Yamaha's 2010 YZ450F could be a watershed design in the history of motocross bikes. Among its many unique design features, the engine's cylinder slopes rearwards, not forward, the inlet port is at the front, not the back, and the exhaust exits from the rear of the cylinder, rather than from the front.
By now you are probably thinking, "What the….", but the reasons behind this radical change all make good sense.
Motorcycle designers have long known that the route to better handing lies in mass centralisation, but using conventional packaging you can only take this approach so far. And, according to Yamaha, mass centralisation via conventional component layout had reached the end of the line.
The heaviest part of a motorcycle is its engine and the placement of the engine is therefore critical to handling. Yamaha's decision to reverse the cylinder head and slope the cylinder backwards rather than forwards brings several important benefits.
First up the relatively light airbox can be located where you would usually mount the far heavier fuel tank. The inlet tract can also be straightened up (important for cylinder filling efficiency), while sloping the cylinder rearwards means the cylinder head (always a heavy component on a four-stroke machine) is now moved closer to the centre of the motorcycle.
Other benefits of the forward-mounted intake include cooler intake air because it is unheated by the engine, and cleaner air, because the dirt and dust kicked up by the rear tyre is further away from the intake system.
The new YZ450F also uses fuel injection, which is more compact than a carburettor, and gives the designers even more freedom with component layout.
These changes mean that a newly designed 'Bilateral Beam' aluminium frame could be adopted. The frame is made up of 16 parts welded together into a single unit, the key element of which is a head pipe made by using a casting method in which the aluminium is shaped while still in a semi-solidified state.
Due to the adoption of the front mounted fuel injection system, the rear spring/damper strut has been positioned in line with the machine's central axis. Other new design features include a new swingarm, longer stroke front suspension and an under-seat plastic-resin fuel tank.
Meanwhile, Kawasaki has announced changes to its own 2010 open-class motocrosser, the KX450F. Engine updates include the adoption of a new shorter skirt piston that's both lighter and stronger than that used before and a new crankshaft that has a higher inertial mass than the '09 model. The intake cam timing has also been revised while the new one-piece stainless steel exhaust pipe is 40mm shorter and helps provide better mid range and top end performance. Chassis changes include revised spring and damping rates for the front forks, different linkage ratios for the rear suspension and a new swingarm.
Another 2010 model that's due later in the year is Suzuki's competition-specific enduro racer, the RMX450Z. It shares the DNA of Suzuki's RM-Z450 open-class motocrosser but with a number of changes including a modified inlet tract and revised cam profiles that enhance its low and mid range power and better suit its enduro role.
To also meet enduro demands, the RMX450Z employs an extra coolant reservoir tank while wider ratios in the five speed transmission improve the bike's bush versatility. There's also an electric starter (compete with a larger magneto to charge the battery), an important convenience item.