Flattracking Finesse?
Yamaha HQ had put themselves in the hands of Yamaha USA when it came top both new models, market forecasts and advertising. They already knew well enough that the old adage…”win on Sunday, sell on Monday” was unerringly true in America. Events and circuits such as the Daytona 200, Ontario, Talladega and Carlsbad had helped sales of road bikes enormously. And yet in America at the time tarmac based racing was still a relatively new sport. By far the most interest in two wheeled racing stateside was dirt tack ovals typical held on mile and half mile ovals.
Harley Davidson was still the preeminent marque and although big British twins had been successful in the late 60s and early 70s the factories had all closed by 1974. In theory Yamaha’s XS650 twin engine should have offered potential with it being similar to the old British twins. However, even when specially reworked engines were opened up to 750 were used the bike often struggled against the home built V twins. Yamaha needed the engine to run 10.5: compression ratios and pay Kenny Roberts some serious money to the best out of the heavy parallel twin in order to gain the 1973 and 1974 dirt titles. The writing was on the wall – the old XS had become fragile when tuned and the Harley factory was out for blood in the following season.
When renowned flat track frame builder Doug Schwerma saw Canadian ace Steve Baker’s TZ700 he immediately began measuring and assessing the four pot stroker engine. Initially Yamaha’s racing manager was reluctant to get involved but Schwerma eventually got his hands on a spare motor in early 1975 and built a brain-out oval dirt bike that saw Rick Hocking take third at the July Ascot Park TT. This result changed Yamaha’s minds and five similar chassis were ordered for Randy Kleek, Skip Aksland, Steve Baker and of course Kenny Roberts. These machines were capable of delivering some 90 plus bhp through narrow rear tyres on bikes with just a notional front brake and only by monetarily cutting the spark to on pot was it possible to calm it on the bends!
At the Indianapolis Mile not everything went as planned with some riders experiencing engine seizures and others having tyre issues. Roberts’ bike was geared for 145 mph in sixth gear but he still found the bike lifting its front wheel when wound hard in third gear! The TZ700 dirt bike had potential and toque to spare scared both its riders and the opposition; no one had ever seen anything like it. By the time Roberts got through the qualifying rounds his team had reduced the rear sprocket by two teeth in the hope of keeping the front wheel down in the final yet despite the taller gearing the bike was still touching 150 mph on the straights of a dirt track! In the race King Kenny played a canny game keeping his powder dry but finding he needed to go to the very outside of the track in order to get reliable traction. He won the race and went down in history saying …”they don’t pay me enough to ride that thing!”… a comment that swiftly had the AMA rewriting the rules to limit dirt track bikes to just two cylinders for the 1976 season.
The ‘TZ Milers’ raced a few more times that season but struggled on slippery tracks. Revisions to engine location and height helped a little but the bike’s power and its delivery were really simply too much – pure and simple. Despite its one and only track win, the brute power, the reputation and the sheer improbability of putting a four cylinder two stroke GP engine in a dirt bike frame Yamaha once again had challenged convention and one!