Gear ratios, cranks lengths and geared unicycles.

I’ve been having some very strong thoughts about how different unicycle wheels sizes and crank lengths affect the way different unicycle disciplines are used/enjoyed and so on.
The main things I wish to address here are how the crank lengths affect the ratio of each wheel size, whether it is possible to put a different gear on an almost standard unicycle without having buy an £800 hub and which combination of crank length/wheel size gives you which ratio.

So the first one I will address is the last one I just mentioned. I ride a 19" trials/street unicycle with 127mm cranks and I know that there are a couple of standard cranks sizes such as the aforementioned 127mm, the 137mm and the 150mm (apologies if I’ve missed out any standard cranks lengths here). What I would like to know here is what the ratios are and whether there is a calculation to figure this out. I also know that different wheel sizes use different crank sizes as standard. So, for example, a 24" unicycle with a 150mm crank length would have a much easier ratio than a 36" with the same 150mm cranks. So what is standard on each unicycle and how close are the ratios between each wheel size, taking into considertion the use in which the different wheel sizes are intended.
I think that has also covered my other point about gear ratios at the same time.

So onto my last point: As I mentioned before, I ride a 19" trials/street unicycle, which I have 127mm cranks on. If I wanted to ride just trials then a 137mm or 150mm would definitely help me there and also give me more torque. But if I wanted a harder gear to ride, for example, a halfpipe/skatepark then it makes for very dificult riding and control if I go down to a 104mm crank length. So is there a way of increasing the ratio but still using an almost standard 19" unicycle without putting a stupidly expensive hub on it. I also am not sure what ratio I am currently riding or which ratio it is I want to go to, I just know that I want it to be a bit harder so I can go faster without loosing all of my torque!
I’m not saying that I want to have a full set of gears on my unicycle, all I want is a harder gear on my current unicycle (or an adapted one) to try to keep clutter down and also not to add too much weight.
All I can think of currently is to put a moving axle through a fixed gear road bike hub with a cog on, have a cog on the crank, then have two more cogs that would transfer the drive from one to the other. This way you could choose cog sizes to change the gear ration. Does anybody know whether this could work or if anybody has tried it at all.

Sorry if this has been a bit lengthy or boring, or if you feel like you’ve just lost 10minutes of your life to my pointless thread. That in mind, any help/info will be much appreciated.

It’s already been discussed in great detail in this ( New concept: Total Gear Ratio ) thread, aka New Concept: Total Gear Ratio, started by Klaas Bil.
Enjoy crunching the numbers and figuring the ratios.


Pete Perron’s many renditions of the jackshaft all used this approach.

Have this free SW from IronJungle.Com crunch the numbers for you and let you play “what if”.

I didn’t fully understand that when I first read through (I did search before my new thread!).
After having somebody else tell me it was a good thread to read over I re-read it and it makes sense now. What I was looking for was this equation to try to figure out the gearing:

outer wheel diameter (including tyre) / 2 (to get radius) / crank length

E.g. (for approx. 19" unicycle with 127mm cranks)

-19" convert to mm = 482.59 / 2 (to get radius) = 241.295

-241.295 / 127 (crank length)= 1.89

So I still don’t understand what that number (1.89) is?! It’s not the amount of revolutions because it will always be 1 revolution.
The guy calls it a ‘Total Gear Ratio’ but surely it can’t be called a gear ratio if there is not gearing involved. My poor brain can’t follow this completely!

OK, so so far I’ve had an idea of how to build a geared unicycle so that I can use the cranks I want and the wheel size I want and still get a harder gear for riding skateparks. Then, as I’m looking through some threads that I’d looked at before, I realised that I’ve pretty much just thought up what somebody else has already done, meaning this:

Which is to be found here:

Sorry for all the double posting. I just looked over some gear ration equations to try to figure out how to change the gearing on a unicycle.

The equation I did before was following Klaas Bil’s TGR thoery. So I came up with 241.295 / 127 = 1.89 for my 19" unicycle. If we now take the 1.89 as the standard rotations of the unicycle (so pretty much forgetting we ever did all of the maths on that bit) and called it 1 (this is because 1 rotation of the cranks gives us 1 rotation of the wheel). So now we can get onto gear ratios to try to change how fast we are going for how many rotations of the cranks we are making.
It is quite a simple equation to get the gear ratios:

Drive cog (crank cog) / hub cog = ratio.

E.g. 25 tooth Drive cog and 25 tooth hub cog would give us:

– 25 / 25 = 1 – Which is 1 rotation of the wheel for every 1 rotation of the cranks.

So if we try:

– 25 / 20 = 1.25 – So now we have an output of 1.25 rotations of the wheel for every 1 rotation we are making on the cranks.

Does anybody know what kind of speeds they can get out of their gear ratios? I know that I am going roughly 7-9Mph with my direct 1-1 ration with my 19" wheel and 127mm cranks.

The 1.89 you are getting is the movement of a point on the outside of the tire relative to the movement of your foot. If your foot is spinning with a velocity of 1 meter/second you will be traveling at 1.89 m/s.

edit: if you gear up the same wheel by 1.25 you would have a total gear ratio of 1.89 * 1.25 = 2.36

You would travel 2.36 m/s if your feet were going 1 m/s in relation to the hub.

Ah right, now that makes a lot of sense. Thanks for that. It can be pretty simple sometimes, it just needs to be pointed out by somebody else!

It’s basically the amount of leverage the uni provides against the ground in who knows what units.

Meters per meter?