by Rebecca Houlihan
I’m a cycling enthusiast, and when the weather is bad I use a bicycle trainer in my apartment. But riding to nowhere has always felt pointless. This got me thinking about how I could use my pedaling to produce electricity. By driving a generator with the movement of the back wheel, I figured I could run a lamp or charge my phone. Realistically, this wouldn’t do much to cut my utility bills (or carbon emissions), but it would give my indoor riding a sense of purpose. Besides, I was curious to see what the project involved.
To skip ahead a bit, I ended up rigging my bike to a 24-volt, 200-watt electric motor, which I modified slightly to generate electricity instead of doing mechanical work. I used the motor (now, operationally, a generator) to charge a 12-volt lead–acid battery. And, finally, I added an inverter to convert the battery’s DC current into an AC current, which is what’s needed to power anything you’d normally plug into a wall outlet, and to store power so you can use appliances even when not pedaling.
Pedal to Metal
I found a lot of the build details on Instructables, the online project-sharing community, where user saullopez52 had done basically what I had in mind. While interning at an educational startup in L.A., Saul Lopez developed the idea as a way to bring environmental technology projects to schools. He thought it would be a low-cost, fun way to provide students with engineering experience. “The exercise component was what made the project engaging,” he says. Plus, he adds, “I like that the project has a lot of room for customization.”
That’s what I did—I customized. I found a combination single-speed/fixed-gear bike that worked well, thanks to its ability to hold a cog on either side of the back wheel. The chain on the right is driven by the pedals, while an added chain on the left spins the motor. On the side that’s driven by the pedals, I used a freewheel, which rotates the wheel when I’m pedaling but allows it to keep spinning forward, without the chain moving, when I’m coasting or pedaling backward. On the left side of the wheel, I attached a fixed cog, which spins in the direction of the chain as long as the wheel is turning.
To keep the bike steady I dedicated a bicycle trainer to the project. A nice thing about commercial trainers is that you can easily detach the bike if you want to go out for a ride. But you can also build your own stand; you just need a setup that allows the rear axle to spin freely while raising the back wheel slightly off the ground. To get the bike stand ready for generating power, I removed the resistance unit, which is the spinnable metal cylinder that rubs against the wheel to mimic the feeling of riding on pavement. (Once you attach the motor, you’ll also feel resistance as you generate a current, but it really doesn’t take much effort.)
With the resistance unit gone, there was space to attach a wooden board extending from the rear of the bike, to hold the motor, battery, and inverter. Because I was using a narrow board (a 2 x 4), I needed to add a crossbar to hold the electrical equipment. (Note: Before attaching anything, you should measure how far the chain extends from the back of the bike. Position the motor so a chain from the left side of the rear hub runs parallel to the wheel, straight back to the motor. With a V-belt, you have to measure precisely; with a chain, you can add and remove links with a chain tool.)
With the motor screwed into the center of the crossbar, I positioned the battery and inverter on either side as counterweights for each other. That helped keep the bar parallel to the ground. I secured them with industrial-strength Velcro, which would hold up when I was moving the contraption around but allow me to fiddle with the parts.
Before linking up any of the electrical components, I tested the connection between the bike and the motor to make sure pedaling actually spun the motor shaft. The shaft of the motor I used is slightly grooved, and the chain gripped well. If you find yourself with a motor that refuses to spin, you can connect a cog to the shaft, guaranteeing that the chain will have a good grip.