I’ve just put my new 89 mm cranks on my 24 inch Nimbus. The results have been er… surprising.

Get the maths out of the way first: 89 mm = 3.5 inches, so the ‘ratio’ is 29%. That’s equivalent to a Coker with 133 mmm cranks (150s are standard, mine has 125s); or equivalent to a 28 with 102 mm cranks.

Mounting requires care, otherwise the uni runs away from you. Riding on the flat is easy, cruising without effort at 8 - 10 mph. Although I haven’t done any serious speed testing, my gut feeling is that there is little top speed advantage compared to the 102 mm cranks I had on before, because of the reduced control and increased fear of UPD. Idling is reasonably easy, but requires care, otherwise it overshoots. It has very little hill climbing/descending ability at all!

The distinctive thing is how easy it is for the pedal to overshoot top dead centre when mounting, idling or decelerating. This leads me to think that there is more to this short cranks thing than simple ratios. I think the **absolute** length of the crank matters too.

I think this is because you can only exert maximum torque on the pedal within a very narrow angle as the crank rotates. Assuming that this angle is the same for all crank lengths (a gross simplification) then the actual distance moved by the pedal as it crosses that angle will vary with the crank length. An 89 mm crank is 52% as long as a 170mm crank, so the pedal is only in the ‘maximum torque’ position for 52% of the distance. Assuming constant footspeed then the foot is only in position to exert maximum torque for 52% as long.

If so, then this brings in reaction times, skill, and ‘muscle memory’. All three of these can be improved with the dreaded P word, but as the 89mm crank is always 52% as long as the 170mm crank, even if you get twice as ‘good’ through practice, the 89s will only give 52% as much control as the 170s.

And if you think about it, if the 52% is a constant, and your skill ‘doubles’, then the gap between the levels of control on the two crank lengths doubles - as far as these immeasureables can be measured. Er…

But in simple layman’s terms… 89s on a 24 are really really silly, totally impractical, but jolly good fun, and mean that you can find challenging riding on the simplest of routes. Everyone should try it once. :0) Ultra short cranks are a fine way of taking yourself back to those heady days when simply riding the uni on an uneven surface was an achievement.

And finally, thanks to Roger for his usual excellent and speedy service. I can’t recommend unicycle.uk.com enough. The steel 89 mm cranks are under 10 quid including postage, as are the 102s and 110s. Put some short cranks on your Christmas list NOW.