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Late ... but great? Kawasaki joins the four-stroke starting gate with the 2006 KX450F

Story by James Holter, photos by Kinney Jones

Kawasaki is the last major manufacturer to throw its hat into the ring of open-class four-stroke motocrossers, getting beat by Suzuki’s well-received RMZ450 by one year. However, the take-time-to-get-it-right approach worked for Suzuki, and if first impressions are any indication, it worked for Kawasaki, too.

Rolled out at the Competitive Edge riding area in Hesperia, California, Kawasaki picked the perfect environment for introducing its biggest entry into the motocross wars—open, fast and loamy.

2006 Kawasaki KX450F Engine Liquid-cooled, four-stroke, DOHC 4-valve single Displacement 449cc Bore x stroke 96.0mm x 62.1mm Compression ratio 12.0:1 Carburetor Keihin FCR40 Transmission 4-speed Rake/Trail 27.1 degrees/4.6 inches Front tire 90/100-21 Rear tire 120/80-19 Front suspension 48mm inverted Kayaba AOS fork 22-way compression damping adjustment 20-way rebound damping adjustment 12.4 inches of travel Rear suspension Adjustable preload 22-way (low-speed) compression damping adjustment Stepless (high-speed) compression damping adjustment 22-way rebound damping adjustment Temperature-compensating rebound adjustment 12.4 inches of travel Wheelbase 58.5 inches Front brake Semi-floating 250mm disc Rear brake 240mm disc Fuel capacity 1.9 gallons Ground clearance 13.6 inches Seat height 38.0 inches Dry weight 220 pounds

Indeed, the new open class is all about power—not just how much, but how it’s delivered. Kawasaki engineers didn’t forget that when they designed the powerplant for the 2006 KX450F. The green beast definitely rips through the sandy clay mix of the Competitive Edge track, but it’s more balanced than brutal. Unlike some of the open-class two-strokes that are quickly becoming the stuff of motocross lore, just because you can rip your arms out of their sockets, you don’t have to.

The liquid-cooled, 449cc four-stroke engine features double overhead cams, four valves and a Keihin FCR40 carburetor. The hand-finished intake ports feed the single cylinder through titanium valves and aluminum valve retainers. Double valve springs are used to provide stable operation at high RPMs and reduce the necessary length of the valve springs, which tightens up the size of the head.

Starting is a breeze with an automatic decompression system. Indeed, the only four-stroke that starts easier than the 2006 KX450F might be the 2006 KX250F.

The engine feeds a four-speed transmission that is all you need on a motocross track. Competitive Edge is a fast, flowing combination of old-school motocross and modern jumps thrown in to keep things interesting. It’s a good mix, and the KX450F could handle it all in third gear.

And a lot of that has to do with the big green bike’s low-end grunt. This bike pulls hard from down low and—almost—lets you forget about the clutch. It’s a very torquey engine.

Just like the KX250F, the bigger KX has a twin-spar aluminum frame. As with other manufacturers, Kawasaki uses a combination of forged, extruded and cast aluminum parts in an effort to balance side-to-side and front-to-back rigidity. Even engine placement was thought through—designed to enhance handling and reduce rear-end squat under acceleration.

Handling is definitely another strong point of the KX. Even in the deep loam of Competitive Edge, it was easy to change lines in mid turn, and the bike leaves a lot of room for rider error if you over- or understeer.

While the smaller green thumper is the first KX to run Showa suspension, the bigger motocrosser sticks with Kawasaki’s traditional use of Kayaba parts. The front end runs Kayaba’s AOS (air-oil-separate) forks that are designed to reduce damping fade, while the tapered aluminum swingarm is held up by a new Kayaba shock with enhanced oil flow properties.

Both ends worked well on hard landings and tracked straight through the rough chop that developed by mid-morning down the sandy straights.

Motocrossers have been treated to steady improvement of component quality on all the major factory motocrossers, and the new KX450F is no exception. The bike comes from the factory with Renthal aluminum handlebars, petal-shaped disc brakes, a slip-resistant seat cover, titanium footpegs and footpeg brackets, and an aluminum skid plate.

If the stock bike isn’t exactly what you want, Kawasaki offers a number of optional parts for the KX450F, including a top triple clamp for an oversized handlebar, aluminum and steel rear sprockets, solid petal disc brake rotors, different fork and shock springs, and a 20-inch front wheel.

The 2006 KX450F is a purpose-built motocross machine that gives motocrossers the transmission, motor and suspension they need to win. The 450Fs may not have the versatility of the 250 two-strokes or the 250Fs for all-around riding, but when the straights are long, the jumps are big, the whoops are deep and the line is stacked, it’s difficult to imagine a better weapon.

Kawasaki brought together four generations of motocross riders to help introduce the KX450F. From left: Jimmy "The Jammer" Weinert, Jeff Emig, Mike Byrne and Ryan Villopoto.	The KX450F uses dual valve springs to keep the valve stems shorter and the head more compact.