Article: 20090301069

Title: Candid Cameron

20090301069
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Candid Cameron
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Cycle World
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Q The image of the new YZF-R1 ’s “crossplane” crankshaft (“2009 Yamaha YZF-R1 : More MotoGP” December, 2008) shows that numbers 2 and 3 crank throws are 180 degrees apart, but the text in that story claims otherwise (“Crossplane crankshaft places each connecting rod throw 90 degrees from the next...”).
Kevin Cameron
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Candid Cameron

Q The image of the new YZF-R1 ’s “crossplane” crankshaft (“2009 Yamaha YZF-R1 : More MotoGP” December, 2008) shows that numbers 2 and 3 crank throws are 180 degrees apart, but the text in that story claims otherwise (“Crossplane crankshaft places each connecting rod throw 90 degrees from the next...”). True, each is 90 degrees from something, but not from each other. I also am somewhat perplexed and fascinated by this crank timing, as I see the engine lighting off as “fire, fire, pause, fire, fire.” Better put, two 270-degree cranks set 180 degrees apart. Gene Jardel Pine Beach, New Jersey

A The intention of the quote you reference was to say that the throws are at 90-degree intervals, but not in the consecutive left-to-right order of the cylinders. The confusion stems from the “90 degrees from the next” statement, which, though lifted from Yamaha’s press materials, should have been better clarified in the story.

When it comes to the crossplane crank’s effect on engine performance, there may be two separate concepts at issue here.

1) The Big-Bang idea: When a 500 GP two-stroke exited a corner, the rider could not smoothly feed throttle because dilution of the fresh charge by leftover exhaust gas made the engine unable to fire until quite a bit of throttle opening. This made power come in too suddenly to be used at high lean angle, so riders would lift the bike up by keeping themselves hung far off on the inside and then, using the “meat” of the tire thus presented to the road, would wind on appropriate throttle.

Because of this, throttle application had to be quite substantial. Big bang, though, made use of the fact that static friction is greater than sliding friction. Honda’s original banger of 1992 fired two cylinders together, then waited 67-68 degrees and fired the second pair. There was then a relatively long delay before the first pair fired again. In lower gears, this long interval allowed the tire to lay down most of a fresh contact patch, free of engine torque. Once this was done, the next clustered firing could exploit its grip.

2) The Inertia Torque idea: When a MotoGP four-stroke exits a corner, its rider can apply throttle very gradually, even at high lean angles. This is because initial power is smooth and proportional, rather than having the two-stroke’s rough transitions from eight-stroking to four-stroking and from four-stroking to smooth and continuous firing. For this reason, very little throttle is being applied during the most tractioncritical part of a four-stroke’s corner exit.

Masao Furusawa, Yamaha’s MotoGP engineer, saw that in an inline four-cylinder

engine with a conventional “flat” (180-degree) crank, all pistons are stopped together twice per revolution. This is a lot of mass to accelerate and decelerate through as much as 100-mph peak velocity twice per revolution, and the result is an rpm flutter imposed on the crank as it exchanges energy with the pistons. Furusawa calls this effect of piston inertia the inertia torque on the crank. He theorizes (no one has published any measurements) that this crank rpm flutter either interferes directly with the tire’s ability to grip or confuses the rider’s ability to sense what the tire is doing.

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Many years ago, Vincent engineer Phil Irving proposed that parallel-Twins be built with their two crankpins offset by 76 degrees. In this way, one piston would be stopped while the other was at peak velocity. Instead of the pistons exchanging energy with the crankshaft, they would exchange it with each other, thereby imposing no rpm flutter on the crank. All that Furusawa has done with the crossplane crank is to space the crankpins at 90 degrees instead of 180, thereby achieving the result proposed by Phil Irving so many years ago.

What is really strange is that during all the years that Honda employed 90-degree firing on its NSR500s, Yamaha riders were better able than they to apply early throttle in corners. When Hondas in 1990 adopted 180degree firing, their off-corner acceleration improved considerably. After much detailed research, Honda in 1992 changed again-to the 67-68-degree Big Bang firing scheme. This gave their riders the ability to begin throttle-up even earlier in corners than could the Yamaha men. -Kevin Cameron

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