PORTLAND, Ore.

L ooking like punk gladiators, teams of young adults on junk bikes pulling chariots made of junk parts frenetically chased each other around a makeshift arena, slamming each other and swinging away with foam clubs.

Spectators tossed beer cans, flour and smoke grenades. Firecrackers popped and obscenities flew during the battle, called the “Ben Hurt Chariot Wars.”

And then, toward sundown, the grown-ups arrived. Seven police cars pulled up and told all 400 or so people they had to leave because they were trespassing on railroad property. They complied. But the contest was over anyway.

Victors had already emerged: Jon Penfold and his teammate Nick Schlabach. Falls, collisions and smashed wheels eliminated the other teams. Penfold’s and Schlabach’s chariot, armored with a bubble-shaped metallic cage, was the last one still able to maneuver.

“You attack, and then run. Attack, and then run,” said Penfold, 29. “It’s a war of attrition.”

Asked later on how he felt after winning, the 26-year-old

Schlabach said: “I wanted to puke.”

Oregon’s largest city has an obsession with bikes. Thousands of people use them to commute to work each day. Lots of people sport fancy road bikes that they use to stay in shape. But it’s not commuting or fitness that motivates the 20-something men and women at the annual Ben Hurt Chariot Wars.

They do it for the adrenaline rush, for the liberation that comes with acting like a kid, and for the parties.

The Chariot Wars are the headline event in a weekend of bike craziness called “Mini-Bike Winter,” contests and parties held at various locations in the city. The term “minibike” derives from the origins of the games. They were started by a group who call themselves the “Zoobombers” and who get their kicks by riding pint-sized kids’ bikes at breakneck speed down steep streets in Portland’s posh West Hills, to the chagrin of many of the neighborhood’s residents.

Another event in the weekend games is called “bike bowling.” Instead of tossing a ball at nine pins, you hurl yourself and your bike against a huddle of plastic traffic drums — the tall orange and white ones used by road construction crews. The goal is the same as in bowling: knock them all over. But the outcome can be painful.

At the recent Sunday “bike bowling” event held at a Portland public park, a young man went flying over his handlebars when his bike hit the drums. He landed on his shoulder and grimaced. It had been dislocated. While other participants kept riding their bikes into the drums, two people yanked on the injured man’s arm, trying to get his shoulder back in place.

In another event, participants ride their bikes off a ramp and into the Willamette River. The water temperature at this time of year is in the 40s.

Safety isn’t thrown completely to the wind during the “Mini-Bike Winter” games, though it may seem that way. There are rules. Weapons used in the chariot wars have to be well-padded. Competitors wear helmets and various other forms of protection.

Many of the competitors have bike repair skills, a handy asset for this kind of mayhem.

Efforts are made not to upset the authorities, at least too much. But run-ins do occur.

This year’s Chariot Wars were held on about 100 square feet of concrete on land owned by Union Pacific Corp.

Ten minutes after Penfold and Schlabach won the trophy, police and a railroad official showed up. They talked with 28-year-old Jon Dutch, last year’s Chariot Wars champion. The railroad official said a train was coming and the crowd had to leave.

“What’s it gonna be, Dutch,” said the official, who declined to be identified. “Do you want one of your brothers or sisters pinned under a train?”

Dutch shuffled his feet. He looked at the crowd of mismatched, overgrown kids in red hoodies and green skirts and purple leggings.

They all got on their bikes and into their chariots, heading for a post- Chariot Wars party that went late into the night. Some managed to make it to “bike bowling” the next day.