One of cycling’s least predictable races, the Keirin, snuck up and bit America’s best rider, Marty Nothstein, yesterday at the Athens velodrome. Competing in track cycling’s most aggressive event, the brash 33-year-old from Allentown, Pa., failed to advance out of the early heats. Since the Keirin was his only race, it also meant he failed to become the first American to win a track cycling medal in three consecutive Games.

Nothstein had won his previous medals in the match sprint (1996 silver, 2000 gold), but decided not to defend the title in Athens. Why? Been there done that, he had repeatedly implied.

After Sydney, Nothstein gave up full-time track cycling and tried a different approach. He joined a professional road racing team and lost 30 pounds from the increased mileage. He competed in only two international Keirin races leading up to the Games. Even without practicing, he was still America’s best rider in the event, and his results qualified him for Athens.

In track cycling, bikes have no brakes or gears, and riders chase each other around a sloping, funnel-shaped track. Athletes are never afraid to nudge, head-butt, or bump wheels to send rivals sprawling across the hardwood. The Keirin involves all this and more.

The first five-and-a-half laps of the eight-lap pack race are spent accelerating in the slipstream of a motorized pace bike, or “derny.” The derny eventually peels off and the riders sprint allout to the finish.

The Keirin, long popular among Japanese gamblers, was added to the Sydney Olympic program in 2000. That year, Nothstein entered as a twotime world champion and placed fifth.

But this year, he entered the race having barely trained for it. He was gambling on himself and liked his chances.

“It’s a lottery,” Northstein explained to The New York Sun yesterday by telephone from the track. “You go in with a plan, and nine times out of 10 it doesn’t work out. That’s the thrill of the Keirin.”

In the first heat, he was nipped at the line by a wheel length. In the repechage round (a second chance to advance to the semifinal), “I decided to make my move and there was a little collision. I lost my momentum for a second. When that happens in the last 200 meters, it’s over.”

In the end, Australia’s 2001 world champion and fast-food junkie Ryan Bayley took the gold. Jose Escuredo of Spain earned silver, and Shane Kelly claimed bronze for Australia. The American team, meanwhile, leaves Athens without a medal in track cycling for the first time since 1976 (excluding the Moscow boycott).

Nothstein flies back to America today to resume his road-racing career with a race this weekend in Binghamton. As for 2008, he said it’s “too hard to predict,” but there’s always the road race and, for the first time at the Olympics, a BMX competition. He hasn’t been there or done that, and Nothstein is always up for a new challenge.