Now that FAST has announced they will be building their own snowmobile, can we safely consider them an OEM? Regardless, the M-10 was the original, effective long travel and the first to introduce the whole coupling concept. In fact, all of the existing coupling designs owe their existence to what Gerard Karpik introduced on the M-10. The M-10 is a true parallelogram, allowing for full two way coupling. It is a falling rate design during the first seven inches of travel but uses a "crossover" spring which is unique to the M-10. This "spring inside a spring" is located on the rear shock inside of the larger diameter coil over spring. While enjoying the ride quality comfort of a falling rate design for the first seven inches of travel, the crossover spring is engaged for the final inches of travel which makes the suspension act like a rising rate unit with excellent resistance to bottoming.
The FAST M-10 has no rear arm "scissors" link. The equal length front and rear arms remain fairly parallel to each other by limiting the movement of the rear arm pivot. When you do bottom, the front and rear arms bottom together and the vehicle remains level with little kicking or pitching, which is true of the two way coupled systems. It is this combination of the falling rate comfort and resistance to bottoming that allows the M-10 to still, in our experience, to provide the best of both worlds - ride comfort as well as resistance to bottoming - in a wide range of operating conditions. Tail landings seem to be a strength in comparison to the other designs.
However, the M-10 is not without its drawbacks. As with other fully coupled designs weight transfer is rather limited. It requires a calibration that is weight specific, and when the rider weight varies by more than 25 pounds it is wise to make adjustments. Top speed is decreased slightly, which may not be regarded as important for the bump environments is was designed for, but is a consideration. (Top speed decreases of any more than a couple miles per hour with an M-10 are caused more by having the track tension too high than anything else.) Varying the points at which coupling occurs is not adjustable without replacing the coupler blocks. Even so, it has been the reference point of all other suspension designs for years.
The "biased coupler" limits the rear arm pivot on the M-10, providing true two-way coupling.