Purchase Hydraulic Spring Perches

Hydraulic Load-Centering

Spring Perch Technology

Usually the fastest car in a class is well balanced (understeer/oversteer), has good mechanical grip (sometimes referred to as traction or bite) and does not impose excessive wear on the tires.  Balance is achieved through careful selection of various rates and settings.  Grip and good tire wear are achieved from minimizing tire force variation.  Tire force variation is the measure of change in load between the tire and the track. Minimizing it has also been termed 'keeping the tire quiet on the track".  It is well recognized that reducing friction in any suspension component aids in minimizing tire force variation.  A 'drivable' car with good grip and low tire wear can win races! 

As a coil spring is compressed it wants to bend in an arc.  When constrained from arcing by a damper with fixed perches, the spring generates a bending load between the perches.  This greatly increases friction forces between the damper piston ring and cylinder wall as well as between the damper shaft and the bushing which guides it.  While a driver feels only the rather low velocity motions of the chassis, data acquisition shows that the shock is constantly experiencing more rapid changes in direction at the rate of 10 to 20 times per second as the tire travels over the course of the track.  Every time the damper changes direction it first has to stop moving.  When stopped the static friction forces attempt to prevent the damper from moving again.  This disturbs the tire contact patch contributing significantly to increased tire force variation, loss of grip, increased tire wear and slower lap times. 

An additional characteristic of a coil spring is that as it is compressed it wants to unwind.  When fixed perches do not allow it to unwind, it's rate increases more and more as it compresses.  As if it isn't difficult enough to maintain balance in the car, now there are unpredictable changes in corner rates and cross weights thus changes balance!

The Hyperco Hydraulic Load-Centering Spring Perch consists of two circular components - the perch (effectively the piston) and a body (effectively the cylinder) that are sealed to each other by o-rings with the cavity between them filled with hydraulic fluid.  The precise shaping of the sealing walls of both the perch and cylinder body allow the perch, along with the end of the coil, to rotate as well as to tilt freely becoming ‘unsquare’ to the axis of the spring thus evenly distributing the load of the spring.  The result is a reduction of up to 96% of the bending load on the shock along with minimizing spring rate change.  Better yet is the increase in drivability, reduction in friction, increase in grip, better tire wear and lower lap times!

Hyperco Hydraulic Spring Platform 7-Post Results                                       12/03

Background

A 7-Post shake rig test was performed at the ARC facility in Indianapolis in December, 2003.  This test was performed on the 2003 Dallara IRL chassis.  The basic car setup was for a 1.5 mile oval configuration.  One of the test items was a prototype Hyperco hydraulic spring perch designed exclusively for the front spring/damper unit of the Dallara chassis.  The Dallara front suspension uses a special 1.5” I.D. front spring which operates on top of the damper body.

The 7-post input profile consists of a constant velocity, increasing frequency sine sweep from 0-16 hz.  The wheelpan velocity used to generate the following data had a peak velocity of 50 mm/sec.

Test Results

The spring perch was tested around a baseline setup which utilizes the new 2004 Penske 8770 front damper.  A baseline run was first made without the perches.  The perches were then installed with no other changes made.  It was decided that the best way to measure the differences made by the platforms was to focus on the initial damper movements. If the reduction in spring side load resulted in decreased damper shaft friction, it should be most apparent at the small initial movements.  

The first comparison involves a time history of front damper displacements both with and without the platform.  See Figure #1

 The blue trace is the baseline damper displacement without the perch, and the pink trace is the damper displacement with the hydraulic perch installed.  The overall magnitude of these displacements is small, +- 0.015”, but there is a considerable and consistent percentage difference between the two cases. 

The second measurement technique involves a comparison of the phase angle transfer function of the pushrod load and damper displacements.  This provides an indication of the damper movement relative to pushrod load.  See Figure #2

A higher phase angle value represents a greater level of damping.  Again, the blue trace is the baseline damper displacement without the perch, and the pink trace is the damper displacement with the hydraulic perch installed.  The hydraulic perch clearly allows more initial movement with respect to the pushrod load, which relates to less damping or less opposition to motion.  What is also interesting is that the phase angle values tend to merge as the magnitude of the displacement increases.  This would indicate that the frictional damping is a much higher percentage of the overall damping at the smaller initial displacements.

On-track testing of these identical test conditions was performed at the IRL Homestead open test on January 27-28, 2004.  On-track testing correlated well with this encouraging rig test data.  A decrease in friction or damping at the “micro” level displacements led to an increase in tire grip, without sacrificing support at the greater displacements related to chassis roll 

                                                         Additiional Background

The evaluation process for oval configuration shaker testing is essentially the same as it is for road course testing. The roll component of the vehicle response introduces an additional dimension for consideration.  The main measure of “goodness” is still the maximum Pitch value,

(lower value is better).  An “average” Pitch value is also recorded, which is a measure of the overall magnitude of the pitch response transfer function.   These values are unit-less, and may be considered as a response gain.  A Pitch value of ‘0’ would represent no relative difference between the front and rear ride heights during excitation. The other noted parameters are Heave and Contact Patch Load variation (lower values also better).  A heave value of 1 would represent unity with the input excitation.

The Pitch is the value that is primarily used to evaluate the car.  The Heave and CPL values are more incidental, and might be used to distinguish between two runs with identical Pitch values.  The Contact Patch Load variation is simply described as the contact load magnification with respect to the input acceleration.  These loads are measured for each respective corner of the car, and their measure may be grouped or summed together for analytical purposes.  These groupings consist of: Front Axle CPL variation, Rear Axle CPL variation, Total CPL variation, and Wedge CPL variation.  The Wedge CPL variation is simply a measure of the (FR CPL+ RL CPL) \ Total CPL. The Wedge CPL variation should give a measure of understeer tendency, with a higher value indicating a greater tendency for understeer.

The phase angle between the push rod loads and the CPL at its maximum response is a measurement used to distinguish damping forces from spring forces at the respective corners of the car.  A force, in phase with displacement, is considered a spring force (displacement sensitive), and a force, 90 degrees out of phase, is considered a damping force (velocity sensitive).  For example, a value greater than 45 deg. indicates that the damper is a larger component of the total force.  Conversely, values less than 45 deg. indicates that the spring is a larger component of the total force.  A 45 deg. value would indicate equal force from damper and spring.