Ramon Benitez had the sound system of his Honda hatchback turned up and Big Audio Dynamite on the tape deck. The hatch was flung open, the better to hear the music while he fiddled with his mountain bicycle. "I'm getting psyched for this," he said, waving a wrench to indicate the crowd of racers milling around the starting line. "You've got to be ready.

"If you wipe out on this course," Benitez added enthusiastically, "chances are you don't get back up."

Welcome to the masochistic madness of mountain bike racing, which survived its debut in the Washington area the last two Sundays as the Bicycle Pro Shop of Georgetown's four-race Konterra Series kicked off.

There have been no serious injuries so far as 50-odd racers careened around the 3 1/2-mile course in an abandoned, 2,000-acre patch of the Laurel Sand & Gravel mines, just outside the Beltway.

But there were spills and thrills aplenty. "You come off your bike on a hill and go sliding down gravel on your butt," said one rider. "That's what we call hamburgered."

It's the lesser of two evils. The alternative is an "ender," which occurs when a rider in the same uncontrolled, forward free fall panics and grabs the front brake, locks the wheel and goes flying over the handlebars.

In most crashes the bike fares better than its rider. "These bikes are extremely durable," said Benitez, a mechanical engineering student who works for the Army Corps of Engineers. "The frames are 6/60 heat-treated aluminum, you've got labyrinth-sealed bearings to keep the dirt out, extruded aluminum wheels, all kinds of good stuff."

All a rider has, by contrast, are skintight shorts, a T-shirt and a light-as-air helmet for protection as he pedals full-tilt at 20 to 25 mph along wooded trails barely wide enough for the handlebars, over roots and rocks and out into the glaring heat of the sand and gravel mines, where bulldozers and rains have carved gullies, hillsides and washes to contend with.

It's rough, dirty, trying sport. Every September the mountain biking community convenes in Crested Butte, Colo., for Fat-Tire Bike Week. "It's at 8,885 feet, and all the trails go up from there," said veteran rider Gregg Vann. "In Crested Butte, the men are men and the women are, too."

Among the crowd of local riders who gathered here Sunday, in fact, were eight or 10 women, who by the end of the races were just as dirty and blood-speckled as the men, and ahead of more than a few at the finish.

It was a bit hard to talk to these women, who were busy with their bicycles, but a word with Thora Westre unearthed the fact she had driven all the way from Denton, Md., on the Eastern Shore, to compete.

Not surprising, said race organizer David Buell, a bearded Alexandria naval architect. There are few local opportunities for mountain bike enthusiasts to compete. Until now, Washington-area riders drove to Canaan Valley or Slatyfork, both in distant West Virginia, for serious competition.

This spring, though, Buell won permission to race in the sand and gravel pits, where the big housing development Konterra is to be built. It turned out to be ideal terrain for mountain biking.

But Buell, who charges a $5-per-rider entry fee for racing, said he never expected the turnouts of the last two weekends.

"If I had publicized this outside the Beltway, I could have had 100 people," a huge turnout for local racing, he said. "Two years ago," added Benitez, "you wouldn't have had 10."

To get a sense of what it's all about, I had Buell rig me up a 15-speed mountain bike and turn me loose on the terrain, following his assistant, Jamie Derby, around the course.

You don't go fast but it's exciting, anyway. Sometimes you don't go at all, as you scramble up hillsides of loose gravel until the back wheel spins uselessly, and then you jump off (mind the toe clips), shoulder the bike and run up the hill. This happens quite a bit in racing, Derby said.

Blackberry vines are entertaining when they rip at your legs, and the free falls down steep hills, when you shoot the bike out in front of you and "ride the back tire down," as Derby put it, are invigorating.

But I liked best the woods work, whizzing along noiselessly on fat tires, banging and clanking over roots and stones and shooting past trees inches from your elbows. It's technical, thought-provoking riding.

There is considerable controversy over the amount of environmental damage mountain bikes do, said Alan Datri, publisher of Urban Nomads, a local newspaper for all-terrain bikers.

Datri said the Sierra Club is a vigorous opponent of mountain biking on public lands, and has succeeded in closing to mountain bikes many areas in the West, where the sport is booming.

From the looks of things in Laurel, mountain bikes didn't make much more of a trail than hikers would. But most of that land has already been wrecked by the mining, so it's hard to judge.

In any case, all indications are this sport is taking off. Derby, a bike mechanic, said mountain bikes sell as fast as the shops can assemble them.

"It's brutal stuff," said John Hargadon, who grew up riding BMX bikes in Washington's Glover Archbold Park and now leads the Konterra series at the halfway point. "But when you finish, you feel good, like any other hard sport."

The Konterra Series continues Sunday at 4 p.m., and concludes July 3 at 4 p.m. Races start at the entrance to Laurel Sand and Gravel at Van Dusen Street, just east of Route 95. For information, call the Bicycle Pro Shop of Georgetown.