Clicker Bleed Flow Area
Bleed circuits typically use a blunt needle tip with a diameter near 1/2 the bleed circuit throat diameter. Needles following this formula produce a nearly linear area change over the range of clicker settings. The default ReStackor clicker needle uses a 1/2 diameter tip with the bleed circuit flow area defined by.
Other needle shapes can be used by setting up table inputs to describe the needle diameter as a function of clicker position. Using tables ReStackor will exactly match the clicker needle flow area curve and improve the calculation estimates of low speed damping performance.
ReStackor Needle Geometry Table Inputs
To run a needle table geometry in ReStackor both of the input parameters d.bleed and MAX.clks must be set to zero.
Setting
d.bleed
and
MAX.clks
to
zero
triggers
ReStackor
to
look
for
and
read
the
clicker
needle
geometry
table.
The
table
must
be
located
at
C:\ReStackor\Work_Dir\clkdat.in.
The
table
has
the
following
format:
-
Line 1 : Title with a length up to 80 characters.
-
Line 2 : The bleed port diameter in millimeters.
-
Line 3 : Text string "Nclk Dclk [mm]"
-
Line 4 : List of clicker position and the corresponding needle diameter. Up to 100 entries can be used in the table.
-
End : The last entry defines the value of MAX.clks.
When ReStackor reads the needle geometry table the inputs are echoed to the output file. You should check the output file to make sure the table was read correctly.
Example Needle Geometries
The inputs to the needle geometry table are best demonstrated through example. The first step is to measure the needle and determine the needle diameter as a function of length and the locations where slope changes occur. You also need to know how far the needle tip moves per click. For the needle in this example 25 clicks are needed to move the needle from the seated position to the location where the needle tip is flush with the bleed port entrance. 4.2 mm of travel over 25 clicks works out to be 6 clicks/mm.
The above measurements define the needle geometry up to the point where the needle tip is flush with the bleed port entrance. This table is adequate for most applications.
The above measurements define the needle diameter as a function of clicker position. Entering that information into the table is adequate for most applications since few riders run their suspension past 25 clicks. The final entry in the table, 25 clicks in this case, is the value of MAX.clks. The needle tip diameter at MAX.clks defines the bleed circuit flow area at the wide open clicker position and the wide open clicker position used in all of the ReStackor plots.
If you want ReStackor to model the full range of the clicker positions including cases where the needle tip is outside of the bleed port you need to determine the needle tip height where the bleed port runs full open. The full open height, h.wo, is defined by the relationship below.
For the needle geometry in this example the needle tip needs to be 0.74 mm above the bleed port to reach the full open position. The port runs wide open at a clicker setting of 29.6 clicks from closed.
The next step is to determine the bleed port flow area as a function of needle tip height, h. For the example the needle tip is flush with the bleed port at 25 clicks and the port runs wide open at 29.6 clicks. Substituting those clicker position into the equations below allow h to be determined as a function of clicker position and d.eff to be determined as a function of h. The values of clicker position and d.eff are then entered into the table. The example table below uses one click increments from 25 to 29.6 clicks, any clicker increment can be used.
Bleed circuit flow metering for the example needle has 4 distinct ranges. From zero to five clicks the steep entrance taper of the needle creates a large change in area as the needle is cracked open. From five to 20 clicks the gentle taper of the needle creates little change in area. Most of the usable clicker settings are in this range where friction loss through the long annular section between the needle and bleed port control the flow rate. From 20 to 25 clicks the rounded tip of the needle is extracted from the bleed port and from 25 to 29.6 clicks the needle is outside the port. While the tip is close to the bleed port exit the blunt tip partially blocks the exit and continues to meter the flow until the tip is a distance of h.wo away from the port. In this external metering range small changes in clicker position produce a large change in bleed circuit flow.
Pointed Needle
For a pointed conical needle the table inputs are much easier and simply require measurement of the needle diameter as a function of length and clicker position. Changes in angle of the cone can simply be measured and inserted into the table. Since the tip is sharp there is no external flow metering for this needle.
Parabolic Needle
To describe the contour of a parabolic needle several points along the contour are needed. ReStackor linearly interpolates the needle geometry between points in the table. To smoothly describe the contour of a curved surface several measurement are needed along the needle length.
Small Tipped Needle
The default ReStackor needle geometry uses a tip diameter of 1/2 the bleed port diameter. For needles with a different tip diameter a needle geometry table can be easily created.
Flow
Area
Curves
The geometry of a simple conical needle produces a nearly linear change in flow area across the range of clicker settings. Pointed needles and parabolic needles produce progressively lower changes in bleed circuit flow area as the needle approaches the MAX.clks position.
Using table inputs ReStackor can accurate descriptions the bleed circuit flow area of any needle shape. All that is needed is a set of measurements that describe the needle diameter as a function of length and clicker position.
Summary Example
The first click moves the needle off of the 3.0 mm bleed port seat and flow is meter by the 2.8 mm shank diameter of the needle. At 22.8 clicks the tip diameter is reduced to 2.5 mm. Over the first 22.8 clicks there is little change in flow area for this needle. Bleed circuit flow rates for this needle geometry are controlled by friction losses through the narrow annular slot between the needle and bleed port.
For clicker position beyond 22.8 clicks the needle tip is outside of the bleed port. The relationships below compute the needle tip height needed for full flow, h.wo, to be 0.78 mm. The needle tip was measured to be flush with the bleed port entrance at NDL.clks=22.8 so the full flow condition is reached at 27.5 clicks. Plugging in needle tip heights, h, from 0 to 0.78 in the relationships below computes the "effective" needle tip diameter, d.eff, to be the values shown in the table.
The needle flow area curve has a large area increase over the first click as the needle is moved off of the bleed port seat. Through the bulk of the clicker range from 1 to 23 clicks there is little increase in flow area. Through this range the bleed circuit flow rate is controlled by friction losses through the narrow annular slot between the needle and bleed port. At clicker positions beyond 23 clicks the needle tip is external to the bleed port and small changes in the needle tip position result in large changes in flow area.
