A GRINDING sound, somewhere between a rattle and a rumble, erupted over the suburban New Jersey hill. The figures, clad in motorcycle leathers and helmets, started to appear, one, two, three, until there were almost 20, crouched on skateboards, like a squadron of roller-villains.

One by one, the skaters rounded a turn, dropping a gloved hand to the asphalt as they scraped their wheels in a slide, taking care that nobody crossed into oncoming traffic and found themselves splattered across the grill of a Buick.

At the bottom of the hill, the leader of the Bergen County Bombers, as this gang of skateboarders is known, wrestled the black motorcycle helmet from his head, revealing a mortgage broker with gray flecks in his beard and the crow’s feet that come from decades in the white-collar trenches.

“It’s a total midlife crisis,” said the group’s leader, Tom Barnhart, 47, of Cresskill, N.J., who started skateboarding two years ago for the first time since the Carter administration. His life, he said, had been in a rut. “My kids grew old, so I got a dog. My dog grew old, so I got a skateboard.”