Reducing weight and size, optimizing strength and rigidity
The main focuses of chassis design and engineering for Yamaha snowmobiles are reducing weight and making components more compact and improving the strength and rigidity characteristics ("strength" refers to resistance to breakage caused by external forces, "rigidity" refers to resistance to distortion caused by external forces).
However, these are often conflicting aims that are difficult to achieve at the same time. There are often trade-offs that must be made in the pursuit of light weight and compactness on the one hand and improving strength/rigidity characteristics on the other. Increasing the strength of a chassis is easily achieved by using stronger materials in more of its parts, but that also increases weight. On the other hand, weight can be reduced by thinning out the parts or reducing their number, but that makes it difficult to maintain the strength and rigidity necessary to withstand the stress loads a snowmobile is subjected to.
As a solution to achieve these two conflicting aims, Yamaha began switching from steel to aluminum chassis in its 2-stroke models from 1995. With the shift to 4-stroke snowmobiles with their inherently heavier engines, aluminum alone was not enough.
There was also the problem of how to join the components. Using rivets to join parts added to the number of parts, which made it difficult to reduce weight.
These factors led Yamaha to explore the possibilities of aluminum casting methods. There are several methods for casting aluminum parts, including gravity (non-pressured) casting, low-pressure casting and high-pressure casting. Yamaha developed its exclusive "CF Aluminum Die Casting" method as a new advancement in the field of high-pressure casting. This new method made it possible to cast thinner and more complex parts. This in turn made it possible to combine what would otherwise be multiple parts into a single cast part, thus reducing the number of parts and enabling the design of lighter, more compact units with good strength and rigidity characteristics. What's more, compared to other types of casting, parts made by the CF Aluminum Die Casting method can be designed for greater flexibility that increases their load tolerance and durability.
Eventually, Yamaha adopted the approach of using different combinations of CF Aluminum Die-cast parts to best fit the different characters of the models in its snowmobile lineup. For example, on the FXNytro and Phazer series models, CF die-cast parts have been employed in a chassis design with built-in chain case and other innovations to reduce weight and increase compactness while also achieving the required strength and rigidity characteristics.
•CF Aluminum Die Casting
The "CF" in the name of Yamaha's exclusive casting method stands for "Controlled Filling." It is a method that enables mass production of high-quality aluminum cast parts with 20% less air bubbles than conventional methods by precisely controlling factors such as the degree of vacuum in the die, the temperature of the die and the speed of injection of the molten aluminum into the die. This casting method achieves a high level of conformity in the solidified aluminum and minimizes impurities, while also making it possible to cast pieces that have varying thicknesses in their different sections. With the CF casting method it was possible to reduce the number of parts forming the front chassis portion of the FX-1 from 18 to just two and reduce its mass by 30%.
It was first used by Yamaha on the RX-1.
Functioning as the body's "trunk"
The chassis of a snowmobile is like the "trunk" (torso) of the human body. It has many components attached to it and it has to have the strength and robustness to tolerate the internal and external forces created by the movements of those components. The latest simulation and analysis technology and repeated running tests are used to develop the strength of such components as the frame and front steering assembly. Key elements in these development efforts are "static strength" and "kinetic strength." The first involves fixing the chassis to a test bench and identifying the distortion and resistance that occurs when force is applied to it slowly in order to find the strong and weak points and develop the right strength balance. The latter involves applying stress loads to the chassis with drop tests and running tests, including high-speed tests.
The desired strength balance of a chassis naturally depends on the type of suspension it is mounted on. In recent years efforts have been made to reduce the amount of the suspension forces absorbed through the link mechanism with simplified designs that distribute them between the chassis and the snow surface. For these and other reasons, development efforts involve analyzing the static strength and kinetic strength conditions of the chassis separately.
Meanwhile, with regard to chassis rigidity balance, Yamaha development efforts have focused particularly on the element of this balance call "torsional rigidity." An automobile chassis is supported at four points, the two front and two rear wheels. In contrast the snowmobile is supported at two points in the front but only one in the rear: the track. Despite this structure, the snowmobile moves with 3-dimensional dynamics that easily cause twisting (torsion) of the chassis, so its torsional rigidity plays a very important role in its handling. In order to develop a chassis with the right amount of torsional rigidity to contribute to responsive handling, tests are conducted with the two front ski mounts stabilized on the test bench and torsion applied at the back of the rear suspension mount in order to identify what types of distortion occur. Then the results of simulation analysis and repeated running tests are used to set the right amount of torsional rigidity at the important points in the chassis.
Another key element of a snowmobile's rigidity characteristics is what is called "longitudinal rigidity." This is an element closely related to shock absorbing capacity. But it is not necessarily a case of "the more rigid the better." The amount of longitudinal rigidity is set in balance with the suspension in an overall design in which some of the forces are deliberately distributed to chassis and the weight of the chassis reduced. This heightens the overall performance of the machine as a strong and nimble body with the chassis as its "trunk" and the moving components as its "four limbs."
In addition to these strength/rigidity characteristics, another important factor is the machine's center of gravity. According to the results of sensory evaluation by experienced test riders, the machine's center of gravity is perhaps the single element that riders are most sensitive to, and it is a factor that Yamaha has paid particular attention to since the 1990s. In general, the center of gravity is set near the center of the machine and close to the rider in order to reduce the effects 'inertial moment' fluctuations on the machine in motion and thus contribute to a greater sense of rider-machine unity. However, simply centralizing machine mass will not necessarily produce a high level of handling performance. Another development aim becomes how to arrange the essential components in a limited amount of space.
One of the biggest issues in developing 4-stroke models is that more weight tends to be concentrated toward the front than was the case with 2-stroke models. As a result, Yamaha's development focus is not simply to achieve centralization of machine mass but also achieving weight distribution that produces a machine balance that feels light and agile. One of the solutions that came from these development efforts is the rear-positioned exhaust system.
•Rear-positioned exhaust system
The role of this system is not only to reduce front-end weight and optimize overall weight distribution. It is also a system designed to bring out the best possible engine performance. By creating a rear-positioned exhaust system in which the fresh air supply drawn from the front is directed straight to the rear helps achieve both solid low- to mid-speed torque and high-speed performance while also providing good exhaust pulse effect to achieve a higher level of engine performance.