Murray Washburn started out racing BMX when he was five. He went to college in Colorado, where they issue you a mountain bike when you hit the state line, at a time when mountain biking was in its infancy. He worked as a shop guy in Colorado for many years, in service and sales. In 1997 he started working for Cannondale and has held a number of roles since then, including sales representative, race mechanic and product manager. Today he is the Global Director of Product Marketing, which is the link between Cannondale’s Engineering departmentt and the outside world. He describes himself as a "supremely unqualified engineer, but someone who loves knowing how stuff works." What is the DYAD RT2 shock?
The DYAD was developed as a collaboration between Cannondale and the suspension wizards over at Fox Racing Shocks. In its most simple form, the DYAD RT2 is two completely different shocks in one. You have a short-travel shock with a small air volume and a long-travel shock with a bigger volume, both of which get their own dedicated damping circuit, designed specifically for the travel.
How does it work?
The oil flow inside the DYAD RT2. (Left) Elevate - Short travel mode. (Right) Flow - Long travel mode.
You've got a handlebar-mounted lever that you can use on-the-fly to flick backwards and forwards between the two. It moves a spool valve which changes the oil flow internally in the shock. The shock itself is kind of a reverse shock. It's a pull-shock, which is fairly different in the world of mountain bikes. In a traditional shock you have an oil bath and then you have an air spring and you push a piston through a fixed oil bath, so the oil bath doesn't move. The DYAD RT2 is really more of an oil pump. So the air piston for the negative chamber is actually the same as the primary piston. When the shock extends it pumps oil from the negative chamber, it pumps oil up into the positive chambers through the damping circuits, and it compresses the positive air spring. You have this kind of back-and-forth-shuffle of oil between the two chambers.
The DYAD RT2 shock is a pull-shock. It utilizes two separate positive air chambers (one big and one small), a shared negative air chamber, and two independent damping circuits. DYAD’s unique in that, rather than moving a damping piston through an oil bath like a traditional shock, the DYAD instead acts like an oil pump, pumping the oil from one side of the shock to the other through the one of the two dedicated damping circuits. Each of the positive air chambers has its own floating piston that separates the oil volume from the air, while the pull-shock piston serves as both the primary oil piston and the negative air spring piston. Oil flow is controlled by something called a spool valve, which moves up or down when you flip the handlebar mounted lever, opening and closing two different oil paths.
In the full travel setting (which Cannondale calls “FLOW”), the two positive air chambers are linked, creating a huge volume air spring which is balanced by the independently adjustable negative air spring. The combination of the large volume positive air spring, the adjustable negative air spring, and the high operating shock pressures creates a linear spring rate that is really similar to that of a coil spring.
In the short travel “Elevate” setting, one of the positive chambers is completely shut off, reducing the air volume and ramping up the spring rate. All of the oil is directed through the other damping circuit, changing the damping characteristics at the same time.
Another interesting thing is the effect this has on geometry.
Since the air volume is reduced in the short travel mode, the bike sags less, making the effective seat angle and steering angle steeper, better for climbing and slower techy stuff. Flip the switch and the BB drops, the whole bike gets lower and slacker for high speed stability.
Why did Cannondale choose this system over a more traditional push-shock?
Mark Weir with the DYAD RT2 in action.
It was the best way for us to utilize this three chamber technology and there are a number of benefits. First of all when you have something that's pulling, versus something that's pushing, it always wants to straighten itself. Imagine you're trying to push something into a cylinder, if there's any torque on it all, any twist, it wants to bind. If you're pulling something out it naturally wants to pull straight. It's very easy to pull a string out of a hole, but it's very difficult to push a string into a hole. The other advantage we have is the way the seals interact with the oil. If you look at a traditional shock, there are very few seals that are constantly in contact with the oil bath, which means that those seals constantly need to be re-lubricated, because they are constantly pushing the lube away and becoming dry. With our design, all but three of the seals are in constant contact with the oil, which means they are wet seals, which means they are constantly self-lubricated. The other advantage is that with a traditional shock you have the oil bath, and then you have an air sleeve that comes around it and covers the portion of the shock that contains the oil for three-quarters of it length or more. That basically acts as a big insulator. As you descend, the air heats up and the oil heats up and there's nowhere for that heat to go. If you look at the DYAD RT2 shock there's just an aluminium wall, which is a really good conductor of heat, between the oil bath and the external air. There's lots of surface area, so it maintains its temperature really well over long descents.
The DYAD RT2 mounted in a Cannondale Jekyll trailbike.
How does the pull-shock affect frame design?
Jerome Clementz and his Cannondale Jekyll
It's not a question of taking an existing design and re-working it for pull-shocks. All of the bikes that utilise the DYAD RT2 were designed from the ground up with pull shocks in mind. So they're not something that would be adaptable to a push-shock-style design. There are some advantages, it attaches very close to the BB, so, when you land hard off something, the downward force on the BB is counteracted by the pulling force on the shock. So that load around the BB is constantly neutralised, as opposed to the force-multiplier effect with a push-shock, where you land off something and the load all goes into one specific point. However, the pull-shock shaft has to be much, much stronger as things are generally stronger in compression than they are in tension. So we use a steel shaft for strength and design our frames around the different loads created by a pull shock.