At the USOC's warm-weather training center, bikers careen down a new course tailor-made for Beijing training By Brian Cazenueve In the race for medal supremacy this summer the U.S. has tried to stave off the competition by creating a world-class practice course at the Olympic Training Center in Chula Vista for riders in bicycle motocross (BMX). That event makes its Olympic debut in Beijing and features races among eight riders who bump, jump and knock elbows for position on a quarter-mile rolling, banked dirt track.