Crank sizes for 29er to equal a 36er

Hey, I was wondering if anybody knows what size cranks I would need for a 29 inch uni to be fairly close speed wise to a coker/36er.

This is assuming the 36er has 150mm cranks, thanks.

1 Like

an answer with a word of caution

You would need 121mm cranks to get the same gain ratio (distance traveled for foot movement)

This is calculated by as follows

(36er crank length)*29/36=29er crank size

in this case 150*29/36=120.83

Unless you are an experienced unicyclist I would caution against going to short (under 110ish) with your cranks in the name of speed. when you have shorter cranks you have less torque and you loose some aspect of control. I had a crash last year riding a 27" (actually 29" diameter) with 102s were I was going about 30km/h and separated my collarbone ripping all the ligaments in my right shoulder

Shortening cranks can only help increase speed if your spinning efficiency increases by changing length. Gear inches increase speed. If you’re interested in what length cranks give an equivalent torque advantage on a 29" wheel to 150’s on a 36" wheel it is 150x(29/36) or about 121mm. I think 125’s are the closest standard size.

Well, with a bit of maths we can calculate a very rough guess at what crank size would be required to get a similar “gearing” in terms of how much push is required from the rider. I think this should, rather approximately, tell us what length you should be looking at.

The crank and wheel together are a kind of lever. But instead of levering to lift stuff up, you’re kind of pushing through the crank and wheel against the ground.

If we calculate crank length divided by wheel size that’ll give us a number for how much purchase this lever gives you, the rider, against the ground. Or, in bike terms, as the number (crank length divided by wheel size) goes up, your “gear” gets lower. As quick sanity check to this reasoning: increasing crank length would make the number higher (=> lower gear) and increasing wheel size makes the number lower (=> higher gear) - that’s the way we know things ought to work.

150 / 36 = 4.17 (rounded to 2 decimal places)

That number doesn’t mean much on its own but we can use it to calculate the crank length you’d want on a 29er to get the same “gear” in terms of how much purchase you get against the road.

Our original number was crank length / wheel size, so logically if we multiply it by another wheel size, we’ll get a crank length out again. If we multiply the constant by 29 then we get:

4.17 * 29 = 121mm (to the nearest whole mm)

This suggests that to get a similar “purchase” as the 36/150mm setup on the 29er, the best of the commonly available sizes to look at would be something like 125mm or 114mm.

This is not an exact calculation since there are other factors I ignored (a higher “gear” in terms of leverage doesn’t always make you go faster, the 36er wheel is much heavier, you may have your own personal crank length preferences, etc). It’s a rough guide, but in general the bigger wheel will still come out being a bit quicker.

Like I said, I’m playing fast and loose with the maths, making dodgy assumptions, etc, so don’t take this as set in stone. But to get a really rough idea of the answers to these questions, in my experience it’s reasonably effective.

Thanks for replying quickly.

Now I will probably get a 29er with 125 mm cranks.

Both a Coker and a 29er will go approximately the same speed no matter what the crank length if you drop them from the top of the Empire State Building. Crank length doesn’t factor in the equation.

Thanks, now I know that if I feel like committing suicide on a unicycle, I’ll get the same speed no matter which unicycle I jump off of the building on. Fascinating…

good to know…thanks for sharing:D :stuck_out_tongue:

this is unless they have diff terminal velocity.

125mm cranks work well on a 29er. You can go a decent speed on them (after practice, I can crank along at 10-12mph cruising speed). 114mm cranks might be faster once you’re used to them, but 125mm is a reasonable place to start as long as you’re careful with them.

150mm on a 29" worked well for me too. A bit slower, but lots of control. Both 150mm and (once I was used to them) 125mm work well for muni for me. I found it was good for my riding to switch to 150s for a bit, develop better technique on the longer cranks, then switch back to 125mm again.

Unless you put the Empire State Building in a vacuum. :roll_eyes:

Keep in mind that you still have to pedal 36/29 times faster on a 29er to keep up with a 36er even if your cranks are 29/36 as long. So, while you may be able to keep up with a relatively slow 36er rider, a fast 36er rider will leave you in the dust, at least until you head up a really steep road.

Aha! But there won’t be any dust if somebody’s already been and used the vacuum … :thinking:

I think maybe I should be asleep.

A few years ago, I did a lot of fairly systematic experiments with cranks and wheels. I have tried cranks from 89mm to 170mm, and wheels from 20" to 36", although not every possible combination.

The simple answer is that you can make a rough estimate of the effect of changing the cranks by looking at the simple arithmentic.

Comparing a 29 and a 36, the 29 is 29/36 as big.

To maintain the same ratio, the cranks have to be 29/36 as long.

However, shorter cranks require a different riding style. The movement of your legs is smaller, and you use fewer muscles. So, if you are doing the same amount of work, you are doing it with a smaller amount of muscle. There are also co-ordination problems maintaining high speed for long periods on very short cranks. Finally, going the other way, if you put very long cranks on a uni, there comes a point at which the length of the cranks is so unwieldy that you cannot pedal smoothly.

This can easily be shown by “reducing to the absurd”.

A Coker has a 36 inch wheel, which has an 18 inch radius. A 150 mm crank is 6 inches long. The ratio of crank to wheel radius (the leverage) is therefore 1:3.

So on a 29 inch wheel, you’d expect:
Radius = 14.5 inches.
1/3 of that is 4.833 inches.
That would be 123mm.

These two unis would have comparable speeds.

But what about a uni with a 6" wheel and 1" cranks? The ratio would be the same, but clearly the speed would not be. However good you are.

Just like reducing a bike down to one wheel.

Sometimes absurdity is more fun but not necessarily faster.

I’ve ridden a good number of miles with Coker riders, with me on a 29er. With 125mm cranks, I’m generally able to keep up with good Coker riders who are using longer cranks, but I don’t have as much fun as they do and I have to work harder. If I were to regularly ride with Coker riders, I would go with 110mm cranks on the 29er; with 110s, I can more or less comfortably keep up with Coker speeds.

It’s faster than walking and running. So running and walking must be absurd too :stuck_out_tongue:

Staying OT, I would guesstimate from experience on both 29’ers and 36’ers (without any fancy equations), that somewhere between 114 and 125mm cranks would give you a similar speed.

At the last two Unicons, 50mm and 65mm cranks on 28" were pretty competitive against Cokers with 100-114mm cranks.

If you wanted, you could get some 110s or 102s to match the cokers somewhat. I could be wrong about that though. :thinking:

off topic, where can you get 50mm or 65mm cranks?

I’ve done many many 1 hour rides, and a few 1.5 and 2 hour rides, on 28 and Coker, flat out, no stops, no dismounts, and deliberately pushing as hard as possible, and so far I have never maintained the same average speed over 1 hour that Paula Radcliffe maintains when running an entire marathon distance. And she’s only a girl…


You’re not pedalling fast enough :stuck_out_tongue:

Fastest times for the Marathon on a unicycle is around the 1hr 30min mark. Fastest times for men running a marathon is just over the 2hr mark. Go figure.