The Motorbike Archives

The key phrase here is "just about." While most of Yamaha's yearly changes are improvements, there have been a few notable exceptions. For example, there was last year's switch to a new compact six-speed engine. It was lighter, smaller and had more gearbox ratios, but it was also awfully short on mid-range power. Riding a YZ250E quickly required lots of revs and frequent clutch fanning—your basic 125-style MX tactics. The old five-speed engine, on the other hand, delivered smooth, tractable power perfectly suited to motocross. For most riders, the new motor was a step backwards.

For 1979, Yamaha's engineers devoted their yearly allotment of magic to the YZ's engine. As a result, the powerband has been utterly transformed. The new F-motor churns out enough mid-range power to make the YZ thunder out of turns like some of the very fastest bikes in the 250 class. But the six-speeder didn't acquire its mid-range wallop at the expense of last year's high-revving peak power. So you can ride the Yamaha' any way you want—short-shift to take advantage of the mid-range or fan the gearbox at high rpm to exploit the substantial horsepower at peak revs.

The F-model's new wide powerband is particularly helpful on short straights. You can rocket out of a turn in third gear, for instance, and the high-revving mill will still have enough power left to let you stay in the same gear all the way to the next turn. There aren't many 250's that can match this performance—last year's YZ250 certainly wasn't one of them. The newfound mid-range makes for better starts, too. Because the power gets to the ground better than last year's high-rpm rush, a delicate hand on the clutch is no longer required.

A new pipe and juggled port sizes help explain the F-model engine's new personality. The dimensions of the exhaust system are lifted almost directly from the factory Yamaha OW pipes. Compared to the E-model, both volume and overall length have been increased. The new pipe doesn't get in your way, but it does protrude from the left side of the bike quite a bit. Although the pipe was never struck in a fall, it developed a crack near one of the front mounts almost immediately.

One other change affecting the YZ's engine is the airbox. A new larger, double-layer foam element resides in a vent-aport-equipped cavern for better breathing and improved air filtration.

Depending on how badly you want to be the guy to lead the pack into the first turn, you can select one of a number of shifting strategies for the YZ, due to decreased dog engagement angles in the close-ratio transmission. First, you can use the clutch. Or you can save a little time and merely back off on the throttle before selecting the next gear. And if you're really desperate to beat the crowd, keep the gas full-on and preload the shift lever with your toe. The YZ will obediently shift itself when the power drops off at high rpm. When it does, you probably won't even hear the exhaust note change, but you'll accelerate with an uninterrupted rooster-tail all the way down the straight. You wouldn't want to make a habit of shifting like this, but it is the most effective (and potentially expensive) way of shortening a straight.

Unfortunately, there is a glitch in the F-model's program, but it has nothing to do with the magical new engine. It has to do with the monoshock. Because when Yamaha's engineers got this far in their development program, their yearly allotment of magic must have been depleted. The result is a step backwards, meaning Yamaha once again has "just about" produced the best motocrosser around.

Last year, the mono's performance was about as good as Japan's best dual-shock setups. But to keep up with the competition, Yamaha changed the shock and the rear suspension geometry. The new shock is shorter, but it has the same shaft travel as last year's, so the rear-wheel leverage ratio was increased to yield 10.1 inches of travel. Damping was increased to cope with the heavier loads on the shock, but the engineers elected to retain last year's spring, which is now effectively softer because of the increased leverage ratio. In addition, the monoshock's adjustability range—one of its most useful features—was narrowed. The net effect of this juggling is an over-damped rear suspension, and no combination of preload adjustment and damping settings provides a significant improvement. In fact, I was hard pressed to discern any difference at all in the way the YZ responded to bumps, no matter where I dialed the 23-position damping adjustment. At least this year's monoshock is lighter and features longitudinal cooling fins to fight off fade.

Without a doubt, the new monoshock encounters the worst problems when confronted with abrupt, sharp-edged bumps. In this situation, the heavy damping won't let the shock respond quickly enough to keep the rear wheel on the ground. On deep, rolling whoopers, the YZ tracks absolutely perfectly, but in the stutters and chops it pitches back and forth as the rear wheel skitters along the top of the bumps, abusing the rider and making the bike difficult to control. Under braking, the misbehaving mono lets the 4.50x18 IRC rear tire spend little time on the ground, making the rear end of the bike hop around erratically over the braking ripples. On really severe thumps, the YZ will move around enough to make you overshoot your line into a corner.

A number of accessory companies have already become aware of the F-model's problem and offer miracle cures. These modifications are worth checking into, since an improvement in rear suspension is all that's necessary to help the YZ perform to its full potential.

Up front, the Yamaha's suspension has no insurmountable problems. The standard-issue fork works quite well after a little dialing-in. We ended up swapping the standard 15-weight oil for 5-weight and found that the change improved fork action quite a bit. Response to sharp jolts was much improved and smoother fork action made it easier for me to deal with the rear end's antics.

The front end's good manners enable the front brake to be used easily and effectively—and this is especially important because the F-model's new full-floating rear brake is uncommonly sensitive. It locks unexpectedly, unlike last year's non-floating brake, and contributes to rear end instability while braking on rough ground. I killed the engine on numerous occasions just because of its touchiness. When Yamaha's engineers began refining the E-model, they didn't confine themselves to just the monoshock and engine. The chassis is very different from last year's as the result of numerous detail improvements. A lengthened swingarm and altered engine-cradle dimensions bring the swingarm pivot closer to the countershaft sprocket in order to keep chain tension more constant. To further insure effective chain control, they installed a new guide and roller system. A solid plastic guide feeds the chain onto the rear sprocket, while fixed rollers take up the slack at the extremes of suspension movement. After several days of testing, however, the lower front roller unbolted itself, letting the chain whip around freely. This seems to be a common problem on the new YZs because we had a similar misfortune befall our YZ400.

The frame of the new YZ is distinguished from last year's by having lower-slung frame rails underneath a redesigned seat, resulting in a one-inch lower seat height than last year. The new seat is also mounted closer to the steering head to help you to get more of your weight over the front end during cornering. The fuel tank looks similar to last year's, but it's actually shorter to make room for the seat.

These seat and tank changes help the rider take advantage of the YZ's quicker-than-ever steering geometry. Both the steering head angle and trail have been reduced. I had thought it impossible to improve on the YZ's steering, but the new F-model steers as well as any Japanese-built 250 so far. It's dead-accurate on either bermed or flat turns.

Though it's nimble in the corners, the Yamaha's quick steering won't get you into trouble on rough ground. As long as you keep your weight to the rear, the monoshocked back-end stays pretty much in line, even in the giant whoopers. And with the gas on, tracking through sand poses no problems.

But real motocross tracks aren't composed exclusively of rolling bumps and sandy terrain. Instead, you must crash your bike across crags and abrupt obstacles. A good motocross bike is supposed to insulate you from this sort of abuse and help you out if you get in a tight situation. The YZ, because of the monoshock's idiosyncrasies, is unwilling to fulfil either of these responsibilities. Instead of bailing you out of tight situations, it creates them. That's not the sort of behavior expected from a top-notch motocross bike.

In many ways, the new YZ250F is a much better motorcycle than the E-model. The new-found mid-range makes the bike easier to ride than before. The gearbox shifts better than before. It even steers more accurately. But each of these improvements is overshadowed by the uncooperative monoshock.

Still, not to worry. Yamaha's engineers won't let the YZ250 hop its way into oblivion. Next year they'll devote their yearly magic to the problem. But until they do, the aftermarket guys will be lightening the wallets of new Yamaha riders with an assortment of mono-cures. And an effective cure will be worth every cent invested, because it will turn a flawed masterpiece into a genuine work of art.

RIDE REVIEW

I'm a sucker for Yamaha's monoshock. The elegance of this solution to rear suspension fascinates me, even though I recognize the handicaps imposed by a mother lode of weight in residence so close to the steering head. But I'm more than just intrigued by the mono's sophistication. The way it intercedes between me and the vulgar bumps of the motocross track also earns my allegiance. Unlike other motocross suspensions, the mono directs the shock waves slam-dunked into the rear wheel to the steering head instead of my tailbone.

With this sort of prejudice, naturally I'm pretty happy with the F-model's rear suspension under most circumstances. But while diving into corners, the rear wheel skipped around like it belonged to a DT- I instead of a YZ. And the combination of the skittish rear wheel and the sensitive rear brake often led me to inadvertently kill the engine while braking hard.

The F-model's monoshock doesn't make this motorcycle unridable, but it does undermine my usual confidence while riding these bikes. And the little hesitations that stem from a lack of confidence multiply into precious lost seconds on a race track.

I'm still convinced that the monoshock was made in heaven expressly for use in loamy purgatory. But unfortunately, the F-model's mono lacks the usual dollop of celestial goodness. —Michael Jordan

Riding the stiff-tailed YZ250F over choppy ground indeed can be tolerated, but I find it hard to comprehend how Yamaha could have made such a sizable suspension mistake. After working with the monoshock concept for so long, after having built motocrossers that swept the 500cc world championship and every major U.S. title, after building a production 250 that does everything else so incredibly well, you'd think Yamaha's motocross design team would have the rear-suspension situation well in hand. But someone must have punched the wrong buttons on Yamaha's suspension computer, because the YZ25OF's new monoshock is clearly a step backwards. Maybe Bob Hannah can ride the wheels off a box-stock 250 and maybe he even likes the new mono's behavior. Hannah has been doing the impossible for years, though, and that's not sufficient reason to ask mere mortals to cope with a grossly overdamped rear wheel.

Of course, the problem is fixable for about $100. But that's not the point. The YZ250F has the worst overall rear suspension of any current 250 motocrosser, and that fact will start people asking themselves what, if any, the advantages are in having monoshock rear suspension. Based on the YZ250F's performance, there are none. —Paul Dean■