First Test: 2005 Yamaha YZ450F

Don't count out the YZ450F in the onslaught of four-stroke newness from every manufacturer. Although it doesn't have an all-new engine or an aluminum frame, Yamaha's flagship wasn't overlooked. The company's Product Information Guide (PIG) details every nut and bolt that differs from the previous year and lists the benefits, and the '05 YZ450F's PIG was as thick as the YZ125's and 250's. Yamaha wants you to know about the changes. Otherwise, if your eyes skim over the fork, you wouldn't strain them to take a second, closer look, but even if you did, you wouldn't notice much on the outside. The 450 didn't get the wholesale makeover the YZ125 and YZ250 got for '05, but if you are looking for a four-stroke 450 motocrosser, what matters is how it works.

We took the big YZ out to sandy and hard-packed RaceTown 395 in Adelanto, California, to see if we could feel the internal changes. We expected the bike would be on its best behavior as Yamaha does R&D testing at the track. Right off the bat the YZ revealed one of its secrets: It feels lighter. Not to a scale, which will register 1 pound less than last year, but to the rider. Somehow, the new engine character and suspension add to that light feeling.

A second striking difference is the bike's smoothed-out power delivery. Yamaha added a little vigor to the very beginning of the power spread by reducing the volume of the intake tract and improving the flow from the airbox. The combustion chamber is reshaped to be a bit smoother, thus reducing the compression ratio from 12.5:1 to 12.3:1. Slight carburetor tinkering complements this change, which results in a more-efficient fuel-air mix. The finicky needle/pilot jet/fuel screw setting that YZ four-strokes have always seemed to require is tamed. The improvements allow a lower start of power and thus the smoother overall spread. The revs build smoothly and consistently but still as quickly as ever despite a little more flywheel effect. The motor warns you more than in the past if you're expecting too much from it, such as when you're a gear high in a turn. Now it gives the rider time to get on the clutch before it stalls. Let the motor sing, and the Yamaha pulls as long and hard as ever without hitting the rev-limiter as abruptly. Its long pull finally signs off—telling you to shift—before that happens.

And these power characteristics are a good thing, too, as the YZ must work through a four-speed transmission. For strictly MX racing, this four-speed has plenty of gears, you just have to get used to them. The YZ's gearbox has near-perfect spacing and an all-new internal primary ratio to spin the gears faster. This change helped reduce some of the compression braking, one of the major problems when you downshift one time too many in a four-speed box. We got along great with the stock 51-tooth rear sprocket, even at a tighter track such as Perris Raceway in Perris, California. A 52-tooth tightened up the gear spacing at RaceTown (where we seemed to be between second and third a lot), letting third work more effectively.

Little has changed in the frame, so any handling differences can be attributed to the '05 suspension. With the new air-oil separate system, the '05 KYB fork is the best Yamaha has ever had. Now it has the control in damping through the stroke with a consistency that is common in Showa twin-chamber forks. The front end is supple at the top of the stroke and runs through the travel, getting stiffer as it gets deeper without any harshness. You can still bottom it, but it is damped. Adding a little to the oil level in the outer chamber really makes a difference. Out back, the shock is a perfect match. It boasts improvements such as a rebound top-out system that cushions the end of the shock's extension. Set up to complement the front end, the back didn't cause any issues once the ride height was set between 100 and 102mm. We got a good setting in the front with a reduction in compression damping (two clicks) and an increase in rebound (one click). That gave the front a more-planted feel in the turns and kept it a little lower on jump takeoffs.

The Yamaha is slim and very easy to move around on. Sure, the seat padding is skimpy and the back section isn't meant to be sat on, but the front portion and its grippy cover make hanging on in the turns worry-free. The footpegs are right where they should be, and the foot controls are tight. Shifting is solid and we never missed a gear, though you needn't do a lot of shifting on this bike. It takes some time for many to quit making unnecessary gear changes and simply ride most of the track in third gear. We used first when riding through the pits, and we hit fourth only on the fastest of straights.

Yamahas have been great with respect to durability, and we expect this YZ to continue the tradition. Small touches such as the front brake line's rerouting show attention to detail. The brakes were solid and drew little attention. The only part about which we could wage a complaint was the chain. It seemed to make more noise than in the past and did stretch during our testing. A Renthal bar is standard, so look to a chain as your first aftermarket purchase.

If it is possible to create a perfect four-stroke MX motorcycle, Yamaha, KTM and Honda appear to be in a race to do it, soon to be joined by others. And if the direction shown with the current crop of '05 models is any indication, everybody is getting closer. In the future, improvements should become smaller and smaller as everyone hones in. With the ability to choose from bikes with smoother, stronger motors and constantly improving suspension at lighter and lighter weights, we're the winners. Yamaha's YZ450F made leaps in performance that go far beyond what the list of changes suggests. At a time when a lot of buying decisions revolve around hype, this 450 is banking on competence.

Opinions

When I rode the '04 YZ450F, I couldn't believe the power. I knew Yamaha had toned it down, but I still felt as if my arms were going to get ripped out of their sockets. The power of the '05 is so much smoother and easier to control than the '04, and with still more power, it was easier for me to use more. I loved the suspension, too. The fork is a huge improvement over '04; the difference is noticeable right off. I felt I was riding with works suspension. It soaked up everything I could plow into! I can't wait to get my own.
J.D. Rogers/6'3"/185 lb/Novice

The big YZ-F has been my primary bike these past two years. I actually like the big power that seems to scare off many riders and have learned the delicate throttle control necessary to tame this beast. Still, I have found the YZ-F to be at its best when I ride it at Cahuilla Creek and Mammoth Mountain, where the altitude detunes it a little. The '05 version has been mellowed some, which definitely makes it easier to ride even without traveling to elevation. The fork was something I modified right away on the '03 and '04 bikes because the stock KYB was harsh in braking bumps and gave little confidence that the front end would stick without a berm. The '05 fork is a huge improvement and now seems to be in the same league as the Showa that comes on the Honda CRFs. These seemingly minor improvements make a big difference in the on-track performance of the 2005 YZ450F.
Sean Finley/5'10"/165 lb/Vet Intermediate

More 450 motocross bike tests:

2005 Honda CRF450R
2005 KTM SX 450