The unusual dimensions of Gronen's margins of victory and heat times tipped off derby officials to illegal circumstances surrounding Gronen's racer. Subsequent X-ray examination of his car revealed an electromagnet in the nose. When activated at the starting line, the electromagnet pulled the car forward by attracting it to the steel paddle used to start the race. Gronen activated the electromagnet by leaning his helmet against the backrest of his seat, which activated its battery power source. This became very evident as Gronen's heat times progressively slowed down as the race wore on, because the battery lost power each time the circuit was activated reducing the attraction of the electromagnet. Usually, heat times get faster each time a racer completes a heat. Videotape of the race also showed a suspiciously sudden lead for Gronen just a few feet after each heat began. The margin of victory for a race heat is normally no more than . Gronen's early heat victories were in the range. (Aluminum insulator plates were added to the starting ramps in 1974 to render an electromagnetic system useless.)