Suzuki's Rm125 What It Takes To Make It Right

SUZUKI'S RM125 WHAT IT TAKES TO MAKE IT RIGHT

GEARING

In order to make the best use of the superior torque band on Suzuki’s new RM 125A, it is necessary to alter the final drive ratio from its standard 4.07:1 to 3.71:1. This is easily accomplished by replacing the 57-tooth rear sprocket with a 52-tooth item. If the RM’s intended rider is particularly light, or if a certain track has some very fast straight sections, a 50-tooth sprocket may be used. When changing the rear sprocket, we suggest that you purchase one designed for number 520 chain. Of course, you’ll then have to obtain a new chain and countershaft sprocket, but the benefit of a larger, sturdier chain is obvious: Reliability.

If you stay with the standard 428 chain, with or without the altered gearing, check it closely each week for cracked links. Twenty-two horsepower is quite a bit for that small chain to absorb for very long.

STEERING

The easiest way to improve the RM’s steering is to replace the front tire with a 3.00-21 Metzeler. Although at the present time you may have to hunt a little to find one (high demand is testimony to their excellence), the results are more than worth it. Metzelers’ dry-track adhesion is fast becoming legendary. The rear tire on the RM does not suffer much on dry tracks, but when it eventually wears out it would be a good idea to replace it with a 4.00-18 Metzeler. These tires are also exceptional in the wet.

The next step can be performed with or without the Metzeler tire in place. We noticed, while looking through the geometry specs on the Suzuki, that its steering head angle was very similar to that of the Bultaco 125 Pursang we tested in our June issue. The Bultaco had straight-leg forks much like the Suzuki’s and similar weight distribution, but came with a Pirelli front tire that has a reputation for being absolutely horrendous on a dry track. Yet the Bui steered with great precision. The difference, we discovered, lay in the triple clamps. The RM’s have a greater off-set between the steering stem and the fork legs. In order to bring the fork legs back to a location similar to the Pursangs’ (to increase the trail until it almost matched the Bui’s), we installed a pair of RM250 triple clamps.

Installation was easy. Everything just bolted on. But when it came time to slip on the 125’s fork legs, we were quickly reminded that the 250’s forks are 36mm thick at the lower triple clamp and are turned down to 35mm at the top. The 125 legs, being 35mm across at both pinch points, presented a small problem. By using .020-in. brass shim stock (available at most auto parts houses), and lapping them in well with valve grinding compound, we achieved a very snug fit between the 125 legs and the 250 lower clamps.

The results of this experiment were very interesting. The machine (still fitted at this point with the standard front tire), steered with the beautiful manners normally reserved for Honda Elsinores. But we had shortened the wheelbase nearly .75 in. when we installed the new triple clamps. This made the RM handle very quickly. On smooth, TT-like motocross tracks the increased quickness is welcome. Corners can be entered at excessive speed, yet directional changes are easy and the front wheel bites hard when asked to. On rough tracks, however, the machine

handled too quickly. Also, rough terrain brought out a small but noticeable amount of fork flex at the lower triple clamp that we hadn’t noticed on the smoother tracks. For rough course racing we would have preferred the steering arrangement in its stock form, thereby sacrificing the new-found cornering ability for the steadiness of the original setup.

Our final modification involved the installation of a complete set of RM250 forks. With the triple clamps already in place from the previous alteration, the RM legs slid right in. An RM250 axle is required, as are wheel spacers, but the change-over is purely bolt-on.

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We realized two advantages with this final modification. First, the cornering capabilities increased tremendously. The flatter triple clamps and offset axle of the 250 front end gave our 125 just what it needed. Second, the improved steering came at no loss of high-speed stability. In fact, stability was enhanced, not by anything connected to the geometry change, but because of the better hydraulic action of the new forks. After we finished with it, our RM 125A test bike was the best-handling Japanese 125 motocrosser we’ve ever ridden, was second overall only to Bultaco’s Pursang, and was the best steering small-bore, bar none.

We felt that cost might be a factor in deciding how you would want to go about remedying the Suzuki’s steering problem, so we’ve given you three alternatives. The front tire change runs about $40. The triple clamps alone will set you back nearly $70, while a complete RM250 front end, including triple clamps and axle, costs in the neighborhood of $ 190.

Keep in mind that the changes we have suggested are in no way essential to one’s enjoyment of an RM1 25 A, nor are they absolutely necessary in order to compete successfully aboard one of these screaming yellow rockets. These changes are designed to make better use of what Suzuki has provided in its stock package, and in so doing, make racing an RM 125 A easier for the rider.

The easier it is, the faster and longer you can go. The faster you go and the longer you go fast, the more you’ll win. The more you win, the more you enjoy riding. And that, friends, is the bottom line.