Introduction: Simple 'Biking Figure' Kinetic Sculpture
I was asked to make a kinetic sculpture for a bike appreciation event. The plan was to make a plywood cutout of a human figure, which would be mounted on a bike with the feet affixed to the pedals. This way when the pedals were spun, the figure would move as if it was pedaling.
Unfortunately the event was cancelled when February in Seattle ended up exactly as wet as you might imagine. I had already started building the thing, however, so I really had no choice but to finish it and post it here. It was a moral imperative.
Step 1: Making the Articulated Figure
The articulated figure was cut out of 3/4" plywood. A Google Image search found me a nice paper doll outline to work from. If you have a projector on hand, that would be the easiest way to transfer the outlines. Unfortunately I didn't, so I just did it by eye. I measured the rough bounding box for each segment and scaled it up by a factor of 9 (chosen to roughly match my own dimensions), then sketched that onto the plywood. I decided to make the head a separate piece, though there was no technical reason for doing so. This let me adjust the angle to keep the head looking directly forward, no matter how the figure ended up being mounted on the bike.
I first started cutting out the segments using a reciprocating saw, but I was unhappy with the quality of the results. It was very hard to get the left and right versions of a segment to match each other. I pulled out an old desktop band saw and used that instead, with far better results. All in all, I used 2 2'x4' sections of plywood, and that included remaking a couple segments.
The knees, elbows and neck were attached using 5/8" bolts. That was much beefier hardware than strictly needed, but I felt it was more in scale with the project as a whole. The hip and shoulder joints had to be spaced farther out, to reflect something closer to the width of a human body. I used 1/2" pipe screwed into flanges which were bolted to each other through the torso piece with 1/4"-20 screws. Short sections of PVC pipe served as spacers, and pipe caps on the far ends held the segments in place.
With the addition of a bike helmet, the figure was done. I tested it on the bike and the pedalling action worked great when spun manually.
Step 2: Mounting the Bike and Motor
For the action to work properly, the rear wheel of the bike needed to spin freely. This meant making a stand to keep it up off the ground, and to keep the bike from tipping over. I grabbed some 2" angle iron from my scrap bin and cut out one 24" and two 18" lengths.
To allow the rear hub to attach firmly to the stand, I cut slots into the ends of the 18" lengths. These were 3/8" wide and about 3/4" deep. After laying out the position with a combo square and scriber, I drilled a 3/8" hole at the end of the slot, and then cut out the rest using a cutoff wheel in an angle grinder. These two lengths were then welded perpendicular to the longer one, making a quick and easy (and even fairly stable) stand.
So now the figure could sit on the bike without it tipping over, and it would pedal nicely if the rear sprocket was turned. All that was left was to get it to turn automatically. Luckily I had an old piece of art of mine that had a sprocket attached to a motor that I could cannibalize. (Normally I am staunchly against stripping pieces out of old projects, but I'm about to remake that one anyway.) Mounting it on the bike stand was a simple matter of drilling holes for the u-bolts and creating a short bike chain to connect it to the rear sprocket of the bike. I also had to remove the rear derailleur and shorten the main chain, to allow the pedals to be driven from the rear sprocket.
Step 3: Results
Making an articulated figure pedal like this certainly isn't a new idea, but people keep doing it because it looks so good given the amount of effort involved. This whole project took just 3 nights for me, start to finish. This was made for a one-off event, so I definitely cut some corners. But a more permanent version of this wouldn't be too much harder, and could be a pretty cool piece of work. Imagine this same thing, but with the articulated figure cut out of thick, rusty sheet. The power source could be a big hand crank, or another bike that people can pedal. It could be scaled up or scaled down. Exploring more of this kind of "whirlygig" style kinetic sculpture is definitely on my list!
FOLLOWUP: The bike event finally happened two months later, and I was able to deploy the Biking Figure as originally designed. It was wobbly and more than a little bit terrifying to pedal the lower bike, but it worked! The two bikes were connected with a ~6 foot section of 2" angle iron welded directly to the seat and chain stays of each. An extra long bike chain connected the rear sprockets. We were in a hurry, doing this for a live audience, so the chain much longer than it should have been. It only derailed a couple of times, though, as long as people pedaled at a fairly constant pace. Were I doing this for a more permanent installation there is a lot I'd do differently, but as a piece of gonzo engineering done live in an alley, it worked pretty well. And it definitely serves as proof of concept for something a bit more carefully designed.