Three weeks ago, Winston resident Chris Martin was having the time of his life, careening around curves on a homemade tricycle, skidding down the Callahan Mountains west of Roseburg at 30 mph. The fun ended when his “drift trike” slid off the road and plummeted over a 40-foot cliff. He landed on his left leg, shattering the knee and damaging arteries. A week later, doctors amputated his left leg just below the knee. Martin, 29, became a casualty of drift triking, a little-known activity in which daredevils speed downhill in three-wheeled vehicles that amount to tricycles for grown-ups. Although not …