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Mid-Valve Damping
ReStackor pro Fluid Dynamics
Motorcycle suspension tuning of an MX mid-valve setup is easy using the spreadsheet interface of ReStackor. The ReStackor-midvalve.xls spreadsheet computes damping force of the combined base valve and mid-valve system giving you the capability to test cavitation limits of bladders or ICS systems and tune shim stack configurations, valve ports, and damping forces produced by the integrated system.
The combined damping force curve produces three distinct regions. ReStackor pro gives you the capability to understand the influence of each valve in each region, tune damping forces in that region and control the suspension velocities where damping forces transition from one region to the next.
ReStackor pro gives you the capability to tune damping forces produced in each region of the damping force curve, the transition velocity in each region and the cavitation limits of the suspension.
The capability of ReStackor pro to model the physical flow processes occurring within the guts of the shock gives you the capability to understand the influence of each component on the overall performance of the shock. That combination of analysis and understanding produces a powerful tuning tool.
Region I: Low Speed
Low speed damping is controlled by the stiffness of the base valve shim stack and the clicker settings. The mid-valve produces little low speed damping. Tuning mid-valve float controls the suspension velocity where damping forces transition from low speed to mid-speed damping.
Region II: Mid Speed
Mid-speed damping is controlled by the combined force of the base and mid-valve. MX suspension setups use stiff mid-valve shim stacks to control bottoming on large hits. Enduro setups use softer mid-valve shim stacks and larger values of stack float to allow the suspension to absorb bumps rather then jump over them.
Region III: High Speed Cavitation
Stiff mid-valve shim stacks and low values of stack float can cavitate the rebound chamber during the compression stroke. When cavitated, compression damping increases and the following rebound stroke produces zero damping during collapse of the cavitation bubble in the rebound stroke.
ReStackor includes the physical flow models needed to define cavitation limits and the suspension performance when driven beyond the cavitation limit. The physics of flow cavitation are all handled internally within the code giving you the capability to simply enter a shim stack, click a button, and determine the suspension performance evaluating all of the following effects.
ReStackor pro gives you the capability to determine suspension cavitation velocities, tune the stiffness of mid-valve shim stacks , shock absorber bladder pressures and the stiffness of ICS springs to control cavitation. The capability of ReStackor to model each component in the suspension system gives you the ability to understand the influence of that component and tune those components to control damping forces and operation limits of your suspension over the entire range of suspension speeds. The combination of understanding and control gives you a powerful tuning tool.
Cavitation
At
high suspension velocities stiff mid-valve compression stacks can drive
pressures in the rebound chamber to near vacuum conditions. When pressure
falls to the vapor pressure of the hydraulic
fluid a vapor vacuum bubble forms. This is known as cavitation. The occurrence
of violent rebound chamber cavitation during the compression, and the capability of a shock to instantly recover from
that cavitation, has been eloquently videoed by Roehrig.
When driven beyond the cavitation limit
the mid-valve cannot deliver the fluid volume necessary to keep the
rebound chamber filled with fluid. 
Excess
fluid that can not pass through the mid-valve is pushed out through the
base valve into the fluid reservoir. This cavitation driven flow surge
increases back pressures produced by the base valve and bladder or ICS system
and partially heal the cavitating mid-valve flow (more).
Fluid Volume Balance
The fluid volume balance built into ReStackor pro gives ReStackor the capability to track the fluid volume in each chamber of the shock, compute the fluid circuit flow velocities necessary to maintain that volume balance and the reservoir pressures produced by bladder or ICS systems under both cavitating and non-cavitating conditions. Thorough formulation of the fluid volume balance gives ReStackor the capability to determine the cavitation bubble volume and the magnitude of the cavitation driven flow surge into the shock fluid reservoir. Quantifying the cavitation flow surge allows stroke limits of ICS pistons to be determined and the potential to damage plastic pistons used in some systems (more).
Force Accounting
For both cavitating and non-cavitating conditions the force on the shock damper rod is simply equal to the difference in pressure on the front and back face of the mid-valve. Pressure in the compression chamber is controlled by the base valve and back pressure of the fluid reservoir. Pressure in the rebound chamber is controlled by the mid-valve pressure drop. The difference in pressure on the two sides of the valve produce the damping force. The fundamental capability of ReStackor to compute shim stack stiffness and pressure drops through the fluid circuits allow the damping force to be computed through a straight forward pressure based force balance under both cavitating and non-cavitating conditions.
The simple concepts of pressure based force accounting and a volumetric fluid balance are fully implemented in ReStackor. The calculations track fluid volumes in each chamber of the shock, accurately compute flow velocities through the suspension circuits, evaluate cavitation limits of the shock and determine the damping forces produced over the entire range of suspension speeds. Combining the basic physical flow models of fluid dynamics with detailed modeling of the shock absorber components and comprehensive integration of the system performance gives you the capability to tune each component and control the system operation.
Suspension Tuning
The graphical user interface of ReStackor produces simple easily understood plots. Inspection of those plots allow you to understand the impact of each modification on performance of the system. This gives you the capability to easily run "what if" scenarios and develop and understanding of the influence of each component in the shock on overall system performance across the entire range of suspension speeds. Referencing the magnitude of damping force changes to the clicker settings allow you to use your ride experience to intuitively understand the magnitude of each damping force change. Integration of the base valve and mid-valve into a single calculation gives you the capability to simultaneously modify the shim stacks on each valve, understand the interaction of those valves, the velocity range and bump height where each valve controls damping, and reshape the damping force curve to obtain the desired ride quality and bottoming resistance. The capability of ReStackor to see inside the shock gives you the capability to understand the influence of each component in the shock and tune those components to control the suspension operation far beyond the limits previously possible.
ReStackor introduces a new era in suspension tuning.
Low Speed Compression Damping
Damping in the low speed regime can be tuned by changing the stiffness of the base valve shim stack or the mid-valve stack float. Changing the base valve stiffness effects both low speed and mid-speed damping. Modifying the mid-valve stack float influences damping in the low speed range and the velocity where the suspension transitions to mid-speed damping. ReStackor pro gives you the capability to tune low speed damping using either approach. Computing the combined damping force of the base valve and mid-valve in ReStackor is a simple matter of entering the stacks and clicking a button to produce the plot below. Having those curves referenced to the clicker ranges of your current setup (Ct.clsd and Ct.wo) makes evaluation of damping force a simple intuitive task.
ReStackor pro gives you the capability to evaluate changes to the base valve shim stack and mid-valve stack float and relate those changes to the clicker settings of your existing setup.
Float values on the mid-valve shim stack control the transition from low speed to mid-speed damping. Lower values of float transition earlier. In the example above reducing the mid-valve float by 0.1mm almost completely eliminates the linear portion of low speed damping force curve.
Mid-Speed Compression Damping
Mid-speed damping is controlled by the combined damping force of the base and mid-valve. Decreasing the mid-valve stiffness moves the damping curve toward a flat linear profile. Changing the mid-valve shim stack stiffness has little effect on low speed damping. This gives you the capability to independently control mid-speed damping.
Increasing the stiffness of the base valve shim stack effects both low speed and mid-speed damping. Crossover shims can be added to the base valve stack to modify that behavior. ReStackor gives you the capability to understand the effect of each valve, tune the damping force produced by that valve, control the suspension velocities where each valve effects performance and understand the effect of shim stack modifications before the stacks are installed on your bike. The capability to determine effects of potential modifications over the entire range of suspension velocities helps to insure that the modifications do not produce an unexpected side effect at other suspension speeds.
Plotting the combined damping force of the base and mid-valve allows the effect of each modification to be determined before the modified stacks are installed on your bike.
High Speed Damping
Stiff mid-valve shim stacks can cause the rebound chamber to cavitate at high suspension velocities. When cavitation occurs damping rates in both the compression and rebound stroke are effected. ReStackor pro has the capability to evaluate cavitation limits, the suspension velocities where cavitation initiates and the damping forces produced when the suspension is driven beyond the cavitation limit.
Shock Bladders
Bladders pressurize the fluid reservoir of a shock and backpressure the fluid circuits to maintain pressures above cavitation limits. Low bladder pressures, stiff mid-valve shim stacks or soft base valve shim stacks can drive the shock absorber into cavitation (more).
Increasing shock reservoir bladder pressure raises the cavitation limit to a higher suspension velocity.
ICS Piston
Pressures produced by an ICS system in the fluid reservoir of a shock are a function of the ICS spring stiffness and the suspension position. As the suspension is driven into the stroke fluid entering the ICS chamber compresses the spring and builds pressure in the shock reservoir. Cavitation limits in an ICS system are a function of suspension position, velocity, ICS spring stiffness and the flow resistance of the base and mid-valve shim stack (more).
Stiffer ICS springs extend cavitation limit to a higher suspension velocity.
Mid-Stroke Harshness
In a motorcycle fork it is desirable to have near zero spring force at the top of the stroke. This allows the suspension to float over small bumps when the bike is under hard acceleration or on a hill climb. To satisfy that criteria, ICS systems produce near zero pressurization of the fluid reservoir at full extension of the suspension. This results in a system that is easily cavitated at the top of the stroke and recovers from cavitation as the ICS system builds pressure deeper in the stroke. Allowing the suspension to flip into and out of cavitation over the coarse of the stroke results in inconsistent damping rates and a number of suspension ills often summed up as mid-stroke harshness.
With stroke depth controlling pressurization of an ICS system and suspension velocity controlling cavitation limits the tuning of an ICS system is not easy. Controlling effects of cavitation requires tuning the base valve stack stiffness, mid-valve stiffness, stack float, and ICS system to all simultaneously operate within the limits of cavitation. Simultaneous control of multiple parameters is difficult when tuning by the seat-of-the-pants. The physics based models of ReStackor give you the capability to test and understand the influence of each component in the system and identify the components that are controlling the system operation at a specific condition. With that understanding the system can be tuned and multiple components simultaneously tuned for control of the overall system operation. The capability to test potential modifications over the entire range of suspension velocities and stroke depths helps to insure that modifications do not produce unexpected side effects. ReStackor gives you the capability to understand and tune your suspension far beyond the limits previously possible (more).
Cavitation limits in an ICS chambered fork are a function of suspension velocity and stroke depth.
Accuracy
Cavitation limits are defined by small differences in large numbers. If the base or mid-valve pressure drop is off by 10 psi one way or the other the impact on overall damping force is small. But, if the base valve pressure drop is 10 psi high and the mid-valve is 10 psi low the 20 psi swing in pressure has a large impact on cavitation limits. Due to this sensitivity cavitation limits are difficult to compute without specific calibration of the calculations to each specific suspension setup. Test rides are required to quantify system performance.
The central focus of ReStackor is in-situ tuning of suspensions. In-situ tuning gives you the capability to test your suspension on the actual terrain and the actual speed that you ride and relate suspension performance and clicker settings to the specific shim stack modifications needed to achieve specific damping rates.
Here, under actual ride conditions, it makes little difference whether your suspension cavitates at the suspension velocities of a 4 inch bump or a 5 inch bump. The fact is it cavitates and cavitation produces a range of suspension ills easily identified by ReStackor. ReStackor gives you the capability to understand the effect of cavitation on suspension performance, the capability to identify the specific suspension components causing cavitation and the tools needed to tune those components for control of cavitation. The capability to do that depends on your ability to implement the art of suspension tuning and interpret the vague details of suspension feel to the specifics of computed damping force, component performance and cavitation limits.
Suspension tuning is and remains an art. ReStackor simply supplies the tools needed to interpret and implement that art.
Suspension Cavitation Limits
The fundamental capability of ReStackor to compute pressure drops through suspension circuits provides the information needed to understand and control cavitation limits. Easily interpreted plots quantify the performance of each component in the shock. Effects of modifications to those components on the overall system operation are easily interpreted. Plots of the pressure in each chamber of the shock allow the onset of cavitation to be easily identified as well as the modifications needed to avoid cavitation. The ReStackor fluid volume balance quantifies the magnitude of cavitation and the effect of cavitation events on overall system performance.
ReStackor gives you the
capability to understand the details of shock operation and the interaction of components within the
shock on overall performance. That understanding of the system operation
and component interaction gives you the capability to easily tune the
damper:
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Stiff mid-valve shim stacks can drive the rebound chamber into cavitation during the compression stroke. ReStackor gives you the capability to tune the mid-valve stack float, stiffness and fluid reservoir pressurization system to control the initiation and recovery from cavitation.
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Base valve stack stiffness can be tuned to suppress mid-valve cavitation through control of the shock compression chamber pressure.
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Bladder pressure can be tuned to control the suspension velocity where cavitation initiates.
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Bladder volumes can be tuned to accelerate cavitation recovery deeper in the stroke.
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ICS spring float can be tuned to accelerate cavitation recovery.
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ICS spring stiffness can be tuned to control the suspension stroke depth required for cavitation recovery.
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The ICS system can be tuned to match a mid-valve shim stack setup or the mid-valve can be tuned to match the ICS system.
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Cavitation of the rebound chamber during the compression stroke results in zero damping during the following rebound stroke while the cavitation bubble collapses. ReStackor gives you the capability to quantify the size of the cavitation bubble produced and the capability to tune the suspension components to control the magnitude of rebound damping loss.
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The cavitation flow surge into the fluid reservoir can over stroke and damage ICS pistons. ReStackor quantifies the magnitude of the cavitation driven flow surge and gives you the capability to tune the system for control of ICS piston stroke depths.
Solid fluid dynamic models implemented within the framework of conservation of mass and momentum in ReStackor provides an engineering analysis tool capable of quantifying the effect of each component in the shock on overall system performance and suspension operation over the entire range of suspension speeds. This gives you the capability to test and experiment with the tuning of each component and understand the influence of that component on overall system performance. Developing an understanding of component performance allows the overall system to be tuned and the specific components controlling suspension operation identified as a function of suspension velocity and stroke depth. Understanding the influence of each component gives you the capability to tune those components and exploit the features of each component to optimize the system performance and control damping forces over the entire range of suspension speeds.
ReStackor gives you the capability to understand and tune your suspension far beyond the limits previously possible.
