Please help me build a motor scooter!

September 9, 2009 3:32 PM

Can you help me build an electric motor scooter?

I have a bunch of old bike parts in a garage, so I should be able to put something together. I found a simple explanation here, but what I am looking for are specifics on where to get each of the materials (especially the motor, the batteries) and how the batteries should be wired to the motor and details on any power regulating circuit that needs to be built.

This is intended for a little kid, so whatever I build needs to be safe, and not TOO powerful.

posted by banished to travel & transportation (6 comments total)

Not a scooter, but the guy who's building the LOLrioKart shopping-cart to electric gokart conversion has a pretty extensive blog. Beyond that, I'd start with a search through the archives of the MAKE: Blog and/or Hack a Day; I know I've seen a few electric bike/scooter writeups. How handy are you, mechanically? Electrically? There are a lot of challenging details here that aren't immediately apparent if you aren't an electrical engineer. As far as parts sources, you should probably get to know DigiKey, Mouser and maybe AllElectronics for electronics and McMaster Carr for physical raw materials (metal blanks, gears, etc). You might also be able to find local scrap yards.
posted by Alterscape at 4:01 PM on September 9

I spent two years (of calendar time) on a project that controlled motors in the little scooters that folks with compromised mobility use.

Here's what you have to do: decide on the power requirements of the motor and the speeds you will need. Select a motor that can deliver the torque and RPM you need. Design a transmission that will prevent the motor from stalling and will translate the higher motor RPM to the design speed you calculate for the wheels. Research and select a power source. (These days things are almost always lead acid gelled electrolyte batteries for an optimum tradeoff between cost and volume/performance.) Design a charger or research and select one.

Design mounts for the motors. Be sure to consider what will happen when/if the young operator wrecks and subjects the batteries to harsh mechanical stresses.

You can buy a FET-based PWM controller that will run off battery voltages. I recommend a 4-quadrant controller with lockouts for low battery, illegal control inputs, and provisions for acceleration/deceleration. Since you are designing this vehicle independent of the controller, you'll have to tune the PID coefficients to make the speed stable. Sadly, this isn't something you can get quick advice on, but I generally turn off the I and D terms and seek good proportional responses first. I haven't had to do this for a year or two, so I've forgotten the nifty tuning approach I used in our units. If you get this far, I'll share.

I should say that since this is a 'man in the loop' product, the I and D terms are superfluous, since the operator can compensate for controller instability with a fine hand. Speed is non-critical in a recreational device.

Lastly, one problem I see with this is that the motor controller can't really discriminate between the load of accelerating up a hill and crushing the leg of a small child, so fairings over any possible trap should factor in your ergonomics.

Our system was able to gently maneuver a 350 pound invalid around the inside of a house trailer and did not accelerate even if said fat person was headed down a ramp. (That's the 4-quadrant thing in action.) We had product liability insurance, two electrical engineers, several mechanical engineers and a product manager, plus consultants. Moving folks precisely and preventing them from killing themselves is an involved process. (Our motor controller also had to be programmable using a PalmPilot, and it all had to cost $100 or less, so I know it can be done.)

It's an ambitious hobby project though, fraught with some unintended consequences types of issues, and requiring some multidisciplinary skills. Be careful. Motors and batteries can be lethal and can certainly injure. We've all got scars to prove it, I'd wager.

I almost think you'd be better off selling all your parts and using the proceeds to buy a little electric scooter.
posted by FauxScot at 4:59 PM on September 9 [3 favorites]

Well, unless you have the money to hire FauxScot, you need to go to these forums and ask around:

http://www.electricforum.com/cvars/

Other people have done projects like yours. Some may claim cheap, but none are easy.
posted by fake at 7:48 PM on September 9

It sounds like you're more at home with the mechanical than the electrical. I would suggest teaming up with someone who knows more about the electrical, and building two devices. One for him, and one for you. Or bring someone local onboard who doesn't mind advising and pointing you in the right directions (this will save you a ton of time from barking up wrong trees).

As to where to find that person, check if there are any membership machineshops in your area (places where you pay a monthly fee to have access to a large workshop space furnished with tools. Like a gym membership for your mind and hands), as plenty of members are as interested in other people's projects as in their own.
Failing that of course, is the internet. (Somewhat related - your Mefi profile doesn't indicate what area you're in)
posted by -harlequin- at 8:54 PM on September 9

I bought my electric vespa-style scooter from a company that also carries conversion kits.
posted by nomisxid at 8:57 AM on September 10

Another forum to check. There are a bunch of e-bike fora there, but they have info on motorcycles & scooters too. Another supplier. I think they've been building & supplying parts for conversions for a long time & now build their own GPR-S conversion using a rolling chassis (designed for ICE, but sourced without one) motorcycle.

Oh- it's for a kid. There's already crossover between the high end (power, not quality) of bike hub motors vs. low end of scooter motors, but for a kid, you probably don't need more than 0.5 to 1kW.

I bought my son a Razor MX350 a few years ago, perhaps when he was 6? He's 9 now & probably double the weight (50 lbs. then, 100+ lbs. now) and he still rides it around our flat yard, but it can't pull him up a hill anymore. The batteries that came with it (2x 12V wired in series) were cheap knockoffs of Panasonic deep cycle lead acid that died quickly.

The lump-in-a-line charger isn't sealed, so don't do what I did & leave it near a garage door that isn't weatherstripped. A replacement (lead-acid 24V trickle charger) was $30-$50.

I don't know if you can beat the price, even if you factor in replacement batteries, buying a motor, batteries, controller and charger. I plan to take the swingarm/motor/rear wheel and build some kind of junk monster device with the kiddo at some point.
posted by morganw at 11:37 AM on September 10