Was that a Sportster that he rode?
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The motorcycles used in the series were supplied by Harley-Davidson (according to the closing credits of the episodes); Sportsters (883cc) were used for the road shots and close-ups, with a Harley-Davidson Sprint SS350 and a Czechoslovakian CZ, and possibly a Harley Rapido 125, used occasionally for hillclimbs, beach scenes and other rough-terrain riding.
According to Bud Ekins, the stunt coordinator for the movie and the series, MGM bought two or three Sportsters for the pilot movie (which was filmed in the fall of 1968; the bikes were probably the 1969 model XLH). The bike used in the hill climb and beach scenes was a CZ, provided by Bud, who was a CZ dealer at the time. The bikes were sold after the movie was completed.
After the movie proved popular, and the series was scheduled, Harley-Davidson provided three Sportsters, one of which was customized with a side-car for close-up shots. H-D also provided a Sprint 350 and later a Rapido 125 (used in the moto-cross in the episode entitled Mating Dance For Tender Grass). The Rapido crashed before the shooting of the episode was finished, putting the stunt rider into the hospital. In that episode, the race's first prize was a green 1970 H-D Sportster, with the new "boat tail" seat/fender assembly. (It was first prize for a real race that was actually held for the episode, and was donated by Harley - even though it had been previously promised to another, paying customer of H-D! The customer did get his bike a week or so later.) In the episode entitled The Great Motorcycle Fiasco, the motorcycle, a "1937 Rudge Ulster with a bronze head," used by actor Keenan Wynn, was owned by either Keenan or Bud Ekins, who recommended Keenan for the part.
Bud said he was the one who customized the Sportsters and other cycles, and said the sissy bar came off of a Schwinn bicycle. He also said that no Triumphs, Yamahas or other cycles were used in the movie or episodes in place of the "Bronson" Sportsters, except the CZ (in the pilot movie) and the H-D Sprint and Rapido (in the episodes).
Bud said he trained Kurt Russell, who appeared on one TCB episode, to ride a cycle with a sidecar for the movie Swing Shift, which also starred Goldie Hawn.