An inherent characteristic of a coil spring when mounted co-axially around the damper of a vehicle with fixed position spring perches is that it will generate a lateral load due to its not being able to distribute load forces evenly around the face of its end coils. This lateral loading manifests itself as a bending load in such coil over applications, and results in greatly increased frictional forces within the damper.
In vehicle applications such as racing cars, this extra friction engenders lowered tire grip levels, and higher tire wear.
The American racing car constructor Indianapolis Competition Products first became aware of this problem in the early ’90’s while trying to solve some handling problems with its Citation Formula Ford cars. It’s early efforts focused on purely mechanical devices and met with mixed results. In the late 1990s the company developed a hydraulic device but progress with this was hindered partially because of the lack of the necessary test equipment to properly quantify results from design changes.
By chance one of ICP’s customers had the necessary connections that would resolve this. ICP vice president Richard Pare explains: “John LaRue who was racing one of the Citation Formula Ford cars that my company had built happened to be an old friend of Kelly Falls who is the General Manager spring manufacturer Hyperco. It turned out that teams had complained to Hyperco about the problem separately from the work we had been doing but in the same sort of time frame. Hyperco had done its own R&D program for a re-design of the spring to see if they could reduce the side loads but the results were inconclusive. I asked John to see if Kelly would like to try out my device if they had a suitable machine to test them and they did.”
Soon after an early version of ICP’s hydraulic device was tested by Hyperco on their ultra high quality spring rate and load test scale, which charts axial load as well as longitudinal load. The results were startling. In some configurations, 98% of the lateral forces were eliminated. An agreement was reached for the two companies to jointly develop and promote the perches, with Hyperco providing the marketing expertise, and ICP providing the design and manufacturing.
Like so many good inventions the HCD (Hydraulic Coupling Device) is simple in design and operation. Essentially it of consists of two circular components – the perch (effectively the piston) and a body (effectively the cylinder) that are sealed to each other with o-rings, and the resultant cavity filled with a hydraulic fluid.
So how does it work? Richard Pare explains: “Careful shaping of the sealing walls of both components allows the perch to tilt which allows the end of the coil to tilt – to become ‘unsquare’ to the axis of the spring. The reason it needs to do that to centre the load is that this is the only way you can get an even force around the face of the end coil of the spring. If you think of the end of the spring as a cantilever, the pivot point is where the end coil closes – where the wire comes back and touches itself. That is the point at which the load is first applied to the spring perch. The purpose of the device is to get the load to distribute itself around the full face of that end coil. Because that end coil is not infinitely stiff, as it accepts the load it has to flex. It’s like putting a load out on the unsupported end of a diving board, the diving board will flex. That’s really all the device does, it allows the end coil to tilt so that the load is evenly spread around its face. In doing that it automatically centers the load at the centerline of the spring and damper unit.
There is a back up to that tilting in the hydraulic element of the device. Because there is hydraulic pressure in a circle, that internal pressure is equal everywhere. That hydraulic pressure is what transfers the load from the part of the device that the spring sits on to the part of the device that is physically attached to the damper. Because that hydraulic pressure is uniformly distributed in a circle and centered on the shaft or body of the damper, it also acts as a correcting force. Achieving perfect mechanical efficiency is only prevented by the residual friction between the seals and the device walls.
Versions of the device are available for use with most widely used damper models and custom made versions for specific applications can be supplied. Although the HCD was first shown in public at the Performance Racing Industry Show in Indianapolis in December 2002, it has already made inroads a number of racing categories around the world. It has been adopted by the majority of Indy Racing League teams and is used by winning teams in the CART Champ car, Toyota Atlantic, Trans-Am, Formula Ford, Sprint Car, Midget, Late Model Cars (Dirt & Pavement), motorcycle road racing teams, Moto Cross Racing and Snowmobile Racing. Essentially all categories running a coil over shock spring assembly will benefit from the hydraulic load centering perch.
New for 2003 is a driver-controlled ride height/wedge adjusting system incorporating HCD technology. This comprises two hydraulic weight jackers with the HCDs as integral components and actuators slaved to a single, “neutral force” driver control. The control system allows both springs to be raised or lowered equally for ride height control, or for just one spring to be changed for wedge (diagonal weight) control. Control of the first iteration of the system is via turn-knob screw pistons with air pressure counterbalance. (The “neutral force” counterbalance being necessary to lower adjuster friction levels for minimal driver effort). Two knobs are employed – one for dual spring control, and one for single spring control. A lever action control system is currently undergoing final development, and is expected to be available shortly.