Improvements in Fork Technology on the 1992 Yamaha GTS1000 | Drawing the Line

A patent issued on a new invention is like a snapshot, a view of the invention frozen in time. If the snapshot captures the moment when the invention is first presented, the invention itself probably changes over time-just the way a child changes even as the photo you carry in your wallet doesn't.

If necessity is the mother of invention, changing necessities must change the invention if it's to remain viable. I call those changing necessities The Mothers of Invention (with full credit given to the late Frank Zappa).

Twenty-five years ago, I applied for a patent on a swingarm-type front suspension system for motorcycles. In 1982, motorcycles were gaining horsepower faster than they were improving in handling. The front fork was stuck in time while speeds escalated. My RADD suspension system (Rationally Advanced Design Development was the company I started to market and refine the invention) looked set to improve strength, stability, suspension compliance and other areas of front-end performance.

The patent was a snapshot from '82. As I built prototypes in '84 and '87 (both of which were featured in these pages), I made many changes and improvements to the original idea. In '92 Yamaha introduced its GTS1000 sport-tourer, which used my design in the first (and still the only) bike produced by a major manufacturer to use a swingarm front end.

When the GTS did not sell well due to funky styling and confused niche positioning (the concensus then and now says the suspension was not at fault sales-wise), internal policies and politics within Yamaha prevented further development of the design. I felt development should continue, and in '95 built another prototype I called RATZ-based on a Yamaha TZ250 race engine-and made enough changes to the design to apply for a new patent. But with development halted at Yamaha, and limited at my end by finances, the RADD suspension could not keep pace with the now much-improved, and continually refined, front fork.

Forks had improved structurally with the inverted configuration, more robust triple clamps and axle clamping, and materials and coatings had reduced friction as cartridge damping units had greatly improved control and adjustability. While I still could offer improvements through the RADD system, the window of possible improvements was getting increasingly narrow.

I put the RADD system out of my mind for several years as life took me on its typical wandering path. Then, a few years ago, I was working on a girder fork design as an optional front end for Indian Motorcycles, and as I worked out its geometry numbers-which gave it changing trail as it went through its travel-I had a Eureka! moment.

To deal with packaging issues, Yamaha engineers had made changes to my design, most important of which was a very short upper arm, where I had specified one as long or longer than the lower arm. I now realized that the short upper arm was responsible for what had been a mystery about the GTS-why it steered more heavily than its geometry indicated. On the GTS, both rake and trail increased through the travel. The numbers I had been given by Yamaha were not correct throughout the travel, and I had not checked them independently.

I began to look at the GTS with a much more critical eye, and saw that I could correct and improve many features of the suspension. On the RATZ 250 prototype I had experimented with twin front brake discs, and I now designed hardware that could use such an off-the-shelf, top-of-the-line setup. By switching the primary functions of the two arms (the upper arm would now carry the suspension loads, the lower arm would become a locator), I was able to reduce the offset of the suspension loads by almost two-thirds-from the GTS's 190mm to only 65mm. Gone was the GTS's offset shock, with the new shock operating near the bike's centerline. I simplified the steering setup while at the same time increasing travel without making the mechanism longer. Perhaps most important were changes to basic geometry and brake-dive characteristics.

I now had a system that could compete with the much-improved fork. I designed new RADD suspension setups around several motorcycles to help choose the base bike for a prototype of the new system, and finally decided on the Suzuki GSX-R1000. I started to design what I would call the GSXRADD and also started work on a new patent to cover the many changes.

The Mothers of the new invention were the major improvements in fork technology, the flaws that I found in the Yamaha GTS design and the realization that the strengths of the earlier RADD system could be retained while incorporating dozens of refinements 25 years on. How did it all work out? As I write this, the GSXRADD prototype is almost complete, and after initial testing you'll see the results in Motorcyclist. The Mothers may take the occasional vacation, but they don't give up.