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 |   |  Yamaha Cross Plane Crank - Parts & Labor
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Yamaha Cross Plane Crank - Parts & Labor

zero, 90, 180, and 270 degrees

May 1, 2009
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When Push Comes To Shove
It's the summer of '83; heat waves dance across the track in front of me like waves of pure energy flowing across the blacktop. It has been a long couple of hours for the whole team. An errant shoelace from another team's Airwalks had wrapped around our right rear axle and the time in the pits for repair had chewed up all of the early lead we worked so hard to build. I've been in the hot seat for far too long, but I'm double stinting it to the checkered flag. There are only two of us on the lead lap, me on the Transformers Big Wheel and the big redheaded kid on the more high-tech Green Machine. The team knee-riding the plastic skateboard looked strong in the opening laps, but a crash involving the Krazy Kar and a neighbor's collie sent their ride into the sewer drain and its recovery had taken them out of contention. In the closing laps the Green Machine had made up a lot of ground with a fresh rider. He just got on, and topped up with Tang and Smarties he's clearly in better shape for the last few laps. I just have to hang on and not make a mistake.

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There are only a few minutes left in the 2.4 Heures du Cul-de-Sac, but those last few laps were all that counted. I had gotten a good run off Dog Dish and coming into Broken Vacuum Cleaner I was carrying more speed than usual. The back end of the Big Wheel steps out violently mid-corner; frantic counter-steer, scraping plastic, and I gather it back up just at the curb, but I've lost precious speed. The Green Machine is now right on my three-position-adjustable seat back. The sound of his hollow plastic front wheel thunders behind my head in a deafening roar. I have to hold him off for just a few more turns. Going into Garden Chair, the Green Machine misses his braking marker. A hollow thud echoes across the cul-de-sac as he plows into my Big Wheel and we both run wide. Struggling to gather it up, my big rear racing slicks scream across the bright gray concrete of the gutter with him on my inside. We glare at each other, side by side for what will come down to a drag race. I kick at the pedals, hunting for forward progress. My sudden charge of adrenaline overpowers the front wheel, leaving me waiting for the momentum of the wheel to bring the other pedal back up into the wheelhouse of my powerstroke. It comes around and with the pedal near top dead center I hit it again. The front wheel breaks loose again. I just can't get the torque to the ground smoothly. The Green Machine edges ahead of me at the line. He gets to spray the bottle of Orange Crush, to hoist the scavenged bowling trophy skyward. I let my aggression get away from me, but it was a learning experience.

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The engineers at Yamaha have obviously felt the same sort of pain. Their motorcycle GP program has been experimenting with engines that can put out power in a more linear fashion. Reciprocating piston engines have a problem with smooth power delivery. It may feel smooth to the average driver, but if we were to look at the frequencies involved in power delivery they would tell a different story. It's most easily demonstrated on a typical inline four-cylinder. The pistons travel in pairs of twos. Engineers do this to keep the rotating masses balanced. If you always have two pistons going up and two pistons going down at the same time, everything stays balanced, rotationally at least. But when it comes to power delivery, just talking about the torque being delivered from the combustion events isn't the whole picture. We must also look at the inertial torque from the mass of the pistons and rods. At the top and bottom of the stroke, the piston and rod inertia has very little torque input on the crank; however, at the two sides, 90 degrees and 270 degrees, the torque is maximized being maximum distance from the axis or rotation (the center of the crank). This causes waves of torque to be transmitted into the driveline, like my legs only being able to push on the pedals at certain angles on the Big Wheel

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What Yamaha has done is create what's called a cross plane crank. A traditional four banger has what's called a flat crank. It has two rod journals at zero degrees and two rod journals at 180 degrees. The new Yamaha Crank has one at zero, 90, 180, and 270 degrees. This crank constantly has a piston producing torque on the crank. It not only solves problems with inertial torque, but also allows the engine to produce torque in a more consistent manner from combustion events.

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So what's the downside of a cross-plane crank? Well, they tend to produce some wicked second-order vibrations. These can be cured with balance shafts or even isolated with cleverly designed engine mounts. However, right now they are only being employed on high-performance engines where it doesn't really matter. But if you've ever wondered why a Ferrari sounds like a like Ferrari, it's because they are using this technology in their V8s-but that's a whole different column.

Michael Febbo
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