Ask Kevin: Hard Riding Beasts - Yamaha TZ750

When the AMA announced that for 1972 the displacement limit for racing motorcycles would be 750cc for all engine types, Suzuki and Kawasaki had three-cylinder production bikes ready to be re-engineered into 750 racers. Yamaha had no such model ready and so soldiered on with its latest 350cc two-stroke Twin, winning Daytona in 1972 and ’73 after the ill-handling, 100-horsepower 750 two-stroke Triples had spun their old-technology tires to shreds.

The EPA had begun to regulate automobile exhaust emissions, so it was obvious that smoky two-strokes had a short market future. That being so, Yamaha did not produce the two-stroke GL750 inline-Four it had readied as competition for Kawasaki and Suzuki.

Yamaha had other surprises ready. One was a 500cc Grand Prix bike, the 0W20, based on the GL750 architecture. This was conceptually two of Yamaha’s racing 250cc Twins set side-by-side, each with a thin gear on the inner end of its crank and that pair of gears meshing with a double-width gear on a jackshaft extending to drive a dry clutch on the right. Just as with the Twins, each pair of cylinders was made as a single liquid-cooled block, but unlike the piston-ported Twins, this 500 had cylinder reed-valve intake. All four exhaust ports faced forward, and four smallish pipes were crowded together to pass under the engine.

It was on such a machine that Finn Jarno Saarinen won the first two GPs of 1973.

After Saarinen’s death at Monza in a smaller displacement class, Yamaha withdrew the 0W20 and began planning for 1974. Central to this plan was to hire Giacomo Agostini away from the Italian MV Agusta team for whom he had ridden since 1966. Yamaha now readied a larger, four-cylinder, 694.9cc reed-valve two-stroke, coded 0W19, which would debut as the TZ750A in the U.S. in March at Daytona.

Australian Kel Carruthers, tech chief of the American Yamaha race team, went to Japan to try the new machine. Perhaps it had been deliberately given the very short 53.5-inch wheelbase preferred by Agostini on his three-cylinder MVs, but Carruthers found the bike to be unstable at high speed. His remedy was one that had worked for him on other fast motorcycles: to extend the wheelbase with a longer swingarm. A 3-inch extension did the job.

Daytona is a difficult place to introduce any new machine because the combination of sustained high speed, extra loading from the banked turns and the 200-mile race distance conspire to frustrate human ambition. Running on Dunlop’s new ultra-wide, belted-bias Speedway Special rear tire, Ago survived challenges and managed not to damage his tires on the sharp fragments of cracked-to-pieces Yamaha exhaust pipes. Like the 500 GP bike, the new TZ750A’s four pipes were crowded together under the engine, and as 10,000 pressure pulses per minute hit each flat-sided pipe, they tried to inflate it to roundness. The resulting flexure covered the track with broken pipes and left Ago’s bike making a harsh open blare. Yet he won the race. TZ750s would win the race every year from 1974 to ’82 inclusive.

TZ750As were twin-shock machines with a swingarm made from sheet-metal pressings. When the AMA series got to Talladega, many big TZs were frighteningly unstable at speed. I saw more than one rider take to his lawn chair white-faced and shaking. Some actually decided that 750 roadracing was not for them and went back to 350cc Twins.

The instability had a cause, and it was not 750cc but rather little wire circlips, whose grooves were not deep enough to keep them in place. When these circlips popped out, either one or both fork damper units stopped damping and terrifying high-speed weave began—a violent Dutch roll at three cycles per second. Don Castro’s Yamaha team bike threw his feet right off the pegs just before start/finish in Friday practice. On Saturday morning, Carruthers had the front ends off all three team bikes, found the problem and fixed it.

These 750s were not easy to ride because two-stroke power does not come in smoothly and progressively. Instead, as the rider turns the throttle, nothing much happens initially, followed by irregular popping that becomes eight-stroking, then four-stroking and finally continuous firing. Because the engine is big, these changes are substantial, and the rider must be ready for them. The irregular firing occurs in all carbureted two-strokes, big and small, because at small throttle, the cylinder is mostly filled with exhaust gas. With little mixture entering, it takes several revolutions to accumulate enough charge for the sparkplug to ignite it. This is no problem on a 125 or 250 because the torque steps are manageable. But if you try to throttle up from full lean on a 500 or 750, the engine will yank the rear tire loose and you will go sideways. If the tire re-grips gently enough that you don’t highside, the result is “only” a big gollywobble. If you are determined to press on, you will do so in an off-corner weave, punctuated by a series of near highsides.

Onlookers were horrified by this, but here came Yvon Duhamel, famed for his bronco-riding ability, in from practice. And he was saying, “I think it’s maybe a little better now,” but you have just seen him wobbling and sliding and can find no meaning in his words. He knew what he was talking about. Experience enabled him to ignore “normal” wobbling to concentrate on detecting actual threats to stability. Attempts to make the ride smooth just slowed the rider down. And there were plenty of slow riders on TZ750As, which sold for $3600 before that 1974 Daytona.

In 1975 came the “Full Size Kit,” which increased bore from 64mm to 66.4. In the kit were cylinders, heads, pistons and new cylinder nuts (the original blue anodized ones broke frequently, one of them keeping Kenny Roberts from a Daytona win).

In 1976, the factory built the 0W31 monoshock 750, with its Vincent-style triangulated swingarm. This was another important step toward solution of 750 handling problems. Tires—Dunlop’s big, round tire and Goodyear’s slicks—were a first step, but stiffening up weak chassis elements like the twin-shock swingarm was next. And long rear-suspension travel, an essential part of the monoshock idea, was the third. With long travel, softer spring and damping rates could be used, which kept the tire on the pavement more of the time, not hopping in the air as it did with the stiff, old, 3-inch-travel twin-shock orthodoxy that dated back to the 1940s.

Despite that, watching along the short straight before Daytona’s Turn 5 (leading from the infield up onto the banking), I could see that tire and damper deterioration was causing the leading bikes to weave threateningly even in middle gears.

At the 1976 Trans-Atlantic Match Races, Steve Baker showed what was becoming possible by sensationally wheelying sideways out of corners. Control! In 1978 at Imola, Bud Aksland said, “Look here” as he depressurized one of Kenny Roberts’ rear suspension units, then unscrewed the gas reservoir and poured out the “oil.” It was a creamy emulsion of leaked gas and oil, mixed with dark wear particles from damper piston, cylinder and seals.

Later, Roberts was testing inJapanand finding it hard to get grip. Often the problem is too much compression damping, which lets bumps knock the whole machine upward instead of just the tire. A tire in the air means no grip and no stability. But reducing the compression damping didn’t do anything, so, at length, Roberts said, “Is compression damping in a part of the damper that we can just leave out? Can we just do away with compression damping completely?”

After some “discussion,” a damper was assembled with a spacer in place of its compression valve. When Roberts tested with that, his lap times immediately improved. Very quickly, having a much-bigger compression valve, capable of letting the rear wheel move really fast, became essential to good suspension. Geoff Fox was an important pioneer in this, but everyone quickly adopted the idea. With a bigger compression valve, it was no longer necessary to gear your bike so you could upshift as it hit the banking out of Daytona’s Turn 5. You could keep the power on across steps or rough pavement. More control, more of the time.

Kayaba's 36mm fork on the TZ750 was designed for the grip of Dunlop triangulars of the classic "hard rubber era." With slick tires, it became common for fork flexure to allow the brake discs to knock the brake pads back in the calipers. When you got to the next turn and went for the brake, the lever came to the bar. What a disappointment! That started a move to bigger and tubular wheel axles and to bigger, stiffer fork tubes.

As tires got bigger, Yamaha made changes to its factory 750s, moving the chain line to the left, away from the tire, and adjusting frame-tube positions. Privateers with production versions used a wet razor blade held in Vise-Grip pliers to carefully—very carefully—pare away the left shoulder of the tire. This made it obvious that tires were going to grow (a 250cc GP bike of 2009 wore the same rim and tire sizes that looked so “huge” to us in 1974), so designers had better leave room for them.

Front discs coned under the heavy braking of twisty circuits like Loudon and Sears Point, pushing pads back in a different way. This made it clear that discs must be flexibly mounted to their carriers so they were free to expand without coning.

Finally, in 1982, Honda was ready to take a swing at Daytona with a designed-for-the-job 1000cc V-Four four-stroke (Formula 750 had been changed to allow 1025cc four-strokes). The FWS1000 had one of the first back-torque-limiting “slipper” clutches so its engine braking would not upset corner entry with hopping and sliding.

It should have been a walk, but the race was won by Graeme Crosby on a hastily built-from-parts 0W750. Roberts, the early leader on a 500cc GP bike, was out with a tightened engine. And the giant FWSs needed to stop too often for tires. This was the last Daytona 200 win by the venerable TZ750 design.

The 750cc era had begun with wild drama, but technological change had gradually softened the sharpness of its problems, preparing tire, chassis and suspension solutions that would be ready when, in the later 1980s and ’90s, powerful, lightweight four-strokes began to enter production.