I saw it the first time...
I was there that Saturday evening at the Indiana State Fair in 1975...
It was the most incredible evening of flat track I've ever seen... and this bike wasn't the only reason. There were 4 other TZs and another three or four Kawasaki H2 triples. The mix of sound was incredible. The two strokes flew. In the trophy dash (a sort of consey race for those not making the final) Don Castro would blow by two Harleys on the straights only to be overtaken in the corner, due to running the four stroke line. The only rider who figured out the line and how to ride them was obviously KR, going in deep and squaring off the corner. There were six frames - Steve Baker, Rick Hocking, Don Vesco, Randy Cleek, Skip Aksland, and Kenny Roberts got them. Vesco didn't show, Cleek had electrical issues, Hocking seized a cylinder, Baker wasn't back into the swing of flat tracking, but Aksland and obviously Roberts did better.
The ban was ridiculous and short sighted. At that time KR was anti about anything 750, wanting GP in the US to go to 500. (If you have been into motorcycle racing long enough, you might remember in the mid-late 80s KR was also against racing four strokes along with some others like Eddie Lawson, they called them "diesels".) I think the real issue had more to do with tire wear/grip problems than with the power. The TZ as a road racer got the same reception, but wasn't banned and went on to have one of the best safety records of any roadracer. There was a great level headed review and actual ride test of Steve Baker's Champion framed TZ in the January 1976 Cycle World magazine.
The magazine article debunks all the current myth that surrounds the development of the bike and the "demon" reputation. Yamaha wouldn't even cooperate with Doug Schwerma to build the bike. They were comfortable with Roberts on the twin until he started getting his ass kicked... then and only then did Yamaha work with Schwerma, long after he'd developed the general bike with Rick Hocking doing the test riding at places like Ascot. Roberts' first ride was Saturday at Indy and he won. No development, just get on and ride. Figure out the gearing in general and go.
It's often led me to wonder what might have been if they'd let it go a bit. Maybe a big bag two stroke cound have resulted with two cylinders firing at once or any variation, with a pressed together crank. The ban took out ALL multis over two cylinders.
That resulted in what we see today on the flat track - a parade of 1980 dirt track technology. Imagine if they'd have banned the TZ in roadracing... we might be watching 500 singles and 350 twins still.
I bought a copy to replace the one I got back in 1976 and lent someone never to see it again.
I've got the article scanned and in an MSword documet if you might want a copy just email me.
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