The aluminum frame used in Yamaha’s YZ125 and YZ250 motocross machines is known internally at Yamaha Motor as the “Katsura Frame.”
It was created by Takehisa Katsura — a veteran engineer who developed Yamaha’s factory “YZM” race bikes in the 1990s, later served as a factory team manager in the 2010s, contributed to MotoGP projects, and still mentors the next generation within the YZ development team. What makes this frame remarkable is that since its debut in 2004, not a single bracket or mounting point has been altered. Even for Yamaha, a company known for long-lived models like the SR and Serow (XT225/250), few could have imagined that a pure competition frame would remain unchanged for over two decades.
The YZ lineup built around the Katsura Frame later expanded to include the cross-country YZ125X and YZ250X models, which broadened its popularity. Affordable and approachable for beginners yet beloved by veteran two-stroke enthusiasts, these machines represent the enduring spirit of the Katsura Frame. This article retraces the evolution of Yamaha’s motocross frames from the 1980s onward — the path that led to this landmark design.
The Era of Aluminum as Structure
The late 20th century saw an explosion of aluminum use in the motorcycle industry. Lighter than steel and capable of being cast into large, rigid components, aluminum offered both functional and aesthetic advantages — it even gleamed when polished, adding perceived value. Starting with Suzuki’s 1965 road-racing prototype, aluminum quickly gained fame as the “material of the future.” It was a time when even soda cans were shifting from steel to aluminum.
In 1988, Yamaha entered the 500 cc Motocross World Championship with the YZ500 (0W83), its first bike to use an aluminum frame. The aim was to combine lightness with rigidity by forming cross-sections impossible to achieve with steel. However, limitations in welding and material technology meant the hurdles of durability and cost were still high.
By 1991, the YZM250 (0WC0) had adopted a twin-spar design similar to road racers, pushing aluminum rigidity even further. Yet in off-road riding, excessive stiffness prevented energy from dissipating, resulting in a bike that was difficult to control — a so-called “hard frame.”
Katsura recalls:
“A twin-tube layout increases rigidity, but high rigidity isn’t the goal. By choosing that form, we’d essentially abandoned the balance of flex.”
That realization marked a turning point. Aluminum was no longer just a lighter replacement for steel — it became a medium fortuning behavior
through controlled flex.
MY2026 YZ125
The “Do It” Order and the Birth of Prototypes
In the early 2000s, Yamaha officially launched a project to switch the mass-production YZs to aluminum frames, with Katsura leading the chassis design team.
“They just said, ‘Do it.’ No explanation. But I knew exactly what it meant — I had no choice but to make it happen.”
Behind that seemingly simple order lay a larger corporate strategy. Yamaha had established YPMI (Yamaha Motor Parts Manufacturing Indonesia) in 1996 and, amid the Asian financial crisis, was moving production overseas while keeping domestic plants busy through higher-value manufacturing. The company’s solution was to push “aluminization” across its lineup. When Yamaha announced its CF Die-Cast technology in 2003 — enabling thin, complex aluminum castings for mass production — the decision to make an aluminum YZ became inevitable.
At the time, few motocross bikes used aluminum for their main frame. Aluminum was light but vulnerable to impact and distortion from welding. Katsura’s team explored how to combine forging and casting to achieve off-road strength.
“I wanted to forge it — casting had only about half the strength. But casting allowed freedom of shape. Both had pros and cons.”
The prototype became a hybrid: forged for strength where needed, cast where shape freedom mattered. Because closed-section tubing couldn’t be forged, they used open cross-sections with internal ribs, machined from extrusions and hand-assembled.
US patent 7,073,617 B2
“We couldn’t do hollow forging, so we built it from extruded sections with ribs. It had this flat, almost planar look — light, but not yet balanced.”
That first prototype was quietly entered in the All-Japan Motocross Championship, ridden by test rider Taichi Kugimura. To conceal its aluminum frame from rivals, it was painted blue — an episode still circulated on social media today.
“They told us, ‘Paint it blue,’ and I remember thinking, ‘Why bother?’ But orders were orders.”
Despite the secrecy, the test provided invaluable data. The chassis felt light and responsive, but its harshness under impact remained unsolved.
Testing, Redesign, and Production Reality
After domestic testing, the prototype was shipped to the U.S. and evaluated at Carlsbad by riders including Doug Dubach.
“They shredded it,” Katsura laughs. “The only positive note was:lightweight.
Everything else was bad.”
Back in Japan, the team resolved to redesign it completely — even at the cost of higher manufacturing complexity.
“We changed the shape to follow the flow of forces. I drew new blueprints through the New Year holidays, and by March we had another prototype running.”
The second prototype prioritized controlled flex over brute rigidity. Yet cracks appeared immediately in both 125 and 250 versions.
“They cracked on the first day — one jump and snap. But we’d found the direction. We stopped chasing uniform stiffness and kept rigidity only where it worked.”
The deeper problem lay in production consistency. Aluminum distorts easily with heat; small changes in welding sequence or jig position could throw dimensions off.
“We remade the jigs for every prototype. Even perfect drawings wouldn’t match reality. The factory said, ‘We can’t mass-produce this.’”
Bridging that gap required new collaboration between design and manufacturing. Katsura, a craftsman from the factory-racer world, had to reconcile performance ideals with production practicality.
“The forging team hated it, the casting team said strength would drop. But combining them covered each other’s weaknesses. That’s when we realized structure itself could create performance.”
They eventually found that shallow weld penetration provided better durability than deep ones, which caused brittleness — a discovery that became foundational for later Yamaha frames.
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Evolving Together with Suspension
Simultaneously, the YZ’s suspension was evolving under engineer Mr. Miyashiro and KYB’s development of the twin-chamber (now AOS) fork.
“We sped up the transition between rebound and compression so damping wouldn’t spike. It matched the frame’s flex perfectly,” says Miyashiro.
The lightweight, flexing frame demanded suspension that could respond equally fast. The synergy of both systems finally unlocked the natural, “honest” feedback unique to aluminum.
Katsura admits with a grin:
“Where the frame fell short, the suspension helped. In the end, it worked beautifully.”
This philosophy carried into production. The head pipe and complex sections were cast as single pieces, while the main spars and swingarm pivots used forged parts — minimizing welds yet maintaining strength. Cast portions varied in wall thickness to tailor stiffness; forged sections bore tensile loads outside the frame, forming the ribbed pivot plate now iconic on 2-stroke YZs.
“When I showed it to Jean-Claude Olivier from Yamaha France, he yelled, ‘Why are there holes here?’ I just pretended not to hear,” Katsura chuckles.
Despite longer welds, he stood firm:
“The load path is different front to back — we made the outer side stronger.”
The Katsura Frame Today
BLU CRU Cup
As aluminum frame development advanced, two-strokes were vanishing from top-tier motocross. In 2005, Takeshi Koikeda won the All-Japan IA1 championship on a YZ250 — the last time a 2-stroke claimed the premier class title. Since then, while YZ125s remain popular for training and YZ125X/YZ250X thrive in cross-country and amateur racing, the crown has eluded them. Katsura remembers:
“Honda had already stopped 2-strokes, and there was a strong ‘we’re done too’ feeling inside Yamaha. I was even working on a new 2-stroke engine, but the company’s mood was shifting.”
Engineer-rider Mr. Sato adds:
“When the YZ250F appeared, I was still racing the YZ125. You could catch them into corners, but they’d pull away on exits. The rulebook itself was pushing everyone toward 4-strokes.”
Yet Yamaha quietly continued to produce its 125 and 250 2-strokes — not out of nostalgia, but conviction.
Katsura reflects:
“Maybe it was good we didn’t stop. Or maybe we just couldn’t. Either way, that’s what kept the culture alive.”
Even in 2025, the Katsura Frame remains at the heart of Yamaha’s 2-stroke family — from the YZ125 training machines to the versatile YZ125X and YZ250X, ridden by everyone from beginners to hard-enduro pros like Cody Webb at the Erzbergrodeo. The white smoke of a two-stroke still signals freedom at the core of off-road culture.
“To be honest,” Katsura concludes, “aluminum is still a material I don’t fully understand. High rigidity isn’t everything. Looking back, it was about keeping only theusable
rigidity.”
The “Katsura Frame” is more than a chassis. It is the accumulated wisdom of engineers who pursuedfeel
over figures — and that pursuit continues to define Yamaha’s off-road DNA today.