Ultimate Suspension for the Miata – Part II

Flow circuit at compression cycle

How the oil flows from the compression side to the rebound side of the piston will be described here. This is caused by increased pressure on the compression side of the main piston, while the pressure on the rebound side is almost constant at the set gas pressure. The oil will reach the compression valves by passing through the port of the separating plate (Figure 3.3-A) extending into the cylinder head and leading the oil into a chamber below the compression valves (Figure 3.3-B). Because of the small restriction of this port, the pressure in this camber will be very much the same as the compression side of the cylinder tube. The piston velocity and how the valves are set determine the pressure in the camber.

The pressure will help to close the check valve in this camber. Depending on the pressure, different things will occur. As the velocity increases, the pressure will rise. In the initial part of a compression stroke, when the velocity of the piston is low, the oil will pass through the adjustable low speed compression valve. In this bleed valve, the restriction takes place in the passage (Figure 3.3-C) between the needle seat (integrated to the needle housing) and the needle. As long as the piston is moving and the bleed valve is not fully closed, some oil will always flow through the bleed valve. If the bleed valve is fully closed, this passage will be blocked.

As the velocity increases, the shim stack on the nose of the poppet valve will start to open and oil can pass between the shim stack and the poppet valve seat (Figure 3.2-D and 3.3-D. The stack configuration will decide the opening pressure. An increased stiffness of the stack will raise the opening pressure and thus raise the damping force. The shape of the nose on the poppet valve gives the shims freedom to bend and lift from the seat, no matter how much preload from spring there is on the poppet valve. This will allow the shim stack to always open gradually and therefore a small amount of oil will pass through the shim stack even with a very low pressure drop over the piston. As the piston velocity increases further, the internal pressure rises. At a certain velocity the movement of the piston creates a pressure difference across the main piston that is equal to the predetermined pressure required to open the poppet valve. The oil is now free to flow between the poppet valve and the seat (Figure 3.2-E and Figure 3.3-E). Due to the oil flow, the nose shims will follow the poppet valve up from the seat.
By using a very stiff shim stack in combination with little preload on the poppet valve, the oil flow through the shim stack will be very limited before the poppet valve opens. This will make the opening of the poppet valve more abrupt and the shim stack will open at a higher velocity. This will change the characteristics of the damping curve. The oil has now reached the low-pressure zone at the gas reservoir (Figure 3.3-F). This volume is in direct contact with the separating piston, separating the oil from the nitrogen gas. Here the pressure is always equal to the set gas pressure.

As the TTX is a through rod damper, there will be no fluid displacement by the piston rod. However, a gas volume is still needed to reduce changes of the static internal pressure due to volume changes caused by temperature variations. The rising temperature of the damper will increase the volume of the oil. Also the damper body will expand as the temperature increases, but not all to the same extent. Now the oil will flow through the compression check valve (Figure 3.3-G) positioned at the rebound valves. However, as long as the low speed rebound bleed valve isn’t fully closed, some oil will flow the through this valve backwards (Figure 3.3-H).  From here the oil flows between the two tubes (Figure 3.3-I). The oil re-enters the main tube on the rebound side through ports placed between the end cap and the inner tube (Figure 3.3-J). The compression flow circuit is completed.

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