Electric Unicycle for Unicyclists?

General
1

Electric Unicycle

Here is a new idea for an electric unicycle; at least it is new to me.

The pedals on a unicycle perform two functions; they deliver power and act as control surfaces. The pedals control the speed of the unicycle and allow the rider to maintain balance. One could separate these functions to develop an electric unicycle that would be controlled by the pedals to give the feel of a real unicycle.

  1. Pure electric unicycle.

The idea is to use an electric motor to drive the wheel. One should be able to adapt electric bike technology to accomplish it, and it would be ideal if the motor were high torque and reversible. The pedals themselves would just have a friction drag at the axel to give some resistance, and would be tied to an encoder, which precisely measures the RPM of the axel and converts it to a digital signal. To speed up you would pedal faster, and to slow down you would pedal slower. The output from the encoder would be used to control the motor, and if we had a very good digital controller one would get precise and nearly instantaneous control of the wheel motion—almost as if they were connected. This idea would basically take a “self-balancing unicycle” and return the control to the rider.

The advantage of this method is that it decouples the speed of the pedals from the speed of the wheel (and it makes riding much less work.) With a digital controller, you could start at a 1:1 ratio and take it up smoothly to any practical ratio such as 1.5:1 or 2:1. This system would give you electronic control of the gearing without using any physical gears.

  1. An electronic unicycle you could ride

If the control system works, one could return to the design table to make one that is actually fun to ride. The only real change is to connect the axel to a generator and use it to feed the electric system. Presumably you would still need a battery to get started, and also to act as a power reservoir.

The advantage of the system is that you would now have a Schlumpf equivalent, but with continuous gearing from below 1 to whatever value the power output would support: perhaps from 0.7:1 to 2:1. It could be shifted on the fly with a dial, and adjusted to uphill and downhill riding. Since the system is electric, you could charge up the battery on easy parts of the ride and use the added energy on the hills. You also might be able to include electric braking to charge up the battery, as Justin Le did with his “electric assist” unicycle.

The disadvantage is the generator efficiency. You will loose some energy converting to electric and back. If the efficiency were too low, the whole thing would be impractical.

I can envision the idea, but its implementation is beyond my skills (or available time). If it worked, it would make a very cool road unicycle, perhaps in a convenient 26” or 29” size.

Comments?

Scott

2

love it :smiley:

I fear if you had motorised unicycles you can pedal, learning to drive it would be really hard if it accelerates depending on how fast you padel…I imagine keeping balance quite challenging…Once you are too fast, what happens? :smiley:

3

Hello Scott,

So glad you posted this.

I see what your proposing in your case 1. I had even started to write a similar post like that just last week. As i wrote it I thought about the lack of pressure to the pedals from the wheel, and was distracted from posting while I thought more about the need for feedback to the pedals.

I can see you given some thought about the easiness of the pedals to turn, and so added resistance. The resistance you suggest, I think is used to reduce the jerkiness of the ride due to the pedals/crank being unloaded, completely isolated from the motor driven wheel. I think we’d find that the resistance adds nothing to the system, that some sort of feedback from the turning wheel is necessary to provide some tactic feel of the resistance of the wheel turning on the terrain.

I had thought about your case 1 and lack of feedback, and so I changed it to have the wheel hub a motor where the axle is connected to the cranks, and the motor case the hub that the spokes are connected to. The encoded turning of the crank, is translated into turning the motor (case) around the axle. (This sort of boggles my mind, but seem like it should work) This way feedback of the wheel turning is directly to the pedals. While this arrangement supports a adjustable gear ratio feature by changing the multiplier from the encoded crank turning unfortunately the rider would need to provide the full power to turn the wheel. I don’t think the rider would feel any assist from the motor.

Anyway, that was a far as I got thinking about this project. I didn’t get to a solution as ideal as your proposal. I hope I’m not misleading the goal with my concern about the need of feedback, and others will pipe in to provide further insight.

4

<snip>

This would work fine and would be a fairly fun project. With practice the brain can adapt to just about anything that controls the wheel spin in order to keep a unicycle balanced, whether it is via pedal torque on the cranks (conventional), friction between shoes and tire (gliding), or coasting with hand brake levers as the control interface.

In this case you’re proposing a pedal rpm as the input control, which is somewhat different from a normal mechanical drive unicycle that is pedal torque controlled, but it would look more or less the same when someone is riding it.

You could also rig up any range of sensors and control the motor via nose twitches, mini joysticks, whistling pitch, or what have you and figure out how to ride it.

This is basically an electric transmission idea that gets floated around a lot in the ebike community but is not practical for the very reason that you mentioned, efficiency. Most small motors under load operate in the 75-80% efficiency range, and similarly the generating end is also in the 75-80% range, so your net pedal to wheel efficiency is on the order of 60%. Plus, you have the weight both of the drive motor and the separate pedal generator.

On the positive side, as you said, it totally decouples your pedaling from both your speed and terrain. So you can pedal at whatever RPM and torque you feel like, and sit there generating power (rpm * torque * efficiency) and then let the unicycle use that power at its own pace to drive the wheel.

The idea probably has a lot more merit in the case of a unicycle than it does with bicycles, since bikes already have an efficient mechanical drive chain that is ~98% efficient and typically spans a 4:1 or so gearing range. But unicycles have a much less than ideal fixed ratio linkage that doesn’t allow the rider to use their full power output, because the cadence is simply too high. So this might offset the rather poor net efficiency of the electric drive.

For instance, take a unicycle trip that involves uphills, downhills, and flats. On the uphills, the unicycle gearing is about right and a rider can put in a good 200 watts no problem. On the flat, the cadence is too high making it hard to keep up and apply much torque to the pedals, so the rider might only be able to put 50-100 watts to the wheel. Then on the downshills, you actually do negative work with a braking force on the pedals. Average power input to the wheel from the rider might be 75 watts or so.

On the other hand, if you took the 200 watts you can do on the uphills and applied that continuously to a generator, even if just 60% of that energy makes it to the wheels again, it still means an average power output of 120 watts over the course of the same trip. So you’d still beat the fixed drive mechanical unicycle even though 40% of your output goes to waste.

If you look at the build pics that I posted on the endless-sphere forum for the electric assist uni, you’ll see that there is plenty of empty space inside the motor. My next project will be to use this space to fit a gearing linkage between the pedals and the hub, in order to do a geared electric uni, but you could just as easily fit a small generator motor inside there. So it’s be feasible enough to build if someone really wanted to.

-Justin

5

Justin,

Thank you for your analysis. I was thinking of your very cool electric assist unicycle when I proposed this idea. I was afraid the efficiency would make it unworkable, and your analysis suggest that there is a significant penalty. From your discussion, I assume no one has made a bike with only electric drive because of the loss in an all electric system.

I liked your electric assist unicycle, but the most serious limitation I find in riding a unicycle over distances is that high cadence limits the speed. If you took a 29" and geared it up to 1.5:1 or even higher, and then installed your electric assist it would be a very nice ride. The electric assist would get you up hills, and the comfortable top speed would be higher than a normal 36". I find it less practical at a 1:1 gear ratio, as I can climb most hills OK and am more limited by my top speed. Do you ever get your prototypes down to Southern California?

The Schlumpf solution to the problem is elegant and I may eventually put up the money to buy one. In the mean time I will consider interesting electric designs.

Scott

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Exactly. The ring inside gears used in most geared electric bike hubs are 135 tooth, so current plan is to scavenge one of those, and combine it with a 95 tooth sun gear and 20 tooth planets for a 1.7:1 drive ratio, allowing the 29" to feel like a 50".

We had the electric down at the ‘U’ games in Berkeley last month and took it up the Mount Diablo ride and a few other events. Where were you?

For sure. I just got my own Shlumph uni assembled today and it is indeed a wonderful piece of machinery.

-Justin

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Perhaps the Huni-Rex could accomodate an adapted rear hub motor - remove the freewheel on the right side, screw a sprocket to the discbrake flange on the left side…
High Torque version of the Heinzmann motor? If only they would tell the hub width on the Huni-Rex website.

Oh, and a little boost for retrofit to any unicycle should be provided by these electric pedals (not in production yet). At the cost of more wobbling, especially with a light rim and tire.

2nd approach for Huni-Rex: use an RC plane motor and clamp it to the fork so that its sprocket drives one of the chains… use a little sprocket so that there is a lot of gear reduction -> high torque.
Or perhaps rather add a third chain. This gets complicated…

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This is nothing against the enicycle (I posted that several times already). The enicycle also balances sidewards, so you could just stop without falling to the side.

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This is nothing against the enicycle (I posted that several times already). The enicycle also balances sidewards, so you could theoretically just stop without falling to the side.

10

Hey!

Today in the local Sunday Mail, here in South Australia, i came across an article to do with electric unicycles.

At Adelaide university, some mechanical engineering students created the “micycle” an electric unicycle, weighing less than 25kg’s and goes about 15km/h. It has an electric motor and uses a control system to keep it’s balance. It also contains a motion sensor, which allows the rider to control the speed by leaning forward to accelerate and back to brake, while foot rests are used to steer.

Other features include fall detection and automatic shut off, audio warnings on motor saturation and low battery signals.

It cost $2800 to make the micycle and the designer is Rhys Madigan, along with 3 other guys.

They say that they did it for fun, and that it was a different type of design challenge.

I found this really interesting, and considering going down to the mechanical engineering exibition in a couple weeks to go see this.

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who is going to contact MIT on behalf of the unicycist community?

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Whatever it takes to get unicycles into the BIG AIR comp at the next X-Games :astonished:

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I did some Googling and found this video:

Also there’s a link to some info here. Looks pretty cool, if not very similar to the other electric unis out there.