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Old 2011-04-15, 07:35 PM   #16
Mikefule
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There are lots of variables here, and some of them are not easily quantifiable.

In terms of "gear ratio" the important thing is how far the unicycle moves for every inch your feet move.

This can be expressed as a ratio of the crank length to the radius of the wheel. As we're dealing with lengths (rather than areas) it works just as well to compare the crank length with the diameter of the wheel, or even (should you choose) the circumference.

We're used to thinking of cranks in millimetres, and wheels in inches.

For those of you who have forgotten, there are 25.4 millimetres in an inch.

As near as makes no odds, a 150 mm crank is 6 inches long.

Put 6 inch cranks on a 24 inch wheel, and the ratio of crank length: wheel diameter will be 1:4.

Now, put some 125 mm (5 inch) cranks on a 20 inch wheel.

Again, the ratio of crank length:wheel diameter will be 1:4.

In each case, if your foot moves an inch, the uni moves 4 inches.

And, in the short term, riding a 20 on 125 mm cranks, or a 24 on 150mm cranks will feel pretty similar.

But the big wheel will roll over bumps more easily. It will ahve more "flywheel effect" so it will be more stable.

On the other hand, the small wheel will be lighter and more manoeuvreable.

And the 1:4 ratio won't work the same outside a certain range. As a thought experiment, consider riding a 1 inch wheel on 1/4 inch cranks. It won't go as fast or as easily as the 20 on 125s!

Now as a thought experiment, try riding a 48" wheel on 12" cranks. Not easy.

On extremely short cranks, you are only using a tiny bit of the available muscle movement in your legs. On extremely long cranks, you won't be able to move your legs smoothly in the huge circle.

But within common sense limits:

Big wheels go faster, but ar eharder to get up or down hill.

Short cranks go faster, but are harder to get up and down hill.
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Old 2011-04-15, 08:31 PM   #17
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Mike, you need to cut your numbers in half. You need to use wheel radius instead of diameter.

152mm (6") cranks on a 24" wheel would be 2.0:1
127mm (5") cranks on a 20" wheel would be 2.0:1

The numbers on my chart represent the ratio between crank length and wheel radius (or virtual wheel radius).



And for those of you wondering, I do plan to eventually make a 48" wheel. It will be built with a regular 36h ISIS hub in such a way that I will have the option to swap that out with a Schlumpf if I want but probably never will. It's fun to see what the numbers look like though. Why 48? because it is a nice round number, what you get if you mate 2 26" (559mm) rims end to end with a 2" tire and the biggest wheel I could use with long (175mm) cranks.

This is long term scheming, maybe next year, maybe the year after that, or maybe I will never get around to it at all. Geared 32 could be fun but my 32" wheel won't ever get geared up unless a 32h hub shell becomes available for the Schlumpf hub.
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Old 2011-04-15, 09:03 PM   #18
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Quote:
Originally Posted by saskatchewanian View Post
Mike, you need to cut your numbers in half. You need to use wheel radius instead of diameter.
I was wondering why, since "wheel_circumference = diameter * 2", but then I remembered that "pedal_diameter = crank_length * 2".
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Old 2011-04-16, 06:35 PM   #19
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It really doesn't matter. You are comparing linear measurements.

You can analyse the same problem in terms of crank:radius ratio of 1:2 or a crank:diameter ratio of 1:4.

You can even analyse it in terms of a crank:circumference ratio of 1: (4 * Pi) if you wish.

All of these will produce consistent results as long as you don't change which statistics you are using part way through.

Double the radius, you double the diameter, you double the circumference.

It would be different if we were mixing lengths, areas, or volumes because then things change exponentially.

I deliberately chose crank length and wheel diameter simply because those are the figures that people use intuitively when thinking about unicycles.
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Last edited by Mikefule; 2011-04-16 at 06:35 PM.
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Old 2011-04-16, 07:33 PM   #20
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To me it just makes more sense to match radius to radius, circumference to circumference etc. keeping it apples to apples. Ratios need to be the same measurement. Cranks are measured in radius, wheels are measured in diameter, either double the crank radius or half the wheel radius to get a direct ratio.


That way a 2:1 ratio would show that your tire moves twice as far as your foot, the force on the cranks is double the force of acceleration, etc.
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Old 2011-04-16, 09:07 PM   #21
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... I meant pi, not 2. At least in the first formula.

Aaaargh.
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Old 2011-04-16, 10:16 PM   #22
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I guess this would be a good time to ask the question. I want to get a 20" trials uni and based on that chart a 125mm crank arm would be a good choice because it gives me good speed AND control over my uni?
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Old 2011-04-18, 09:49 PM   #23
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I ride 175 cranks on a bike, but on a uni, it feels like too much throw. anyway, the wierd thing is I use 150s on my 26, 29, & 36er (sometimes 125 on 36er)! 150's just seem right all around even on different wheel sizes. I don't ride 20", but if I did, I would expect I'd ride 135/137mm cranks. I don't understand why I like the 150's oddly enough.
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Old 2011-04-18, 11:39 PM   #24
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BlueFlyer83:
A trials would be comparable to a 24" Muni w/ 165 or 170's. You want the length for control while on obstacles and you don't care about rolling speed. You don't worry as much about pedal strikes since you mostly hop on/over the obstacles instead of rolling through like on a Muni. So go w/ 135-140.

If you are going to do street or flat on your trials you may want shorter cranks to get speed for jumping stairs and faster flips. If that's the case you may prefer 114-125. Plenty like the same longer length as "regular" trials. I've seen street riders get tons of speed for jumping stairs (8 set) w/ 145 cranks on their trials uni.
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Old 2011-04-19, 12:24 AM   #25
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Quote:
Originally Posted by skilewis74 View Post
BlueFlyer83:
A trials would be comparable to a 24" Muni w/ 165 or 170's. You want the length for control while on obstacles and you don't care about rolling speed. You don't worry as much about pedal strikes since you mostly hop on/over the obstacles instead of rolling through like on a Muni. So go w/ 135-140.

If you are going to do street or flat on your trials you may want shorter cranks to get speed for jumping stairs and faster flips. If that's the case you may prefer 114-125. Plenty like the same longer length as "regular" trials. I've seen street riders get tons of speed for jumping stairs (8 set) w/ 145 cranks on their trials uni.
Thanks for the advice!
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Old 2012-03-19, 10:25 PM   #26
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good thread.
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Old 2018-05-30, 11:33 PM   #27
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Just added 27.5 wheel size to Eric's creation. I wasn't aware that he had already posted the 32" wheel... to be honest that was my personal driver to do this.

I was a tad too lazy to colorize all of his categories... This version has fewer bands of colors.



In case the image refuses to post:

https://www.dropbox.com/s/a0zor7aab1...lsize.GIF?dl=0
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Old 2018-05-31, 09:52 AM   #28
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Quote:
Originally Posted by Biped View Post

I was a tad too lazy to colorize all of his categories... This version has fewer bands of colors.
The colour bands are only for guidance and are very subjective anyway. So many variables affect the ideal ratio.
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Old 2018-05-31, 01:22 PM   #29
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Quote:
Originally Posted by Mikefule View Post
The colour bands are only for guidance and are very subjective anyway. So many variables affect the ideal ratio.
Off course it is only for guidance and subjective. But I find it pretty much sums up what you would get from a: "what cranks should I ride on a XX" unicycle?" thread in this forum, all in one compact picture. Which is rather nice, I think.
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Old 2018-05-31, 10:53 PM   #30
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140mm cranks on a 20" puts us in the "pedal strike" zone, according to the chart. A beginner wondering which cranks to get on their 20" might avoid 140mm because of this chart. I have 140s on my trials unicycle. I am able to force pedal strikes by riding in fairly tight circles, but for everything else, I almost never have pedal strikes. The benefits of the longer cranks for me outweigh the occasional pedal strike (which almost never causes a UPD, anyway).
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