The history of the KX parallels the history of cutting-edge technology.
As groundbreaking new mechanisms were introduced in the late 1970s through the 1990s, followed by the transition to four-strokes from the 2000s onwards, the models in the KX series have changed to reflect the new technologies they actively incorporated.

One After Another

The KX series had been making steady progress, but eventually came into a period of turbulence. With the very first KX in 1973, the engine and chassis were almost equally important in terms of development, but from that point forward Kawasaki decided to focus more on the chassis. Revolutionary technologies like the Uni Trak suspension that had been developed previously for the KR250 and KR350 road racing machines, and disc brakes—which had been seen as unsuitable for motocross machines—and other technologies, first appeared on factory racebikes between 1979 and 1980 before being adopted in production models one or two years after their first appearance.

Early Uni Trak suspension (left) consisted of a seesaw-type bellcrank located under the seat and two pushrods. This evolved to a single pushrod design (centre) and then Bottom-Link Uni Trak (right). Uni Trak continued to mature though a series of improvements to the linkage system.

Left: The first front disc brake appeared on the KX250SR and KX125SR at the opening round of the 1980 All Japan Motocross Championship, attracting much attention in the paddock.
Right: Starting with the 1982 KX250 and KX125, production models began featuring front disc brakes. Rear disc brakes followed later for the 1986 model year (photo shows a 1986 KX250).

Meanwhile, development toward liquid engine cooling proceeded at a rapid pace, including the decision to locate the radiator under the fuel tank as part of the move. As chassis sizes differed greatly for the varying displacements at the time, the KX125 had its radiator positioned vertically on one side. In the same period, a rear fender with an integrated number plate was adopted for three seasons, which helped to both boost the Uni Trak’s appeal and improve its serviceability.
In the 1980s, the Kawasaki Integrated Power-valve System (KIPS) was developed. It could simultaneously control a two-stroke engine’s exhaust port and resonator, providing both increased power and better controllability in the low-rpm range. The KX125SR was the first motorcycle to feature the KIPS system, and with it, Jeff Ward won his first title (1984 AMA MX125). The period also saw a marked evolution in the KX chassis, with the introduction of front and rear disc brakes, Bottom-Link Uni Trak rear suspension, an inverted front fork, and others. These technologies were the precursors that led to the modern motocross machines we know today.

Left: Graph showing the characteristics of an engine with KIPS. Opening and closing the valves according to rpm resulted in a smooth power curve from low to high rpm.
Right: Engine from a 1986 KX125. KIPS was a feature on the 1985 production KX series, but it was not until the 1986 model year that the KIPS logo was engraved on the cylinder head.

A Revolutionary
New Frame Arrives

In 1989, the KX125SR and KX250SR appeared in All Japan Motocross competition and became the most iconic machines in the history of the KX series. Their twin-tube frames—with next-level rigidity balance—became known as the “perimeter frame” and replaced the previous single-backbone frame. The perimeter frame’s effectiveness was proven when Atsushi Okabe won the 1989 All Japan Motocross Championship 250cc title. The frame was swiftly added to the 1990 model year production KX125 and KX250 models and would go on to be adopted for all Kawasaki off-road bikes thereafter.
When development of this perimeter frame began, plans were considered for a downdraft intake with an airbox positioned between the twin tubes and the fuel tank. Kawasaki had already been working on airflow for some time, and in 1984 a cool-air intake with a duct through the tank to direct fresh air into the airbox was introduced to the KX racebikes. This idea was also applied to the ZXR-7 road racing machine and was added as a feature on production models like the 1989 ZXR750.

Top/Bottom left: 1989 KX250SR (Okabe’s All Japan Motocross Championship machine). Its twin-tube frame was revolutionary at the time and became known as the “perimeter frame” because of its shape, which wrapped around the fuel tank.
Bottom right: Perimeter frames were fitted to production models starting with the 1990 model year KX250 and KX125 (photo shows a 1990 KX250).

The Transition
to 4-Strokes

Eventually, motocross racing entered a time of transition with two- and four-strokes lined up on the same starting gate. This was when Kawasaki brought the KX250F-SR, which featured a newly designed four-stroke DOHC engine, to the All Japan Championship. After Tetsuya Mizoguchi won in his very first race with it, he went on to put together an excellent season and lift the title (2003 MFJ MX125). A production model KX250F was release the following year, and by the time the KX450F joined the lineup in 2006, the motocross world was completely dominated by four-strokes. Both 2006 models featured lightweight aluminium perimeter frames.
In 2007, Kawasaki swept every class in AMA Supercross, which is one of our main areas of racing focus. The three musketeers to pull off the feat were James Stewart on the KX450F in Supercross, Ryan Villopoto on the KX250F in SX Lites West, and Ben Townley on the KX250F in SX Lites East. Of them, Stewart’s performance stands out. He won his first major title in his sixth year as a pro, and had a successful transition from 125s to 250s and then to 450s. The most dramatic success stories of the transition from two-stroke to four-stroke were the result of machine development, team management and the efforts of the riders themselves.

Top left: The four-stroke KX250F-SR that was introduced in the All Japan Motocross Championship in 2003, winning the title in the first year that four-strokes were introduced. The next year, the production model four-stroke KX250F motocross machine was released.
Top right: The KX450F made its debut for the 2006 model year. The aluminium perimeter frame was featured on the 2006 KX450F and KX250F models.
Below: The KX450F that led James Stewart to the 2007 AMA Supercross Championship title.

Following this, the lineup went on to add new technologies like fuel injection and electric start on the full-size KX250 and KX450 four-strokes, and the lineup was rounded out with small-displacement two-strokes to create a path from beginner to expert. As the performance of motocross machines has reached a point of maturity in recent years, there have been fewer drastic model changes. Nevertheless, as we celebrate the 50th anniversary of the KX series, Kawasaki has no intention of changing its “Built to Win” development concept.