ReStackor Fluid Dynamic Models

Finally computer software to tune a shim stack

ReStackor pro models 10 components of the valve flow circuit

There are ten tunable features in the fluid circuits of a shock absorber modeled by the numerical shock absorber analysis of ReStackor. If each of these features were tuned to be fluid dynamically perfect there would be no flow losses through the shock fluid circuits and the shock absorber would produce no damping. Tuning shock absorbers is the process of tuning each feature in the fluid circuits of a shock to produced controlled fluid dynamic flow losses. Through control of the flow losses you can tune the shape of the damping force curve and control the damping forces produced at specific suspension speeds.

The numerical modeling of shock absorbers in ReStackor pro uses basic fluid dynamic principals to deteremine the flow losses through each feature of the suspension circuit. This allows you to tune each feature, control the flow loss and the shape of the damping force curve across the entire range of suspension speeds. Accurate control of flow loss is the key to shock absorber tuning .

ReStackor pro models both mid-valves and base valves of suspension systems

There are two types of valves used in suspension systems: mid-valves and base-valves. The relationship of oil flow rate through the valve and damper rod speed are different for these two types of valves. ReStackor pro uses keywords BVc, MVc and MVr to specify the different valve types and the direction of flow through the valve. The differences in flow rates and its impact on damping forces produced by base valves and mid-valves is discussed here.

ReStackor models both mid-valves and base valves used in suspension systems.

ReStackor uses real world commercial suspension fluid viscosity data

Peter Verdone has compiled an excellent data base of the viscosities of commercial suspension fluids. ReStackor pro uses Verdone’s Silkolene viscosity data to establish the relationship of SAE wt to fluid viscosity. Effects of fluid temperature on viscosity are corrected for in ReStackor calculations using the Andrade equation. This allows ReStackor to match reported viscosity data at cSt@40c and cSt@100c as well as reliably compute viscosities at room temperature of even sub-zero temperatures (more).

ReStackor accurately computes effects of fluid viscosities on suspension performance.

Accurate modeling of fluid viscosity allows ReStackor pro calculations to accurately compute skin friction losses in the valve flow circuits over the entire range of suspension speeds and operating temperatures. This allows ReStackor to reliably compute flow rates through suspension bleed circuits and accurately reference shim stack changes in terms of how many clicks stiffer or softer each change will be.

Bleed circuit "clicker" flow rates

ReStackor pro accurately computes flow rates through the bleed "clicker" circuits. This allows you to take a set of clicker settings that work for whoops and a separate set of clicker settings that work for rocks and use ReStackor to retune the shim stack to match those specific flow resistances at specific wheel speeds. This retunes the shim stack to use of a single clicker setting at all conditions. The capability to relate wheel speed to damping rate is a key feature of ReStackor pro and allows riders to target specific shim stack changes significantly streamlining the tuning process.

The bleed port flow area is determined from the clicker settings based on a simple cone shaped needle geometry. A blunt tip needle geometry is assumed that blocks 25% of the flow area at the wide open position. Through adjustment of the input parameters N.clicks and d.bleed the user has complete control of the flow area of the bleed circuit.

ReStackor pro accurately models flow rates through the clicker bleed circuits.

The capability to accurately compute the pressure drop and flow rate through an orifice is a basic fluid dynamic problem dating back to development of the Bernoulli equation in the 1700’s. There is a huge industrial importance for accurate fluid metering and huge development efforts have been spent to arrive at a set of equations capable of accurately calculating flow rates through orifice metering circuits. Fluid dynamic relationships developed through these efforts account for effects of flow geometry, orifice length, flow velocity as well as fluid viscosity and are capable of accurately computing flow rates over the entire range of suspension speeds from ultra low to ultra high speed. ReStackor uses these well anchored Reynolds number based equations to accurately compute flow rates and fluid resistances through the bleed circuits.

Valve Leak Flow

ReStackor calculations assume the valve piston seals, stack seat and check valve are in working order with zero leak flow. If your suspension is known to have a leak this can be modeled in ReStackor using either the bleed clickers or stack float to simulate a leak.

Valve port entrance loss

There are two styles of valve ports used in suspension systems. Discrete side entrance ports drilled into the side of the valve or annular slots created by the gap between valve ports and the stack. For either style the fluid flow area at the entrance is the critical parameter controlling flow losses. Flow losses at the port entrance are caused by viscous interactions in the fluid as the fluid accelerates from the stagnate conditions of the reservoir to the high velocities in the valve port. ReStackor uses Reynolds number based flow loss coefficients to quantify these losses.

ReStackor pro calculations model conventional or side entrance port geometries.

Valve port turning losses

Downstream of the entrance additional flow losses are generated in the valve ports due to turning the flow. These losses are created by a combination of momentum loss from changing the direction of the flow and potential flow separations at higher suspension speeds. ReStackor estimates turning losses using the flow area of the entrance and valve port based on a sharp 90^o bend. This simple model has been demonstrated to adequately model turning losses in suspension systems through comparison with dyno test data. The turning losses can be reduced by modifying the piston valve to increase the port entrance area or the valve port diameter. ReStackor pro models both of these effects. This allows you to use ReStackor calculations to experiment with various modifications to valve port geometry and evaluate effects of potential changes at different suspension speeds before committing to irreversible modifications of the valve port geometry.

Example ReStackor calculations evaluating effects of valve port restrictions are included in the XR650 sample application page of the ReStackor web site.

Shim Stack deflection

Shim stack lift on the valve face is caused by a combination of static pressure and jet impingement forces. At low suspension speeds static pressure is the dominate force controlling stack lift. As suspension speeds increase jet impingement become progressively more important as well as the flow losses incurred in the valve flow circuits to generate the high momentum jet. Accurate calculation of damping rates requires careful accounting of the jet impingement forces acting to deflect the stack.

Thorough modeling of the stack structure stiffness and accurate accounting of the jet impingement forces allows ReStackor to accurately compute flow losses created by acceleration of the fluid through the shim stack gap at the valve edge. ReStackor pro has been demonstrated to accurately account for these effects through comparison with dyno data.

Jet turning loss

Jets from the valve port deflect off of the shim stack face and exit the thin annular gap between the shim stack and valve face. Jet stagnation at the impingement point cause the boundary layer to reinitialize resulting in high skin friction for the flow across the shim stack face.

Reinitializing the flow at the stagnation point also creates distortions within the fluid velocity profile creating increased flow resistance at the valve edge gap. ReStackor pro uses the impinging jet velocity profiles in Blevins’ “Applied Fluid Dynamics Handbook[1]” for description of the velocity profile.

Impinging jet effects are a function of the jet velocity and are continuously changing over the range of wheel speeds. Suspension valves using large ports produce lower flow velocities and have reduced jet impingement effects. The above model allows ReStackor to model these effects in a natural way.

[1] Blevins, R.D.,"Applied Fluid Dynamics Handbook", Krieger Pub, Malabar, Florida 2003.

Stack exit turn

After exiting the valve the flow impinges on the shock body and turns to enter the downstream fluid reservoir. ReStackor calculations account for this turning loss and the losses created as the fluid accelerates through the annular gap between the shock tube body and the largest shim.

Fluid dynamic flow losses

ReStackor pro uses physics based fluid dynamic relationships to account for pressure losses and flow restrictions of the fluid circuits of a shock absorber. These physics based models allow geometric features of the flow circuits to be modeled and give you the capability to tune each component of the flow circuit, determine effects created by those modifications and the damping performance of the suspension.

Fluid viscosity
Valve port entrance area
Valve port area
Valve diameter
Valve port width
Valve port center of pressure
Bleed port diameter
Clicker settings

Shim stack stiffness
Shim stack diameter
Stack edge stiffness
Stack face bend profile
Stack float
Stack preload