LOUDON, New Hampshire — Motorcycle engines buzzed and rumbled, impatient, behind the painted starting line stretched across a patch of scorched pavement. Anticipation and gasoline fumes permeated the atmosphere. Then, a sudden flash of color. The green flag flapped, flying through the glaring June sunshine. They were off. Throttles twisted. Clutches popped. The pack of hungry bikes, eager for asphalt, shot down the front stretch toward the first turn. Eleven curves and one hill later, they all came thundering back around the straightaway in a blur of helmets, leather and hammering pistons.