There are two kinds of cyclists in Manhattan: the quick and the dead. This narrow island between the Hudson and East rivers is possibly the least bicycle-friendly place on the planet. Delivery people, couriers and casual cyclists face the challenge of crossing six-lane avenues choked with heavy trucks and honking taxis. And even if you make it to the oasis of Central Park, you must contend with packs of riders on high-end racing bikes who seem to be engaged in a perpetual Tour de France.

As a mild-mannered graduate student from California who was spending the summer in New York City, I sensed that I needed some high-tech edge to compete in this torrent of two-wheeled traffic. I soon learned that Shimano, the Japanese manufacturer of bicycle components, produces the ultimate accessory for recreational cyclists: a digitally controlled automatic transmission that takes all the guesswork out of gearshifting. I called up Shimano's U.S. headquarters in Irvine, Calif., and convinced the company to let me evaluate this voguish velocipede for Scientific American.

Just to be clear, I don't have an aversion to hard pedaling. On the West Coast I use my bike to commute to school and cart my groceries. I don't own a car and have no use for a moped or scooter. Bicycling is how I get my exercise. But even as a bike-lane veteran, I'm often stymied by the gearing on a standard bicycle. Take my commuter ride, for example, a hybrid road/mountain model made by Specialized Bicycle Components. It has three chain rings--small, medium and large--bolted to the pedals; working with the bike chain, each of these toothed rings can drive one of seven sprockets on the hub of the rear wheel. In theory, that gives me 21 gears to choose from. The problem is that some of those 21 configurations are redundant. For example, pairing the biggest front chain ring with one of the larger rear sprockets delivers the same gear ratio as combining the smallest chain ring with one of the smaller sprockets.

Furthermore, the chain is stretched diagonally in some gear combinations, which can wear down the chain links or tear the teeth off the sprockets. In reality, I get only about seven gears out of my bike--and a bit of a headache from trying to figure out the proper pairings. Often I find myself in the wrong gear, which is bad for the bike and terrible on the knees. Legs have an optimum rate for going around in circles; ideally, you should be cranking the pedals at about 90 revolutions per minute.

The solution for cars is an automatic transmission, which shifts the vehicle's gears so that the engine can hum along in its own preferred range (2,000 to 3,000 rpm) no matter what the car's speed. Not surprisingly, automatic transmissions for bicycles have been in development for more than 30 years. The largest market for the most up-to-date models is Europe. Jasen Thorpe, consumer marketing manager for Shimano's American division, notes that in the U.S., "We have our Lance Armstrongs and Greg LeMonds," but the Europeans are more likely to rely on bicycles for their everyday needs.

In the U.S., Shimano is after the leisure market. Racing cyclists want complete control over acceleration, but the typical weekend rider will often take convenience over performance. The bicycle that Shimano sent me was a Bianchi Milano, an Italian model with a somewhat retro look. It was equipped with a seven-speed Nexus internal hub, a transmission system that looked like an oversize beer can enclosing the bike's rear axle. No shift levers were on the handlebars; instead the gears were selected by Shimano's Digitally Integrated Intelligence (Di2) system, which was controlled by a computer chip in a gray, molded-plastic enclosure mounted on the bike frame.

"Take this bike to the coffee shop," Thorpe suggested. Appropriately, the Bianchi Milano sported a decal of a cappuccino cup with the words "Caf Racer." I cruised down the bike path paralleling Manhattan's waterfront, admiring the variety of refuse floating between the piers. I was impressed with how smooth and reliable the automatic transmission was. One of my concerns had been that I would have to cross a busy street starting from a dead stop and that the bike would leave me stranded in high gear and heaving wild-eyed on the pedals as a yellow cab careered toward me. But no--every time I came to a stop, the transmission would start from the lowest gear. As I accelerated, it geared up as naturally as I would have done myself. There were no alarming clunks as the gears shifted, either.

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.