Karen's coaster car had been hanging on the garage wall since the day she crashed while starring in a chase scene in a Bonnie and Clyde home movie. The affair made great film, but she lost some of her skin and all of her enthusiasm for driving the contraption her father had made.

Her little brother Mark, now seven, had been fascinated by the thing since toddlerhood, but was put off by one excuse after another until the recent afternoon when he set out to build his own car out of an old Hot Wheel and various odds and ends.

I understood the passion that was driving him. The building of Karen's car and another modeled on a "slingshot" dragster, which is still in development, was the outgrowth of my frustrated boyhood. I had the misfortune to grow up with a guy named Mike McCullough, whose Navy captain father had built the world's neatest coaster car in the machine shop of his aircraft carrier. It had everything: geared steering, torsion bar suspension, drum brakes, spoke wheels and pneumatic tires. Its performance would, and did, shame a Soap Box Derby champion. Its only shortcoming was that it was built of aircraft duraluminum, so it eventually succumbed to a telephone pole.

But before its demise that car blighted my youth.It made the one I had built out of an old Star wagon look like a penny waiting for change, and raised my coaster-car standards so far above my skill and means that I never tried to build another one until I had children of my own.

A couple of runs with Mark at the wheel showed that the slingshot model still needed serious work, so we hauled out Karen's car.

Dust and rust aside, the main problem with it was a broken steering shaft. Steering mechanisms have always been the curse of coaster-car makers, because it takes sophisticated machinery to shape a metal yoke and kingpin for the front axles to pivot on. The traditional one-piece axle with a central pivot leads to oversteering followed by overturning, which is why all Soap Box Derby courses are dead straight.

The solution is to go to a shop that repairs riding mowers and beg the front-end parts off a junker. The owner won't want to be bothered, but take along your own tools and the kid for whom you're building the car and he will find it hard to turn you down. Talk to him nice enough and he might let you have some wheels, too.

Karen's steering shaft having been proved inadequate, the replacement was a massive threaded rod, plus a padded steering wheel from a wrecked go-cart. Then the brake pedals had to be moved forward to accomodate Mark's legs, which had grown six inches while nobody was looking.

He suffered impatiently through the two afternoons all this took. It could have been done in one afternoon, but since it was to be his car, he enforced his car, he enforced his right to do the sawing and drilling and bolting. His mother, who thought the whole thing was a lousy idea, was dragged out to see the finished product. See how precise the steering, how sturdy the frame, how quick the brakes.

How do you like it?

"He will run in the street," she said. "If you want me to like this stupid machine you have been fooling with instead of finishing the porch and the bathroom you will have to take off the wheels and plant flowers in it. Karen got hurt and he will get hurt."

"He'll only be using it when I'm along."

"That sure is comforting," she said. "While you two are playing in the traffic I will go see whether anyone has fallen off the unfinished porch."

The first test runs were down the driveway, which soon was black with skid marks. The car is a two-seater made of boards, an old metal hammock frame, the front end of an Ariens mower, 10-inch Sears industrial wheels, a plastic chair, an outgrown car seat and about ten pounds of machine bolts. The steering is geared down through a bicycle chain and sprockets to make the response less sudden. The curb weight is something like a hundred pounds; with Mark at the wheel and his father on the rear seat the grand total approaches four hundredweight.

Once the boy seemed to have the hang of it we moved on to a sidewalk slalom course -- the obstacles being mounds of dog leavings -- and then to the mild downhill cul-de-sac in the next block.

He proved to be a natural driver. We went on to the park for some cornering practice and then to a steep hill ending in the parking lot of his school. The course included a sweeping right turn, a parked-car chicane and esses into a wide runout.

He took a perfect line through the corner and was steady through the chicane. By the time we entered the esses our speed was approching 25 miles an hour. At that velocity and under such a load, it turned out, the spring-dampened steering tends to oscillate in response to minor corrections. The car began to swerve with increasing wildness, until I was thrown off. Then it settled down, and while bouncing and rolling along the pavement I caught glimpses of Mark steering expertly through the final turns.

It was dark by the time I managed to hobble home. "See?" I told his mother. "Not a scratch on him. And anyway, I can type with one hand."