It was late at night and I was riding home, tired. The mild slope of the Boston University Bridge seemed like a pothole-strewn mountain, and the curve of Putnam Street to Harvard Square, with nary an inch to spare for cyclists, took forever. Then at Mount Auburn and Massachusetts Avenue, I saw a flash. A cyclist flew by, and I got a jolt of adrenaline.

The bike was like a greyhound, narrow and forward-leaning, the essence of motion. Its rider had a U-lock stuck in his back pocket and a black T-shirt over his shoulders. He sat straight up in the saddle, easily smoking a cigarette. Though I'd stayed back, he'd sensed my presence, and the cranks started to churn. His bike - unencumbered by derailleurs, levers, or even conventional brakes - leapt forward. In a moment he was gone, and I was on my own again.

I'd just met a member of the fixed-gear tribe.

As a rule, the machines we make move from simplicity to complexity. Your grandmother's pen became your father's Selectric typewriter, and if your laptop has anything less than a gig of RAM, you're feeling short-changed. Bicycles have gone through a similar evolution, starting as wooden "boneshakers" and ending up as carbon-fiber rockets. But they're one story where the clock doesn't always run in one direction.

"When I started riding fixed in '91, '92, there were only two or three other kids doing it," said James Norton, 39, co-owner of Revolution Bicycle Repair in Jamaica Plain. James was a courier then - he says he worked pretty much full time until last year, when he opened the shop, then on Atlantic Avenue in Boston - and he and his friends had gotten tired of breakdowns. Taking inspiration from track bikes, they removed everything that wasn't necessary. Much of the early equipment was improvised to a degree that boggles the mind, but it worked.

Within three to four years, James said, three-quarters of Boston's couriers had switched to fixed-gear bikes.

And it didn't stop there, far from it. Riders in New York and other cities had been doing the same thing, and, nearly 20 years later, what was once the exclusive reserve of couriers is now seriously mainstream: informal counts reveal fixed-gear bikes pretty much everywhere.

Sometimes it's just a rusty conversion, other times a dazzling silver Swobo with a faux-leopard cover for its lock and chain. In certain spots of Boston, there are even more fixed-gears on a rack than conventional ones.

The attraction is simple, literally. Fewer parts, fewer breakdowns. With a fixed-gear bike, the cog on the back wheel is attached the same way the one on the crank is - solidly. If your bike moves, so must the pedals. That provides a wonderful sense of connection to the road, but at a price. Coasting, that childhood favorite, is out, and learning to brake with the legs takes time. For many, though, the feel of the ride is unbeatable.