Ultimate Suspension for the Miata – Part I

Design Criteria

When designing the new TTX, the goal was to come up with a damper which came with some new features not available on other dampers. 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. Furthermore,  compression high speed adjuster gives you new possibilities to reshape the compression curve and 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.

Starting in 1996, the Öhlins TT44 quickly became tone 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 T44 dampers. 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. Öhlins patented six of the most important design criteria for the TTX through various decades of development.

 

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 optimized – the right amount of reservoir damping. The hysteresis is minimized, 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. Using reservoir valves always increases the internal pressure. The friction from the piston rod 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 match- ing of dampers will be improved.

3. Full adjustability

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 re-shimming 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; 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 tremen- dously 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.

6. External clocking

The clocking of the adjusters on the TTX in relation to the top eye should be possible to change without opening the damper. This allow optimum installation of the suspension setup on any car.

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 situ- ation 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. 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 pop- pet 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 open- ing 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.

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