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Yamaha FZ 750 Geneses
John Nutting fails to exercise his constitutional right to remain silent after crimes against the red line aboard Yamaha's FZ750...
The head honchos at Iwata will be the first to admit that Yamaha haven't had a very good decade, so far. Corporate enthusiasm tuned to beating the daylights out of Honda turned sour when it dawned on the Yam execs that Yamahas are not Hondas in the minds of punters. The name Honda is virtually synonymous with motorcycle in most parts of the world and Yamaha found to its cost that to attempt to break the mould is just a recipe for breaking the bank. In a word, Yamaha sales slumped.
The repercussions of the collapse echoed long after the replacement of the company President. Revised plans were initiated to put the company back on its feet; a degree of caution characterised the approach. British importers found that, despite the enthusiasm for the RD500LC and the FJ1100, the men from Iwata were sceptical of sales potential, so model allocations for Britain were low, reflecting the squeeze being applied at all levels, not least in the PR department. The result was that Yamaha caught a little bit of a cold in that sales of these two highly desired bikes didn't reach their full potential.
It's not a mistake the factory intends to repeat with the FZ750. Yamaha claim that the 5-valve in line four is to be a cornerstone of their range for the next ten years. A touch of cynicism might be acceptable here; after all, the bike trade has seen the pace of change accelerating by the second over the last two to three years, to the extent that to dwell on a new model for any length of time is to miss the impact of its successor.
But in an age where 'planned obsolescence' is translated into barnacled cam lobes and anaemic cam chains, Yamaha is keen to impress on us the longevity of this new engine, quoting oil-change intervals of 7,500 miles (!) and valve gear that'll only need the once over at 25,000 miles (!!) It all seems to good to be true, but if it is, we're on the way to the maintenance-free machine.
Like the Suzuki GSXR750, which we've yet to try out, the FZ750 makes a claimed lOObhp. That gives the hike a specific power of 133bhp per litre, putting it above Kawasaki's GPz900R's 125bhp/l, and it is only beaten by Honda's VF500F vee-four's 140bhp/litre.
Without the means of substantiating that claim on a dyno the only way of finding out if Yamaha were good as their word was to give the bike a good thrashing in Portugal.
Result was that it clocked 145mph at 10,800rpm in top gear on a deserted road south of Lisbon. By any reckoning that's pretty good for a 750 and it relates well to the 137mph top speeds accredited to the 90bhp 750's.
Now this top speed may be connected to the bike's remarkably slim profile. Long proponents of the idea that across-the-frame fours should be as narrow as possible, Yamaha has gone a step further with the FZ by exploiting water-cooling so that the pots are closer together and disposing of ignition components on the end of the crankshaft by using the end faces of the outer crank webs as part of the pick ups. Overall width of the motor is just 16.3 inches, less than the factory's own Japan-only air-cooled XJ400 four and about the same as Honda's vee-fours.
The speed may also be something to do with the neat fairing, claimed to offer a drag coefficient of just 0.34. It is certainly to do with power, and what's interesting is that it hasn't been attained by sacrificing flexibility. Though the FZ750 makes its best power over 7,000rpm, storming out of corners with the rear wheel drifting, it still pulls well from a walking pace in sixth.
This is where all the criticism of the complexity of five-valves-per-cylinder hits a block. Ten years ago we had 750cc racing engines putting out 90bhp if they were lucky, and only then with power bands a couple of thou wide valves. Valve springs need to be stronger to keep the cam followers on the cams without damage.
Four-valve layouts have gone a long way to raising the mechanical limits and improving the combustion chambers shapes which ideally should be a simple sphere. Honda first used four valves on its racers of the sixties to lighten the valve gear and raise the rev limits. But combustion was poor because of the awkWardly shaped piston crowns.
Yamaha claims to have tried to obtain the perfect combustion chamber with a seven valve arrangement that offered 20,000rpm limits. But, as you'd expect, fitting this number into a small combustion chamber proved too expensive for a production machine and eventually after trying six valves, five valves offered the best compromise.
The benefits are broad. By laying out the three inlet valves so that the middle one is out of line, and at a different angle from the cylinder axis, but still operated from the camshaft by a bucket follower like the other two, the combustion chamber shape needn't be complex. Indeed, Yamaha claims an 11.2 to 1 compression ratio which is remarkably high and shows that the combination of a slightly dished piston crown and centrally placed spark plug in a symmetrical chamber is the way to go.
Three small valves can flow more air than two and in the FZ's case they need less lift and opening timings to achieve it; that's the reason why the motor is more flexible than you'd expect. The three valves are individually lighter, so the point at which they start to float is higher up the rev range. Yamaha placed the red-line of the FZ at 1 l,000rpm with a rev limiter operating at 1 l,800rpm.
Because the valves are lighter, they need less strong valve springs so not only is the power absorbed by the valve gear lower, the camforms needn't be so radical. Wear is reduced as a result.
Straight, uninterrupted inlet ports have always been the goal for obtaining high power from four-strokes. Trouble is they're difficult to fit into bikes without getting in the way of the rider. But at the suggestion of the race team during the FZ's early development the engine was laid forward so that the air box was where the front of the fuel tank would have been. This meant that both the inlet ports would be straight and the spacious airbox got large helpings of cool air.
Yamaha's engineers say that the first prototype engine developed 130bhp at 13,000rpm, a not unlikely figure because they're claiming 330bhp at 13,000 for their two-litre vee-six car racing engine, and the production lump needed detuning for the road.
On the road, the FZ is smoother than most straight fours and as expected the hydraulic clutch and six speed box are slick, though at the expense of slack in the drivetrain at low speeds.
It's also very low with a sporting riding stance that tucks the rider behind the screen sufficiently well to keep his helmet out of the breeze. Steering is light and vice free despite the use of a 16-inch front wheel, a remarkably steep Steering Head Angle of 64.5 degrees and short trail of 97mm. This is possible because, says Yamaha, pushing the engine-forward puts more weight onto the front wheel -almost 50 percent with the bike unladen.
For the scratchers, the news is that there's so much cornering clearance that they'll be in the gutter before anything hard drags despite the use of stickyish Dunlop Japanese tyres in fat 120/80V16and 130/80V18 sizes. The rear is an 18-incher so that racers can fit the more widely available slicks in this size.
Without doubt, Yamaha is hoping that this bike will form the basis of racers for the world endurance series and TT Formula events. With this in mind there's a tuning kit on the way that'll bring the power back to around 130bhp, making it more than competitive. In almost every respect, the bike's a detuned racer, but, like the RD500LC, the quality of the engine and running gear is such that it offers a refined package for the road.
Source Which Bike 1985