hi all,
can i ask how to choose crank lengths as reading this forum i think i have chosen wrong!
i built my nimbus 26" using qu-ax 170 cranks as when i rode downhill bikes i used 170 crank arms so thought it best to stick with that.
reading this forum you all run a lot shorter,have i done wrong?
Depends on your terrian, but 170’s are long for a 26. I ride a 24 with 150’s and a 29 with 165’s.
There really is no right or wrong crank lengths, just some combinations that tend to work better for certain tasks. Here is a chart I made a while ago for a different thread.
Use what you like, make up a justification for them later. Just like the rest of us. 
There was a thread about 170 mm cranks a couple of weeks ago. I don’t think anything was really settled but consensus might be that they’d be most useful for the most technical riding, negotiating rock gardens and fallen logs and such. Without gears, we have to spin pretty quickly to keep up speed, and momentum can get you over a lot of obstacles. Shorter cranks make it easier to pedal smoothly and keep the cadence up, if we’re rolling along more than stopping, hopping, and dropping. Most of the time we don’t need that much leverage. The open question seems to be when, if ever, we do.
But that’s mountain unicycling. On smooth pavement, you’d just be making things hard for yourself with them.
Crank lengths on unis tend to be completely different to those used on bikes - generally we want to spin faster as otherwise we never get anywhere, and for that you want shorter cranks.
Thinking about this, I now have 6 different crank lengths in my garage - 170, 172.5 and 175 on bikes, 125, 140 and 150 on unis.
I had 165s on my 24" when I started learning, and found it felt more natural spinning the bigger circles, probably like you say because I’m used to the longer cranks on an mtb. However now I’m a little better I’ve changed back to the standard 150mm cranks. It still feels a bit odd but I can see the benefits & will stick with it.
If you have the money, I’d suggest trying a couple of crank lengths to see what works best for you, but if cash is an issue just stick with what you have. Experiments can come later once your balance (bank and riding!) is more stable.
Adapt pretty quickly
You will adapt pretty quickly to the 170’s, and after about a week of riding they will seem normal. You can always go shorter at a later date if you feel the need.
I’ve used sizes from about 80 mm to 170 mm and ridden wheels from 20 - 36 inches, and in my early riding career I spent a lot of time and effort (and money) experimenting with different combinations.
End result: I use 150 mm for general riding on medium to large wheels and 125 for faster general riding on small to medium wheels, and sometimes 114s on my 28 just because.
Short cranks are faster on the smooth and flat. Long cranks can be unwieldy. I found 170s to be horrible on my KH24 Muni, but love it on 150s.
Although shorter cranks are faster, they use less of the muscle movement, so they focus the work on a smaller part of the muscle which may be a bad thing if you are constantly changing speed or going up and down hills. My theory is it is better to learn to spin faster with slightly longer cranks because it gives you more versatility.
Short cranks are at their worst on steep descents - assuming you don’t use a brake - and can even be dangerous in this situation.
But as someone said above, make your own choice then come up with a post hoc rationalisation like the rest of us.