Though the new Yamaha is called a YFZ, its specially designed bottom end is actually closer to that found on Yamaha’s WR450F off-road (as opposed to motocross) motorcycle, which is a five-speed with electric start. The YZ450F motocross motorcycle is a kick-start-only four-speed. As compared to the WR bottom end, Yamaha engineers made the gears in the quad wider (for durability) and redesigned the cases to accommodate those gears. The crank diameter was reduced and the stroke is shorter. Many other components on the quad motor are straight from the YZ motorcycle.
The liquid-cooled, DOHC single has five titanium valves — a first in ATVing. For good top-end response, the intake cam is from the YZ motorcycle, and for the strong torque a quad needs, the exhaust cam is from the WR motorcycle.
According to Yamaha, it’s the best engine for racing. It will likely be faster than any current production ATV motor, regardless of displacement.
The motor can be bored, but — because the walls of the lightweight, solid-aluminum cylinder (borrowed from the YZ motorcycle) are ceramic composite coated — boring is a little more difficult and time consuming. Even at 439cc, though, the motor should be competitive and a total screamer.
Yamaha engineers based the piston on the unit from the YZ motorcycle engine, but the wrist pin diameter was increased. The minimal piston skirt reduces friction, saves weight and allows faster revving, according to Yamaha.
At 11.9:1, the YZF's compression ratio is less than both the YZ450F and WR450F engines. Yamaha engineers explained that the YFZ has a much larger contact patch to the ground than a single skinny bike tire and that a lower compression ratio was required for the engine — any more compression, and the engine would need race gas. Yamaha says that it might produce a GYT-R kit to increase compression ratios for racing. In stock form, the YFZ450 needs premium pump gas to avoid detonating.
Dialing in a machine for smooth response throughout the rpm range is a tricky proposition. For carburetion, Yamaha engineers used a 39mm FCR carb from the YZ and WR motorcycles. According to Yamaha, it’s about three times as costly as the more conventional CV carb, but its quicker response is worth the additional cost. FCR carbs are often used by Pro ATV racers to improve low-end response and create a lighter throttle pull.
An over-sized foam air filter provides plenty of clean air to the 450’s carb. The airbox uses a no-tools design, and is not held in by an airbox lid. Yamaha says with the lid removed, a properly jetted carb and its GYT-R performance exhaust, a 15 to 20 percent boost in performance can be expected.
We didn’t ride this quad, yet, but we hope that the 450’s gear ratios are better spaced than those on the first Yamaha Raptor. According to Yamaha, the ratios on the YFZ are similar to those on the motorcycle. These ratios allow for running a 37-tooth rear sprocket and increasing ground clearance. Yamaha engineers — wanting to avoid similar gearing problems it faced with the Raptor’s tranny — designed the YFZ450 quad with wider and stronger gears. Clutch steels are also thicker than those on the motorcycle.
The 10.1-inch rear-wheel travel is higher than on most sport quads and the 9.1 inches up front is average. More wheel travel is possible with wider A-arms which are already an available accessory from Yamaha. See the sidebar “Make It Faster, Dress It Up” for other Yamaha (GYT-R) factory accessories for the YFZ450.
Yamaha says the YFZ’s low-speed ride is more firm than on a Raptor or Suzuki Z400, and maybe a little plusher than on a Polaris Predator. However, the suspension, at least with the stock settings, is more plush than on most race quads. Yamaha says steering is precise and predictable, maybe a little quicker than on a Raptor. See the Barry Hawk sidebar (pg. 39) for a more detailed ride report.
The frame is mild steel rather than chromoly, but the geometry mimics the most successful aftermarket frames and all the rails are round tube. Square frame tubes are easier to weld but not typically as strong, pound for pound, according to Yamaha.
The 450’s steel, gull-wing-type lower A-arms — as found on a few aftermarket brands — allow longer shock travel, and, like the rear linkage, don’t overwork the shock. To reduce weight, the upper arms are aluminum.
Though the lack of bump steer isn’t the end-all, be-all of steering performance, none was visible when compressing and extending the front suspension by hand.
We were a little surprised, though, that the front end doesn’t rake back a little more. By tilting the front of the frame upward, the A-arms deflect back slightly when the suspension compresses. We expect this design to soak up the hits better and reduce unwanted feedback through the bars, but its steering won’t be as precise. The final verdict will be our first ride test and you’ll read all about it in the October issue.
Overall dimensions of the YFZ are neither the largest nor smallest among sport quads. The wheelbase is longer than all sport quads except the Polaris Predator and Bombardier DS650, an indication the 450 is stable at high speeds and carves precise turns. Overall width (with the stock A-arms and axle) is comparable to the Z400 and Cannondale. The seat height is comparable to the Banshee, so the machine shouldn’t ride as tall as the Raptor.
Yamaha engineers said the pivot point of the cast aluminum swingarm is positioned to reduce squatting under acceleration. When the rear suspension compresses under power and you’re going through deep whoops, the ride gets bumpy and traction can suffer. The YFZ’s aluminum subframe is cast under Yamaha’s exclusive vacuum process, which reduces air bubbles and allows for a thinner-yet-stronger component.
Yamaha engineers tried to equal the feel and stopping power of YZ motorcycle brakes. They designed dual-piston calipers specifically for the YFZ quad. The rear brakes are borrowed from a Raptor. The front brake lever is also from the YZ motorcycle and it features an adjustment for lever-to-grip distance. The clutch lever features a large thumbwheel at the perch to allow clutch freeplay adjustment on the fly.
Tires, front and rear, are Dunlop radials, and although the fronts have a tread pattern similar to the Raptor’s, the sidewalls are stiffer to reduce flex in hard cornering and give more precise feedback through the handlebars, according to the engineers. The rear radial is an all-new design, built specifically for the YFZ.
What does it weigh? Yamaha’s specs say the YFZ is the lightest sport machine — by a substantial margin. At 350 pounds, dry, it’s 20 pounds lighter than any other full-size sport quad. By comparison, current Pro Production race quads weigh roughly 375 pounds.
We picked up the front end and it felt lightweight. The rear end is biased only about two percent heavier than the front, according to Yamaha. Of course, swingarm length, wheelbase, available traction and power characteristics are also important factors in determining how light the front is when powering out of a turn. Credits Given to www.Atvnews.com