Kurt Schaefer
On the forth of July I went down to Cambria to attend a forth of July bash that my friend Ann was hosting. It was great fun, but one unexpected piece of fun was that Ann had just gotten one of the Zap electric bike conversion kits for her bike. Amazing! We all took turns taking the bike for a spin. It was quite zippy. There's a certain euphoria that comes from zipping up a hill without any effort at all. The fact that the motor is silent, and unobtrusive made it so that passers buy would simply not notice that you weren't peddling. It rather felt like you were walking around with your feet not quite touching the ground. Special and amazing, but no one stops to stare.
Ironically I had been day dreaming about an electric kick scooter project on the trip down. That little taste of what an electric bike could do was a galvanizing experience. I knew I had to give it a shot. So weekend before last I spent most of Saturday testing out various DC motors at the surplus stores. I took a meter, and some batteries with me so that I could test current draw, and get a feel for how much "Umph" they had. I found a pair of servo motors that seemed to fit the bill. I also picked up a 12 battery, and a big pushbutton switch.
The building begins
The
basic design was to use two motors to drive both ends of a friction spindle
which presses against the back tire. I spent that Sunday machining the
friction spindle out of some 1" leaded free machining steel. I used the
rotary table and a end mill to cut 360 little teeth around the outside
of the spindle. (A strait knurl would have been faster, but I didn't have
one on hand.) I also spent some time brazing together some mounting brackets
to hold the motors in position, but I didn't quite get get the motors onto
the bike by the close of Sunday. I was mounting the motors on Kate's bike,
and I hadn't factored in the time it took to do the routine maintenance
that the bike needed in order to be road worthy.
So during the next week I decided to re-do the somewhat ugly motor mount. I designed a more elaborate mounting scheme, made up some patterns to cast, and by the next Saturday I was raring to go. I did two separate melts that day, and cast three pieces that made up the motor mounts, and the pivot (which then mounts to the frame of the bike.) I machined the plates flat, match drilled them, and bolted them together with length of 3/8 rod. Here's a close up of the two motors actually mounted to the bike frame. And you can take a peek a little bit further in at the frictiondrive spindle and the motors and mount.
After the motors were mounted I strung wires along the bike frame up the the handle bars, and installed the big push button. Hey look the button even matches the bike! This was a pretty crude first test, so the button was actually just taped in place!
We
take the first ride by star light
So late Sunday night the bike was ready for it's first test ride. It worked!
Mark, Adam and I spent some time riding it up and down the street. It wasn't
very powerful, but it was a start. The neighbors were out looking at the
stars through a very large telescope. I offered to trade a ride on the
electric bike for a peek through their telescope. I got to look at Hercules
which is apparently a globular cluster. (Or do I have that all wrong? I'm
not much of an astronomer.) Here's a shot of the bike in all it's glory:
The
motors that I had didn't have any ratings on them, and we were operating
them at 12 volts. After much riding around they were still stone cold,
so I decided that we should try them at 24 volts. I went and picked up
another battery, and some 24v relays so that I wouldn't have to be running
quite so many wires up the the handle bars and back. After an evening of
crimping, wiring, and switch mounting I had wired up a two speed 24v bike.
(One of the speeds is with the motors in series, the other speed is with
the motors in parallel.) The new 24v bike had much better power. It's quite
fun to ride. Not quite as good as a Zap system, but pretty close. The Zap
system is lighter, and a much more "turn key" solution. The major drawback
of my system is that the motors stick out enough that the interfere with
your heals as you pedal. Very annoying, although you can peddle fine with
your feed a little bit further forward on the pedals.
All in all the project has been very quick and enjoyable.
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