Takashi Yasui: KX development engineer during an era of radical motocross evolution.

"If only we could use disc brakes on motocrossers…"
The wistful remark, part of a casual conversation between KX engineers, was followed by a sigh of resignation. With engine and chassis performance making steady progress (in the form of increased power and the change to long-stroke suspension, et al), the relatively ineffective drum brakes were rapidly becoming outclassed. An improvement in performance was desperately needed, but following in the footsteps of road bikes and switching to disc brakes presented a number of obstacles.
"Difficult? No… It was impossible – that was the level of the problem we were dealing with," reflected Takashi Yasui (motocross design team member at the time).
"To meet the needs of increased brake performance we made do at first by using larger-diameter brake drums. Early motocrossers were using 120mm diameter hubs – these quickly grew to 200mm, where we started facing weight issues. Of course we tried using different brake shoe materials and a twin leading shoe configuration, but these did not yield significant improvements. Switching to disc brakes seemed to be the only solution. What made us hesitate was concern about how performance would be affected when the caliper got covered in mud. But then, in a prime example of Kawasaki challenging spirit, we decided "Let's go for it!" Our first test part was made using components scavenged from a mid-sized road bike – I believe it was a Z400FX. We used its 28mm single-piston rear caliper and tried various brake discs and pad materials. Compared to the cast iron discs used on road bikes, we found that a stainless steel disc with half the thickness worked the best in off-road conditions. We settled on a disc diameter of 240mm, and for our initial disc brakes, positioned the caliper in front of the fork."

Equipped on the KX250SR and KX125SR, disc brakes made their debut at the 1980 All-Japan Motocross Championship.

In Autumn of 1979, the first trial of a bike equipped with a front disc brake took place at the course in Hyogo Prefecture's Yokawa Village. That first ride on the test machine left a lasting impression on Mikio Tatewaki (an International A-class rider with Team Kawasaki at the time).
"The drum brakes being used back then didn't work at all, so we had to use three or four fingers when braking. Usually, we'd use our thumb and index finger to operate the throttle and the remaining three fingers to actuate the brake lever. When that still wasn't enough, we'd use four fingers and squeeze for all we were worth. With a disc brake, gently touching the lever with the index finger was sufficient. But what impressed me the most was how brake force could be modulated with a subtle change in pressure. Disc brakes let us change our riding style, making it possible to dive into corners much harder. My most vivid memory is of passing Katsuhiko Seo (All-Japan Senior Champion in 1977-78) in a race. It wasn't due to any skill on my part; it was all thanks to the disc brake. Discs did have a weak point: if they got muddy, braking performance dropped considerably. One time in the rain at Sugo, I flew straight over the outside banking and had to retire from the race. I think I was running the number 10 plate in the 250 class at the time…"
The KX250SR and KX125SR fielded in the 1980 All-Japan Motocross Championship featured liquid-cooled engines and Uni-Trak rear suspension – the most advanced technology of the time – but it was the front disc brakes that had everyone talking, ensuring an endless parade of people streaming through the Kawasaki paddock.

Development wasn't all smooth sailing. The most effective combination used a stainless steel disc and sintered pads, but matching them proved more difficult than expected. The more effective the brakes became, the more vibration and brake squeal became issues, with calipers and brake discs even cracking on occasion. Then we also had to worry about brake hose routing. We didn't have a lot of data telling us how the hose would move when attached to a front fork whose stroke measured almost 300mm – and we were unable to foresee that the violent motion would cause the banjo bolts to loosen. Motocrossers bump into each other, so a warped disc or other damage was always a concern."

1982 KX250: the first appearance of a disc brake on a mass-production motocrosser.

Still, disc brakes' strong points greatly outweighed their demerits, and the time from start of development to implementation was extremely short. Kawasaki's release of the 1982 KX250 and KX125, the world's first mass-production motocrossers equipped with front disc brakes, caused a big sensation. Rear disc brakes followed on the 1986 models, completing the basis for the brake system used on motocrossers to this day.
"The change to rear disc brakes came later because in motocross racing almost 90% of stopping power is generated by the front brake, so drum brake performance was sufficient for the rear. However, when supercross became popular in the 80s, riders started using the rear brake to intentionally change the bike's pitch in mid-air and to initiate brake turns in tight corners in order to take the straightest lines possible. The spread of these riding techniques can be attributed to the greater control offered by rear disc brakes over drum brakes."
Having been at the forefront of many of the changes in an era of developmental upheaval lends credence to Yasui's testimony. "Any advantage we gained by being the first to implement disc brakes has long since disappeared. But taking the initiative and grappling with a task deemed "impossible" – especially since it fostered the kind of out-of-the-box thinking that led to Kawasaki's continued success – I think that's something we can be proud of."