ReStackor User Manual

Finally computer software to tune a shim stack

ReStackor User Manual

The entire input file to the 11,000 lines of ReStackor code is shown below. There are four sections to the input file:

Shim stack configuration

Lists the diameter and thickness of each shim starting at the valve face.

Shock absorber configuration

Shock body and damper rod diameter along with the keyword Vspec specifying the valve configuration as a mid-valve or base valve configuration.

Valve port geometry

Details of the valve port geometry and number of ports.

Operating conditions

Maximum force to be applied to the shim stack and the maximum wheel velocity. This section also specifies the clicker position for the bleed circuit and the oil viscosity.

The outputs from ReStackor are discussed in this separate thread.

If there are any errors in the input the pop up code execution window will display those errors when you hit the "run" button in the upper left had corner, or you can click on the "Edit_Output" button to view the detailed output from the ReStackor calculations.

The ReStackor spreadsheet interface uses excel macros to write the calculation inputs to a file. The excel macros reference specific cells in the "Plots" worksheet to extract the inputs. Due to these specific cell references you cannot move the inputs cells around on the "Plots" worksheet. It is a good idea to have a backup of the ReStackor spreadsheet incase you muck something up on the one you are working on.

If you need to make your own calculations within the ReStackor spreadsheet you can add another worksheet, or "tab", to the ReStackor spreadsheet. Within that new worksheet you can add data to any cell you want. Only the "Plots", "stack" and "ReStackor" worksheets are accessed by the excel macros.

Stack Configuration

The stack configuration is simply a list of each shim in the stack along with the shim thickness. The first shim is on the valve face. The definition of each parameter is given below.

ID [=] The inside diameter of the shims in millimeters. All shims in the stack have the same inside diameter.

Float [=] Stack float. The gap in millimeters between the first shim and valve face, usually zero.

Positive values of float lift the shims off of the valve face using the classic definition of float.

Negative values of float preload the stack. Negative float values can be used for modeling of digressive damper valves using dish, Ohlins X-rings or washers to preload the stack.

Shim # [=] Up to 50 shims can be used. If you only need 10 shims just leave the rest blank. Don't forget to enter the clamp shim at the top of the stack.

The ReStackor code doesn't use the shim number, it is simply here to help you track shim positions within the stack. The Shim # must be sequential. You can't mix up the order of the shims and expect the code to straighten it out for you.

Diam [=] The shim outside diameter in millimeters. A couple of requirements here:

The shim OD must be greater than the ID, duh.

The shim OD must be less then the valve diameter.

If the shim OD was greater than the valve/body diameter you could never assemble the shock. In general the OD needs to be a couple of mm smaller than the valve diameter. If not you will generate huge pressure drops as the flow attempts to squeeze through the thin annular gap between the shim and valve body. The ReStackor calculations will force the flow through that gap and if it is zero the calculations will probably blow up.

Thickness [=] The thickness of each shim in the stack in millimeters.

Damper Geometry

The damper geometry section specifies the basic dimensions of the shock absorber. There are four parameters:

D.rod [=] The shock absorber rod diameter in millimeters.
D.valve [=] The inside diameter of the shock absorber body in millimeters.

w.seat [=] The sealing seat width of the valve ports in millimeters.

Vspec [=] The keyword Vspec specifies the type of valve to be analyzed. There are three types of valves:
- BVc [=] Base valve. The compression damping valve.
- MVr [=] Mid valve rebound. Generally the rebound damping valve.
- MVc[=] Mid valve compression stroke.
- Ukey, Ckey [=] The Vspec keyword is also used to extract your user ID or enter a ReStackor code key as discussed below.

Suspension systems use two types of valves: mid-valves and base-valves. A base valve is generally located on the opposite end of the shock from the rod entrance. As the damper rod is forced into the shock body, fluid is forced out through the base valve. Since the damper rod is a small diameter the fluid flow rate through the base valve is generally low. The base valve is usually used for compression damping and is specified by the Vspec keyword BVc.

A mid-valve is located on the end of the damper rod. As the damper rod is forced into the shock fluid flows through the the mid valve to fill in the volume behind the mid-valve. On compression the entire face of the mid-valve is pressurized. On rebound only the annulus between the outside diameter of the valve and rod diameter is pressurized. Due to this difference in face area the forces generated by the mid-valve are different under compression and rebound. ReStackor uses the keywords MVc and MVr to differentiate the compression and rebound stroke of the mid-valve.

User Key

ReStackor also uses the Vspec keyword to extract your user key and enter your code key. The code key, that you can purchase from paypal, will unlock the Shim ReStackor and ReStackor pro sections of the code. Before you can purchase a code key you need to extract your user key and enter your user key at paypal when purchasing your companion code key.

Ukey [=] The pop-up code execution window will display your user key ID. You will need this key to purchase a code key from paypal.

Ckey [=] The pop-up code execution window will prompt you to enter your code key.

Valve Geometry

The valve geometry section describes the valve port dimensions and any flow restrictions created by the valve ports.

r.port [=] The radial distance from the valve center to the inside edge of the valve port in millimeters. This parameter describes the inner radius of the shim stack face pressurized by the valve port. Large values of r.port pressurize less of the shim stack face and produce less stack deflection.
d.port [=] The radial distance from the inside to outside edge of the valve port in millimeters. This parameter, coupled with w.port, describes the area of the valve port and the area of the shim stack face pressurized by the valve port. The sum of r.port, d.port and w.seat should equal the valve diameter.
w.port [=] The circumferential width of the valve port at the outside perimeter of the valve, millimeters. Measured from the inside edge to inside edge of w.seat. Large values of w.port increase the flow area at the stack perimeter and reduce the valve flow resistance.
N.port [=] The number of valve ports.
D.thrt [=] The diameter of the valve port minimum area. For free flowing valves, like the KTM valve, d.port and w.port are adequate to define the valve port flow area. For the KTM valve D.thrt can be set to zero. For valve ports with flow restrictions, like the Honda and Marzocchi valves, D.thrt defines the diameter of the minimum port area.
N.thrt [=] The number of valve port restrictions. For the Honda and Marzocchi valve above Nport is equal to four. In general N.thrt and N.port are the same. ReStackor uses the separate input for N.thrt to handle the special case where multiple side ports on the valve feed one valve port.
D.bleed [=] The valve bleed port diameter.
N.clicks [=] The number of adjuster clicks from closed to full open. For some clickers the valve needle is fully withdrawn from the bleed port a couple of clicks before the needle hits the clicker stop. These last couple of clicks obviously do nothing to effect the bleed port flow restriction. If the needle stop is at 24 clicks and you know the needle is fully extracted at 20 clicks you should enter 20 clicks for N.clicks.

ReStackor uses the parameters d.port, w.port and N.port to define the valve port flow area and provides the additional parameters D.thrt and N.thrt to handle the special case of a flow restriction within the valve port. Normally the valve port is fed by the annular slot formed between the shim stack and valve body. The flow area of this slot is typically much larger than the valve port and produces no flow restriction. For some special valve configurations the ports are fed by a perimeter side port and there may be more than one side port feeding the valve port. The additional parameter N.thrt allows ReStackor to handel this special case when multiple side ports feed a single valve port.

Settings

The settings section of the ReStackor input file describes the operating conditions for the calculation. Suspension velocity, oil viscosity and the clicker settings are specified.

n.clicks [=] The number of clicks out from the closed position.
SAEwt [=] The shock absorber oil viscosity in terms of SAEwt. The relationship of SAEwt and cSt@40c used by ReStackor calculations is here along with the effect of temperature on oil viscosity.
T.oil [=] The shock absorber oil temperature in Fahrenheit. The effects of oil temperature on viscosity are modeled in ReStackor using using the Andrade correlation.
F.max [=] The maximum force to be applied to the shim stack. Shim ReStackor calculations display the stack structure and compute the shim edge lift and stack face flow area at applied forces from zero to F.max. F.max is specified in pounds force.
u.wheel [=] The maximum wheel velocity in inches per second. ReStackor pro calculations compute the damping force as a function of suspension velocity up to the value specified by u.wheel. If the maximum wheel velocity produces a shim stack force greater than F.max additional calculations are made internally in the code to determine the stack deflections at the higher applied force. Decoupling F.max and u.wheel in the ReStackor input file allows you to display the stack structure at a low applied force level, like examining where a crossover gap closes, while computing the damping forces of the suspension at a much higher wheel velocity.

The relationship of bump height, bike speed and suspension velocity for different wheel sizes is given here. With an estimate of the suspension velocity, say a three inch bump hit a 30 mph, you can design a crossover gap to close at a specific wheel velocity on a specific bump height. This allows you to design a crossover gap to float over two inch bumps and transition to the high speed stack on a three inch bump.

ReStackor Input File

The ReStackor inputs have been purposely designed to use simple geometric dimensions describing the basic features of the suspension. This allows input files to be easily created with simple inputs you can measure in your garage. For computing the nuances of fluid dynamics more complex inputs could certainly be created describing details of surface finish, edge sharpness, turning angles and port lengths. While these details could improve the codes capability to compare with the absolute damping force measurements of dyno tests, those measurements are to some extent irrelevant for suspension tuning.

For tuning it doesn't really mater if your suspension produces 50 lbf or 56 lbf of damping. All that really matters is the fact that your need 10 lbf more damping. Here, in scaling the suspension force up or down, all of the internal nuances of fluid dynamics, such as edge sharpness and surface finish, scale directly with suspension flow rate. This scaling effect allows ReStackor to accurately determine the shim stack changes needed to increase the damping force by 10 lbf regardless of the absolute value of damping force.