volume10 The Birth of Kawasaki's Original Rear Suspension: Uni-Trak

From Twin- to Single-Shock Suspension: Uni-Trak is Born

1978 KX250 with twin-shock rear suspension.

1978 KX250 with twin-shock rear suspension.
Pursuit of greater suspension performance led to the development of Uni-Trak.

"The first time I saw it at the office, I thought it was the spring for a national railroad rolling stock car."
Even now, a glint can be seen in the eyes of Satoshi Masuda (motocross design team member at the time) as he reminisces. The massive spring was for a single shock rear suspension system being tested. At a time when thinner twin shocks were still effective, a spring that large was a definite break in convention.
Originally called the Bell Crank, the noteworthy Kawasaki device was later christened the Uni-Trak. Its first iteration featured a seesaw layout, where swingarm movement caused the vertically arranged shock absorber to be compressed via pushrods and an upper link. In 1979, at the start of development, the Uni-Trak used two pushrods mounted to the bridge joining the left and right spars of the swingarm, but this arrangement was soon replaced by a single pushrod arrangement. Material, too, changed: round pipes made from steel gave way to an aluminium rod with a rectangular cross-section.
On the front line during the motocrossers' period of intense development, Takashi Yasui (motocross design team member at the time) kept close observation of Uni-Trak's evolution, from its inception through its various iterations.
"Uni-Trak development started with the KR250 and KR350, but road racer and motocross design was done in the same room, so the drawings on our colleagues' desks often gave us ideas that we incorporated in our design. For motocrossers in the late 70s, longer suspension stroke was a key developmental concern – Uni-Trak was the ideal solution. The progressive characteristics offered by the linked suspension were just what motocrossers needed. As development progressed the new suspension system offered a number of merits, including mass centralisation, improved rough road handling and improved traction when accelerating, but heat became a problem. The larger we made the linkage ratio, the greater the damping required of the shock absorber – which meant much more heat was generated. With twin-shock suspension, it was easy for the shocks to be cooled by the wind, but with Uni-Trak the shock was located directly behind the engine, where it was out of the breeze and where it was more susceptible to heat. As load increased, performance would drop off due to overheating. We tried a number of things to deal with this problem: we added a finned aluminium reservoir tank, connected it using a hose and located it near the radiator."

Uni-Trak evolved as improvements were made to the linkage arrangement

Uni-Trak evolved as improvements were made to the linkage arrangement:
original Uni-Trak (left), single pushrod (centre), Bottom-Link Uni-Trak (right).

From Race Debut to Mass Production

When the first Uni-Trak-equipped factory racers made their debut, they caused a big sensation. Toshio Fukumoto, Shuichi Nomiya and Mikio Tatewaki in Japan, Jeff Ward and Jimmy Weinert in the U.S., and Brad Lackey in Europe all saw positive results campaigning the new racers. Their success in the early 1980s marked the first wave in an onslaught by KX series models.
In 1980, the first mass-production KX models were equipped with Uni-Trak. The original Bottom-Link Uni-Trak followed in 1986, and with numerous improvements to the linkage arrangement, evolved into the type seen today.
Placing the shock absorber directly behind the engine offers numerous benefits from a chassis design perspective, but engine designers disliked the layout because the shock would get in the way of the intake system. On the first Uni-Trak-equipped models, engine designers were able to avoid the shock and link by locating the airbox on the left side of the bike. However, with Bottom-Link Uni-Trak, not being able to use a straight intake passageway is an ongoing dilemma that designers still face today.

Satoshi Masuda: Kawasaki's suspension specialist.

Satoshi Masuda: Kawasaki's suspension specialist.

"Chassis and engine designers have reached a standoff, with neither willing to give ground. One of the reasons we tested downdraft carburettors when development of the perimeter frame started was to try to solve this problem."
Engine characteristics are important, as is suspension performance. When it comes to space, motorcycle design requires a fair amount of give and take. Through the development testing he has conducted, Masuda has a deep understanding of both sides of the argument.
"I was originally in charge of carburettor settings. But when my boss came to me and told me, ‘for motocrossers, suspension is more important than engine performance,' I ended up switching to suspension testing. Our main priority used to be power, and we didn't even have anyone working on suspension full-time. But as Uni-Trak evolved, the importance of suspension settings grew. The more we worked on the suspension settings, the more performance we were able to draw out, which made the job both fun and rewarding. The KX series is highly acclaimed for its chassis performance – I think this is a good example of Kawasaki's decision to shift priority to suspension having a positive effect on the finished bikes. I'm proud to say that I worked not only on Uni-Trak rear suspension, but also on the inverted front fork."
30 years have passed since the introduction of Uni-Trak, a name almost as well known as Kawasaki. There have been few major changes since the Bottom-Link Uni-Trak became standard equipment, but it would be difficult to assert that this is the final version.

(Interview by Shintaro Urashima)