August 5th, 1973 was a historic day for Yamaha Motor Co., Ltd. as well as the motorcycle industry. It was the Finnish round of the 250cc Motocross World Championship and the day that Hakan Andersson rode to wins in both motos on a Yamaha YZM250 factory machine equipped with a revolutionary new suspension. These victories brought Andersson his and Yamaha’s first motocross world title.

It was a moment that decisively proved the competitive potential and future possibilities of the single-shock “Monocross Suspension,” a device that would soon become the new standard worldwide for motorcycle suspensions.

The story of this great achievement had actually begun the previous year. Having sparked a worldwide off-road riding movement with the release of the DT-1, Yamaha decided that in order to achieve a new level of technological capability, taking on the challenge of winning the world’s premier off-road competition was a necessity, and began competing in the Motocross World Championship. It would be no easy task to win the title, however. Standing in the way was a formidable group of rivals that included the European manufacturers of Maico, Husqvarna, CZ as well as Suzuki, which had already established a formidable presence in the sport.

As a latecomer, Yamaha was in a position where it would certainly not win using the same means as the competition. On top of that, accomplished riders would not choose to ride with Yamaha unless the bike they would ride had some kind of revolutionary new technology.

Yamaha’s development team had already succeeded in winning world titles in road racing and had accumulated a wealth of engine development knowhow. Realizing how the chassis—and suspension performance in particular—is essential in motocross, Yamaha set to work ahead of the other manufacturers to learn about new suspension-related technology. Instead of depending solely on established suspension manufacturers, Yamaha resolutely began suspension development efforts of its own. That same year, new “Thermal Flow” rear shock absorbers were equipped on its factory machines. To solve the problem of inconsistent damping force resulting from the rise in damping oil temperature when in use, the new Thermal Flow rear suspension’s shock absorbers had three times the volume of oil of standard shocks and the oil tanks were fitted with fins for added cooling capacity. This improved performance and race results, including a win in the 250cc class at Round 10 in Sweden and the 500cc class at Round 11 in Luxembourg. But, the Yamaha teams were still a long way from winning a championship.

 

That first prototype machine with the single rear shock unit was immediately sent to Japan. After arriving in Iwata in October, it was taken to Yamaha’s test course at the Tenryu River near the headquarters to be tested by contracted rider Hideaki Suzuki. His first impression was that the machine lacked power but it was easy to ride. Still, he was worried about the lap times the bike would run. Like the test riders in Europe, Suzuki was used to riding machines with two rear shocks and he didn’t feel like he was riding fast. But the times told a different story. On a roughly 1 km course, he was posting lap times about three seconds faster than usual! The riders and engineers were amazed at the potential this new suspension had shown. The development team immediately set to work to improve the technology and get it into practical use as soon as possible.

“The first thing we concentrated on was refining the mechanism that produced the damping force,” recalls Nomura. “We decided that we should accumulate knowhow ourselves about the key parts of the unit, and we used things like the process for changing carburetor jets to develop an exclusive mechanism for producing damping force.” Another key point for a shock absorber is how it is mounted to the chassis. For this shock absorber that would be positioned basically along the fuel tank rail, the positioning of the air cleaner became a problem. “We made countless design sketches. In the end, there was simply no more time for drawing plans, so we began making the shapes with Styrofoam and clay to work out the possible spacing. It was during this process of trial and error that we got the idea of making a triangular swingarm,” he adds.

March 18th was the day of the 1973 All Japan Motocross Championship season opener. When the new Yamaha factory machine went out for the practice sessions, the race venue was soon alive with exclamations of the crowd’s confusion about what they were seeing: “Hey! That bike’s got no rear suspension!” Because the new suspension was positioned inside the frame under the fuel tank, it was invisible from the outside. What’s more, these new Yamaha bikes were jumping higher than any of the competition.

When the races were over, Torao Suzuki had won the 125cc class race and his older brother Hideaki Suzuki had done the same in the 250cc class. It was an overwhelming victory for Yamaha; these machines with their new Monocross suspensions dominated the podium in both classes. The Japanese media quickly dubbed it the “Flying Suspension,” and the news of its performance quickly spread around the world.

After its stunning debut in the All Japan Motocross Championship, the now-publicly-known “Monocross suspension” was put to the test in the 250cc and 500cc classes of the Motocross World Championship, the pinnacle of motocross racing. It would first show its advantages in the 250cc class.

Before the opening round of the 1973 series in February, Hakan Andersson saw the new YZM250 for the first time at the course in As. Looking back on the experience, Andersson mentioned how he was surprised at how he’d managed to set lap times similar to the twin-shock bike because the characteristics of this new suspension as it had been developed and set-up in Japan didn’t immediately mesh with him. There was a difference between Japan and Europe in the way rebound and compression damping was used.

After the necessary adjustments to the settings had been made to accommodate for this difference, a machine mounting the Monocross suspension finally made its GP debut at the third round of the World Championship in Belgium.

Andersson finished 3rd in the first moto and won the second one. The racing success continued in the next round in Yugoslavia where he won both motos on the new machine. At the next round in France, Andersson got off to a bad start in the first moto that left him at the back of the pack, but he managed to roar back to finish 2nd and nearly win the race. He then took a runaway victory in the second moto.

The overwhelming success of the Monocross suspension and Yamaha’s motocross machines spread to North America as well. Amidst the rapid growth in popularity of off-road riding following the release of the DT-1, Yamaha Motor’s sales company in the U.S., Yamaha Motor Corporation, U.S.A. signed a contract to bring Dutch national motocross champion Pierre Karsmakers to its racing team in 1973. As Karsmakers gradually got used to the American style of competition, he managed to clinch the AMA 500cc Motocross title that same year. Then in 1974, he entered the new AMA Supercross series and became its first champion riding a YZ motocrosser with a Monocross suspension.
“I think we worked probably six to nine months on [the Monocross suspension that arrived from Japan] to get it tested to where it was very, very good,” he recalls. “But the biggest problem we had actually with the monoshock was that the rear became so good; it got better, and better and better all the time...we couldn’t keep up with the front forks.
“When I had a proposition for something or wanted to change something, [Yamaha] always listened and had meetings about the problem and we always tried to make the product better...I always felt that I was part of the family with Yamaha.”

Thanks in large part to Karsmakers’s efforts, Yamaha became very successful in America. One of the great stars leading the Yamaha charge was Bob Hannah. In 1976, he became the AMA 125cc National Motocross Champion and then won the Supercross title three years in a row (1977–1979). Another star was Broc Glover, who became the youngest AMA 125cc National Motocross Champion ever and contributed to many articles in the media about Yamaha’s “takeover” of America. As the passion for off-road riding continued to grow in the U.S., the Monocross suspension became a shining icon of Yamaha’s presence. And, this was only the beginning of what would become an era in which Yamaha reigned as the leading brand in motocross.