Which motorcycle is regarded as the most successful model in American flat-track racing history?

The Harley-Davidson XR-750 is regarded as the most successful motorcycle in American flat-track racing history.

Harley-Davidson developed the XR-750 for AMA racing in 1970 after rule changes disadvantaged the company's older iron-head Sportster-based racer. The original XR-750 had an iron cylinder head, while a lighter alloy-head version followed in 1972 and became the iconic configuration.

The XR-750 achieved an extraordinary record in American dirt-track competition and became closely associated with riders including Scott Parker. It also became famous outside racing through Evel Knievel's jump performances. The Indian FTR750 later became a major rival, but it does not replace the XR-750's historical reputation as the benchmark American flat-track motorcycle.