The Motorbike Archives

Even if few other people recognized the fact, Hakan Andersson knew he was riding a phenomenal motorcycle when he won the 250cc Motocross World Championship in 1973 and broke Joel Robert's string of five consecutive titles. Anders-son was aboard the prototype Yamaha monoshocker, which was not destined to see the light of production until late 1974. When it did appear, the new model—christened the Monocross—was not exactly a factory replica, and it needed refinement. But because Yamaha Motor Corporation took a chance with the 250 and gave it a unique suspension design, they established a reputation as a manufacturer willing to experiment with relatively radical ideas.

In the last five years Yamaha has applied a relatively conservative process of refinement to their radical idea: where other companies have instituted leap-frog development programs—adopting different strategies and introducing entirely new machines every couple of years—Yamaha has laid claim to one specific design and set out on a steadfast course of improvement. They have, moreover, applied this philosophy to every aspect of the YZ250 by continually making significant but often subtle modifications, including expanding the reed-valve induction, adding a larger carburetor, repositioning and enlarging ports, adding a gear, beefing up the swing arm and changing the frame material. Throughout the metamorphosis, the YZ's basic personality has remained recognizable; it's just gotten better.

Last year, Yamaha engineers attacked every minor and major component with their regular enthusiasm, and the result was a faster, better-handling, lighter (by 15 pounds) motorcycle. Cycle called the 250-E and the Honda CR250R the best 250 motocrossers available. The YZ-F is the first 1979 model to be tested, and we'll tell you up front that the Yamaha is the yardstick by which every other 1979 250 motocrosser will be judged. It's possible that another manufacturer will introduce a better motocrosser in the coming year, but it's unlikely to be much better.

All the engine modifications—inspired primarily by Yamaha's highly successful 1978 racing program—are aimed at giving the YZ a wider powerband. The E-model was peaky and resembled nothing so much as a 125 with a gland problem. Its porting dimensions were strikingly similar to a road racing engine's. For example, a TZ250 — designed to run at 10,000 rpm and above—has an exhaust port height which is 51 per cent of the engine's stroke and an exhaust port width 71.3 per cent of the bore diameter. Last year's moto-crosser had comparable 51.6 and 71.4 per cent figures. It was hardly a surprise that the YZ-E was pipey. This year's model has had its exhaust port and the transfer ports lowered. The rest of the cylinder remains the same. Likewise, the die-cast aluminum cylinder head with its polyspheric chamber, is unchanged.

To save production costs, Yamaha has followed the practice of using a paint-like coating instead of polishing the ports. Last year, the cylinder's coating showed some bare spots. The YZ-F's cylinder is evenly coated, which indicates the production line is paying more attention to quality control.

Following last year's design, the two-ring piston is knurled on the bottom 33mm to retain oil during its downstroke — when it is under the most severe pressure—which helps prevent seizures. The long rear skirt also adds needed piston support. Having shown their durability, the connecting rod, the big-end bearing with its lightweight silver-nickel cage, and the piston pin are unchanged.

Contributing equally to the YZ's broader powerband is its new OW-type exhaust pipe. Its extremely wide configuration is strikingly different from the previous pipe. The fat center and reverse cones help produce the mid-range torque the motocrosser needs. On the track, the YZ's power comes on quick and strong from low mm. Bogging slightly in corners is no longer a problem—a light slap at the clutch lever gets the YZ rocketing out of any turn.

Yamaha engineers knew a wider-ratio transmission would also be beneficial with the new power characteristics. First is higher-2.14 instead of 2.38—to take advantage of the low-rpm power. Second and third gears have remained the same. Fourth is only slightly higher, up to 1.20 from 1.22. Fifth is now a direct one-to-one ratio, and sixth is a higher overdrive ratio, up to 0.87 from 0.92. In practice, the new ratios work well with the wider power-band. Gear selection is not as picky, and each gear has a greater spectrum of usefulness. Second gear starts are still standard, and the YZ willingly runs through full-power gear changes. Shifting and clutch actuation are precise, as they were on the E-model. Some credit here goes to last year's move to a 10mm longer shift lever which provides more leverage when shifting.

The 38mm Mikuni carburetor and six-petal, reed-valve induction system function extremely well. After having the reed cage widened 10mm on the E-model, the Yamaha has not needed updates in this area. The carburation provides instant and clean throttle response in all rpm ranges. There is a larger venting system in the side cover which allows a greater volume of air flow; a new double-thickness air cleaner also helps.

There have been no changes to the YZ's capacitive-discharge ignition system. Moreover, aside from last year's four millimeter narrower CDI rotor, the ignition has been modified very little recently. Yamaha has developed an efficient, lightweight assembly and sees no need to change it. Besides providing maintenance-free service, the CDI helps the two-stroke's performance thanks to its very rapid voltage rise time.

All the engine changes are effective in combination, making the YZ-F more forgiving and easier to ride than the YZ-E. The Yamaha R & D men have performed the rare feat of gaining midrange without sacrificing top end. The YZ is just as fast as ever. In all respects, they've made a good bike better and proved that careful attention to and refinement of a specific design is better than trying to follow all the trends, all at once.

There also have been several changes to the YZ chassis, most of which combine with the new power characteristics to make the 250 handle more precisely than ever. The steering head has been pulled in to give the YZ a 29.5-degree rake, one degree steeper than the E-model's. In the process, the entire frame backbone, especially the steering head, is more heavily gusseted. The swing arm has been lengthened 35mm, which benefits handling in two ways. First, it increases rear wheel travel 15mm, up to 265mm (10.5 inches). It also extends the wheelbase to 1450mm (57.1 inches) and adds to the YZ's straight-line stability. This extension is in keeping with the general move toward longer-wheelbase bikes. The typical machine's wheelbase has increased in response to its increased wheel travel.

Manufacturers are discovering that tall, short machines with 30 or 31 degrees of rake don't handle as well as tall, long bikes with 29 degrees of head angle. Effectively, making a bike taller necessitates a general increase in scale. The problem is that a long motorcycle doesn't steer very quickly; a steeper fork angle solves that problem.

There have been a few other modest changes to the frame and swing arm. Besides being 35mm longer, the needlebearing-mounted swing arm is now constructed slightly differently than last year's; the member which extends approximately from the monoshock to the swing-arm pivot is offset to the left, and it has a straight-wall left side and curved right side. Moreover, Yamaha has again moved the swing-arm pivot closer to the countershaft; it is now 102mm (4.0 inches) between centers. The swing arm is still aluminum; the box-section unit has proven to be so durable and rigid that aftermarket replacements have become scarce. Last year, Yamaha opted to replace the 1.6mm-thick mild-steel frame tubing with 1.4mm chrome-moly, which is much lighter and at least as strong.

There are indications that the red-hot suspension revolution is cooling off. In the last five years wheel travel has increased from an average of six inches to a not-uncommon 12 inches. But most authorities agree that much beyond a foot of travel is impractical for several reasons: notably awkward seat height, suspension unit flex and violent changes in steering geometry when the suspension is working. Yamaha, with the 1979 YZ, has increased the 250's suspension travel modestly, but has concentrated on other aspects of fork and shock performance. The E model had its fork tubes and sliders lengthened 20mm, the extra length being translated into more bearing contact area. This year Yamaha increased travel 20mm, bringing axle movement to 270mm (10.6 inches). The air-charged unit still uses standard oil damping.

Previously, the steel monoshock body incorporated small cooling fins running lengthwise from the rear of the shock. The F-model uses an aluminum body with large cooling fins, modifications which lighten the unit and dissipate heat faster. Since Yamaha adopted the DeCarbon set-up (named after its designer) for all their monoshockers, the suspension has yielded virtually fade-free performance. A small thermostatic valve controls the damping; it adjusts the damping orifice as oil viscosity changes with temperature. Ordinarily, the longer and harder a shock (or fork) is run, the hotter and consequently thinner the oil gets. With the monoshock, the rise in temperature begins to close the valve, which keeps the damping characteristics consistent. Additionally, the YZ monoshock has manually adjustable damping and spring pre-load. The damping valve can be reached through a small hole in the backbone near the steering head, but the absorber must be removed to adjust the pre-load.

On the track, the YZ's suspension performance is positively brilliant. With air pressure set at 11 psi, the fork is set for a 170-pound rider. It soaks up bumps of all sizes, damps smoothly without any static friction off jumps, yet offers a precise, controllable feeling in turns. The mono-shock's performance draws equal praise. With its stock spring rate, damping setting and nitrogen charge, the mono inspires confidence; it has fluid and instant response to all types of terrain, and even expert riders rarely bottom the YZ-F's rear suspension.

Particular situations extract varying responses from the mono. In whoops, especially deep sandy ones, the YZ tracks remarkably straight as long as the rider keeps the gas on. As with all long-travel bikes, though, the Yamaha wants to dance all over the trail if power is not maintained. In this case, the Yamaha rewards aggressiveness: a little throttle holds the YZ in line, and most everyone will be surprised at how fast and stable the YZ is through whoops if he'll trust the bike. A trick here, for beginning or intermediate riders, is A use the rear brake and throttle simultaneously, which slows the bike but also keeps it straight.

When hitting berms, riders invariably use their own styles. The YZ, however, is more controllable if the rider slides into a berm. If he simply charges into a turn and hits the embankment, the suspension loads and unloads quickly, and the bike tends to stand upright. There are various ways to counteract the response, but Cycle test riders preferred the sliding method, which effectively pre-loads the suspension.

In all cases, chassis geometry and construction result in neutral and precise response. The new head angle encourages steering rather than sliding through a corner, which is often the faster approach. Though at first the longer wheelbase makes the YZ feel less maneuverable, one soon learns to take advantage of the 250's tractable powerband and slide through sweepers under perfect control. Frame and swing arm flex are now officially relegated to history books as pre-YZ-E ailments; under the most rigorous load conditions the Yamaha retained its composure.

Every year Yamaha has been improving their dirt machines' brakes. Wet or dry, the YZ's front unit is first-rate. It requires only a feather touch, resists locking, is never grabby and is extremely powerful. Although people have considered Yamaha's traditional rear brake set-up too touchy, the YZ-F's is completely satisfactory. Much of the credit for the improved performance goes to the full-floating design. A full-floating brake has its torque arm tied to the frame; under braking loads, the force is transferred to the entire motorcycle, which goes a long way in eliminating wheel chatter. The YZ-F's brake shoes are still very powerful, and overly heavy boots lock up the rear wheel. Loosening the brake rod adjusting nut, however, gives some extra brake pedal free play, and this, combined with the new floating design, results in good control. The brake rod itself has been relocated to pivot on the footpeg mounting bolt, a placement which keeps it from being bent when serious berm-busters drag the cases.

IRC Motocross-series tires have established a reputation .r providing satisfactory performance in all-around use. The YZ-F's are a bit slippery on very dry ground, but not enough to pose a problem. On wet ground or in sand, the fairly hard GS-45Vs are good. Real competitors will surely arm themselves with specialty tires for the wide extremes of mud or sunbaked adobe, but the IRCs are an inexpensive alternative to several sets of tires. Detail items also came under the scrutiny of Yamaha's R & D department. Indentations are now formed into the YZ's gas tank to prevent contact with either fork pipe. To compensate for the ever-increasing seat height caused by longer travel suspension, the seat's foam has been reduced in thickness, and a better, stiffer grade installed; it is very comfortable and resists collapsing. A handlebar with a slightly different bend, modeled after the OW works bikes' bars, is now used. There have been a few complaints about previous front fenders packing with mud and bending; the F's has been reinforced around the mounting points. To meet a recent FIM regulation, the YZ has its side numberplates mounted farther to the rear. Instead of last year's spring-loaded chain tensioner, the YZ-F has a fixed-block unit attached to the lower rear of the swing arm. Three other fixed rollers help maintain tension, one on the swing-arm pivot, one just below and to the rear of the pivot and one mounted higher up the frame to protect the carb-to-airbox rubber tube when the monoshock is fully compressed.

Yamaha has done an excellent job refining the YZ. They've adopted several innovative (or at least unusual) designs and stuck with them; even though some innovations were at one time better in theory than practice, they did not give up. Through exhaustive refinement, the engineering department has nearly perfected the center-axle, air-charged fork, the monoshock, and the reed-valve, six-speed engine.

The factory has done just about all it can to offer a production race bike pros can win with. But various economic factors are catching up, and fast. The YZ250F costs 16 per cent more than last year's E model; suggested retail is $1829. Expensive or not, the YZ is absolutely first-rate - and it's worth every dollar.■