We were recently at KW Suspensions North American headquarters for some new product training and we got to take apart some of their latest technology, the innovative solid piston damper. The solid piston damper is a radical departure from conventional shock technology and is for sure worth a close look at how the insides work.
We had a quick look at the prototype solid piston damper at last years SEMA show. The concept was very fascinating to us and we were eager to see actual working shocks.
We got to see the insides of a 4-way adjustable solid piston shock from a Porsche 911 cup car. The shock is standard equipment on them. Here are the shocks guts minus the outer tube. Compared to other shocks, the solid piston is quite simple. That is one of the beautiful parts about it.
The solid piston shock is technically a twin tube. It has an inner and outer tube with the shock rod and piston contained in the inner tube. This is where the similarity to a conventional twin tube ends. In the KW solid piston shock the outer tube serves as a fluid transfer passage. The piston is connected to the shock rod. Unlike a conventional shock, the piston itself is solid with no valves! The piston serves to move fluid back and forth inside the shock and serves no function to control damping at all! Here is the shock rod, piston and inner tube, the outer tube has been removed.
On compression the piston moves the fluid and it flows through the black manifold pictured on the right. It then goes through a one way check valve and the compression valves. The fluid then flows out of the manifold and through the space between the inner and outer tube, through holes into the inner tube and to the other side of the piston. On rebound the fluid flows through the inner tube through the holes, through the space in between the inner and outer tube, then into the manifold through a one way check valve, then the rebound damping valves, then out the manifold and to the inner tube on the other side of the piston.
So the piston acts like a push me pull you for the fluid. This has many advantages. The first one is that there is a lot of fluid flow volume available to generate damping control force. Normally a lot of the compression damping is done by the small amount of fluid displaced by the shock shaft that flows out to the remote reservoir through the high and low speed compression valves. With this small volume of fluid generating the control force, the orifices must be very small and the valves tightly shimmed to develop hundreds or thousands of pounds of control force. This causes a big drop in pressure across the valve system. With the pressure drop you have the possibility of cavitation with localized boiling and aeration of the fluid. To combat this the shock must be pressurized to a high pressure which has issues with potential leaks, high seal sticksion and high gas reaction force which gives the suspension a potential rising rate.
With the solid piston shock the fluid volume of entire body of the shock is flowing through the valves giving the valves many times more fluid flow to develop the control force. With more fluid, the orifices and shims do not have to be as small or tightly preloaded to develop the force. More fluid flow, the more sensitive the valves are with less hysteresis. With greater fluid flow, the solid piston shocks run a lot cooler as well leading to more constant damping and more fade resistance.
With the solid piston doing the push me and pull you, there is very little pressure differential across the valves so there is less potential for cavitation! With less potential for cavitation the shock can run at a lower pressure. In fact, the remote reservoir can be a lot smaller because its only function now, is to hold enough gas to make up for the displacement for the shock shaft.