Those crafty Swiss people, with their tiny multipurpose knives and perforated cheeses — who knows what they're capable of? They hide up in their lovely mountain chalets, sipping cocoa and plotting devious things, like powering city buses with spinning wheels. Well, that's what they were plotting 60 years ago, but still.
Yes, way back in the 1950s the Swiss were experimenting with flywheel-powered city buses, which they called gyrobuses. The reasons for investigating these at all are still valid today: they wanted something more efficient and cleaner than gas or diesel buses, and while they liked the characteristics of overhead-wire electric trams, the cost of running overhead wires all over new routes was prohibitive.
Battery technology still left a lot to be desired, just like today, so Bjarne Storsand of Maschinenfabrik Oerlikon (MFO) came up with the idea of using a spinning flywheel to store energy. The buses were powered by a rear-mounted electric motor making about 70HP, and that motor was fed by a generator spun by a one-and-a-half ton, 5-foot diameter flywheel rotating at about 3000 rpm. The flywheel was stored in a housing filled with hydrogen at reduced air pressure to minimize resistance on the spinning wheel. And knowing there's a pressurized vessel full of hydrogen and a massive spinning chunk of metal just makes bus rides more exciting for everyone.
This big flywheel was mounted horizontally in the middle of the bus, and the spinning gyro effects made handling interesting, though it did give a nice, level ride. The wheel was spun to 3000 rpm initially, and then at each bus stop, three tall conductors would engage with power lines to drive the flywheel's generator like a motor and put more energy into the flywheel.
Braking would also transfer energy back into the spinning flywheel, which was capable of driving the bus about 4 miles or so (~6 Km). This worked out fine, since the stops were closer together than that.
These flywheel gyrobuses were used in limited quantities in Swiss and other European cities, though the biggest application of them was in Léopoldville in the Belgian Congo (now Kinshasa). The Congoese system used 12 gyrobuses and was in operation from the mid '50s until war put an end to it all in 1959.
All the gyrobuses met their ends soon after, not the victims of war, but rather boring old economics. The buses were just not efficient enough to run, and gas and diesel were just too cheap still to make their continued development worthwhile. They also had their share of mechanical issues, spending lots of time in the shop, though interestingly the safety concerns of the massive, energy-rich flywheels in an accident didn't seem to be a factor in their demise.
Today, there's one surviving gyrobus left, in the Flemish Railway Museum in Antwerp. Flywheels are appealing ways to store energy, and while I'm not sure they make sense for most applications, I kind of would love to see some flywheel-powered buses running around. But with clear, external flywheel housings, because that would be exciting.