productive.
As a rough rule of thumb, for MUni, choose the longest cranks you can comfortably spin for short periods. Sometimes you need high rpm in a burst, either to get up a short rise, or to spin out on a descent too steep to ride down under complete control.
Another rough rule of thumb: change crank length about 1 size at a time - don’t go from 150s to 102s just like that. Give the new size time to “bed in” so that you are comfortable with them. Don’t be afraid to experiment.
Have fun. That’s why we do it.
