ONCE A YEAR, the best downhill mountain bike riders gather high in the French Alps for a race know as Megavalanche. It’s a madcap 18-mile race down the side of a mountain. En route, riders pass through alpine meadows, boulders, snow and even glaciers with breathtaking views along the way. With more than 6,000 feet of vertical descent and a physically taxing course, it’s probably easier to fall down the mountain than ride.