Friday, January 27, 2006


Q: What is a 3WDM?

A: "3WDM" is my own personal shorthand for the Three Wheel Dorkmobile™. It's an electric scooter I built about a year ago based on a Mongoose Slalom 132 mountainboard. I've replaced the rear truck with a small BMX bicycle fork, which holds a wheel with an in-hub electric motor. A battery pack and some control electronics are mounted to the underside of the deck.

Q: Why build your own electric scooter? Why not buy one for $99 at Wal-Mart?

A: This one's special. Unlike most scooters, electric or otherwise, there isn't any sort of handlebar: the remaining truck provides steering like on any other skateboard. Nor is there any sort of cheesy handheld throttle. Rather, a set of force-sensing resistors mounted between the deck and the frame detect the weight distribution of whoever is riding the thing, and direct the control electronics to accelerate or decelerate accordingly.

I couldn't find anything like this commercially available. If you find one, I'd love to know about it.

Q: So how do you ride it?

A: Very well, thanks. The basic idea is I set it on the ground, get settled standing on it, and use the keychain remote to turn it on. Then I shift my weight forward to speed up, and backwards when I want to slow down. If I keep leaning backwards it will begin to move in reverse, until I lean forward again. When I'm stopped, or nearly stopped, I use the keychain remote to turn it off.

Q: So sort of like a Segway™?

A: Yeah, sort of. Except the Segway (at least the one I tried) is a lot more sensitive to weight changes (some might say twitchy) because it would fall over otherwise. The 3WDM is more like snowboarding on pavement.

Q: How much did/does it cost?

A: If I had to build one from scratch that matches it's current configuration, it would cost around US$1500 in parts, and about 6 hours of labor (most of that spent soldering). I've spent a bit more than that if you include parts that I bought and later upgraded or didn't end up using, and of course countless hours designing and rebuilding it to get it to its current level of (cough) refinement.

Q: How much does it weigh?

A: The current weight is around 42 pounds (about 20 kilograms).

Q: How fast does it go?

A: The top speed is around 12.5 miles per hour (20 km/h).

Q: How far will it go on a single charge?

A: I haven't tested the new battery pack's range. Extrapolating from the old pack, I'd say around 10 miles on gentle slopes.

Q: Why three wheels?

A: Well, two wouldn't really be stable, and four means that you have to drive two of them on the same axle to prevent torque steer. That means having either two motors or a differential (if you want to be able to maneuver around tight corners). A single drive wheel (in the middle) avoids these complications, while the pair of wheels up front gives you stability and the ability to turn. Incidentally, it's much more difficult to ride backwards (which probably comes as no surprise to you vehicle engineers out there).

Q: Will you build me one?

A: No.

Q: Why not?

A: Lawyers. But seriously, it's not something I've tested enough to inflict on the general public. There is no redundancy in the propulsion—and more importantly—braking system, and I haven't really analyzed any of the myriad other failure modes. It becomes possessed by evil spirits when it gets wet, and sometimes turns itself off when riding over bumps. In short, it's not ready for prime time.

Q: Can you tell me how to build one?

A: I'm not going to write the 3WDM Cookbook if that's what you mean. But with basic electronics skills can look at the reference designs for the various parts and put them together with some simple op-amp glue circuitry.

Q: Have you thought about patenting the idea?

A: Yes, but someone beat me to it.

Q: Did you steal their idea?

A: Nope. A friend and I came up with the idea back in college and started developing it before the patent was issued. Still, we don't have any documentation that we were first to invent, so we couldn't invalidate the patent by any stretch.

Q: Don't you owe them money?

A: Hmm, ask a patent lawyer. When I originally built the thing their patent had expired due to not paying their maintenance fees. But it looks like they've had it reinstated since then. In any case, I think they'd have trouble showing damages.

Q: Why don't you mass-produce and sell them?

A: That would be fun. But to sell more than a couple of them I would have to cut the size, weight and cost in half without sacrificing range, speed or torque. Not to mention improving the safety, reliability, and manufacturability of the design to consumer-product standards. And that's not something I can do while holding down a day job, if in fact it can be done at all. Plus I'd have to acquire the relevant patent rights, and I don't have that kind of money.

Q: Couldn't you raise some VC money or something?

A: Probably not. It's not Web 2.0, and the only other guys doing this sort of thing haven't been a huge financial success.


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