Yamaha's Mono Shock RA Rear Suspension

The story of the new Mono Shock RA rear suspension could have been straight out of Hollywood. There is a cast of engineers who believed in the project, there are uncontrollable forces of nature, adrenaline-charged racing action, and ultimate success in the face of near elimination.
Back in the mid-1990s, Yamaha’s snowmobile development team was putting the finishing touches on its entry into the long travel suspension segment, the ProAction. By 1995, Mr. Masao Furusawa, the Snowmobile General Engineering Manager, was looking at his ProAction design and examining ways to make its stroke longer and make it lighter overall. He devised and patented a new design to achieve those goals by using only one shock absorber and a single linkage between front and rear arms.
Mr. Furusawa took his concepts to suspension designers. They worked with Furusawa’s design parameters and ultimately fabricated the first prototype pieces for pre-testing. As the long-stroke, single-shock design showed real potential it was given the green light for further refinement and development.
In 1998 the concept was solidified and needed a way to test its capabilities. The answer came in the form of the emerging popularity of snocross racing in North America.

The Testing Ground: Snocross
Yamaha re-entered Snocross racing at the factory level in 1997. This season was highlighted by Chris Vincent’s Pro 600 class title. Racing engineering development fell under the guidance of Mr. Thomas Imamura. Imamura is a young snowmobile enthusiast that has been involved with snowmobile engineering for many years. The first goal was to take the Mono Shock suspension concept and develop it to meet the rigors of Snocross racing. He refined and re-engineered the mono shock system into a highly tuned racing suspension. That season Nathan Titus piloted the Mono Shock mod sled to a 2nd place overall point’s championship.
Behind the glitz and flash of snocross racing, Imamura had taken ownership of the suspension development program. “We wanted to test the mono shock suspension in all forms of use,” Imamura said. “We tested it in snocross and then we tested it on the trail.”
Trail testing showed some problems. “The Mono Shock system had become a finicky suspension as it became more and more race specific. Ride quality and handling would change on different snow conditions.” remembered Jim Kedinger, Yamaha Testing Engineer. “We stopped development since the suspension didn't have the weight transfer and confidence needed on the trail.” Without an official green light for his project, Yamaha was forced to make a crucial decision.

Flying Under the Radar
Yamaha assigned Imamura to a completely different project, but unofficially, he was secretly developing a consumer version of the original mono shock design. He stayed after business hours and worked on weekends to develop the suspension. “My first goal was to increase the Mono Shock’s trail capability by making the suspension easy for the customer to set up and adjust.” Imamura recalled. “But the greatest challenge was to lessen the natural internal stresses placed on a mono shock link system and greatly reduce the weight.”
Imamura secretly ran hundreds of computer simulations before finding the perfect geometry that matched the trail application. Now he was confident to bring his project back to life.

Testing Becomes Reality
Based on Imamura’s new design, the first prototype pieces were fabricated and cleared for pre-testing in January of 2002.
The Mono Shock RA design was put through more rigorous testing than any design Yamaha had worked on previously. With the knowledge gained through snocross, and the private testing Imamura had done, the test team was able to quickly fine tune the suspension for the 2005 RX-1.
“We had test sleds running for 3 years,” explained Jim Kedinger. “This allowed us to put thousands and thousands of additional miles on the Mono Shock, than perhaps we would have from a normal development cycle.” In the end, the Imamura’s design achieved all of the original design goals from 1995.
The suspension is lightweight, thanks to using fewer parts and a mono shock design. In fact, the Mono Shock RA is 14 pounds lighter than the previous suspension. “Plus, it is extremely easy to adjust for individual riders and varied terrain. That makes it hundreds of times better for a trail sled application than the Race suspension ever was,” added Imamura.

The Final Chapter
In the face of official cancellation, Mr. Imamura refused to let the project languish on the shelf. He took his own time and resources to not only keep it alive but to bring it to market. If not for Imamura’s love of snowmobiling and his personal belief in the mono shock concept, we would not be getting ready to ride it this winter. And that is the True Development Story for Yamaha’s new Mono Shock RA rear suspension system.