Yamaha Yz125 Monomods

YAMAHA YX125 MONOMODS

Walt Fulton

Now handling and long-travel come in a kit.

IN THE SEPTEMBER '74 issue of CYCLE WORLD, we ran the first test on Yamaha's answer to long-travel suspension. While some manufacturers use cantilevered (lay-down) shocks, others have moved the whole shock body forward to increase travel, thus offering the rider easier and more precise control over the roughest of ground. Yamaha attacked the problem in a novel manner. Utilizing one large shock mounted under the tank, they were able to improve swinging arm rigidity and offer more than six inches of travel at the axle. That one shock absorber's largediameter shaft also made it much stronger than conventional shocks.

Our exclusive test of the Monocross 250 brought several interesting points to light. One concerned the bike's need to be ridden fast and hard in order for the shock to work properly. Since the first YZ, however, nitrogen gas pressure has been reduced and damping improved, which makes for a much easier ride at less than all-out racing speeds. In fact, this system works so well now that we expect to see it fitted to most of Yamaha's off-road and racing bikes in the future.

Just recently, the YZ125 was introduced, or, more accurately, unleashed. Like its predecessor, it has some minor teething problems, but they are not as severe. For heavy riders the shock characteristics are satisfactory, but lightweights (we're referring strictly to poundage!), will find that the 125 rides stiffer than is necessary. The standard setup has a tendency to pitch the rear end into the air on choppy surfaces when the throttle is closed, and, consequently, requires some alteration.

Regardless of the shortcomings of the monoshock, they are far outweighed by the advantages. Besides, the little bugbears can be easily corrected. And, while the shock is apart, you can seize the opportunity to increase travel.

The kit that alters damping characteristics is a separate item from the increased-travel kit. Both are available from Orange County Cycle, 13666 Harbor Blvd., Garden Grove, CA 92643. We took last month's test 125 YZC monoshocker to the folks at OCC and learned many things about the monoshock system of suspension from service manager Tom White.

Tom showed us the various pistons that have been used during the experimentation that eventually yielded the final monoshock piston that OCC uses. Internally, the changes are miniscule. The one-piece OCC piston replaces the two-piece stock unit. This in itself does little to alter the 125, but is very important on the larger Yamaha Monoshocks. The standard oil is discarded and the damping chamber is filled with Bel-Ray oil of a viscosity that is dependent upon the rider, his particular style, and the type of riding he does. The shock must be carefully bled of all air bubbles and assembled with exceptional care. The gas chamber is then filled back up to 200 psi (which is what the 125 shock comes with from the factory), and the unit is ready for the next step.

$14.95

Without the spring in place, the monoshock unit has about six inches of travel. Were one able to utilize this full six inches, it would translate to almost 12 inches of travel at the rear axle. But the monoshock unit, when fully assembled, is limited in travel by the spring. When a monoshock bottoms, it isn't the shock that has reached its limit, but rather the spring that has become coil bound. When fully compressed, the spring allowed the shock to travel just a little more than three inches. This translates to about 6.5 inches at the axle. After studying the situation, it became apparent that the only way to increase

travel would be to use a longer spring.

At its fore end, the monoshock unit uses a screw-on spring locating collar, then a slip-over pip-type collar that locates the shock properly into the frame just behind the steering head. At the very tip, in order to keep everything secured tightly, two nuts are screwed on and backed against each other. The folks at OCC surmised that if they made the collar and nut one piece—and machined threads into the collar rather than just slipping it on—they could secure the spring to the shock unit, and the shock unit to the frame, with the actual hexagonal nut nearly an inch beyond the end of the shaft. This would allow them more room for a spring, which in turn would provide more travel before its coil compressed completely.

But there was still more room for improvement. At the spring's base is an aluminum spacer. This serves no other purpose than to keep the spring away from the nut that seals the shock unit. By replacing the stock spacer with a much thinner one, more room was available for the longer, lighter poundage spring. Presto, 8.2 in. of axle travel!

Now, with the back end finally performing up to snuff, the front end's shortcomings became even more apparent. The front end was imprecise in its standard form; fork action was mushy and not very confidence-inspiring.

We installed a Number One Products 433 kit that was designed for the YZ250. Since the 125 has the exact same forks as the 250, we thought that this would be the answer. It certainly helped the damping and control at the front end, but there was still a good deal of harshness when the forks were suddenly "jolted" into action by landing off a jump or hitting an unexpected pothole.

In further discussions with Brian Fabre at Number One, he mentioned that although the number 433 kit worked well on the larger YZs, a special kit had been developed for the lightweight YZ125. The new kit reduces the compression damping to a much greater degree than the first kit did. With the finalized damping components in place, the harshness at the front end diminished, while the damping improvements previously obtained remained unchanged. The new kit is numerically designated as a 1033. We used 20-wt. Bel-Ray in our forks.

Back to the race tracks we went. The YZ not only handled more precisely, but delivered an extremely plush ride to boot. The Number One kit did an admirable job with the front end, but the change at the rear was the real capper. Without a doubt, this was the best Monoshock Yamaha we had ever thrown a leg over. To be quite honest, we were unable to detect the increased travel because we were so overwhelmed by the newfound comfort and tractability. But we know that the extra travel is there, available whenever called upon.

The Number One Products kit will soon be available through your local dealers. The OCC kit is available only through them. If you don't live in this area, send your shock unit to Tom White at OCC. Be sure that you package it well and include your name and return address. The shock unit will be returned to you C.O.D. within about two weeks.

If you own a YZ125 Monocross, you already have the quickest one-two-five around. Now, with these modifications, you can have the best handling one, as well. When you put those two together, it's a tough combination to beat.