TOKYO — Connor Fields plunged down the mountainous ramp at the start of a BMX race, a semifinal in the Olympic competition most considered him the favorite to win. At the bottom, he began to peddle furiously, from the left side of the track, near the front of the pack as it barreled, the riders clustered together, down the first straightaway.
All Friday, the first turn—a sharp bank left with a steep incline that required riders to steer up, lean left, turn their wheels and fly back down—had proven problematic, even for athletes who see crashing as an occupational hazard.