My first build out of real necessity (not something like, I really want a delta robot)! Scooty Puff is a brushless motor powered wonderfully fast Razor style folding scooter.
The problem:
I needed a way to get around in Dallas, specifically from where I live to work and back.
The constraints:
- It had to fit in one of my suitcases. This rules out bikes.
- It has to go some reasonable range.
- I can carry it on a train.
Nice to have things:
- Speed.
- Pneumatic wheels to not rattle my joints apart.
- Brakes.
All signs point to scooter! Name: Scooty Puff, after Fry's ride in the Futurama episode "The Why of Fry."
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| The original Scooty Puff Jr. |
When designing, I had a lot of help from Jamison and his experience making scooters, and referred to Shane's Pneu Scooter and Charles' RazEr rEVolution and Instructable on electric scooters quite a bit.
Parts List
- EMP C6374 200 Kv brushless DC motor, originally designed for big model aircraft.
- 6"x1.25" (150x30) pneumatic caster wheels. I originally used a pneumatic tire for the front wheel and a flat-free tire for the back wheel. Later I find out why this is a bad idea.
- 4" x 1.580" x 0.180" aluminum channel, a bit less than 2 feet of it. This is the body.
- The longpack, a 6S 5Ah lithium polymer battery.
- A 24V 250W brushless ebike controller, "without hall." These are shady controllers from China but they work.
- 3/16" thick aluminum plate for making the motor mount ("motorpod").
- 1/8" polycarbonate to cover the channel
- Odds and ends for spacers and standoffs and washers and whatnot.
Design Considerations
As always, a link to the Solidworks design files are linked below in the appendix.
The Rear Wheel
One tricky part was getting the sprocket mounted on the rear wheel. The hub of the wheel is plastic with spokes, so there was not a lot of material I could remove for, say, holes in the spokes.
So, after asking some friends around the Invention Studio for advice and taking inspiration from Pneu Scooter's hub motor, I came to a solution. I would use a flat free wheel. The sprocket is mounted onto a plate, and that plate onto the wheel with screws that go through the entire wheel assembly, like so.
This way there are no threads pulling on the plastic hub. Now this solution works all hunky-dory, until unforeseen material properties smacks me in the face. First, the tire does not have as firm a grip on the hub, so it tends to pull away during turns. This creates the terrifying feeling of the rear wheel slipping around corners. Second, the flat free tire wears down really, really fast.
How is it supposed to last the summer with these terrible characteristics? I had to switch to a pneumatic wheel, with a redesign of the sprocket mounting plate. Two plates now, one on each side of the wheel, are screwed onto the hub (yes, threading into plastic). The sprocket mounting bolts go through both plates to hold the plates in compression, minimizing the chance of the plastic threads pulling out.
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| Wheel in the flesh! |
This way there are no threads pulling on the plastic hub. Now this solution works all hunky-dory, until unforeseen material properties smacks me in the face. First, the tire does not have as firm a grip on the hub, so it tends to pull away during turns. This creates the terrifying feeling of the rear wheel slipping around corners. Second, the flat free tire wears down really, really fast.
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| This wheel used to be round. |
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| Just one week of use. |
The Motorpod
Heavily inspired by Razor Wind's wheel pod, it is a self contained unit for the rear wheel, motor, and power transmission.
I used the EMP/Turnigy C6374 because not only is it appropriately sized, but it also has a bearing on both sides of the motor. This makes the can more resilient to shaking and vibration and other nasty real world conditions.
Motor Selection
I used the EMP/Turnigy C6374 because not only is it appropriately sized, but it also has a bearing on both sides of the motor. This makes the can more resilient to shaking and vibration and other nasty real world conditions.
Controller
I use a cheap sensorless ebike controller from China. After reading Charles' Beyond Unboxing of the controllers, I thought they would do well after a bit of modification, and they do. The mods:- Reduced shunt resistance to 2 milliohms. This, in theory, boosts the controller's wattage from 250W to close to 1000W
- Due to Charles' reports on the bus caps getting warm, an additional 4700uF of capacitance.
- Swapping the power FETs with IRF3207s to more than halve the on resistance (Rdson)
The rest of the scooter was designed to be made with the waterjet, manual mill, and lathe. After two weeks of machining, it looks like this.
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| A scooter! |
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| Scooterbros |
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| With a 3D printed fender |
- 4.25 mile range
- 28 mph theoretical top speed
- 25 mph observed top speed
- 2 hour 15 minute charge time from empty to full, limited by charger power
- 26.1 Watt-hour/mile efficiency
