2010 Yamaha YZ450F - Final Answer?

A backwards cylinder is just the beginning

By Brian Catterson, Photography by Dirt Rider Magazine, Karel Kramer

Interacting with Japanese engineers is always entertaining, but this press dinner was getting more comical by the moment. After discussing the various and sundry details that make the 2010 Yamaha YZ450F revolutionary, the conversation inexplicably turned to television game shows. We all agreed that the original Japanese Takeshi's Castle was way better than the American rip-off Wipeout, and that MXC is better than both. Then one of the engineers mentioned Who Wants to be a Millionaire? "Is that your final answer?" we all said in unison, bridging the language barrier. Which got me thinking...

American bicycle giant Cannondale made its brief foray into the motorcycle market a few years ago with a line of dirtbikes that had one revolutionary feature: a backwards cylinder head. Air entered through a filter behind the front numberplate, and was routed around the steering stem to the intake tract, where it mixed with fuel via another then-revolutionary feature: fuel injection.

After setting the sag, moving the bars forward and adjusting the levers, I was ready to hit the track-hopefully not literally. Pulling the cold-start button under the front of the gas tank, I kicked and kicked and...nothing. Unlike the latest Honda and Kawasaki, on the Yamaha you absolutely must find top dead center first. Do that and it starts right up. Forget and you'll go a lap down before rejoining the race.

Lift the YZ off its stand, roll it around the pits, and it feels about as heavy as any other 450cc 'crosser. But underway, all that weight seems to disappear, centralized between your calves. This helps make the bike stable at speed, maneuverable in corners and neutral in the air. Even on a track rough from two days of riding, the YZ never threatened to swap. Braking over stutter-bumps was especially good, with no undue jarring through the bars-credit the new, longer-travel fork, tapered handlebar and dual-S-bend bilateral-beam frame. Yamaha has been getting its suspension settings right for years and the 450 is no exception.

The 2010 Yamaha YZ450F also has fuel injection and a backwards cylinder head, with air entering through ducts in the radiator shrouds. The airbox is located above the engine, while the gas tank slots in between the airbox and seat. A serpentine Tornado exhaust (which wasn't named after MotoGP racer Colin Edwards, never mind what he claims in the promotional video) exits out the back, spiraling where the airbox would normally be to give it the necessary length. A heat shield keeps the pipe from roasting the rider's rump, and there's no worry about the piggyback shock heating up since it's farther from the pipe than on the '09 model. Eliminating the old rubber boot connecting the airbox to the carburetor let the shock be centrally located instead of slid over to one side. And because the pipe no longer exits the front of the cylinder, the right-side radiator could be repositioned lower, at the same height as the left.

That's just half the story, however. Contrary to current practice, said cylinder is also tilted backward, away from the front wheel. Much has been made of a crankshaft's rotational inertia, and its influence on handling. But until now, no one has considered the rotational inertia of the camshafts, spinning high above the crank at half speed. By tilting the cylinder back, that inertia moves closer to the bike's center of gravity, where it's least likely to affect handling. It's the same sort of thinking that led to the award-winning Husaberg FE450's "upside-down" engine, which positions its crankshaft above its gearbox with its cylinder jutting forward almost horizontally.

The brakes are excellent, too. I briefly suspected the front felt too sensitive ... until I realized the problem area was a slick, downhill entry to an off-camber corner. What brake wouldn't feel touchy there?

The new "backwards" engine is every bit as good as the chassis. Unlike past five-valve YZs, the new four-valver is especially strong off the bottom. Throttle response is instantaneous-almost too much so. It takes a steady throttle hand and deft clutch work to get the most out of this motor. You've got to take care rolling it on exiting corners or you'll find yourself getting behind the bike-hopefully not literally. The wet, deeply rutted corners at Budds Creek proved especially challenging as the YZ tended to dig in and wheelie toward the inside of the turn.

Final answer? Or just a nod to Cannondale's and Husaberg's ingenuity?

The engineers didn't stop there, however. Yamaha's big-bore YZs have traditionally employed five valves-three intake and two exhaust. But the 2010 model has an all-new cylinder head with four titanium valves, boosting low- to mid-range power. Cylinder dimensions are more oversquare, with a 2mm bigger bore and 2.6mm shorter stroke. The connecting rod length was reduced by .5mm, helping to make the cylinder 15mm shorter. And the cylinder was repositioned slightly forward of the crankshaft centerline, so the connecting rod is vertical at the point of greatest combustion force. Clever.

Living up to its hype, the 2010 YZ450F truly is revolutionary. With James Stewart in the saddle, the competition has every reason to worry.

To sample their latest creation, Yamaha invited the moto-press to Budds Creek in Maryland. Former home of the Motocross des Nations and the United States Grand Prix, and host this year to the wettest outdoor national on record, the track undulates in and out of a small valley with some of the steepest hills and craziest off-cambers of any MX track.

tech
Spec
Evolution

Latest and greatest iteration of the YZ400/426/450 line, with its cylinder spun 'round backwards.

Rivals
Honda CRF450R, Husqvarna TC450, Kawasaki KX450F, KTM 450SX, Suzuki RM-Z450.

TECH SPEC
Price $8090 (as tested)
Engine type l-c single
Valve train DOHC, 4v
Displacement 449cc
Bore x stroke 97.0 x 60.8mm
Compression 12.5:1
Fuel system EFI
Clutch Wet, multi-plate
Transmission 5-speed
Claimed horsepower na
Claimed torque na
Frame Aluminum perimeter
Front suspension 48mm Kayaba inverted fork with adjustable compression and rebound damping
Rear suspension Kayaba shock with adjustable spring preload, high/low-speed compression and rebound damping
Front brake Nissin two-piston caliper, 250mm disc
Rear brake Nissin single-piston caliper, 245mm disc
Front tire 80/100-21 Dunlop D742F
Rear tire 120/80-19 Dunlop D756
Rake/trail 26.9 deg./4.7 in.
Seat Height 39.3 in.
Wheelbase 58.7 in.
Fuel Capacity 1.6 gal
Claimed wet weight 245 lbs.
Colors Team Yamaha blue/white, red/white
Available Now
Warranty 30 days

Contact
Yamaha Motor Corp. USA
6555 Katella Ave.
Cypress, CA 90630
800.962.7926
www.yamaha-motor.com

Verdict 4.5 stars out of 5
The biggest breakthrough in motocross since the 1998 YZ400F that ushered in the Four-Stroke Revolution.

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By Brian Catterson
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