Crank size on a muni.

ive ridden a 24" and i think they have 160s and those felt pretty nice.

longer cranks are not just for hills!

Of course I meant 165mm on a KH24. It was a simple typing error. I prostrate myself before you all in abject apology. Mea culpa.

Long cranks are more versatile.

You can learn to spin long cranks (that’s why road and track cyclists usually have something like 165 - 170 mm cranks) but there will always be an absolute limit on the torque you can apply through shorter cranks.

Years ago, I did lots of research on this. I now have a house full of cranks: 80, 89, 102, 110, 114, 125, 127, 150, 152, 165 and 170 mm, and I used to have 20, 24, 26, 28 and 36 inch wheels. I tried off roading on a 24 with 102s and found it possible, and sometimes fun.

Long cranks help you to keep control on steep descents. They don’t help all that much on steep ascents.

Long cranks give you more power to stop from high speed. On the other hand, they can be a bit clumsy when tight manoeuvring is called for.

Short cranks make it easier to spin, but you can learn to spin longer cranks.

I have now stabilised my fleet with 165s on the KH24 (tractor MUni!); 125s on the 26x2.3" (Holy Roller, faster cross country MUni) and 114s on the Bacon Slicer (700c, skinny high pressure slick, for road and easy cross country).

You can improve your leg strength and apply more torque through shorter cranks, or you can improve your leg speed and spin longer cranks.

Bikers typically have 175 cranks, because they typically don’t need to spin fast ever, what bikes call spinning fast is 100 rpm, which is nothing on a unicycle. However, they also typically run much higher gears than a muni.

If you can spin well, and have good leg strength, there’s very little difference between 150 and 165s, the only thing I would say is if you’re doing a typical muni ride, it’s probably a bit more tiring on 165s. I guess that’s why KH changed to 150s, because he does long rides and rides fast, which is exactly where it might make a difference.

To be honest though, for an alright rider there really isn’t much difference between 150 and 165s, I find it takes about 5 minutes to get used to the other size for a ride, and there’s only a slight difference in speed. The difference is way way less than say the difference between a 2.6 and 3" tyre, or a 24 and a 26. (although those differences aren’t super big either).


I’m sure you can increase your leg strength. However, applying torque is not just about leg strength. My legs are strong enough to lift me right off the ground. I can hop! (The wonders of Morris dancing!)

So if I push down harder on the pedal, I might just rise higher from the saddle.

Unless, of course, I improve my timing and my technique. I can learn to throw my weight onto the pedal, or I can learn to pull up on the saddle/handle. Either of these will increase the amount of torque I can apply - especially uphill where I am trying to keep the wheel turning, rather than trying to stop it turning.

But ceteris paribus, short cranks will always transfer less torque than long cranks, and long cranks will always spin that little bit slower.

I think of it like an engine: you have short stroke engines that rev higher, and long stroke engines that produce more torque. You can improve the carburation, the timing and the combustion chamber, and use the best materials available, but the short stroke engine will always be optimised for higher revs compared to the equivalent capacity long stroke engine.

On a unicycle, the trick is to find a stroke that fits the legs you actually have. A midget would find 170s unwieldy; a top basketball player would find 102s so short that he was barely twitching his muscles to pedal. Find the best size of cranks to fit the legs you have.

I defer to you as a far better and more experienced rider, but my experience is that the better I get, the more I go for middling sized cranks and just ride.:slight_smile:

I think it’s the opposite. The longer the cranks, the faster you spin, or can spin, and vice versa for shorter cranks.

It is interesting that you should think that. So when your feet have further to travel, they travel faster?

In racing they impose a maximum wheel diameter, and a minimum crank length.

I xould never use anything longer than 155 on a 24

the main reason for this is because im really short; but i think you can easily get enough tork with 150s

well because you slightly shorter, you weigh slightly less. so I’d imagen that 150s would be equivalent to a tall person using 165s

Maybe we’re not understanding each other here, or getting caught up in semantics or something. You said "long cranks will always spin that little bit slower."

Here’s my example: If you ride a 36er with 89mm cranks, going 12 mph, you’d be pedalling (spinning) much slower than I would, if I were riding a 36er using 175mm cranks going the same speed, right? For me to be able to keep up with you, using the 89mm cranks, I’d have to be pedalling/spinning almost TWICE as fast!

he meant longer cranks make the wheel spin slower.

That’s what I suspected, but usually the term spinning-at least in biking terminology- refers to pedalling, not tire rotation.

Spinning is not the same as foot speed. Spinning is how many revolutions per minute. Foot speed is how fast your feet (or pedals) are moving through the air.

A 36er rider with 89mm cranks going 12 mph has the same cadence (RPMs) as a 36er rider with 175mm cranks going 12 mph. Both are spinning the pedals at the same speed.

If you instead consider foot speed then the rider with the 175mm cranks has a much higher foot speed than the rider with 89mm cranks.

In cycling, spinning is considered the cadence you are pedaling. Crank length isn’t considered.

A while back there was discussion about the constant foot speed hypothesis where the foot speed was considered. Those are different strokes.

Longer cranks are more difficult to spin higher RPMs than shorter cranks. The foot speed is higher with longer cranks and the leg movement is more which leads to the rider bouncing on the seat at higher RPMs. So longer cranks are considered slower.

In unicycling, as in bicycling and ticycling, “spinning” refers to the rate at which the cranks go round. For example, pedalling at 120 rpm is spinning twice as fast as pedalling at 60 rpm. It is nothing to directly do with wheel size, gearing or road speed.

A 36" directly-driven wheel doing 12 mph will be rotating at 112 rpm, whether it has 89mm cranks or 170 mm cranks.

However, the rider’s feet would have to travel further, and therefore faster, because they would be describing larger circles.

Therefore, within certain limits, shorter cranks enable the rider to spin faster, and longer cranks make it more difficult for the rider to spin fast.

As a lever, a crank allows the rider to apply the pedalling force to the wheel. The amount of force available is restricted by the rider’s weight (she pushes down the pedal against her own weight - equal and opposite reactions and all that) and ultimately, a long crank, which is a longer lever, will allow the rider to apply more torque.

So the sacrifice for having shorter (and therefore faster) cranks is that the levers are shorter, and there is less torque.

There is an upper limit to how much force you can apply. Therefore, if you can learn to spin faster, and keep the longer cranks, you should get the best of both worlds.

There are common sense limits, though. 10 mm cranks on a Coker would not be faster. 300 mm cranks would be so unwieldy that any gains in torque would be more than offset by the loss of rhythm and smoothness. The ideal crank length varies depending on your ability, style, size, weight, technique and many other variables.

Way to much “over-thinking” going on here, imo. I think my earlier example is correct and logical. If you have TWO people each riding a 36er at 12 mph, with one using 89mm cranks and the other with 175’s, the rider with the longer cranks will be pedalling Faster at 12mph than the other rider with the 89’s. No question about it. Of course both wheels will be rotating at the same speed, but the foot speed will be much faster with the longer cranks.

Not just on a unicycle, anything you pedal. My son and I have been racing BMX. I’m tall enough that I pretty much maxed out the length of cranks I use, but as my son has grown, we’ve bumped up the crank length as he’s able to turn them. Cranks that are too long push the knees too high and beyond the point that the leg has efficient power.

BMX Crank Length chart example.

CRANK LENGTH: Remember that as you decrease crank length, you reduce leverage and may need to lower the gear ratio.

RIDERS INSEAM under 19" 20" 21" 22" 23"
CRANK LENGTH 135mm 140mm 145mm 150mm 155mm

RIDERS INSEAM 24" 26" 27" 28-29" 30-31"
CRANK LENGTH 160mm 165mm 170mm 175mm 180mm

So while charts are just a guideline, and unicycling is a bit different, but it’s odd to me that it doesn’t seem leg length gets as much consideration as it should in these discussions.

I’ts just a question of definitions:

  • Angular speed, measured in rpm, the same with different cranks, changes with unicycle speed and wheel+tyre size)


  • Tangential speed (of the pedal-foot), measured in mph or whatever, changes according to cranks lenght.

Formula: foot speed[m/s] = angular speed[rps] * crank lenght[m] * 2pi

Ok, maybe needless, but here it is :stuck_out_tongue: