Solar Bike Project

Solar Bike Project

"Our first search was to find an efficient electric drive motor and a way for it to drive the bike. Since we wanted to be able to go up hills at optimum efficiency, it was attractive to use a chain-driving motor or “mid drive” that allows for gear changing via the derailleur. The best of these is made by Ecospeed of Portland, Oregon, which we saw operating firsthand on one of Brent Bolton’s bikes. We took careful measurements of the motor’s size and from these Russell made a foamcore model, which we used to figure out where in the hell the thing would fit on our existing bike’s frame. An obvious spot was in the triangle behind the captain seat and in front of the rear crank. But would it fit? The foamcore model proved it would, so we mounted the EcoSpeed motor to a quarter-inch thick aluminum plate connected to the frame with four stout U-bolts. Ah, it worked.

Key to using both human power and an electric motor is to make the motor independent of the crank, so the riders wouldn’t have to spin their legs. Each power source needs a freewheel, so we found an oddball freewheel gadget once made for Vision tandem bikes. It was called the IPS or Independent Pedaling System, basically freewheels that made the chainrings for the captain and stoker independent of each other. Thus we could drive a chainring on the rear crank from the motor, and another chainring on the same crank with the pedals, and make them independent (the motor having its own freewheel). We had some problems keeping the chainline aligned at first with the new freewheel, but eventually using a chain tensioner and structural reinforcement we got it right.

Every solar vehicle needs batteries, and to choose the right type we carefully calculated the amp draw and distance requirements we’d need to make for an efficient system. We’d need batteries to store surplus solar power to keep us going during cloudy weather, in tunnels, or when shaded by trees. More to the point, we knew that hill-climbing would demand more energy per hour than our solar panels could produce, so we needed as large of a buffer as we could carry. That battery would have to be charged and discharged quickly, maybe 50 amps of current drain over some tens of minutes, so that narrowed the available (and affordable) battery chemistries.

In the end we settled on a new design of the old standard lead-acid battery. We began testing our system using two Optima yellow-top deep-discharge batteries, 55 amp-hour capacity. The batteries are heavy, at 42 pounds each, and they store only about 15 watt-hours per pound. But they’re just what we need – they will quickly accept a full charge in just two hours of morning sunlight and then charge up again in a couple of hours at the end of the day. In the meantime, we set about designing our system so that it would run almost-completely on solar power in the middle of each day."

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