When Russ Barlow is riding his free-ride mountain bike, the city becomes a playground. For him and other urban biking enthusiasts, a lawn-covered hump of ground becomes a jump, a concrete wall becomes a balance beam, and a staircase becomes a drop.

Barlow and others like him take on urban obstacles when the trails are too wet or too dark to be enjoyable. Most local riders use big heavy free-ride mountain bikes, with pillowy suspension systems and disc brakes, the same bikes they use for big stunts on the trails.

"As long as the streets are dry it's better than sometimes getting so muddy," says Zac Dubel.

They ride in the evenings, when downtown is nearly deserted.

Barlow accelerates down the sidewalk on Grand Avenue, turns and launches himself down the stairs behind the library.

There are two sets of eight stairs, linked by a 5-foot-long landing. He lands three steps from the bottom.

Three other riders try out the stairs more tentatively, rolling over most of them.

Then 15-year-old Ashlon Durham launches over the first flight of stairs, touching down on the landing.

"Did I just do what I think I did?" he asks another rider, Ryan Erickson, 21.

"You almost died," Erickson says.

"Sweet," Durham says.

It's a small group tonight. The informal gatherings downtown can number as many as 12. Sometimes 30 people show up for nighttime rides around the Western Washington University campus, Barlow says.

Durham, a freshman at Bellingham High School is the youngest. Barlow, 34, who works at Clark's Cycle, is the oldest. Dubel, 32 a father of two, works at Kulshan Cycles, and Erickson is a pre-loader at UPS. (The health benefits help out with the bike injuries, he says.)

What bonds these guys is a common fascination with what can be done with muscle, bicycles, obstacles and gravity.

This Thursday night they begin at dusk, and continue into the dark. Outside Bellingham High School, they hop up and down on benches and planters. They jump down a mulch-covered slope leading down to a parking lot for the Whatcom County Sheriff's Office. Where Dupont Street reaches a bridge across Whatcom Creek, Barlow and Dubel ride the bikes along the tops of the concrete barriers on each side of the street, using them as balance beams, or in mountain biker talk, skinnies.

Erickson heckles.

"Skinnies are the devil! If you play a skinny backwards, it says I am the devil!" he calls.

None of them can recall anybody complaining about their evening antics in public places.

As the days lengthen and the rains taper off, the group rides urban obstacles more rarely, because most of the time everyone would rather be on Galbraith Mountain, the hot spot for local mountain biking.Still, urban bike riding is a growing national trend.

Mark Peterson, national advocacy coordinator for the Kona Bicycle Co., based in Ferndale, says that participation in this kind of riding is growing steadily nationwide. Many serious urban riders use small, 26-inch wheel bikes, often with a fixed gear.

Kona is selling more of such bikes each year.

"If you live in a city there's actually a lot of cool terrain you can ride with an urban assault bike," Peterson says.

You won't see many of them in Bellingham.

Kevin Menard, co-owner of Transition Bicycle Co., a Ferndale-based business that makes free-ride, dirt-jump and skate-park bicycles, says urban biking is most popular in places far from trails.

From the Whatcom Museum parking lot, a curving stairway leads to the Maritime Heritage Park amphitheater, with eight sets of four steps leading to eight landings. They jump each of the stairs, bouncing down to the amphitheater.

Jump, roll, jump, roll, jump, roll, jump, roll, jump, roll, jump, roll, jump, roll, jump, roll.

And repeat.

Urban riding has a leisurely rhythm. People come to terrain features, try them, and try them again.

"It hones your skills in a different way," Barlow says. "You can stop at each obstacle, whereas on the trail you hit it as you go by."