Rick at Cogent Dynamics is an innovator, who has used his considerable talents improving the suspension of the DR650. Lately, he has written several posts to enlighten us about some of those improvements, particularly concerning the rear suspension. Many thanks to Adv Grifter for compiling them into one essay
Originally Posted by NC Rick-
"I will try to give a bit on insight into this subject. The dampers we are discussing are mostly gas charged monotube units originally designed and called DeCarbon after the inventor. This design of shock uses a full diameter damper piston running in the body and control the oil flow and thus damping using deflective disk shims over ports in the piston along with a bypass path we call "bleed". The damping force comes from a pressure difference in the damper chambers within the shock. To simplify; design considerations must account for the displacement of the shock shaft entering the shock body, compressability of the oil (actually mostly gasses trapped in the oil), and maintaining pressure within all the shock chambers above the point of where oil turns back to vapor of gasses come out of suspension. There are many other considerations such as friction and such but lets keep this simple.
We need room in the non-compressible oil to account for the shaft displacement as well as expansion or the fluid from thermal changes. The most simple way to do this is to introduce a gas bubble into the oil. This design is called an "Emulsion" shock. The gas in any of these will be pressurized to allow for the pressure drops across the damper piston when it is in the bump direction. With the emulsion shock, the gas will become mixed with the oil in most any use forming an emulsion of gas and oil (like a foam). The disadvantage there is that the damping fluid becomes much more compressible, placing limits on the dampers performance. You can also see how the shock may be less consistent with that design. A reservoir offers the ability to separate the gas charge from the oil. The OEM DR shock uses a rubber bag in the reservoir to keep the gas segregated from the damper fluid.
In the case of a shock like our Cogent Dynamics Mojave for the DR, an internal reservoir is used. At the top of this type of shock there is a dividing element (we use a piston with a special seal and low friction band) to keep the gas charge separated from the damper fluid. This is a big upgrade from the emulsion design shock. A disadvantage of any of the internal reservoir shocks is that added length is needed in the shock body. An external reservoir removes this problem allowing for more stoke and fluid volume as well as area to cool the damper.
Cogent Dynamics manufactures shocks with remote reservoirs and has built them like that for the DR over the years. This photo shows an example of a Cogent DR shock from a few years ago:
Remote reservoir shocks offer a fairly easy way to add adjustability to the damper. We can employ a system to restrict the fluid flow from the compression chamber in the shock that is being displaced by the the shock shaft as the shock is compressed by a bump. Restricting this flow provides additional damping. Another very relevant design feature is that monotube dampers can be designed with a secondary compression valving system that also takes advantage of the displaced fluid we are discussing. Having extra resistance in the compression direction can function to improve the shocks performance in many ways. By adding compression force at this auxiliary valve, we do not need to make as much at the main piston, lowering the pressure drop across the main damper piston making it harder for the cavitation bubbles to form. With a good design, we can run lower internal pressures reducing friction and extending damper fluid life.
As many of you know, Cogent Dynamics makes the total custom shocks but we also have a really good conversion of the stock DR650 shock. The remote reservoir of the OEM damper offers many of the above advantages. There are disadvantages as well but we won't delve into those in this post. The OEM shock on the dr has a form of the auxiliary compression valve as I explained however is is very simple in design due to the obvious cost constraints in the DR target price point. It also has a system to give some compression adjustability.
Cogent Dynamics is presently undergoing design of a new remote reservoir that uses a sophisticated auxiliary valve with similar technology to our DDC valve for the forks. We intend to offer a high end DR650 shock that will represent the best damper technology and quality available for the DR650. The photo above shows a remote reservoir with both high and low speed compression adjustability which we will drop in favor of a single adjustable compression adjuster backed up by a sophisticated and tuneable auxiliary compression valve. Most of the reservoirs with high and low speed adjusters are of a very simplistic design which do not really add much ability for a rider to tune and are not making the best advantage of the possibility for performance.
I hope this helps to demystify the reservoir thing a bit. I apologize for my poor writing skills. I left out a lot of what could be said to keep things more simple and stop my fingers from bleeding but am happy to answer questions."
Cogent Dynamics Inc.