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ReStackor User Manual

Finally computer software to tune a shim stack

Mid-Valve Damper Tuning Plots

The two figures below contain the bottom line of ReStackor mid-valve analysis. The first figure shows damping force as a function of suspension velocity, the second figure shows damping force as a function of stroke depth. The suspension damping performance, cavitation limits and the effectiveness of bladder or ICS systems are all contained on these two basic plots giving you the information you need to tune a mid-valve setup.

ReStackor_midvalve.xls spreadsheet computes the combined damping force and cavitation limits of a mid-valve setup.

The remainder of the plots in the ReStackor-midvalve.xls spreadsheet are simply there to help you understand why the damping force curves are shaped the way that they are, the effect of each shim stack on the shape of the damping curve and the influence of bladder or ICS systems on the damping profile. Understanding the influence of these systems gives you the capability to tune the components, individually or in combination, control the shape of the damping force curve and the performance of your suspension over the entire range of operation.

Each plot produced by the ReStackor-midvalve.xls spreadsheet and the influence of each of these systems on the shape of the damping force curve are discussed below.

Cavitation Limits

The mid-valve compression shim stack is typically much softer then the base valve stack. Due to the mid-valve float and the softer stack little damping force is produced by a mid-valve in the low speed range. As suspension speeds increase, higher oil flow rates through the mid-valve produce a rapid increase in damping force as well as the pressure drop across the mid-valve.

For the sample ReStackor calculation shown the pressure drop across the mid-valve matches the base valve at a damper rod velocity of approximately 60 in/sec. Above that velocity the mid-valve pressure drop rapidly increases resulting in progressively lower pressures in the rebound chamber. When the chamber pressure falls to the vapor pressure of the fluid the fluid in the rebound chamber will flash, or vacuum boil, into the gas phase. This is the cavitation limit.

Cavitation limits are controlled by the backpressure supplied by bladder or ICS pressurization systems in the shock fluid reservoir. These systems backpressure the fluid circuits of the shock and attempt to maintain chamber pressures above the cavitation limit over the entire range of suspension velocities. If the back pressure supplied is insufficient the rebound chamber will cavitate. ReStackor calculations track the fluid pressures in each chamber of the shock through the entire suspension motion making evaluation of cavitation limits and suspension velocities where cavitation may occur easy.

Bladder Pressurization

As suspension speeds increase pressures in the rebound chamber are driven to progressively lower values. For the sample ReStackor calculation the bladder system successfully prevents cavitation up to damper rod velocities of 190 in/sec. Above that speed the mid-valve pressure drop forces cavitation of the fluid in the rebound chamber.

When cavitated, flow that can not pass through the mid-valve is forced out through the base valve. This cavitation driven flow surge into the fluid reservoir results in an increase in reservoir chamber pressures and increased flow resistance through the base valve shim stack. Both of these effects are easily identified in the ReStackor plot as the suspension is driven beyond the cavitation limit.

ICS Pressurization

An ICS system produces a much different cavitation limit behavior. For an ICS system the fluid reservoir is initially un-pressurized at the top of the stroke and becomes pressurized as fluid is transferred into the reservoir as the suspension is driven deeper into the stroke . Lack of reservoir pressurization at the top of the stroke in an ICS systems result in a suspension that easily cavitates at the top of the stroke. As the ICS system builds pressure deeper in the stroke the suspension recovers from the initial cavitation. The process of an ICS system flipping into and out of cavitation over the coarse of a stroke produces a number of unusual effects in a ICS chambered fork (ReStackor Web Site).

The behavior of cavitation in an ICS pressurized fork can be understood by reviewing the plots produced by ReStackor. At the top of the stroke the shock reservoir is at the initial bleed pressure of the fork, 14.7 psia. Over the first 1.5 inches of stroke there is little change in the reservoir pressure due to the float on the ICS spring. Beyond 1.5 inches the ICS spring engages pressurizing the reservoir and the shock compression chamber.

With no pressurization of the fluid circuits at the top of the stroke the rebound chamber cavitates at the start of the suspension motion producing near zero pressures in the rebound chamber. This cavitation is caused by the stiffness of the mid-valve shim stack. With the stack closing off the valve ports the mid-valve can not deliver the fluid volume necessary to keep the rebound chamber filled with fluid. Due to the insufficient flow a vacuum cavitation bubble forms. Preventing cavitation requires higher pressures in the shock compression chamber (Pc) to force the volume of fluid necessary through the mid-valve to keep the rebound chamber filled with fluid.

As the suspension is driven deeper into the stroke pressures produced by the ICS system increase. The pressure in the rebound chamber remains fixed at the vapor pressure of the hydraulic fluid as long as the cavitation bubble is present in the rebound chamber. This produces a high pressure drop across the mid-valve and rapidly fills in the cavitation bubble. While the cavitation bubble is filling the high pressures produced by the ICS system act directly on the face of the mid-valve and react a higher then normal damping force to the damper rod. When the bubble is completely filled pressures in the rebound chamber suddenly spike up. The spike is caused by the jump from the near vacuum conditions of the cavitation bubble to the near incompressible hydraulic flow conditions of non-cavitating flow. This jump in pressure pressurizes the back side of the mid-valve and causes a corresponding jump in damping force.

While the cavitation bubble is filling , flow rates through the mid-valve are increased. Higher flow through the mid-valve results in a decreased flow through the base valve. The steadily decreasing base valve flow created by filling of the cavitation bubble produces the arc like shape of the compression chamber pressure profile from 1.5 to 3.9 inches. Once the cavitation bubble has completely filled the flow rates through the suspension circuits sharply jump to the near incompressible conditions of hydraulic flow. This restores the flow through the base valve and results in a sudden jump in the base valve pressure drop as shown in the plot. ReStackor calculations show the pressures in both the rebound and compression chamber sharply jump at the end of cavitation.

Suspension Velocity Effects On Cavitation Limits

Determining the suspension velocity where a given stroke depth will cavitate is easily determined with ReStackor by inspection of the damper rod velocity plot. For the six inch suspension stroke depth used in this example the suspension can not recover from cavitation at suspension velocities above 170 in/sec.

Below velocities of 170 in/sec the suspension initially cavitates then recovers from cavitation over the coarse of the six inch suspension stroke. Above the cavitation limit of 170 in/sec the suspension fails to recover.

When cavitated, the flow surge created by cavitated mid-valve pushes additional fluid into the shock reservoir. The increasingly larger flow surge as the shock is driven above the cavitation limit creates the continuous increase in shock reservoir pressure due to compression of the ICS system. Pressures in the shock compression chamber step down slightly at the jump from non-cavitating to cavitating flow. This step in pressure is caused by the reduced flow through the base valve when the cavitation bubble is filling. As the suspension is driven further away from the cavitation point the cavitation bubble shifts from the filling process to the growth process creating the compression chamber pressures shown. (more, ReStackor Web Site).

Relationship of Stroke Depth and Suspension Velocity

Cavitation limits are a function of suspension velocity and suspension position for an ICS chambered fork . The above plots of chamber pressures as a function of suspension position and suspension velocity allow easy identification of cavitation limits. To further simplify that process ReStackor logs the cavitation threshold at each suspension damper rod velocity and suspension position. Using this data a plot is easily produced of the cavitation threshold at each suspension position and damper rod velocity. Operation above the curve at deeper stroke depths or lower suspension velocities will not cavitate the suspension. Operation below the line results in cavitation of the rebound chamber.

Cavitation Severity

ReStackor reports the severity of chamber cavitation as the fraction of rebound chamber filled with fluid [Nr]. For a deep suspension stroke the rebound chamber may initially cavitate at the top of the stroke and then recover as the bladder or ICS system builds pressure deeper in the stroke. The process of bubble formation and filling over the suspension stroke can be viewed through the behavior of [Nr]. If the value of [Nr] is equal to one the chamber is completely filled with fluid and operating under non-cavitating conditions. When [Nr] falls below one the chamber is cavitating.

Cavitation effects both the compression and rebound stroke. When the mid-valve is cavitated compression damping forces are reduced. For an ICS system under cavitating conditions compression forces are reduced through the first portion of the stroke and increased during the cavitation bubble filling process. If the rebound stroke is initiated before filling of the cavitation bubble the rebound stroke will produce no damping until the cavitation bubble has collapsed. The collapsing bubble actually produces a suction force pulling the mid-valve and damper rod toward full extension. Once the cavitation bubble has collapsed the remainder of the rebound stroke produces normal damping.

The fraction of rebound stroke losses can be determined from the value of [Nr]. For the example below at a suspension velocity of 350 in/sec the suspension losses 25% of the rebound stroke. For the right hand figure examining the suspension stroke the velocity is 250 in/sec.

Rebound chamber cavitation severity is reported in ReStackor outputs as the ratio of fluid volume to chamber volume. The calculations also determine the fraction of ICS stroke used.

The ReStackor parameter [Nics] reports the fraction of the bladder or ICS stroke used. For an ICS system the stroke fraction is the ratio of stroke used to the coil bind travel length of the ICS spring. For a bladder [Nics] is the fraction of bladder volume used.

When the rebound chamber is cavitating excess fluid that cannot pass through the mid-valve is forced out through the base valve. The cavitating surge causes the value of [Nics] to increase as the suspension is driven beyond the cavitation limit of 160 in/sec. Under severe cavitating conditions the surge can cause the ICS system to bottom out and potentially damage plastic ICS pistons used in some systems. ReStackor gives you the capability to determine the ICS stroke used and tune the system components to control the ICS stroke.

Damping Force

The damping forces over the stroke length computed by ReStackor is shown below. There are eight curves on the figure:

Ft.clsd [=] Combined base and mid-valve damping force with the base valve clickers in the closed position. Rebound clickers on the mid-valve are set at the position requested on the MVc input sheet. To determine the effect of the rebound clicker position on compression damping forces you can run additional calculations with the rebound clickers set at various positions and compare the results.
Ft.clk [=] Combined base and mid-valve damping force with the base valve clickers set at the position requested on the BVc input sheet.
Ft.wo [=] Combined base and mid-valve damping force with the base valve clickers set at the wide opened position.
Fbv.clsd [=] Damping force produced by the base valve with the base valve clickers closed.
Fbv.clk [=] Damping force produced by the base valve with the clickers set at the requested clicker position.
Fbv.wo [=] Damping force produced by the base valve with the clickers wide opened.
Fmv [=] Damping force produced by the mid-valve at the requested clicker position.
Baseline [=] If you click the "Baseline" button on the main plot page the ReStackor spreadsheet will save the current calculation results on the Baseline tab of the spreadsheet. The baseline curve shows the Ft.clk damping force from a the previous calculation saved by the "Baseline" macro button.

ReStackor pro cavitation analysis shows mid-valve cavitating above suspension velocities of 150 in/sec.

The open and closed clicker settings are shown on the plot for reference. This allows you to user your "real world" riding experience to understand the magnitude of damping force changes. The curves also show you the variation in cavitation limits as clicker settings are changed.

Including the base and mid-valve damping forces on the curve help you determine the relative damping force delivered by each system. For this case the damping force of the mid-valve dominates the damping force.

Damping Force w/ and w/o Cavitation

The right hand figure shows the damping force produced by the base and mid-valve both with and without cavitation. At the cavitation threshold velocity damping forces produced by the mid-valve jump up. This jump is caused by the near vacuum cavitation bubble pressures on the back side of the valve and the high pressures produced by the ICS system on the front side of the valve. At the threshold velocity damping forces produced by the base valve are reduced by the mid-valve backflow filling the cavitation bubble. That backflow increases the flow rate through the mid-valve and results in the damping force jump shown.

At higher suspension velocities (350 in/sec) the cavitation driven flow surge pushes additional fluid through the base valve which results in higher damping forces from the base valve and reduced damping forces from the mid-valve. This is also shown in the plot.

Details on the ReStackor web site step through the entire cavitation process examining the initiation and collapse of the cavitation bubble and its effect on the damping force behavior exhibited above.

Suspension Velocity Effect On Damping Force

The suspension damping coefficient computed over the suspension velocity range is shown below. For an ICS system operating in the cavitation range damping forces are a strong function of suspension stroke depth. For tuning purposes ReStackor reports both the damping coefficient at the requested stroke depth and the stroke averaged damping coefficient. The stroke averaged coefficient is integrated from the top of the stroke to the requested stroke depth over 240 increments at each suspension velocity. For an ICS systems the difference in damping force at a specified stroke depth and the stroke averaged damping force can be substantially different. For a bladder pressurized system the two parameters will be similar due to the lower change in pressures produced by a bladder pressurized system.

There are nine parameters on the plot:

Ct.clsd [=] Stroke averaged damping coefficient of the combined base and mid-valve damping force. Base valve clickers are in the closed position. Rebound clickers on the mid-valve are set at the position requested on the MVc input sheet. To determine the effect of the rebound clicker position on the compression damping you can run additional calculations with the rebound clickers set at various positions and compare the results.
Ct.clk [=] Stroke averaged damping coefficient with the base valve clickers set at the position requested on the BVc input sheet.
Cx.clk [=] Combined base and mid-valve damping coefficient at the requested stroke depth. Base valve clickers are set at the position requested on the BVc input sheet and mid-valve clickers set at the position requested on the MVc sheet.
Ct.wo [=] Stroke averaged damping coefficient with the base valve clickers set at the wide opened position.
Cbv.clsd [=] Damping coefficient of the base valve with the base valve clickers closed.
Cbv.clk [=] Damping coefficient of the base valve with the clickers set at the requested clicker position.
Cbv.wo [=] Damping coefficient of the base valve with the clickers wide opened.
Cmv.clk [=] Damping coefficient of the mid-valve at the requested clicker position.
Baseline [=] If you click the "Baseline" button on the main plot page the ReStackor spreadsheet will save the current calculation results on the Baseline tab of the spreadsheet. The baseline curve shows the Ct.clk damping coefficient from the results saved by the "Baseline" macro button.

Damping coefficient curve computed by ReStackor shows the combined damping coefficient of the base and mid-valve and the cavitation operation limits of the suspension.

The above example shows the performance of an ICS system at a stroke depth of four inches. Below suspension velocities of 170 inches/sec the ICS system is able to closeout the cavitation bubble before the end of the four inch stroke and the suspension is producing the normal non-cavitating damping rate.

When the suspension velocities exceed the cavitation limit of 170 inches/sec the ICS system is unable to close out the cavitation bubble before the end of the four inch stroke. This results in a jump in damping force. That jump is more clearly illustrated by plotting the suspension damping rate at multiple stroke depths.

Cavitation limits of an ICS system are a function of stroke depth and suspension velocity.

At the top of the stroke the ICS system produces no back pressure and the rebound chamber is easily cavitated. As the stroke depth increases the increasing backpressure of the ICS system requires higher suspension velocities to cavitate the system. The progressive increase in ICS system back pressure at deeper stroke depth results in progressively larger jumps in damping force as the system is driven into the cavitation limit. The jump is caused by the high pressures of the ICS system operating on the front side of the mid-valve and the near vacuum conditions of cavitation on the back side of the valve. Those pressure forces directly act on the damper rod and produce the damping force jump shown. Details on the ReStackor web site step through the entire cavitation process examining the initiation and collapse of the cavitation bubble and its effect on the damping force behavior exhibited above.

Stroke Averaged Damping Force

For tuning purposes ReStackor computes the averaged damping force at each stroke depth. The average damping force is computed by integrating the damping force over 240 increments through the specified stroke depth to compute the stroke averaged damping force. The averaged damping force is shown by the dark blue line along with the damping force produced with the clickers wide opened and closed.

ICS Spring Force

The pressure forces produced by an ICS system are present under both the compression and rebound stroke. The ICS force produced is a spring force not a damping force. Since ReStackor is focused on the calculation of damping forces the calculations only report the damping force and do not include the spring force of an ICS system. For this reason a 1.2 kg/mm ICS spring will produce exactly the same damping force as a 2.1 kg/mm spring under non-cavitating conditions.

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 it maks little difference in the overall damping force. 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 can easily be the difference between cavitation or not. Due to that sensitivity cavitation limits are difficult to compute without specific calibration of the calculations to the specific suspension setup you are tuning. Without calibration, test rides are needed to quantify system performance.

The central focus of ReStackor is in-situ tuning of your suspension. In-situ tuning gives you the capability to test ride your bike on the actual terrain and speeds that you ride and relate suspension performance to clicker settings and the specific shim stack changes needed to achieve specific changes in damping performance.

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 component tuning causing cavitation and the tools needed to tune those components to control cavitation. The capability to do that depends on your capability to implement the art of suspension tuning and interpret the details of test ride suspension feel to the specifics of computed damping rates, component performance and cavitation limits. That process in an art and proceeds outside the known numerical scales, here ReStackor is merely a guide.