So here we are 10 miles outside Fredericksburg, about an hour's drive from Washington. It's 8:30 in the morning and we're parked next to a cornfield. When what happens? An 8,900-pound rent-a-truck drives up. You could carry an elecphant in the thing. And what happens? A 90cc Kawasaki gets out.

A 90cc Kawasaki is a little motorcycle. You could carry it in the back seat of your family car. James Shepard of Arlington, a truck driver, started from home with his son, Dale, 11, in the fron seat of his pickup truck and the 90cc Kawasaki in the back end. When what happens? The truck dies.

The Shepards, like 50 other families, were headed for WHite Oak Motocross Park, there next to the cornfield outside Fredericksburg. For two years. Dale, who's about 5 feet tall and weighs maybe 75 pounds, had been riding his little motorcycle in the alley behind his house. Then he heard about the Scholastic Motocross Club, open to all school-age riders.

Now, motocross is not checkers. You race a motorcycle over a twisting, rolling, up-down-and-sidways track that causes the motorcycle to do wonderful things such as slide, bounce, fly and (yes) crash. It is you and your motorcycle locked in combat with the land and the laws of physics. It requires strength (to wrestle the bike around a 120-degree turn). It requires agility and balance (you stand more than you sit). It requires coordination and concentration (you may be called on which means you're using both hands, both feet and the weight of your body while the bike urns fiercely at a big tree). You should also have great confidence in your ability to heal quickly. Motocrossers fall off a lot.

Anyways, here's Dale Shepard, 11, and we first see him when the back door of the giant rent-a truck goes up. For $25 a day, 21 cents a mile plus gas, James Shepard rented the truck at the same service station at which he left his expired pickup. That was in Springfield. And for the next 50 miles, Dale Shepard rode in the back of the rent-a-truck to hold up his 90cc Kawasaki.

"We had to come - had to - because of Dale," the father said. "He's been dreaming about his since the last race."

The last time the seventh-grader at Commonwealth Christian Elementary School raced, he and his little bike demonstrated conclusively the truth of the motocrosser's fatalistic motto: "He who lives by the dirt occasionally eats the dirt." No one knows the exact count, but one Scholastic club official said, "In four laps, he must've fallen 30, 40 times. But he never quit. We gave him the Hard Luck Trophy."

"It's on top of the TV," said Dale.

What does he like about motocross?

"The races," he said, his tone full of wonder at the questions some people ask.

Dr. Elizabeth H. Moores rode a motorcycle once. She rode it into a fence. Now she's content to be a passenger. A teacher of vocational and industrial education at Thomas A. Edison High School in Alexandria, Dr. Moores in the winter of 1975 came up with the idea for the Scholastic Motorcross Club.

"My son, Billy, who's 14, was riding and I thought he shouldn't be out there racing with old men and beards," Dr. Moores said.

The club's first races were in March of 1976. More than 400 riders, including some from New Jersey and Pennsylvania, have been members. The club's emphasis, Dr. Moores said, is on first-timbers, those riders with no motocross experience. The first-timers are given riding instructions and safety tips before every race.

"It's a family sport," Dr. Moores said. "You've always though of 'motor-sickles,' but this is a good middle-class sport. There's no riff-raff involved. All good, clean kids, who due to size or inclination have chosen not to participate in the traditional varsity sports."

Dr. Moores is an activist. Because motorcycle racing is nosiy and requires a significant parcel of land, racers must scramble for somewhere to run. Dr. Moores thinks government should supply a place - as it supplies baseball diamonds, baskeball courts - so she has written to her U.S. senators and representatives in addition to dealing with the Fairfax County park authority.

She also believes motocross should be a varsity sport in high schools. It is in California and Michigan.

"We've got the kids who want it," she said. "The problem is finding sponsors in the schools. I called all the vice principals and only a small percentageof the schools in Fairfax County could find a sponsor."

All that - the public track, the varisty sport status - is somewhere in the furture (or in Europe, where a two-wheeled society has made motocross a consuming passion second only to soccer). For now, motocrossers here are dependent on the good will of men like Russell Sullivan, who built the White Oak track on his farm last year at a cost of $41,000 - a number he'll never make by charging the Scholastic Club $500 a day four for five times a year.

"The competitive spirit's what I like; the guts these kids have," Sullivan said. "At this age, motocross gives them a direction to go, something to do."

The winners this weekend were Ray Pulley, Scott Bland, Bob Siegle, Jimmy Fones, Jeff Talley, Mark Jennings, Scott Kennedy, John Graumann, Kevin Meadows, Mary Untiedt, David SUmmerlin - and Dale Shepard, whose hour-long ride in the back of a giant rent-a-truck, holding up his little bike, earned him another trophy, this one the True Grit Trophy.