Introduction

The TTX damper is the culmination of three decades of Öhlins’ successful participation in world championship events winning more than 100 World Championships. Many years of work together with some of the world’s most successful racing teams together with advanced dynamic analysis methods developed at Öhlins Racing headquarter in Sweden has given Öhlins the unique knowledge needed to design the TTX damper.

The Öhlins TTX damper, originally developed for formula racing, is designed to handle the demanding damping characteristics needed for all types of tracks, from street courses to super speedways

The TTX damper is fully adjustable with maximised damper response together with qualities you’ve never seen before when it comes to "settings".

Low and high speed compression and rebound damping are externally adjustable and fully independent. The adjustment range is huge with equal increments of force throughout the adjustment range. Even the shape of the damping curve can easily be changed. All adjusters affect the flow from the main piston, not the piston rod displacement volume.

Some classes may need the TTX 40 with 3 or even 2 adjusters. For this puropse there is a needle housing that uses the existing parts in the 2-way adjuster and transforms it to be a 1-way adjuster suitable on either compression or rebound or both. Part number for this needle housing is 05953-03. Preload changes is made by adding shims on one or boths sides of the coil spring. Shims that should be used in the 1-way adjuster are: 00610-16, 00615-16,00625-16 and 00630-16.

The compression damping forces of the TTX damper are not, as in a conventional damper, caused by a pressure drop on the rebound side, but by increased pressure on the compression side. This reduces the risk of cavitation and makes any reservoir valve or high gas pressure unnecessary. So, no balancing of reservoir damping to main piston damping is needed to avoid cavitation and improve damping response. Maximum response and minimum risk of cavitation will always occur. With no reservoir valve, the internal pressure of the damper unit will be kept to a minimum. The low amount of hysteresis results in excellent short stroke/high force performance. Also, a very low gas pressure can be used without any loss of damping performance.

Along with the damper comes a unique Valving Reference Program (available for download free of charge at www.ohlins.com/Automotive/Products/FormulaSportscar). This computer model of the damper will allow you to find damping curves without a dynamometer. It will reduce building time tremendously and allow exact damper adjustments in pit lane. The TTX product will revolutionise the work for mechanics and engineers in the racing business.

This manual text is based on TTX dampers starting with Öhlins part number TTX NE0. These are through rod type dampers loaded with several new concepts. As always, all dampers are tested before they are delivered to the customer. In keeping with Öhlins long tradition of perfection, quality is outstanding and long life is to be expected.

Welcome to the World of Öhlins.

After the Öhlins TT44 was introduced to the market in 1996, it very quickly became one of the most popular dampers in formula racing. For some period, more than 95% of the cars in The Champ Car World Series were using TT44 dampers.

There are several reasons why the TT44 became so popular. One reason is that it came with some new features not available on other dampers. One of them is the powerful low speed adjusters, totally independent and with the compression adjuster restricting the oil flow from the main piston, not only from the piston rod displacement. Another is the compression high speed adjuster, giving new possibilities to reshape the compression curve.

When designing the new TTX, the goal was to come up with a damper which would be just as big a step forward as the TT44 had been. Highest priority should be not only to design a damper with excellent performance, but also a damper easier to work on and use than any other product available.

During the development several patent applications were made.

Six of the most important design criteria for the TTX are listed below.

1. No reservoir valve
The damper design should be with no reservoir valve. In dampers where reservoir valves have to be used to avoid cavitation, one more parameter has to be optimised – the right amount of reservoir damping.

The hysteresis will be minimised, as no reservoir valve has to be used. All damping comes from the pressure drop over the main piston. Damping forces from a reservoir valve always causes more delay in the damping force build up. See chapter Hysteresis for more information.

Using reservoir valves always increases the internal pressure. The friction from the piston rod seal/seals can be kept low because of the low internal pressures.

2. Main piston flow
Another criterion was to have all the adjusters regulate the flow from the main piston. This will give the maximum pressure area and because of this, the maximum oil volume to regulate.

The larger the pressure area is, the lower the internal pressure will be for a given damping force. The lower the internal pressure, the less flex there will be. The flex is caused by expansion/compression of the damper body and compression/expansion of the oil. The result is excellent short stroke/high force performance.

With a large volume of oil passing through the valves , it becomes easier to control the restriction of the oil. In other words, the matching of dampers will be improved.

3. Full adjustability
On the TT44/TT40, it was never possible to use a high speed rebound adjuster in combination with a high speed compression adjuster. On the TTX, we wanted to be able to combine those two while keeping them completely independent from each other, as with the low speed compression and rebound adjusters.

Poppet valves preloaded by coil springs were picked to become the high speed valves, as they can be made very compact in size and precise in opening pressure. This type of valve very often gives an abrupt opening characteristic, resulting in a sharp "knee" in the damping curve. To make the "knee" more rounded and to be able to change its shape, some shims are added to the face of the poppet valves. By changing these shims, the shape of the "knee" can be affected.

4. Simple valve changing
Even if the adjustment range of the external adjusters is huge, sometimes there might still be a need to change the valving of the dampers. In other words, change one or several of the following parts: poppet valve/valve seat, coil springs and nose shims. As this very often is done at the track and has to be done quickly, this job has to be simplified as much as possible. Compared to reshimming a conventional damper, any of the changes in the TTX will be a lot quicker. The result exceeded our demands.

Also it should be possible to fill the damper without a vacuum-filling machine, as this otherwise would be a limiting factor.

5. Through rod damper
A through rod damper has some technical benefits. One is packaging, which is a main issue on formula cars. The reservoir volume can be very small, as there is no piston rod displacement. Here no external reservoir is needed. Also there is no gas force pushing the piston rod out of the damper body. (The word "nose pressure" is sometimes used for this force.) Here the nose pressure is zero. This has several advantages. The nose pressure doesn’t vary due to temperature changes and you don’t have to fight the gas force when installing the damper on the car or in the dynamometer.

Designing a through rod damper gives the possibility to separate the rod bushings and keeps the distance between them constant. If coilover springs are used, the amount of friction will be tremendously reduced.

As the piston area for compression and rebound are identical, the damping forces will be the same if the same valving is used and the adjusters are set the same. To some degree, this simplifies the use of the damper.

For all the above reasons, race teams have been interested in through rod dampers. Also, when introducing the TTX, we wanted it to be something very different from the other products out on the market.

6. External clocking
Another strong side of the TT44/TT40 was the possibility to clock the reservoir bracket at any angle. This function we wanted to keep on the TTX damper, to ensure an optimum installation on any car. Just as on the TT44/TT40, the clocking of the adjusters on the TTX in relation to the top eye should be possible to change without opening the damper.

How the Damper Works

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.

Flow Circuit at Compression Side

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.

Flow Circuit at Rebound Cycle

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.

1. First the oil has to get to the rebound valves. The ports between the end cap and the inner tube (A) will lead the oil to the volume between the tubes (B) from where the oil will reach the chamber below the rebound valves (C). The pressure here will be roughly the same as in the rebound side of the cylinder tube due to small restrictions of the oil flow. The pressure will help to close the check valve in this camber.

2. See above in chapter Flow circuit at compression cycle for more detailed information as the rebound valves are identical to the compression valves.

a) Unless the low speed rebound valve is fully closed, the oil will first pass through this valve (D).

b) The second valve to open is normally the nose shim stack (E).

c) If the pressure level reaches the opening pressure of the poppet valve, the poppet valve will open (F).

3. Now the oil has reached the low-pressure zone at the gas reservoir (G), where the pressure is equal to the gas pressure

4. The oil will now flow through the rebound check valve (H) positioned at the compression valves. Some oil can, in the same way as described above in Flow circuit at compression cycle, flow backwards through the low speed compression valve (I) unless it is set to the fully closed position.

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.