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Damper Internals
If you disassemble a typical
rear damper, you will see there is a piston attached to the end of a
shaft. The chamber that the piston moves in is filled with (almost)
incompressible hydraulic oil. The viscosity of the oil causes restriction
through small orifices, and this resistance causes a pressure change
across the piston when it moves, producing a damping force. Situated at
the end of the housing opposite the shaft (or in a reservoir if there is
one) is a floating piston, ( IFP ) this separates the oil from a
high-pressure gas, nitrogen or air. Some volume of gas/air is necessary,
because as the shaft plunges in and out of the housing, the volume of oil
displaced by the shaft must go somewhere
( Displacement ) The oil displaced by the shaft
moves the IFP and compresses the gas.
( The static spring rate of a pressurized damper is
almost zero, but the contained pressure ranges from 75 to 300 psi.) The
gas/air, and therefore the hydraulic oil, is pressurized in order to
reduce aeration. ( Aeration happens when gas bubbles form in the oil due
to very rapid pressure loss caused by the oil passing through an orifice
at high velocity. ) Increasing the air/gas/oil pressure increases the rate
of re-absorption of the gas bubbles. Because these gas bubbles are
compressible, the characteristics of the damper change, until the bubbles
are re-absorbed into the fluid. Aeration is not necessarily a bad thing,
and it can be used to an advantage. Altering gas/air pressure changes
damping characteristics, and the location of the gas/air volume is
significant due to the action and correct operating state of the damper,
as piston speed in the compression stroke increases, the pressure on the
shaft side of the piston decreases, and the opposite with rebound travel,
the pressure on the shaft side of the piston increases. The pressure on
the IFP side of the piston is almost constant …… unless there are
additional orifices in the housing.
Oil Flow
Movement of the shaft forces
hydraulic oil through various orifices in the piston, shaft or housing.
There is a stack of several thin steel shims covering holes or ports on
both the top and bottom of the piston. In one particular design, the ports
connect with slots on the top and bottom faces of the piston. With this
arrangement, the oil passes around one stack of shims, through the ports
in the piston, and then through the narrow slot. However, that slot only
opens up when enough pressure differential is applied to the shim stack to
force it away from the top or bottom face of the piston.
Another way
for oil to pass through the piston is through small ports that bypass the
fluid path through the shim stack. These holes may be in the piston or in
the sides and bottom of the shaft. If there are holes in the shaft, a
tapered needle can be mounted in the hole to vary the orifice. The tapered
needle extends through the shaft to an external adjuster which is most
often rebound control. Using a reservoir allows added orifice’s to the oil
flow, and only the oil displaced by the shaft moves through these
additional orifices. With the introduction of one way check valves oil
flow can be diverted to specific areas where valving is situated that
inhibits rider induced movement.
Fixed Oil Flow
During low speed
damping, the pressure increase across the piston is not large enough to
force the shims away from the piston face, the only oil flow path is
through the fixed orifice …. SPV dampers force oil flow through the stable
platform valve. Fox Propedal force oil through the shim stack. Force is
proportional to velocity squared, ( force versus velocity ) doubling the
shaft speed quadruples the damping force. As long as the piston speed is
low enough that the shims do not open, the force versus velocity remains
unchanged. (The force will have an offset due to the added gas pressure
times shaft area. This pressure is almost constant and has no effect on
damping.) Reducing the area of the orifice by adjusting the tapered needle
( more rebound ) increases the damping force for a given shaft speed. In a
standard shimmed damper this is the only way to adjust low speed damping
other than by changing the gas pressure .….check valves are now used in
some bicycle shocks that inhibit oil flow during low speed
compression.
Low speed and high speed damping refer to the speed of the
damper shaft relative to the damper housing, not to bicycle speed. Low
speed damping adjustments affect dynamic weight transfer and the motion of
the sprung mass relative to the trail surface. High speed damping
adjustments affect the motion of the unsprung mass (wheels and tires)
relative to the trail surface.
Variable Oil
Flow
When the piston moves rapidly enough to lift the shim
stack off of the piston face, additional oil flow is created. Increasing
the piston speed forces the shims to open further, this increases the
orifice flow volume. The shim stack is a variable orifice. Because the
orifice size changes with shaft speed, the damping force is no longer
proportional to velocity squared. The shims all have different diameters,
in side view the shim stack resembles a pyramid, this and the varying
diameters of the shims produce a linear force versus velocity
characteristic. Adding shims increases the rate of the force versus
velocity line. Changing the shim stack changes the force versus velocity,
and has no effect on low speed damping.
One Way Oil Flow
When the shim stack
opens, oil is still flowing through the fixed orifice also. Therefore,
changing the fixed orifice area affects damping through the entire shaft
speed range, although directly effects low speed damping.
Changing/increasing/adding shims only affects high speed damping. ( Fixed
orifice adjustments are referred to as low speed adjustments and shim
stack adjustments are referred to as high speed adjustments )
Note, that SPV dampers such as Manitou, 5th
Element & Curnutt, and check valves found in Fox Float R and Van R
eliminate the two way oil flow of the fixed orifice, by introducing one
way valves ( of various designs ) prevents oil flow bleeding back through
the rebound needle seat ( opposite direction to rebound oil flow )
therefore inhibiting low speed compression and eliminating low
amplitude forces like riders induced bob, the downfall is small bump
compliance is effected. In this situation and with no bleed off of oil
bypassing the fixed orifice a position sensitive valve can be
incorporated, introducing a threshold platform that
basically prevents the damper piston from travelling further into the
compression stroke than necessary.
Gas/Air Pressure
Increasing the gas
reservoir pressure increases compression damping but increases rebound
damping more. As you go screaming down your favourite trail, the piston is
moving back and forth rapidly through the same volume of oil. This oil
becomes aerated when the piston moves in one direction. This same volume
of aerated oil is forced through the piston orifices when the piston moves
in the opposite direction, causing more aeration. Rebound travel increases
the pressure on the shaft side of the piston, increasing the rate of
absorption of gas bubbles. Compression travel decreases the pressure on
the shaft side, decreasing the rate of absorption.
Assuming equal shaft
speed and damping in compression and rebound, slightly more aeration is
produced from compression travel than from rebound travel. Because the gas
reservoir chamber is on the opposite side of the piston from the shaft,
the pressure and the rate of absorption is relatively constant on that
side, additional orifices in the reservoir can add to complications. (
aeration comes about quickly ) Increasing the gas reservoir pressure has a
more significant effect on the shaft side of the piston, because the rate
of gas bubble re-absorption is generally lower on the shaft side and the
percentage of gas bubbles is generally higher on the shaft side. Because
the working volume of oil is less aerated during the compression stroke,
more damping force is produced in rebound. The effect of gas pressure is
less pronounced on compression because the rate of gas bubble
re-absorption is generally higher on the IFP side of the damper
piston. |
