Make no mistake about it, Yamaha isn't one of those companies that develops an off-road bike, gives it a lukewarm update the second year and then forgets about it to concentrate on the motocross side of the house. The tuning-fork engineers do spend a lot of time and energy on perfecting the YZs and YZ-Fs, but reading down the list of changes to the 2005 WR250F and WR450F shows they still care about their trail-oriented four-strokes.Sure, it might take a year for the trickle-across technology—such as the 48mm inner fork tubes—to arrive, but often the changes are concurrent, most notably the same-as-YZ revised intake tract and smoother combustion chamber surface.The big news is Yamaha has finally addressed the WR line's chronic chubby feel. The new bikes are slimmer than ever before and carry their claimed 1-pound-lighter weight lower, thanks to a serious redesign that included lowering the engine cradle by raising the steering-head tube 10mm on the frame, shortening the shock 3mm and sacrificing a minor 10mm of travel. The 450 saw its seat height drop 20mm overall. These small adjustments equal big changes in the overall feel of the bike. But wait, there's more.The tank was also given a trimming, reducing the capacity a half gallon to 2.1 gallons, but it now shares the radiator shrouds with the YZ-F. Perhaps the biggest weight savings came from tossing the stainless steel ex-boat anchor, otherwise known as the silencer, for a svelte aluminum version that is both light and quiet. With the insert installed, the blue bikes whistle an 82-decibel tune; remove it and they both issue a still-muffled sub-96-decibel bark. Yes, we can now say quiet, performance and Yamaha in the same sentence!WR450FOverall, the engineers basically answered many of the gripes riders have had since the first WR400F: too heavy, too wide and too unruly in the woods. Besides the common changes to both WRs, the 450 had its carburetor and ignition settings revised to make more power and do it right now. To tame some of the big girl's notorious bite, the designers left the same piston in the bike after the chamber makeover, dropping the compression ratio to 12.5:1 from 12.3:1.For those living in California, the big addition to the WR450F engine is an Air Induction System (AIS). Besides adding more tubes and hoses under your seat, the device routes clean air from the air-filter box to the exhaust port to lower the emissions. This means that for the first time ever, a WR-F is green sticker-legal. And Yamaha did it without stifling the output of the 449cc powerplant. That translates to just leave it alone; you'll be wasting time fiddling with bypass kits and all that. The only things we did before we attempted any serious riding was to replace the WR throttle-stop screw with one from a YZ450F (part No. 5JG-14591-00-00, $11.20), remove the 82-decibel insert in the exhaust and disconnect the famous gray wire under the tank.Enough tech talk, the bottom line here is a faster-than-ever WR450F. Fans of the class mark your calendars: 2005 is shaping up to be the year of the 450 enduro. With the impending arrival of Honda's CRF450X in the class, revised KTMs, Husqvarna back on solid ground, a new Gas Gas and Sherco adding a model to its line, it was do-or-die time for the blue woods wonder. And it looks as though the Yamaha got its training plan in order and it'll be a tough customer to beat. By this stage of the game our 2004 WR450F was as comfortable as a pair of well-broken-in boots. However, a stint on the '05 made it just feel fat and lazy. The new WR450F spins up to the power zone so fast that at first you'll think you're on a 250F, but you won't think that for too long. It has all the vigor of a 450 thumper, and that burst arrives more quickly than you can say accelerate. It's a good thing the clutch action was improved and the pull made easier; in the tight stuff you need to be one with that left lever to control the straining pony. It lunges out of corners, up hills and across any open space. This speedy revving hides the soft underbelly of the 450's power: the bottom-end. It still shares the rpm-happy delivery of the YZ-F and comes on strong in the midrange, and it keeps building into a healthy upper end. Sure, it will cruise all day on the lower throttle settings, but turn the right grip more than halfway and the pace becomes a full action/adventure feature.With all the hoopla over the lower center of gravity, we must talk about turning. Changing course was never a WR strong suit, but the '05 will let you flick it around much more easily. You must still overcome the gyroscopic effect of 449cc's worth of rotating mass, so in the really twisty areas the big-bore can be a handful. If that's OK with you, the new Yamaha will be a delight; the cg now feels as if it's located between your ankles. The bike drops over for turns without much thought. Or a simple stomp on the sensitive but not-quite-a-light-switch rear brake, a shift of the waist and you've turned. It's that easy most of the time. Climbing snaking uphill trails requires more clutch dexterity to keep on track and prevent the front end from heading upward. But most fire-breathers require that attentiveness, too.We spent the most time trying to sort out the WR450F's suspension. On our initial foray at the press intro at Hungry Valley SVRA in Gorman, California, things were nice until we reached a section in a loose sand wash. Going into corners was disconcerting. The front end pushed, and the rear end wanted to wag. After setting sag (at 100mm) and turning the fork compression out four clicks from stock and the rebound one click out, the front was willing to settle into turns better and track straighter in the sand. We even put the bike through its paces on a new, fast, sweeping and mostly natural-terrain MX track, Competitive Edge in Hesperia, California, which features sandy dirt and few jumps. The WR-Fs share fork size and basic frame design with their YZ-F cousins, so flex isn't an issue, and—surprisingly for an off-road machine—neither is bottoming. Coming up short on a double took care of that test. Both ends soaked the miscue with a slight kick off course, but nothing catastrophic. Yeah, this is an off-road bike that appears capable of holding its own on a natural-terrain MX track—fine for open Western conditions or a WORCS race, but likely too stiff for the roots and rocks more common to the Northern and Eastern states. In the bike's defense, it cushioned whoops, braking bumps and rocks we found in Hungry Valley to comfortable levels in a controlled manner. Once in a while we did encounter some twitchiness; it doesn't seem as planted as our '04 did, perhaps due to the lighter feel of the bike.Our first few dates have been civil; time will tell what course our relationship with the 2005 WR450F will take, but we are looking forward to measuring the big blue bike against the rest of the growing 450 class. The latest from the Yamaha factory has plenty of promise; it has already demonstrated excellent performance capabilities while remaining quiet and with emissions gear included—something we're mighty pleased to see.WR250FPerhaps the biggest bummer with the WR250F is that California owners will have to wait until 2006 to see AIS and thus a green sticker on the little WR. Although the system exists and Yamaha was ready to equip the bike with the unit, the paperwork hit a red-tape bottleneck and it could not get the required stamps of approval in time. That said, there's one word to describe this bike: fun. Receiving nearly all of the same updates as its bigger stablemate yet weighing in 16 pounds less, the WR250F feels more compact and is certainly less brutish. You can ride the 250 harder without getting into trouble nearly as quickly as with the bigger WR. A word of warning: Don't ride the smaller 250 in a tight section (or anywhere besides deep sand) and then hop back on the 450; you'll be ruined. As on the WR450F, before we tackled any serious riding the WR250F got its throttle-stop screw replaced with a YZ-F version, the 82-decibel exhaust insert removed and that gray wire under the tank disconnected.Unique to the 250, and just the opposite of the 450, a taller, flatter piston head was incorporated to keep the 12.5:1 compression ratio for an improved delivery at low rpm. When the surface of the combustion chamber was made all smooth curves rather than angular outcrops, the volume of the combustion chamber actually increased, so a 2004 piston would have resulted in a lower compression ratio. Yet in spite of efforts to boost low-rpm response, the WR250F is still fluffy down there and demands to be ridden like a 125.But don't mistake this as a weak engine. This little bike rips; the engine revisions add up to a major boost of muscle. You just need to keep the throttle open—a lot. Unlike the 450, the 250 takes a little while to build up steam if you let it drop off the pace. This soft bottom is nothing new to seasoned WR250F riders. In our experience, a free-flowing but still quiet silencer usually helps cure it.With its major downfall of girth removed, the Yamaha is a serious threat to its main rival, the CRF250X. Although the Honda has more torque and a cleaner response off idle and at low rpm, the WR-F feels as though it has more gusto throughout the rest of the powerband. It accelerates hard, cleanly and for a long time, yet it doesn't tire out the rider or require effort to change direction. The suspension didn't present the same mysteries as the 450's, either. Four clicks out on the fork compression, set the sag at 100mm and it was smooth sailing.Did we mention this is a quiet bike? Under 96 decibels and still raging—we like that. For trails that require weaving between tight trees or even just exploring a single-track, this is probably the better tool from the Yamaha store. For sure, this will be the bike that has the staff bickering like two kids forced to share the same toy.