IS YAMAHA’S BACKWARD ENGINE A CRACKPOT IDEA? No, it isn’t. The Yamaha engineers didn’t come up with this idea out of the blue, and they aren’t doing it just to be different. Way back in 2006, Yamaha tilted the YZ450F cylinder and head back 4 degrees, which moved the weight and rotating mass of the engine about 20mm closer to the bike’s center of gravity. The 4 degrees was almost imperceptible to the human eye, but every rider raved about how much lighter the 2006 bike felt in roll, pitch and yaw (compared to the 2005 model). Yamaha’s engineers were emboldened by the 2006 experiment and got the R&D budget to start over from scratch. Their goal was to minimize the gyroscopic effects of the engine’s rotating parts.
WHAT IS THE SCIENCE BEHIND THE YZ450F’S UNUSUAL LAYOUT? Even though many riders don’t like it and can’t understand it, the scientific theory behind Yamaha’s backwards-engine design is solid. Yamaha isn’t tilting the engine rear-
ward just to be different, but to move a significant amount of the bulk and rotating mass of the drive- train (cams, valves, cam chain and piston) toward the center of the bike. We won’t bore you with class- room lectures on the relationship between reciprocating forces and gyroscopic stability. Suffice it to say that a body in motion, like a crank- shaft, rod, piston, camshaft or timing chain, not only wants to stay in motion, but its sphere of influence spreads across the complete machine. Yamaha’s backwards engine moves the reciprocating mass toward the center of gravity in an effort to harness the gyroscopic effects. For example, as the camshafts spin, they develop their own gyroscopic inertia. And, like any gyroscope, they fight against the forces of gravity to maintain their own stability. You can knock a gyroscope around, but it will always return to its vertical axis. Thus, spinning camshafts, rotating cranks, pump-action valves and reciprocating pistons are moving objects that exert their influence on the overall handling package. As you lean the bike to the right, the camshafts are trying to stop the bike from leaning. That is what spinning objects do. And, that is what Yamaha is trying to mitigate. HOW DOES ROTATING MASS WORK? Here is a simple way to visualize the rotating mass of a four-stroke engine: Think of a figure skater, like Nancy Kerrigan, doing a spin on ice. As she begins to spin, she has her arms outstretched, but when she wants to spin faster, she moves her arms closer to her body’s center of gravity. By holding her arms against her body, Nancy Kerrigan reduces her rotational inertia. If figure skaters can control the speed of their spins by harnessing rotational inertia, then motocross designers can do the same thing. Yamaha’s engineers wanted to get the gyroscopic forces of the cams, crank and piston closer to the center of gravity so that the spinning parts would have fewer negative effects on pitch (rotation around the side-to-side axis), roll (rotation around the front-to- back axis) and yaw (rotation around the vertical axis).