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.