Hi All, sure this question (or versions of this question) has been asked to death, however, does anyone know in regards to speed if there is much difference between 26inch 100mm crank vs 29inch 150mm crank?!
Lots of maths hurts my head so can’t figure it out on my own.
Thanks
Nick
All other things being equal, the 26er with 100mm cranks would be faster it’s “gear ratio” is about 1/3 higher.
Hi Nick,
On a 26" with 100mm cranks, for each rotation your foot moves ~628mm (or 24.7 inches), and the wheel moves 81.7 inches.
On a 29" with 150mm cranks, for each rotation your foot moves ~942mm (or 37 inches), and the wheel moves 91.1 inches.
It’s already fairly clear which will be faster, but just for the sake of it I’ll keep going. There’s probably better ways of doing this, but my brain is dead and I can’t think of them so I’ll do it the slightly longer way 
The least common multiple of 25 (rounded from 24.7) and 37 is 925. Multiplying the revolution distance by these same numbers, we find that:
On the 26", for every 925 inches your foot travels, the wheel travels 3022.9 inches, or roughly 252 feet.
On the 29", for every 925 inches your foot travels, the wheel travels 2277.5 inches, or roughly 190 feet.
And so, as @Duff said, the 26" is faster “all other things being equal,” because the same pedaling will make it go just under 150% of the distance of the 29" (if my calculations are correct).
Hopefully that helps!
Thank you so much!! Really appreciate that, was thinking of going for a 29inch uni with 137 cranks or 150s as I feel 100s would be too difficult on a 29er but I already have a 26er with 100mm cranks and was trying to decide if it was worth going up in size. You answer and Andrew’s helped me decide
Thanks for the detailed answer when proof in numbers even if it did make my head hurt trying to keep up, but appreciated the summary at the end
I think there’s more to it than simply the numbers. Theoretically you could go really, really fast on 25mm cranks, but its pretty self evident that our physiology is not suited to pedal such a small rotation. Also, as your cranks get smaller you get a higher center of gravity.
That’s true, and there’s a limit to how fast your feet can move. I considered crunching the numbers valid on those crank/wheel sizes, though
Which one is actually faster all depends on how fast you are pedaling. The ratios of how much distance your feet move and much distance the wheel covers don’t necessarily translate to which is faster. You will be faster on a 29” than a 26” at the same cadence, because the 29” wheel covers more distance per revolution. A 26” with 100mm cranks is only faster than the 29” with 150’s if the shorter cranks allow you to pedal faster than you could on 150’s, and if you can sustain that cadence. If you like pedaling slowly than a larger wheel will help, but if you are good at tight, fast spinning for long time periods then the 26” might be good. What type of unicycles are you looking at and what type of riding do you plan on doing?
I notice that overall, my daughter rides faster on 150s than on 125’s. Sure, she can probably reach a higher top speed on 125s, but she had trouble keeping up with me on hill climbs and overall was slower with the shorter cranks. So I think it’s an individual thing to a large degree.
True, I was thinking foot speed and not cadence. I personally think you would still have a higher average speed on the 29" (with both higher acceleration and more momentum) on an average ride. Top speed will be higher on the 26" if your feet can move at the same speed despite the smaller rotations.
[Diameter in inches] x 12.7 = [Radius in mm]
So
(26 x 12.7)/100 = 3.30
(29 x 12.7)/150 = 2.46
Here are a bunch more pre-calculated:
That said, speed is much more than the raw gearing. Being able to put those shorter cranks to affect depends largely on you. For me though, the 26" setup would be faster.
FWIW a 26" with 100mm cranks is fairly highly geared for a unicycle compared to their default setups. Consider a 36" generally ships with 150mm (yes, people use shorter but that is typically the standard crank length they ship with).
(36 x 12.7)/150 = 3.05
Thus the 26" with 100mm has a higher gain ratio. Which makes sense to me. I am typically as fast on 26" setup like that, though it also helps that that is my favoured and most used setup. 
Awesome feedback and advice- really appreciate all the comments!! Most useful.
I don’t plan on ever doing distances more than 20miles and might only ever do that distance a couple of times a year, mostly will be doing 10-12miles and muni riding so it sounds like 26inch would be best and alternate between cranks
I think the same! I don’t have a 36er anymore so won’t comment on that. If I go my fastest, I go maybe 14km/h on my 24. But on my 29er, it’s 15km/h. So, it’s not very different!
My 24" is way easier to mount though, and the 29er is nicer to use if it’s a longer distance.