The compression damping cycle describes the situation when the rod and piston unit moves into the damper body shortening the length of the damper. While the rebound damping cycle describes the situation when the rod and piston unit moves out from the damper body extending the length of the damper.

The terminology "compression side" of the piston here refers to the oil volume in front of the piston when the external piston rod is moving into the damper body (compression cycle). The "rebound side " of the piston refers to the oil volume in front of the piston when the external piston rod is moving out of the damper body (rebound cycle).

When the rod and piston unit doesn’t move, the internal pressure in the whole damper unit is equal with the set gas pressure. When track conditions cause the vehicle suspension to move, the damper piston will attempt to move through the damper oil. In order for the piston to move, oil must flow from one side of the main piston to the other. The restriction of the valves causes a pressure difference between the two sides of the piston, resulting in damping forces. In the TTX, this pressure difference comes from increased pressure on the forward side of the piston and not reduced pressure on the backside, as in conventional dampers.

Unless a different valve configuration is used compression to rebound, the compression and rebound valves are identical. On both sides there are three type of valves used for adjusting the damping characteristics.

• Bleed valve
• Shim valve
• Poppet valve

The compression bleed valve is in parallel with the compression poppet valve and the rebound bleed valve is in parallel with the rebound poppet valve. The poppet valves are pushed against their seats by preloaded coil springs. The preload is externally adjustable. The amount of preload of the poppet valves determines the pressure differentials across the main piston necessary to make the poppet valves open. For more information about the bleed valves and the poppet valves, see chapter External adjusters.

The shim valves are placed on the nose of the poppet valves. These shim stacks affect the opening characteristic of the poppet valves. The shim configuration can be changed to achieve different opening characteristics of the poppet valve. See chapter Internal adjustments for more information.

Also, there are two check valves installed in the damper, making the compression and rebound valves fully independent.

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.


1. The oil will reach the compression valves by passing through the port of the separating plate (A) extending into the cylinder head and leading the oil into a chamber below the compression valves (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.

2. Depending on the pressure, different things will occur. As the velocity increases, the pressure will rise.

a) 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 (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.

b) 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 (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.

c) 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 (E). Due to the oil flow, the nose shims will follow the poppet valve up from the seat.

Note:
In practice, the piston often does not reach a velocity high enough to cause a sufficient pressure drop and open the poppet valve.

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.

Note:
The opening characteristic of the poppet valve is always abrupt, unlike the gradual opening characteristic of the shim stack.

3. The oil has now reached the low-pressure zone at the gas reservoir (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.

4. Now the oil will flow through the compression check valve (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 (H).

Note:
The compression check valve is placed together with the rebound valves.

5. From here the oil flows between the two tubes (I). The oil re-enters the main tube on the rebound side through ports placed between the end cap and the inner tube (J). The compression flow circuit is completed.

Below is a description of how the oil flows from the rebound side to the compression side of the piston. The rebound cycle is very similar to the compression cycle, but the flow will be in the opposite direction and the oil will move through other valves. During the rebound stroke, the pressure of the rebound side of the main piston is increased, while the pressure of the compression side is kept almost constant.

Note:
The compression check valve is located together with the rebound valves.

5. Finally the oil re-enters the main tube on the compression side through a port in the separating plate (J). The rebound circuit is completed.