The Nexus transmission uses planetary gearing similar to a car's automatic transmission. The bike chain drives a sun gear on the rear axle, which transmits power to planetary gears situated around it. The planetary gears, in turn, mesh their teeth with a ring gear, which drives the rear wheel. In a car, the components of the automatic transmission are much heavier and driven by hydraulics, but the systems are otherwise the same.

The Di2 controller continuously monitors wheel rotation using 14 magnetic sensors inside the rear hub. When the device detects acceleration, it sends a signal to a small electric servomotor that rotates a sleeve on the axle to shift the planetary gears. Because all the gears are enclosed within the hub, road grit can't get inside to abrade the metal parts. The electronic transmission actually needs less servicing than the manual version, which can break down if the cable to the shift levers slips or frays.

When I first got the bike, I was concerned to see that it had no batteries--not even a switch to turn on the electronics. A call to Shimano cleared up the mystery: the Di2 is always on because it is powered by a generator inside the hub of the bicycle's front wheel. Producing electricity from the wheel's rotation, the same generator can also power a lamp. And the lamp will automatically turn on as soon as the photoelectric eye on the Di2 enclosure (another mystery at first) decides that it is getting dark.

Another worry still nagged at me, though. What happens if something goes wrong with your digitally controlled bicycle? Where would you get it repaired? This was no idle question. I could coax only six gears out of the bike: cruising along in sixth gear, the transmission would strain and click as if it were trying to shift higher but couldn't. With an ordinary bike, I could just pull out a multitool and adjust whatever needed attention. But the hub covering the Nexus transmission prevented any attempts at tinkering. So I took the bike to a repair shop.

Allen Schulmann, owner of Sid's Bike Shop in Manhattan, was as impressed as I was by the Di2. "That's really cool," he marveled after taking it for a jaunt. He wasn't too fazed by the prospect of fixing it. Scientific American, though, didn't give me a budget for repairs, so I didn't ask for an estimate.

Even as a grad student, I might be able to afford a Di2-equipped Bianchi, which costs about $800. Unfortunately, it would probably get stolen or stripped if I left it anywhere on campus. But if you live in an area with good bike trails or commute lanes and you don't care how your bicycle shifts gears--as long as it gives you the right one at the right time--then this system is for you. If you're a cyclist in Manhattan, though, shifting gears is the least of your worries.

This article was originally published with the title Easy Rider.

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