My FunWheel: A Onewheel work-alike
Updated Aug 10, 2020 including parts list and model of axle assembly.
Hat tip to Mitch
Lustig, he wrote the VESC balance
code, that is what makes this possible. Also special shout to
During COVID-19 quarantine in 2020 I decided to build myself a one-wheeled
skateboard, aka self-balancing scooter, aka funwheel. The machine itself is not
my ultimate goal, it is only one step towards my goal, which I will not reveal
here. When I started, I had no CAD skill, and no desire to overcome what I felt
was a large learning curve to get that CAD skill. Instead, I wanted to hone the
ability to build a working electric powered personal transporation machine, and
a funwheel seemed like a great way to do that. It was! Objective achieved!
This build was a complete in-place build, the construction started by mounting
the motor into the frame with nothing more than a pair of matched holes, and
proceeded to iterate until completion. It was not the most efficient way to get
a funwheel, but my goal was to learn what problems I would need to overcome,
and that certainly happened. As a follow-up to building the machine, I decided
it would be a good idea to learn CAD after all, and so I retroactively modeled
my build using Autodesk Fusion 360. The result of those renders are what you
Would I do it twice like this? Overall yes, the result has been a very ridable,
very fun machine. Generally this was a fun build, and I wouldn't change
anything if someone set out with the same goals I had. I learned a lot and so
will you. However, for a great Funwheel, if there's one thing I would do
differently, it would be to have a motor mount that attaches to the frame,
rather than mounting the motor right into the frame. This is a tough call
because having the motor mounting straight into the frame is so simple and
easy, but complicates tire changes. Also, I would use different batteries. The
Samsung 25R was great for learning, and the 20A per cell output rating (60A
combined in this 3p configuration) means that I can lose a cell (or two!) and
still limp home. Normally that would not be desirable, because you lose
capacity for this, but I had some pack challenges along the way and arguably
the safety of the Samsung 25R saved my board and my home from fire due to the
challenges I encountered. At one point early in the build there was a twisting
motion in the board that was being transmitted into the battery pack, causing a
pack fracture that eventually resulted in a non-catastrophic meltdown; the 25Rs
allowed the board to limp on. Good? Bad? Hard to say. However, with battery
pack experience and a bit more clearance for the pack itself (so that the pack
does not experience any twisting) a better choice would be Samsung 30Q, or Sony
VT6A, among others. More power density allows more range. A 60A continuous
discharge is really much more than is needed, I feel that even 20A continuous
discharge rating would be adequate. Stiffening of the bays with a battery box,
wrapping the packs with a hard shell such as fiberglass, strengthening the pack
with epoxy like Tesla does it, building a stiffer frame from the start, using
thinner material for the bay floors, or increasing the thickness of the bays
slightly would all relieve this pack twisting problem. I ended up increasing
stiffness by using a wider board, stronger fasteners with much more holding
power, and adding 1/16" of clearance by putting washers between the top plate
and the frame. Early on I overdischarged my packs due to a failure to correctly
configure my voltage cut-off and I ended up having a small fire that ocurred as
I was putting the cells on to charge. I learned my lesson, and now I do not
discharge below 3.0V. Some would consider even that to be too high.
Here is a video of me riding it. It weighs 10.7kg/23lbs and I get about 20Wh/km, or 32Wh/mi. It isn't as capable as I want yet, but with some motor tuning and an additional voltage boost (up to 14s/50.4V) it should get there. Currently speed is not an issue, I am easily attaining 20mp/h (30km/h) on flat ground. Steep hills are a challenge when the battery is below 40% because of a low-voltage glitch that causes motor ticks (note! This was later fixed by decreasing motor observer gain); this is not a real issue for general ridability. Additional enhancements will round out the board's usability:
- An integrated BMS, so I can stop opening the lids every time I want to charge the batteries. (Update: I fixed this by simply rewiring the board so that the batteries could be unplugged and charged while still in the board.)
- A reliable on/off switch. Currently I'm using a power cut pull-key to disable the board reliably. (Update: This was removed and replaced with a main disconnect that doubles as a charge point.)
- Oh yeah, and footpads! (Update: I did end up making a front footpad, it is good but also unreliable because changes in temperature cause the activation range to change, so I sometimes need to change the footpad trigger voltage depending on the temperature. This also means a rider that weighs less than I may not be able to ride the board, because they will not reliably trigger the footpad.)
Here is a more or less complete parts list with sources.
- Motor controller: Trampa VESC 6 MkIV
- Motor controller accessories from Trampa: Bluetooth module for real-time datastream and on-ride reconfig. Dust jacket. Both highly recommended.
- Motor: PHUB188
- Balance code: Thanks Mitch! Balance code is integrated into the VESC open source project.
- Batteries: 36 Samsung 25R 18650 batteries in a 2x6s3p arrangement.
- Frame: Aluminum sourced from home depot (1.5"x1.5"x1/8" L, 1"x1"x1/16" L) and some plywood scavanged from an old cabinet.
- Inspiration: Mohammad from the Fungineers
- Tools required: Chop saw, drill. Some relatively easy soldering for the motor hook ups. Useful: Step-up hole drilling bit, small crimping tool for JST-XT 2.0mm (VESC). Various types of rulers and markers. A grinder. Sandpaper. Drill press. Multimeter. Level.
- There is no foot pad (yet) which makes it a run-away hazard. The device mainly uses the roll sensor to detect a shut-off condition. It is dangerous and advised that users practice mounting and dismounting before doing so powered. See many videos about how to jump off a Onewheel; jump works, but I don't like it, instead I prefer the tilt & slide method of dismounting.
Motor controller settings
There is a spreadsheet with various tuning options. The Trampa IMU appears to be pre-filtered, and so due to bad interactions between the Trampa filter and the Balance filter, I needed to drop the accelerometer confidence decay down to zero. However, I was late in tuning a good value for Mahoney Kp, and so tuning here is still ongoing. The board rides really well, but improvements are still possible especially for handling rough road texture (like on a boardwalk or sidewalks with tightly spaced relief cracks.)
* VESC Tool & firmware: 2.06
- Rear bay consists of the rear half of the battery pack, the motor controller, and motor, sense, power, wake-button wiring, and BLE wiring if there's a Bluetooth module.
- Front bay consists of the front half of the battery pack, and the main power fuse.
- Front top plate supports the fang wheels, the pair of little wheels at the front.
- Metal corners are softened by grinder. An out of control funwheel can cause a fair bit of damage to an infant, toddler, sports car, garden bed, or anything else you don't want scratched or squished. Consider adding a bumper of some type. My funwheel did in fact crash into me during early testing and tuning, and my ankle hurt like heck.
- Main rail length is 24.5"
- The rear bay lower bracket is not the right length. On the actual machine the length is 7.5", so that the rear bay lower bracket meets the rear of the main rail, just as the front bay lower bracket meets the front of the main rail.
- I haven't figured out how to express wiring in a good way with Autocad Fusion 360, so the rendering has only the basic large electronic components, none of the wiring.
- "Fang wheels" are essential. When the board nosedives, the fang wheels buy you a half-second or so in which to start running. That extra half-second makes all the difference in the world. The control logic is experimental even when well tuned. You will be happy not to get thrown when the PID control loop can't keep up with the oscillations due to some bumps.
- It is built out of at-hand supplies. I didn't want to get into 3D printing.
- Main member is a pair of aluminum L that are 1/8" thick and just over 2' long each. They are 1.5x1.5" on the long side of the L. This worked out perfectly, for the frame hight, but could have been less wide. 1x1.5" would have worked really well too.
- I weigh about 200lbs. The 1/8" thick aluminum main bar works really well, feels solid. I did try a 1/16" main bar prior and that was just not strong enough.
From PHUB188 vendor page, with annotations.
- what they don't tell you: Stopping the hub from turning is easier said than done. I screwed a "tight clearance" 10mm wrench that I happened to have lying around onto my frame.
- voltage is 24V 36v 48v 60v: I'm running 12s so 40.8/43.2/50.4 but more would be better. 14s would be perfect.
- optional power is bewteen 250w and 800w: aiming for the high end would be good. Riding over bumps can overwhelm the motor at low power. Recent ride attained 1000W up a hill.
- speed can be 20km/h to 35km/h: Yeah I am pretty sure it can go that fast, but honestly I chicken out at about 20km/h. So much could go wrong, including in the machine, that I just don't want to ride that fast. Hospitals are no fun. But on smooth asphalt with my protective gear on I have attained 30km/h.
- actully width of tire is 170 mm: Actually width of tire is not even close to 170mm, the tire is marked as 6 inches wide (152.4mm). What they really mean is that the unusable area close in to the motor is 170mm wide, that is the interior distance between the axle stops. Also notable is that the motor is slightly fatter on the left side, by about 5mm, due to the power/sense wires and the air valve both being on the right side. There is a right and left side, because the tire has a "direction of rotation" arrow on it, so I assume we ride forward in the direction of rotation (I certainly have a dominant foot and I think most people do, so safe to assume there is a "forward".)
- length of shaft is 200 mm: Seems to be. An extra 10mm (5mm per side) would be nice.
- diameter of hub wheel is 150 mm: The number you really want is diameter of the tire. I decided on 280mm of clearance between front and rear bays for the tire. That is pretty tight and leaves just barely enough room for flex and wobble. Juuust enough room to fit my gloved fingers in between the back topplate and the wheel, useful for when the board is disabled and needs to be carried.
- diameter of axle is 14 mm: I can't get my caliper to read less than 14.8 on this, so I would expect to drill a hole a bit wider. Aren't step up bits great?
- Power table: Notable is the top current at 60.39V is 30.54A
Pay attention to this graph! The shape of the power/efficiency curve can be felt in riding. If you wish to avoid nosedives, getting familiar with this curve would be good.
Early on I solved the problem of the motor turning in the axle mounting hole by putting an ultra-thin 10mm collet wrench to work holding the axle. This eventually stopped working when the wrench started to round out the axle. The axle metal is quite soft, so I decided that drilling through it and the mounting nut together was a good solution. Using a guiding jig and successively larger drill bits, I was able to grow the diameter of the hole in-place to eventually get to a nice tight-fitting and perfectly aligned hole for a hitch pin. This has worked really well and remains the way on my board today.
- 7.5Ah/324Wh battery pack
- Two packs split across the board, each 6s3p.
- No on-board BMS; cut-off is managed by VESC, remove packs to charge.
- High/Nominal/Low voltage: 50.4V/43.2V/38.4V
- Sustained discharge of 60 Amps!
The fang wheel is not as depicted. Instead, I found this wheel at the local home depot, named a "Richelieu Hardware 1-9/16 in. 20 kg General-Duty Rubber Rigid Casters":
Richelieu Hardware Link
Home Depot Link
I'm happy to talk about my project. Hit me up at firstname.lastname@example.org !
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