Scooty Upgrades

Edited 08 Sept 2012 to add more pictures of the brake assembly.
Brakes that Work

Due to the failure of the caliper brakes I redesigned the brakes to use band brakes. These brakes work by tightening a band of braking material around a rotating drum. The caveat is that they don't fit on the front wheel, so they have to be placed on the back wheel. While it does eliminate the possibility of going over the handlebars through overzealous braking, it does significantly reduce braking force, something I'm not pleased about. Danger of going over the handlebars can be eliminated through practice. Maximum rear wheel braking force will never change.

Cramming brakes onto the back wheel in addition to the sprocket was a mess. Thankfully, I was able to use the existing bolts going through the rear wheel to also attach the brake drum. The brake band was attached to the side of the motorpod, which was redesigned to include a back footrest. This made my ghetto extended fender obsolete.

It worked enough to survive a couple rides in the rain.

The new motorpod used the existing mounting points as the old one, so it worked pretty well.

Apologies for pictures of the screen, my internet wasn't functional at the time.

Showing the bolts that go through the entire wheel, connecting the hub and sprocket.

The existing 70mm brake hub had a couple holes that almost fit the existing holes. However, after tightening the screws on the wheel, the hub distorted. While it was still usable, it made braking force highly inconsistent. It was either little braking or locked up wheel. Thus, I made a new hub from a scrap hunk of aluminum.

I love the countersink look.

With this new, significantly more concentric brake hub, the brakes work fantastically.

The entire motorpod. The brake band was removed for disassembly so it'll fit in a suitcase.

With the brake band. The wheel is changed because the previous one had a flat was a real pain to remove the inner tube.

Showing the end of the bowden cable.

Nice and compact.

A New Front Wheel

While at Tech, I had a little accident. When the manufacturer warned the wheels I used were not designed for motorized applications, they meant it.

The rim on one side is completely gone.

The impact from falling on the hub smashed it up.

Yeesh.

Luckily I came out of it with only scrapes and sprains.

Learning from my mistakes I changed to a 6 inch steel hub pneumatic wheel (McMaster part number 2717T41). This wheel had already proven itself for the Velociryder. Of course, this meant redesigning the front fork to fit the wider wheel.

While making a new front fork I needed to quickly hack a usable front fork.

teehee

Solidworks and waterjet and annoying milling of steel later is a new front fork.

So much more metal

Chain Stretch

Three days before the end of my internship the chain stretch in my scooter reached a breaking point. After stretching nearly a half link, or 1/4", the chain no longer stays on the sprocket. Time to cut slots to take up the slack and lubricate the chain to keep it from stretching so quickly.

Scooty Brakes (and a stint through the rain)

Using your shoe as a brake is okay for testing, but the wear gets ridiculous. So, I put a few holes in the front fork to mount a plate and the plate to mount a set of brake calipers on the front wheel.

The plate mounts right on the front fork, held on by 6 4-40 screws. The plate is 1/4" steel.

The calipers I used were inexpensive bike calipers from Amazon. While I knew braking onto a plastic rim is not such a good idea, I thought it would still work okay.

Then, parts and assembly, after a particularly nasty ride in the rain back from work.

Ghetto extra waterproofing with masking tape

Still alive!

Yes, that is water on the inside on my caps.

Attaching the brake to the brake plate. Haha what clearance.

Due to the calipers blocking the screws, I had to slowly inch the parts together, screwing down half the screws, moving the caliper around, and screwing down the other half.

Derp.

And then, I got excited and forgot to take more pictures. Long story short, after some testing, I decided they were worse than failure. Not only do the brakes not work, but the brakes and rims must be mortal enemies because they destroy each other. The pads exhibit significant wear and some chunks coming off already, and this is from a nice walking speed.

Overexposed for your dark caliper viewing pleasure.

 On the rim, it seems these bits have melted on. I'm not sure which surface made them.

Can't brush this off. Its melted on.

I can't imagine can guess what would happen slowing from full speed. Rim failure and tire blowout, or I eat through the pads so much they stop applying pressure, or both.

Well, back to the CAD board. This time, I'll use one of these.

70 mm rotor band brake

Scooty Puff Build

*Note: somehow the styles on this post crapped out and all the headers are tiny. I can't figure it out*

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."

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.

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.

This wheel used to be round.

Just one week of use.

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.

There is a hole on the plate opposite of the sprocket for the valve stem. Thus, three of the sprocket bolts go through the entire assembly, the last one only goes through one plate.

The Motorpod

Heavily inspired by Razor Wind's wheel pod, it is a self contained unit for the rear wheel, motor, and power transmission.

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)

This give the controller enough beef to accelerate uphill.

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. 

A scooter!

Scooterbros

With a 3D printed fender

The stats:

• 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 

Lets see how long this thing lasts with a 3.5-4 mile long round trip commute every weekday this summer.

Appendix

Complete design files on github: https://github.com/aaronbot3000/scooty-puff