Sealing the fate of suspension damper systems

Dave Wilson talked to Greene, Tweed about better ways to design suspension damper systems for the modern racing car

Suspension systems are a vital element in the performance of the modern day racing car. The difference between a good and bad suspension design can literally mean the difference between winning and losing a race.

One important element of the suspension system is the damper. Most are telescopic, monotube units filled with gas (nitrogen) and a damper fluid. The gas is sealed in a compartment at high pressure and exerts a constant pressure on the fluid column, eliminating cavitation and foaming. The efficiency of the damper ensures accurate wheel damping.

In most high-performance systems, the function of the damper is divided into two separate but connected systems to minimise space and maximise efficiency. In such a design, the dampers may not be fitted in line with the wheels to save space further and not to interfere with the aerodynamics of the vehicle.

The design shown in Figure 1 comprises three main units: a separator unit, containing gas on one side and hydraulic fluid on the other, a connecting unit, and a damper unit containing hydraulic fluid. The damper rod connects to the suspension system and is free to move inside the damper unit. As the damper rod moves inside the cylinder, the gas is compressed and acts like a spring to provide damping action.

Essentially, there are three areas in which the performance of such a damping system can be improved: in the separator piston, the damper piston and in the damper rod seals and bearings.

In the separator piston itself, a piston is usually combined with a seal in order to separate the gas and the oil. Here, sealing efficiency is very important. So too, however, is low friction, since the friction of the piston and seal determines the responsiveness of the system.

Traditionally, the sealing solution has comprised an elastomeric seal, either circular `O- ring’, or square in cross section, running with reduced squeeze, housed within an aluminium piston in conjunction with a filled PTFE bearing strip. The problem with this solution, however, is that the seal tends to have a high friction, thus adversely affecting response time. Clearly, what is needed is a better solution in which the friction is improved upon and the response time is enhanced.

As an alternative, the designers turned to the use of their ACT ring where the seal itself is housed in a separator piston manufactured from their Arlon 1555, rather than the more traditional aluminium. The sealing efficiency is improved due to a more stable seal configuration, and the new seal is not susceptible to roll. The new 965 elastomer, from which the seal is made, also offers a reduced coefficient of friction over standard nitrile and fluorocarbon based elastomers.

What is more, use of Arlon 1555 for the piston eliminates the need for using bearings entirely, thus allowing for tighter running clearances that allow for controlled reduced seal squeeze, as the amount of piston lay down is reduced. Overall weight of the piston may also be halved, reducing hysteresis and increased system response time.

The development team has also examined ways to improve the design of the damper piston. The piston can have an elaborate and sometimes complex profile or shape in order to restrict the flow of damper fluid, thus providing the damping effect. The main purpose of the seal and bearing is to prevent metal to metal contact. In this case, the sealing efficiency is a less important design criterion than it is in the separator piston seal. However, friction is an important parameter, and ultra low friction is a main goal of any design. Generically, most damper pistons use a metallic piston and an elastomeric energised step cut plastic plain bearing.

In this area, the company recommends that the damper piston be manufactured from Arlon 1555, and the O-ring seal replaced with an Enercap seal. As the damper piston then has enhanced tribological properties, the need for the step cut plastic wear ring of the former design is eliminated. Sealing efficiency is therefore improved without compromising the friction performance, as the seal contact area is also reduced. Seal squeeze can be tailored to achieve optimum performance as tighter running clearances can be obtained, reducing the piston lay down effect. Component weight is also significantly reduced, thus decreasing hysteresis and system response times.

Finally, the design of the damper rods seals and bearings may also be examined, in order to optimise their performance too. The damper rod itself is directly connected to the vehicle axle, thus rocking with wheel movement. The other end of the rod is connected to the damper piston, and the oil in the damper unit is at a pre-charged pressure, thus eliminating the need for rod seals.

As the damper unit is often used in conjunction with a spring, this applies a bending movement on the damper rod, thus a bearing is required to counteract the applied side loading. Typically, chrome-plated damper rods are used that run though an aluminium bearing and seal housing together with elastomeric energised cap seal(s) with a PTFE bearing. Various exclusion devices are currently used such as rubber wipers and PTFE based scrapers.

The Greene, Tweed solution is to replace the energised cap seal with an MSE seal and to manufacture the bearing from Avalon 57 (a specialist, low friction, low deformation under load, reinforced PTFE). Experience has shown that breakout and running friction is reduced significantly with savings up to 50% when compared to existing solutions. Leakage performance is not compromised and installation into the seal groove geometry is quicker and easier.

{{Greene, TweedTel: Nottingham (01159) 866555Enter 500}}

{{Figure 4: A comparison of the typical physical properties of Arlon1555 with other contemporary materials

Units Arlon Nylon 6 TFE Titanium Aluminium

Specific Gravity – 1.32 1.16 2.2 4.54 2.8Hardness Shore 85 80 55Tensile Strength Nmm2 105 83 65 620 600Tensile Elongation % 35 28 7 25 30Rel. Thermal Expansion – 11.9 80 100 8.5 23

Comparison of various physical properties of materials.

*typical values only refer to specific grade informationof exact spec.}}