LONDON — The capacity crowd at the Olympic Velodrome on Tuesday roars in anticipation. Six highly trained and fit cyclists set off on a race for gold.

And leading the way is a 65-year-old man on a motorbike.

One of the more incongruous sights at the Olympics has been Peter Deary, who drives the derny, a two-stroke motorized bicycle that leads the riders in the keirin race.

The keirin originated in Japan, where fans bet on the races, and was added to the Olympics in 2000. It is a dynamic event run at blazing speed with many crashes. But its early stages are much more pedestrian.

The derny driver leads the riders for the first five and a half laps of the race; they may not pass him. Then he pulls off the track, and the riders sprint to the finish for the final two and a half laps, about 600 meters.