TZ750 REVISITED: The Chassis

I spent ten years (1974-83) with the Yamaha TZ750 but find I have no nostalgia for it. What looked so promising and purposeful (it was!) in 1974 and 1976 (the year of the factory 0W-31 redesign) now looks like a hodge-podge of anachronisms.

Listing them reminds me of all that has been accomplished since its day, and of the lines of development which it forced into being.

Stand back and look. The engine is far back in the chassis because the tires for which it was designed were hard-rubber-era Dunlop Triangulars. Triangulars were a huge advance when they appeared on factory GP bikes in 1964, but at Daytona in 1972, they and Goodyear’s waffle-pattern round-profile offering, were shredded by the new Suzuki and Kawasaki 750 two-strokes, forcing tire makers to move the slick-tire concept from four wheels to two. Having the engine that far back sacrificed acceleration–how can you steer out of corners if the front wheel has left the room?

Then there’s the frame itself, made of one-inch steel tubing. We knew all along that it was flexing a bunch because it kept cracking in the same places. Why was that? Because when we put grippy slick tires on a chassis in effect designed for 1964, it wound up like a steel spring.

Yamaha coped with that via a much-needed innovation–their soft, long-travel ‘Monoshock’ single-shock rear suspension. Yes, the chassis wound up, but that softer suspension coped fairly well in early laps, absorbing and turning to heat the many upsetting forces delivered by the pavement. It showed the way to something better, even though in retrospect its damping sophistication had much in common with a door closer. You have to start from where you are!

The big TZs were delivered on wire-spoked wheels, but while those wheels were usable, everyone switched to one of the many “children” of Elliott Morris’s 1973 cast magnesium one-piece motorcycle wheels.

TZ750s had disc brakes front and rear, but the discs of that time were bolted solidly to thick aluminum or magnesium carriers. Mind you, disc brakes were a huge step forward–in 1972, my homemade 750 Kawasaki H2-R, with its previous-era Fontana four-leading-shoe double-throwdown drum front brake was losing at least a hundred feet to disc-braked bikes, and that was just into Daytona’s turn one! Onto the scrap heap with that antique braking system, even though it is gleaming with classical beauty. Great thing about race bikes is, there are no style points. Everything actually has to work.

But as riders realized just how much harder they could brake with slick front tires, fresh problems arose. Dump heat into the disc fast enough and its hot outer diameter region expands, dragging its colder inner diameter outward. Do that hard enough, and the colder ID is permanently stretched. When the disc cools, its OD region returns to its original size, but now the ID is too big for it. So the disc assumes a slight cone shape–enough to push the pads back in their caliper bores a bit. So when you went for the lever, at least half of its available stroke was used to flatten the disc. Riders do not like that threat of no brakes, so later-designed brakes gave up bolting the disc to its carrier, and instead adopted a floating connection that allowed the disc to expand freely as it grew hot with brake heat. Button floaters.

Look at those skinny little fork tubes! We thought they were huge in 1974, and Yamaha had made them beefier than they looked by giving them a whacking great 5-mm wall thickness! That made each tube weigh 4 pounds ten ounces (just over 2 kg). A person seeking respect in a bar-fighting situation could do worse than to carry A TZ750 fork tube.

In 1976, Kawasaki’s suspension supplier did their bit for power-to-weight ratio but the resulting lighter, thinner-walled tubes were put to a fierce braking test by Gary Nixon. Gosh, where did all this extra trail come from? He’d bent them back. That sent fork makers back to their stress manuals, where it is revealed that making tubes larger in diameter but thinner-walled is an efficient way to combine stiffness with light weight. Soon, Suzuki factory bikes had 37mm tubes and the race was on; some forks today have 50mm tubes. And along the way, there were games to be played; if the fork is too floppy, chatter appears, but if it’s too stiff, riders say it lacks ‘feel’. So every team carries a veritable forest of fork legs and boxes of fork crowns in small increments of offset. Take two with meals.

What are those big black brake calipers? Surely they’re not…iron? Pull the two mounting bolts and heft one. There’s no mistake, these four-and-a-half pound lumps are iron, same as came on the lightweight RD sportbike. This is improvisation. Riffling through the pages of Handbook of Chemistry and Physics, we find that iron is two to three times stiffer than aluminum. And sure enough, when Yamaha in 1980 ‘improved’ TZ calipers to aluminum, riders hated their springiness, which gave the lever that “You’re about to have no brakes” feeling. The trick stuff was set aside and those great iron blocks went back on TZ750s everywhere.

Since then, a lot of caliper design experience has taken place, so your basic $8500 Brembo MotoGP caliper weighs just one pound and the lever feels fine.

Another traditional feature of the big TZ’s chassis is that it’s a twin-loop design, with structural members (little skinny tubes) passing completely under the engine, which has no structural function as engines do in nearly every modern design. In this respect it dates back to 1950, when the McCandless brothers designed and built Norton’s first twin-loop chassis. Skeptics have called that chassis the “Feather-duster” but with it, the late Geoff Duke could spot the Gilera-4s three cylinders and still beat them. The McCandless idea influenced design for decades.

If you make little drawings of Yamaha racing chassis 1980-1990 into a flip book (I did, just for fun) you can see the twin loop idea morph quickly into today’s Antonio Cobas-inspired twin aluminum beam variety, which has no structure under the engine.

So, if anyone asks you if the Yamaha TZ750 was an influential design, the answer is a vibrant ‘yes’ because in its innovation, in its cost-cutting use of existing technologies, and in the new possibilities its high performance revealed, it provoked designers to cut loose from the past and draw a different future.

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