The All-American Soapbox Derby is alive and, well, rolling along.

A decade after a cheating scandal and sponsor pullout nearly put an end to the "gravity grand prix," the Derby has evolved into a neighborly affair that's at once highly sophisticated and laid-back. It's fun again, as it was when it began in 1934, with fresh-faced lads driving wheeled wooden crates. One big difference is the growing number of girls who are competing -- and competitive.

The sophistication is in the designs made possible by Fiberglas, super-slick paints and deeper understanding of the physics of gravity-powered coaster cars. The fun comes from the determination of the sponsors and parents to never again let the Derby be perverted: As far as is humanly possible, they're keeping it of, for and by kids.

The Downtown Jaycees, sponsors of the Greater Washington Derby, will hold a get-acquainted meeting for prospective car-builders and their parents this Saturday in Springfield. Most of the old hands are already working on their racers for this year's local contests, from which the junior and senior winners will go on to the finals at Derby Downs in Akron. But plenty of time remains to build a car for the July 16 races on Eastern Avenue.

The question that leaps to mind when one sees the highly polished, virtually seamless modern Derby cars is, of course, whether boys and girls between 10 and 15 can possibly build such beautiful machines. The answer from Paul McGowan, 17, a former Washington champion and pit boss for his sister and brothers, is: You better believe it.

A bystander, looking over Paul's 1979 Washington championship racer, commented. "Your father must be a fine craftsman."

"He sure is," Paul replied. "It's a shame he didn't get a chance to work on this one."

The cars actually aren't as good as they look, he said, as he showed off the seven sleek racers that take up most of the space in the Rockville family's garage. "Fiberglas covers anything. The construction is fairly simple, actually, once you're set on the design. What it takes is time."

A senior-division (ages 12 to 15) car is essentially a Fiberglas cocoon spun around a framework of narrow laths fitted to the body proportions of the driver. The driver doesn't get into the machine so much as put it on, wriggling in one hip and shoulder at a time. When in position, the kid is flat on his or her back, with only the helmet visor showing.

A junior division car is a much simpler matter: It's essentially a one-design class, and prefab body kits are available. "You can be competitive from go," Paul said.

Paul has high hopes for the car he's supervising for sister Martha, 15, this year, which looks so narrow you could barely stuff a baby into it. "The thing about girls is that they don't have shoulders," he said. "That means we can really taper it off in the rear end, and keep the cross-section down. The less cross-section, the less wind drag."

On form that should make Martha the one to beat this year, since in the 1982 Washington races she was runner-up by a fraction of a second -- to her brother Peter, now 13 and, as a former champion, ineligible for further Derby competition.

"The fact is that the car doesn't win races, the kid does," said John P. McGowan, father of the family and, confusingly, also known as Paul. "Design and construction are important, but there aren't any real secrets. Everybody has to follow the same rules, and all you have to do is take a careful look at a well-built car to be able to see how to build a competitive one. And anyway, we all help each other out. Weights are equalized, wheels are swapped and matched and heats run in various lanes to eliminate all possible variables. The final races generally are decided by thousandths of a second, out of 30 or so. That amounts to just inches over a thousand-foot course, and the difference is the driver."

McGowan, an architect, is overseeing his second generation of, Derby-car builders, having helped his younger brothers back home in Pennsylvania, and he's endlessly enthusiastic. "I was much older than my brothers, but working together on those cars helped make us close, and we still are. It has done the same for this family.

"It's not just fun, it has taught us to respect and rely on each other, and the kids have learned all sorts of things. There are the skills -- like research, conceptual design and mechanical drawing -- and working with wood and metal and Fiberglas and paint. Another important thing is that they have to do all the calling around to find out where we can get materials and hardware, which is a good way to learn how to deal with all sorts of people and problems. Theyre better at setting up work shedules, and keeping to them than a lot of professionals I know. We travel around to community race rallies, which is fun in itself, gives them racing experience and has won them friends all over the East and Midwest."

"I have two more moms." Martha said, "one in Greensboro, North Carolina, and another in Hickson, Tennessee, that we met at rallies. There are such nice people you meet at the races, they make you feel right at home."

The McGowans are the sort of people who'd be welcome in anyone's home, but Martha may need a little extra mother-support from time to time: Mrs. Margaret McGowan doesn't fully share the Derby obsession, and will sometimes go so far as to point out that the lawn needs mowing and that those marvelous skills might occasionally be used in fixing up the house. "And there are other places I'd like to visit besides Akron," she said. "We've been there five times, and the people are wonderful, but..."

And there's the matter of money. Slice it as fine as you can, a Derby racer is going to cost at least a couple of hundred dollars. "That really isn't much, considering what even a good bicycle costs these days," said David E. Tivel of McLean, whose son Jonathan won the Washington Derby junior division at age 10 in 1980 and will be racing as a senior this year. "And I'd be hard-put to think of anything else that returns as much value to a young person, dollar for dollar." One spinoff was an elementary-school paper by Jonathan on Derby design and tactics that would be respectfully received in many college engineering survey courses. It explains such arcana as monocoque construction and weight distribution for maximum efficiency on various track profiles.

The Greater Washington Soap Box Derby Association, operating on a limited budget but not wanting anyone left out for lack of money or skills, is seeking sponsors and construction advisers. Sydnee Schwartz, the Jaycee's honcha for the Derby, who was barred from competition when she was (and because she was) a girl, is also stretching the shoestring to make sure every kid who wants to race gets a chance.

SOAP BOX DERBY CLINIC -- At 10 Saturday in the auditorium of the Washington Gaslight Building, 6801 Industrial Road, Springfield (I-95 south of the Beltway). For information, call: D.C., 291-4964: Bethesda, 229-0118: Washington, 291-4694; College Park, 474-4591; Rockville. 279-7174; Prince Georges. 372-8443; McLean 893-7942.