Just 12 years ago Yamaha dropped a bomb on the motocross world: Not only would the company release a four-stroke motocrosser to the public, it would laugh at what was considered to be four-stroke doctrine at the time. For those of you too young to remember, four-stroke motocross machinery-when there was any-came from Europe or was handmade by specialty shops, and the rules allowed four-strokes up to 550cc to race in the 250cc class against two-strokes. Yamaha opted for a five-valve 400, and the public couldn't imagine how a four-stroke that small could compete. Yamaha proved everyone wrong, and started the 250cc four-stroke revolution. Since then, though, Yamaha hasn't set the world on fire, but now the company appears ready to attempt a similar lesson in 2010.
Without reinventing internal combustion, the '10 couldn't be a more radical departure from existing Yamaha motocross technology and practice. The wheels and one brake are carryovers from the existing YZ line. All other parts of the 2010 YZ450F are new and redesigned. As it did with the original YZ400F, Yamaha chose not to bow to convention. The 2010 chassis and engine were designed together for what Yamaha calls an "ideal rigidity balance." This chassis is made up of cast, forged and extruded parts for a total of 16 components formed into what Yamaha has termed a "double S-beam" design. To us mortals it means that the side spars have S-curves when looking at them from the side and also when looking down from the top. The open spars at the top of the frame look narrower near the steering head than examples from other brands. Up front the Kayaba fork boasts 10mm more stroke, and the triple clamp offset has shrunk from 25mm to 22mm. Thankfully, the four-position handlebar clamps return to fine-tune the riding position.
A right-side cutaway of the...
A right-side cutaway of the 2010 Yamaha YZ450F makes it clear just how unique the bike is. Note the straight shot that the fuel injection has at the intake valve, how protected the air filter and airbox intake is from dust thrown by the rear wheel and how centered the mass of the fuel is located in the chassis.
The body parts and engine look more exotic since the air intake and airbox are at the front of the frame with the filter unit being under the fuel tank in a direct line below the gas cap. Though the intake location is designated by the EFI and the desired performance, the bonus will be the clean air reaching the filter. Much of the dust a normal filter location must deal with comes from the bike's rear wheel. The YZ will draw relatively clear air from near the steering head. Yamaha leaned the top-end of the engine back slightly, and the tunable 44mm Keihin throttle body fuel injection gets a straight shot to the now four-valve cylinder head. Yamaha claimed that with FI, two intake valves worked better than the three it has used for 12 years. In addition to the new valve layout, Yamaha went with lightweight, hollow cams with more radical ramps and lift numbers.
Yamaha did much more than lean the cylinder and head back; the engine is all-new with a shorter stroke, larger bore and the crank set deeper in the engine cases so the portion of the cylinder that shows looks like it is 20 percent shorter than the 2009 cylinder. In addition to being set deeper, the crank is offset to the rear of the cylinder. This is common in street engines, and it both saves on friction loss and minimizes side loading on the piston against the cylinder wall.
With the induction in the front, that means the exhaust pipe must exit to the rear, and what a pipe it is. To gain tuned length, what Yamaha calls a "tornado style" pipe makes a twisting loop in the center of the frame. With no conventional airbox to deal with, the shock is angled forward and centered in the frame. As you can imagine, adjusting shock spring preload is a snap. That shock uses a hammerhead-style reservoir cast to shock body sideways. Additionally, fuel is carried closer to the center of the chassis in a plastic tank that is subtly translucent, so you can judge fuel levels externally. Like other FI models, the tank has an ignition-powered (no battery) fuel pump mounted in the bottom.
We haven't ridden the bike, but the ergonomics should be much like the YZ250F, which we have ridden, and the bike should be very easy for the rider to move around front-to-back on. For sure the YZ450F will have more visual "Wow!" than any bike at the track, and that will hold true of the standard blue model or the optional white-shcemed bike with black rims. An EFI tuning tool that requires no laptop and a variety of accessories will be available. We can't wait to see if it feels as good as it looks.
 Here is a better look at the...  Here is a better look at the tornado. The bolt-on flange should resist air and exhaust leaks. The pipe has two junctions in the header area to ease removal. |  The fuel tank is a light gray...  The fuel tank is a light gray plastic with the fuel pump mounted in the bottom. You can see the fuel level from the outside of the tank. |  |
 The frame is a relatively...  The frame is a relatively radical departure for Yamaha, and the slim, double-curve spars make it unique among other current brands. The frame and engine were designed as a package to gain the flow character Yamaha wanted. |  After years sticking with...  After years sticking with five-valve designs for the cylinder head, Yamaha is moving to a four-valve arrangement. It claims that two intakes work better with the EFI. |  There is no mistaking the...  There is no mistaking the difference in the cylinders from the old bike to the new. |
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 |  With no airbox to avoid at...  With no airbox to avoid at the rear of the engine, Yamaha was able to center the shock in the chassis and lean the top mount forward to get the mass of the shock lower in the bike. |  From the left side the engine...  From the left side the engine looks conventional aside from the canted back cylinder and the large ignition to provide power for the EFI. |