Electric unicycles reimagined

[mhpr262]

Pretty much all the of current models have a hub motor and the battery pack enclosed in the shell. While that makes for a tidy and neat package I see four major problems with that design:

- The hub motor is way larger than it has to be for the actually required power. That means more expensive magnets, more expensive copper wire, more sealing issues and thus more weight and more cost, both for raw materials and for manufacturing.

- the battery packs within the shell are battered badly in everyday riding, due to vibrations and crashes, not to speak of the possible problems with moisture when riding in rain or through puddles. They are also not easy to switch out with most of the current designs, especially when you still have to ensure adequate protection from the aforementioned issues (and you do!).

- heavy motor and heavy batteries really add a lot of weight, which makes the wheel harder to carry and to maneuver. Worst of all, the hub motor doesn't only add overall weight, it adds rotational weight, which does not only have to be accelerated backward and forward during balancing, but also sped up and slowed down in regards to rotation speed and is thus doubly "damaging".

- All that weight is really low down - literally at your feet. An electric unicycle basically balances by going back and forth underneath your body, similar to how you balance a long stick vertically on your fingertip by moving you hand. And a lot of the weight the EUC has to accelerate backwards and forwards in its attempt to balance you is its own. The higher the center of gravity is, the easier the balancing becomes, just like a long stick is easier to balance on your finger than a short stick, and putting a weight on top of the stick makes balancing still easier. So anything that moves the center of gravity up will make it much easier for the wheel to balance you (and itself). It will therefore use much less energy and gain range.

 

So, how could those issues be addressed?

I propose the use of a smaller, external motor that drives the wheel by means of a belt and high reduction gearing achieved by different belt discs. With a 16'' wheel I estimate the reduction could easily be 20:1 or 30:1 just by means of the two discs. Instant torque at low rpms is a strength of electric motors, but their real strength is their ability to run effortlessly at really high rpms. High rpms coupled with massive reduction gearing would mean a much smaller, lighter motor could be used (like, half the size of a coke can). Then take the batteries out of the wheel and put them in a convenient backpack or sturdy messenger bag that the rider wears and I am pretty you could build a wheel that weighs less than 9 or 10 lbs. It would be much more efficient too, as much less energy would have to be spent on balancing with the overall rotating mass of the wheel hugely reduced and the higher center of gravity that comes from the batteries in the backpack. The batteries in the backback or bag would be protected from vibrations, dust, dirt and moisture and would be easily exchangeable. Also, a thief could steal the wheel or the batteries, but it would be much harder to get both at the same time.

The only downside of this arrangement: The battery would have to be plugged into the wheel before you could start riding, and you would have an electrical wire connecting you to the wheel while riding. On the other hand one doesn't move around much while riding so I doubt it would be much of an issue.

 

Let me hear your thoughts!

 

[esaj]

Rockwheel used the geared smaller motors in their older models, and they did have both high speed & torque, but the gears (apparently) wore down fast, had issues with "accuracy" (ie. the wheel balance wasn't as "precise" as with hub-motors) and they seemingly never got much popularity. I think they've switched to hub-motors since.

The "carry-on" batteries might not be a bad idea, but I think most people still prefer to have the wheel as a "single piece". Carrying several kilos of batteries (a single 16S pack weighs around 0.75kg) on your back all the time might become tiresome. Not to mention if you fall down, the batteries might take a worse hit than when inside the shells.

[US69]

yup!

batteries of my 1360wh powerhorse weight about 6.8 to 7 kg....

i would not prefer to have these in my backpack :-) also the brushless "big" motors have made their way!

there was a nice looking solowheel "orbit" which worked nearly the same like that....but batteries still in case....it was reported as a really worse working EUC and never made the way to consumers hand i think...

[esaj]

15 minutes ago, KingSong69 said:

there was a nice looking solowheel "orbit" which worked nearly the same like that....but batteries still in case....it was reported as a really worse working EUC and never made the way to consumers hand i think...

There's the Fastwheel Ring (which is basically a Solowheel Orbit -"clone"), but we've never (to my knowledge, and I think I've read all the messages published in the forums ever) had a review of it. I think it's been available for at least since the turn of the year?

[mhpr262]

 

1 hour ago, esaj said:

Rockwheel used the geared smaller motors in their older models, and they did have both high speed & torque, but the gears (apparently) wore down fast, had issues with "accuracy" (ie. the wheel balance wasn't as "precise" as with hub-motors) and they seemingly never got much popularity. I think they've switched to hub-motors since.

The "carry-on" batteries might not be a bad idea, but I think most people still prefer to have the wheel as a "single piece". Carrying several kilos of batteries (a single 16S pack weighs around 0.75kg) on your back all the time might become tiresome. Not to mention if you fall down, the batteries might take a worse hit than when inside the shells.

if the gears wore down fast it was most likely because somebody tried to save money by making them out of cheap stuff. The gears in high-performance RC cars for example take a brutal battering that nothing in an EUC could possibly match, but they still last hundreds of hours. All you need is a really good, hardened steel pinion gear and a fiber reinforced, super tough plastic spur gear.

The weight is certainly an issue - I imagine this would be a good solution for commuters though who ...

- will only be riding moderate distances, like 2-5km.

- will have to be carrying a bag anyway, with their documents, lunch or other stuff.

- will really appreciate the reduced weight of the wheel itself when they ride on a bus or train and only use the whell for "last-mile" transport. If one uses the wheel for leisure riding or touring a heavier wheel is of course much less of an issue (they will still be less efficeint, though).

[HEC]

1 hour ago, mhpr262 said:

- All that weight is really low down - literally at your feet. An electric unicycle basically balances by going back and forth underneath your body, similar to how you balance a long stick vertically on your fingertip by moving you hand. And a lot of the weight the EUC has to accelerate backwards and forwards in its attempt to balance you is its own. The higher the center of gravity is, the easier the balancing becomes, just like a long stick is easier to balance on your finger than a short stick, and putting a weight on top of the stick makes balancing still easier. So anything that moves the center of gravity up will make it much easier for the wheel to balance you (and itself). It will therefore use much less energy and gain range.

I think you've got this wrong way around. More weight being further / higher from COG means more energy needed to ballance the wheel as more mass would travel longer angular path and also the inertia will be higher for such "displaced" weight. That's why much taller people have lower range on EUC than shorter people of the same weight (apart of the obvoius higher draft / air resistance as well).

[mhpr262]

More mass being further fromm the COG also changes and moves the COG along with it. What you say would only be true if the EUC balanced you by grabbing you by head and shoving you forward and backward. But it moves your feet UNDER you, so by moving the COG higher up you have LESS angular path and less inertia.

 

[mhpr262]

On second thought, I think we may be confusing two issues here. What you are referring to has more to do with centralization of mass, not really something with the cog. But external batteries would also definitely help with centralization of mass and would let the EUC balance its rider more easily.

[steve454]

5 hours ago, mhpr262 said:

I propose the use of a smaller, external motor that drives the wheel by means of a belt and high reduction gearing achieved by different belt discs

That sounds like the torque converter used on some minibikes back in the 70's.  A friend of mine had one and it had great torque and speed.  The two halves of the pulley would make the belt move up or down, that changed the gear ratio from minimum to maximum and all points in between.  I think the continuosly variable transmission is modern cars works the same way, and is very efficient.

[esaj]

20 hours ago, steve454 said:

That sounds like the torque converter used on some minibikes back in the 70's.  A friend of mine had one and it had great torque and speed.  The two halves of the pulley would make the belt move up or down, that changed the gear ratio from minimum to maximum and all points in between.  I think the continuosly variable transmission is modern cars works the same way, and is very efficient.

I think also the moped-scooters use something similar.

[OliverH]

On 29. Juli 2016 at 8:27 PM, mhpr262 said:

Pretty much all the of current models have a hub motor and the battery pack enclosed in the shell. While that makes for a tidy and neat package I see four major problems with that design:

- The hub motor is way larger than it has to be for the actually required power. That means more expensive magnets, more expensive copper wire, more sealing issues and thus more weight and more cost, both for raw materials and for manufacturing.

- the battery packs within the shell are battered badly in everyday riding, due to vibrations and crashes, not to speak of the possible problems with moisture when riding in rain or through puddles. They are also not easy to switch out with most of the current designs, especially when you still have to ensure adequate protection from the aforementioned issues (and you do!).

- heavy motor and heavy batteries really add a lot of weight, which makes the wheel harder to carry and to maneuver. Worst of all, the hub motor doesn't only add overall weight, it adds rotational weight, which does not only have to be accelerated backward and forward during balancing, but also sped up and slowed down in regards to rotation speed and is thus doubly "damaging".

- All that weight is really low down - literally at your feet. An electric unicycle basically balances by going back and forth underneath your body, similar to how you balance a long stick vertically on your fingertip by moving you hand. And a lot of the weight the EUC has to accelerate backwards and forwards in its attempt to balance you is its own. The higher the center of gravity is, the easier the balancing becomes, just like a long stick is easier to balance on your finger than a short stick, and putting a weight on top of the stick makes balancing still easier. So anything that moves the center of gravity up will make it much easier for the wheel to balance you (and itself). It will therefore use much less energy and gain range.

 

So, how could those issues be addressed?

I propose the use of a smaller, external motor that drives the wheel by means of a belt and high reduction gearing achieved by different belt discs. With a 16'' wheel I estimate the reduction could easily be 20:1 or 30:1 just by means of the two discs. Instant torque at low rpms is a strength of electric motors, but their real strength is their ability to run effortlessly at really high rpms. High rpms coupled with massive reduction gearing would mean a much smaller, lighter motor could be used (like, half the size of a coke can). Then take the batteries out of the wheel and put them in a convenient backpack or sturdy messenger bag that the rider wears and I am pretty you could build a wheel that weighs less than 9 or 10 lbs. It would be much more efficient too, as much less energy would have to be spent on balancing with the overall rotating mass of the wheel hugely reduced and the higher center of gravity that comes from the batteries in the backpack. The batteries in the backback or bag would be protected from vibrations, dust, dirt and moisture and would be easily exchangeable. Also, a thief could steal the wheel or the batteries, but it would be much harder to get both at the same time.

The only downside of this arrangement: The battery would have to be plugged into the wheel before you could start riding, and you would have an electrical wire connecting you to the wheel while riding. On the other hand one doesn't move around much while riding so I doubt it would be much of an issue.

 

Let me hear your thoughts!

 

You can put the belt with a serpentine drive inside the rim and can use four motors.. But a belt will have a delay in acceleration/ deceleration. You can set it as strong as on motors in cars (crankshaft/ valve train), but the available electric motors (cots) will not have such strong bearings to withstand the forces. The radial or also axial coil design of the bldc motors have an advantage: being service free and cheap to manufacture.

[Tomek]

nice to think outside the box, but, uhm, I agree with @HEC that the whole premise of the idea is a bit flawed

1. the lower the center of gravity, the better - e.g. monocycles (big motorised wheels where you sit inside) have super low c.o.g. and are very stable. ( to explain - imagine a frame of reference where the wheel is fixed - as if fixed to ground - and the rider is being balanced on top of it by being turned by the motor - a small, chubby rider would be easier to balance on top than a tall rider with the same weight - that's because moving the tall rider requires more momentum M=distance_from_the_wheel_axis x force_applied_is_the_same_for_both_riders)

and also:

2. the motors of EUC's are not too powerful. on the contrary the (relatively) recent 800W motors are providing finally just enough power to reach decent speeds and prevent easy overleaning of the unicycle

3. "rotational weight" of the wheel/motor also generates more angular momentum = more stable wheel when riding (that's why Msupers are so stable, they have bigger, heavy wheels AND c.o.g. is lower from the wheel axis than on smaller wheels)

4. hub motors are most efficient, every gear or transmission will add resistance, noise and also involve more parts that would be likely to break

5. the (primary) battery in a backpack would be a suicide - think what would happen if you accidentally pull and break the cable (edit: and we're talking reaaally thick cables here at such length, with such currents...)

 

On 7/29/2016 at 9:23 PM, esaj said:

There's the Fastwheel Ring (which is basically a Solowheel Orbit -"clone"), but we've never (to my knowledge, and I think I've read all the messages published in the forums ever) had a review of it. I think it's been available for at least since the turn of the year?

I've read somewhere that it never got further than a prototype - too many engineering problems

...but also there the motor and batteries were positioned as low as possible to lower the c.o.g.

[Keith]

+1 with @Tomek,

Yes you can get 800W or more out of small high revving motors ( some model heli's, geared of course, pull more than 5kW - 100Amps at 50V). Geared motors are the simplest solution in model cars and helicopters, but in model planes, gears were dropped ASAP when brushless out-runners made big diameter direct drive motors viable - the efficiency is way higher  and weight lower. A geared motor has to be much higher revving, I.e.higher kV, which means less turns of wire, made much thicker to carry the higher current required to get the power from a small motor.

For EUC's It is at zero speed, I.e. When just balancing, that a geared solution really performs at its worst,  poorer resolution, gear wear, noise, and, potentially very high current as the motor is stalled with no back EMF to hold the current in check. If the hub motor could be smaller then the weight would be saved by concentrating the motor coils close to the rim (where they can generate the most torque.) and making the hub centre as light as possible. Interesting that isn't done!

Inmotion have started reducing the hub weight by using more aluminium, that seems a more practical solution.

[Tomek]

13 hours ago, Keith said:

If the hub motor could be smaller then the weight would be saved by concentrating the motor coils close to the rim (where they can generate the most torque.) and making the hub centre as light as possible. Interesting that isn't done!

I guess that's what Solowheel Orbit tried, but there seem to be some problems with engineering it in a robust enough fashion.

Also, Solowheel Extreme managed to remove some material in the center of the hub and place all the electronics there - I really love the elegance of this.

edit:

I guessed you meant smaller in overall size, not in diameter - concentrating the magnets and weight close to the tire and leaving the middle empty. Making the diameter of the motor smaller would reduce the torque. Rockwheel used small motors, but they were geared...

[Jason McNeil]

@Keith, @esaj & @Tomek covered most of the thoughts that I had on the topic as well—sorry guys, I ran out of up-voting points allowance today  

• No matter what measures are taken to try to enclose the belt/pulley system, contamination & fine particle abrasives will inevitably cause havoc
• Materials will be placed under high stress loads in situations like jumping off a curb, where massive changes in forces are imparted instantenously 
• Unsuitable with extremes of humidity & temperature
• It will almost certainly be noisy 
• Dissipating the waste heat in a energy dense motor could be problematic 
• The angular momentum energy of a hub motor is preserved in steady-state
• Both peak & continuous power potential of a large diameter hub motor should be many times higher than the smaller out-runners  
• As Keith touched on, Inmotion's V8 will be replacing elements of the hub motor with Titanium & magnesium saving an impressive 1kg  

The silence, durability, elegance, maintenance-free nature of the hub-motor can probably be continually evolved where it's only 3-4kg, bringing a top-end 16" Wheel to under 12kg. 

21 hours ago, Keith said:

Yes you can get 800W or more out of small high revving motors ( some model heli's, geared of course, pull more than 5kW - 100Amps at 50V)

When operating at these outputs, don't these motors have like an Airship's volume of air passing them every minute to keep them cool?

[Keith]

2 hours ago, Jason McNeil said:

When operating at these outputs, don't these motors have like an Airship's volume of air passing them every minute to keep them cool?

@Jason McNeil, good point that had slipped my mind, even with a 6ft diameter fan wishing round high power 3D helicopters often have an additional fan built into the main gear and the motor mount doubling as a heavily finned heat sink, not to mention the total flight time with a typical 5Ah battery being only 3 minutes.

Bottom line, when trying to reinvent the EUC drive train its amply covered by that well known abbreviation - K.I.S.S.

P.S. Failings of the geared motor route were also discussed on the Airwheel F3 thread here: 

 

[Mono]

If I am not mistaken, you need to have a stiff connection between motor casing and pedals, which would be a good reason to place the motor close to the pedals.

[mhpr262]

On 31.7.2016 at 8:43 PM, Jason McNeil said:

@Keith, @esaj & @Tomek covered most of the thoughts that I had on the topic as well—sorry guys, I ran out of up-voting points allowance today  

• No matter what measures are taken to try to enclose the belt/pulley system, contamination & fine particle abrasives will inevitably cause havoc
• Materials will be placed under high stress loads in situations like jumping off a curb, where massive changes in forces are imparted instantenously 
• Unsuitable with extremes of humidity & temperature
• It will almost certainly be noisy 

Belts are used without any issues at all even in powerful motorcycles. There is a conversion kit even for my 1250ccm Suzuki Bandit. Good for 40,000 miles or more. Harley Davidsons have been running belts for decades without any issues, in any climate, both primary drive (crankshaft to gearbox) and secondary drive (gearbox to rearwheel). And belts are very quiet.

There are plenty of high-performance skateboards just as quick as an EUC that prove every day that heat dissipation is not an issue with those motors. They may be much more power dense, but their smaller size and their "open" design make it much easier to get rid of excess heat. many modern ones have casings that double as ventilation fans, too. A great many electric skateboards use outrunners ... and belts, btw.

I am not sure how importnt angular momentum is - maybe it helps going over obstacles, but I am sure my version could accelerate the wheel a lot quicker. That is where the real balancing takes place.

Output of a large hub motor COULD certainly be much higher, but how much does one actually need? i don't see EUCs going much above 1500W, ever. Only a small minority of riders will ever want to go so fast on an EUC that more power would be required. That is easily achievable with a small outrunner. I don't see power as a motor issue, mor as an expensive power electronics issue with the controller.

[noisycarlos]

On 9/1/2016 at 11:14 AM, mhpr262 said:

Belts are used without any issues at all even in powerful motorcycles. There is a conversion kit even for my 1250ccm Suzuki Bandit. Good for 40,000 miles or more. Harley Davidsons have been running belts for decades without any issues, in any climate, both primary drive (crankshaft to gearbox) and secondary drive (gearbox to rearwheel). And belts are very quiet.

There are plenty of high-performance skateboards just as quick as an EUC that prove every day that heat dissipation is not an issue with those motors. They may be much more power dense, but their smaller size and their "open" design make it much easier to get rid of excess heat. many modern ones have casings that double as ventilation fans, too. A great many electric skateboards use outrunners ... and belts, btw.

I am not sure how importnt angular momentum is - maybe it helps going over obstacles, but I am sure my version could accelerate the wheel a lot quicker. That is where the real balancing takes place.

Output of a large hub motor COULD certainly be much higher, but how much does one actually need? i don't see EUCs going much above 1500W, ever. Only a small minority of riders will ever want to go so fast on an EUC that more power would be required. That is easily achievable with a small outrunner. I don't see power as a motor issue, mor as an expensive power electronics issue with the controller.

I wonder though if the belt introduces latency or a delay between the motor moving and the wheel moving. Depending how bad it is, it could affect the balance big time. 

This latency wouldn't affect bikes or skateboards, since the motor only pushes the wheels forward, and you only control the amount. So it doesn't matter if the wheel reacts a second after you moved the throttle.

But with eucs a second can be the difference between a save and faceplant. 

Of course, i don't know how big or small the latency is, it would be a matter of testing.

 

[lizardmech]

I don't think any of the ring models ever actually shipped. From what I could tell they were all trying to hack together off the shelf parts but belts had high failure rates. I don't think any of them were willing to build a new motor controller or custom motor for them so they never work well.