b'February 2004Pressure Balancing of Shock AbsorbersIntroductionThe pressure balancing of shock absorbers is a new method of optimizing a particular damper set-up for a particular track or target velocity to reducehysteresis and improve grip while still avoiding cavitation. Poor response of a damper can be equated to a lag in pressure across the main pistonresulting from compliance in the damper including the compressibility of the oil. Hysteresis is reduced when pressure differences across the piston arereduced and when overall internal pressure levels are lowered. Reduction of hysteresis improves response of the damper and therefore enhances grip.What is Hysteresis?Hysteresis is lag in response of a damper usually due to compressibility of damper fluid from high pressures in the damper.What is Cavitation?A damper cavitates when the rebound pressure of the damper, while in compression, drops below thevapor pressure of the damper fluid or creates a vacuum. When this happens, dissolved bubbles in thefluid expand and cause voids in the damper fluid. These voids prevent the damper from functioningcorrectly because in order for fluid to flow through the piston, pressure must first build to a certainlevel. The pressure that is generated with shaft displacement must first collapse these voids beforefluid flow happens which causes a lag or dead spot in the rebound damping during change ofdirection of the damper. The vapor pressure of most hydraulic damper fluids is approximately 0 psi.Rebound pressure of a damper can drop below 0 psi if a very heavy main compression stack is usedon the piston without sufficient reservoir pressure, a head valve, or remote compression adjuster.This happens because the needed amount of fluid cannot pass through the piston in the time neededto replenish the void area and maintain oil pressure due to the stiffness of compression shims used.Instead of flowing through the piston, the column of fluid above the piston simply passes into thereservoir instead of being resisted and forced through the main piston to create force. If a dampercontinues to be used when cavitating, it will not produce consistent performance and the oil willeventually break down because of excessive shearing forces.What is Pressure Balancing?Pressure balancing is the method of making external and internal adjustments to the damper to achieve a symmetric pressure differential of compres-sion and rebound pressures (i.e. the pressure differential across the main piston). This coincides with the percentage of overall compression forceattributable to the head valve being 7-13%. At 7% head valve damping, the damper will have the lowest hysteresis and quickestresponse while being able to run low gas pressures (50-70 psi). At 13% head valve damping, lower gas pressures may berun with less of a chance of cavitation then the 7% build. Ideally, 7% damping should occur at low velocities while thepercentage ramps to 13% to avoid cavitation over bumps or high velocities while still running low pressures. A headvalve producing less than 7% overall compression damping will cavitate unless higher gas pressure is used. A headvalve producing over 13% will most likely never cavitate at any pressure used but will have increased hysteresis andslower response. This can be checked with a pressure-tapped shock as most dyno load cells will not have resolution atsuch low forces and friction may overshadow the results. When the pressure difference above and below the staticpressure line is the same, it can be reasoned that they will both equalize in the same amount of time without one laggingbehind the other. This improves response time of the damper which improves grip. Also, the less the overall pressuresare, the quicker the pressure can be equalized and the quicker the response. The overall pressures in a head valve orcompression adjustable damper tend to be less than a damper without because the needed gas pressure to avoid cavita-tion is less as well.Pressure balancing is done by measuring pressure at 4 different locations inside the damper and making adjustments tohead valve build, adjuster settings, main piston build, or a combination of all three to reduce internal pressures.Pres-sure tap locations are:(1) Rebound side of the main piston (oil pressure)between shaft bearing and piston(2) Compression side of the main piston (oil pressure)between main piston and head valve or CD housing(3) Head Valve (oil pressure)on exhaust side of head valve between compression shims of head valve or CD adjuster and reservoir piston(4) Reservoir (gas pressure)in gas reservoirBy evaluating pressure levels at these locations versus time while cycling the damper at a constant velocity, a goodpicture of dynamic pressure changes results.A graph of pressure versus time gives one direction in tuning thedamper components to balance the damper.'