Takashi Yasui: motocross project leader at the time the perimeter frame was first proposed.

A pile of rough sketches covered the desk. Among them, a drawing of a conceptualised motocrosser with a road-bike-style dual-beam main frame. Probably a designer's hasty scribbling, but the sketch stopped Takashi Yasui (motocross project leader at the time) in his tracks. "This configuration could actually work," he thought. Kawasaki had been searching for a way to revolutionise their motocrossers. That moment in the summer of 1988 marked the start of development of a twin-tube frame that would do exactly that.
"Kawasaki was already credited with a number of industry-leading innovations, like disc brakes and Uni-Trak suspension. But this time we were contemplating a frame that flew in the face of convention. Looking back at the advances made in motocrosser chassis technology, in the 1960s suspension stroke was 150mm. This increased to 250mm in the 1970s, and in the 1980s was over 300mm. With the increasing length, greater rigidity was necessary, hence the change to inverted forks. Naturally, in order to accommodate an inverted fork's wider outer tubes, the frame too needed to be more rigid. We had been looking for something to replace our steel single backbone frame. We thought that changing to a twin-tube configuration would afford the high rigidity we were looking for and would also allow the airbox and fuel tank positions to be reversed, making possible the use of a down-draft carburettor."
The revolutionary twin-tube frame design pursued by Yasui would not be called a "perimeter" frame until later.

Shinichiro Yokoyama: still designing motocross racers. Currently the motocross project leader.

Nowadays, computer analysis is used right from the design stage, but at the time development started with building a test frame using readily available materials, getting a test rider's feedback, then building it again. Designer Shinichiro Yokoyama (motocross design team member at the time) reflects back on the start of the project…
"We built a trial frame using some 40x25mm square tubing we had on hand. Test rider Shuichi Nomiya's impression after trying the new frame was that ‘the weight is a disadvantage, but straight-line stability is excellent.' With the potential of the new design confirmed, we knew the direction we needed to take and moved forward."
Structurally, the design offered high torsional rigidity, but it was weak longitudinally, which meant that after landing jumps or heavy braking the frame showed signs of sagging. Reinforcement of the downtube and the area around the headpipe solved the problem. Chassis design is always a fine balance of two contrary elements: driveability and stability. While driveability can be achieved later through tuning, stability has to be ensured from the beginning. Since this was a brand-new design and not an evolution of the current frame, compromise in this area was not an option.
Reducing the size of the high-tensile steel square tubing to 30x20mm, and forming the section used to mount the rear shock as a separate aluminium piece helped achieve weight goals. At the beginning of 1989, the first mass production test machines were built. Frames for the 125cc and 250cc engines had slightly different dimensions, but one of the mass production design requirements had been that close to 90% of the parts be shared. When the 1990-model KX125 and KX250 rolled off the assembly lines, the development team knew they had accomplished something great. But some unfinished business remained: the new motocrossers still had to prove themselves race-winners.

First perimeter-frame racer:
Okabe's 1989 250cc championship-winning KX250SR.

In 1989, the International A-division of the All-Japan Motocross Championship followed the irregular schedule of six 125cc races at the beginning of the season, followed by six 250cc races in the second half. The Kawasaki Racing Team was comprised of Atsushi Okabe, Shigeki Hanada and Tomoyuki Naganuma. Okabe, who had been the 125cc champion in '85, '87 and '88, was a favourite in both classes. At the season opener in March at Safety Park Saitama, the KX125SR's novel new frame was the centre of attention, but Okabe's first win would have to wait until Round 2 at the Kasaoka Special Course. The '89 125cc class would have eight riders vying for the championship, and Okabe narrowly missed taking the title. However, in the 250cc class, Okabe achieved his goal, snatching victory in the last round at Tajimi in another closely contested battle to earn the championship. It was Okabe's first 250cc title, and the first 250cc championship for Kawasaki since Masaharu Takezawa's victory in the 1976 championship, 13 years earlier.
"I hardly remember the first win," Yokoyama recalls. "For me, what is unforgettable was the disappointment of coming so close but ultimately missing the 125 championship, and then the joy when we took the 250 title."
While Yokoyama claims not to remember the details of the '89 All-Japan Motocross season, the site of the first race comes easily to his lips. Perhaps only those who were involved in the design of the epoch-making new frame can really understand the amount of work that went into getting it ready. Although it went from planning sheet to podium in under a year, the new twin-tube frame's naming took longer. Finally, it was a suggestion from the U.S., the largest market for KX series machines, which was chosen: the "perimeter" frame. Unlike the twin-spar frames used on road bikes, the motocross-use frame wrapped around the tank, making "perimeter" a very apt name.
After its highly successful debut season, the perimeter frame, a key component of the KX series machines, evolved quickly. Six generations of the high-tensile steel frame have been used over the years. '90-'91 models featured the original frame. On '92-'93 models, the shock tower was changed from aluminium to steel. On '94-'98 models, the round downtube was replaced by a monaka-style rib-reinforced box. '99-'02 models featured a revised main frame. From '03 onward, the frame's cross-section changed from square to D-shaped. '04-'05 frames were redesigned for the 4-stroke KX250F.
The latest iteration of the perimeter frame, first appearing on the '06 KX250F and KX450F, is a highly acclaimed all-aluminium construction. Kawasaki's motocross racing success is due in large part to the perimeter frame conceived 21 years ago. Had the project not started that summer, Kawasaki's illustrious racing resume might be a completely different story today.