A peice about shock function...

[Jeremy Wilkey]

Readers,
Here is a peice I had written up for a plan that never came to pass.. I'm working on lots of other stuff, but here is a peice about shocks.. Let me know what you think.. I would like to thank MACE for his consultaion on format and content..

Regards,
Jer


SHOCK FUNCTION.

The most important aspect of motorcycle handling is the rear suspension. The rear suspension is composed of many factors: Linkage motion ratio, spring rate and damper design. Each of these topics can be broken down into many small sub categories that ultimately become more and more complex as they are understood further. We could discuss any of these topics at length. Interestingly, experience has shown that most riders benefit most from a quick lesson in how a shock actually functions.

Definitions:

• Shock Body: The sealed unit that contains all the internal components.

• Shock Shaft: The rod that connects the swing arm (or suspension linkage) to the valving piston. The volume of the rod produces internal oil displacement as it enters the body.

• Shock Piston: The primary damping component of the shock. The piston seals to the shock body around its outside diameter and forces oil through its orifices and shims based on wheel movement. A shock piston and its shim stack are variable orifice mechanisms (as described below).

• Reservoir: A chamber connected to the main shock body by a fluid passage. The reservoir provides a holding space for fluid displaced by the volume of the shock shaft as the shock shaft travels into the body.

• Bladder or Reservoir Piston: The mechanism that seals the gas charge from the oil. The bladder serves to pressurize the whole system. The pressurized system forces the oil to travel through the shock piston, preventing cavitation.

• Shock Fluid: Hydraulic fluid typically consisting of synthetic oil stock with appropriate viscosity (flow properties) and thermal (heat resistance) properties.

• Seal Head: Creates a seal between the shock body and the shock shaft.

• Compression Adjuster: A small variable orifice device that limits the flow of fluid between the shock body and reservoir. The adjuster functions at a specific speed range, and with the implantation of a check valve offers no restriction in the rebound direction or shock shaft extension.

• Rebound Adjuster: An adjustable orifice using a needle and seat only. This adjuster serves as a bypass to the shock piston. On most current model off-road bikes, it bypasses in either direction; however, that is not always the case.


• Orifice: An orifice is an opening for fluid flow sized to restrict the flow of fluid depending on the speed of the fluid

• Variable orifice: A variable orifice changes its size based on the speed of the fluid. The piston shims and ports create a variable orifice and allows a shock to respond with more or less compression depending on shaft speed.

• Damping: Control of suspension motion by resistance of fluid flow through shock internal passages.

• Active (compression/rebound) damping: (Describes the nature of the fluid flow). The active description is used when referring to a swept volume of fluid. (The fluid stays fixed in position; the piston travels through the fluid).

• Passive (compression) damping: (Describes the nature of the fluid flow). The passive definition is used when referring to a displaced volume of fluid. (Displaced volumes are pushed across the valve mechanism and regulated).

• Cavitation: A pressure differential inside a shock that results in a vacuum in the opposite direction of piston movement. This vacuum causes the remaining fluid to vaporize. When the shock travels back the other direction it recondenses the vapor but results in reduced damping and hence very poor suspension performance.

To understand how a shock functions, it is easiest to look at the system in a series of cases.

• Case I: The shock starts as a pressurized system (with nitrogen). The bladder pressurizes the entire fluid body in the shock body.

• Case II: As the shock is compressed, the piston travels inside the shock body. The shock rod is now occupying increasing space inside the shock body. Since the system is closed, something must compress. The volume of oil occupied by the rod is displaced through the compression adjuster and compresses the bladder. Since the piston was in the shock body from the beginning, it displaces no additional fluid. Since the nitrogen is pressurizing the entire system equally, the fluid is swept through the piston, and the piston pushes no fluid into the reservoir. The volume of fluid passing through the piston ports is equal to the swept volume of the piston in the body MINUS the swept volume of the shock shaft. If the nitrogen were to leak or the shock was to be improperly charged, the pistons variable orifice might offer more resistance than the bladder. If this happens, the piston will push fluid into the reservoir, and the space behind he piston will not fill completely. When the shock begins its rebound stroke, the piston will have a distance of travel where no fluid is behind the piston. This section of travel will offer no resistance to the spring, and will obviously result in a miserable suspension action.

Although all this definition of components and their function may seem excessive, clarifying helps the rider to relate at a higher level, taking some of the mystery out of tuning.

 

[Murf]

Thanks Jeremy, I have been looking for information, and although I had all ready picked up some of what you posted, I did pick up some things.

 

[Jeremy Wilkey]

Murf,
Your very right.. we've coveed this at least 1 billion times over the last 2 years I've been doing this... But maybe not that simply put.. Its good review for the newbies.. :)


I can't really talk about what it was for as I don't like burning bridges.. For the techincal type here its underkill, but it may have made a cool article.. for those who get tired of reading about how to put on stickers or service airfliters....



:p


Regards,
JEr

 

[svi]

Here's a similar thing I found on a car website:

Statement of Non-Liability

Motor racing is dangerous. I disclaim any responsibility for your
actions and the resulting consequences.

You may also use this as classroom material.
---------------------------------------------------------------------

The development of externally adjustable dampers, or shock absorbers,
is one of the hottest areas of race car chassis engineering. The
adjustments available from these dampers provide a quick, simple method
of changing the behavior of the car in transitions and over rough
pavement. Adjustable dampers can also provide the combination of a
smooth ride for daily driving and firmer damping for competition
driving simply by rotating an adjuster. There are two common types of
adjustments available. This article will focus on the physics behind
the results produced from each type of adjustment.

We will now delve into the deepest, darkest depths of the damper
(alliteration accidental). We will start off with a basic description
of the guts of modern racing dampers, then discuss the types of
adjustments available from them and the results of using those
adjustments.

DAMPER INTERNALS

I will assume you have not seen a racing damper disassembled. As with
many other things, it's what is inside that counts. There is a piston
attached to the end of the shaft inside the damper. The chamber that
the piston moves in is filled with (almost) incompressible hydraulic
oil. The viscosity of the oil causes resistance to oil passage through
small orifices. This resistance produces a pressure differential across
the piston when the piston moves, thus producing a damping force.

Near the end of the housing opposite the shaft (or in the canister if
there is one) is a sliding piston that separates the oil from a high-
pressure gas, usually nitrogen. Some volume of gas is necessary because
as the shaft moves into the housing, the volume of oil displaced by the
shaft must go somewhere. The oil displaced by the shaft moves the
gas/oil separator piston and compresses the gas slightly. The static
spring rate of a pressurised damper is almost zero, but the static
extension force can be rather large. The gas, and therefore the
hydraulic oil, is pressurised in order to reduce the severity of
aeration.

Aeration is the formation of gas bubbles in the oil due to very rapid
pressure loss immediately after the oil passes through an orifice at
high velocity. Increasing the oil pressure increases the rate of
reabsorption of the gas bubbles. Because these gas bubbles are
compressible, the characteristics of the damper change unless the
bubbles are reabsorbed into the fluid. Aeration is not necessarily a
bad thing because we can use its special characteristics to our
advantage. By the way, aeration happens even in 450psi pressurised
dampers. That is why changing gas pressure changes damping
characteristics. The location of the gas volume is significant because
as piston speed in the bump direction increases, the pressure on the
shaft side of the piston decreases. Rebound travel increases the
pressure on the shaft side of the piston. This changes the rate of
aeration recovery. The pressure on the canister side of the piston is
almost constant unless there are additional orifices in the canister.

Movement of the shaft forces hydraulic oil through various orifices in
the piston, shaft, and canister. One type of orifice is a deflected
shim stack. There is a stack of 4 or 5 thin steel shims covering holes
on both the top and bottom of the piston. The holes connect with slots
on the top and bottom faces of the piston. Oil can pass around one
stack of shims, through the slots and holes in the piston, and through
the narrow slot (orifice) that 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. The slots on the piston faces are
positioned so that oil passes through different holes in bump than in
rebound.

The other type of orifice is a small hole that bypasses the fluid path
through the shim stack. This hole may be in the piston or in the sides
and end of the shaft. If there is a hole in the shaft, a tapered needle
can be mounted in the hole to vary the orifice area. The tapered needle
extends through the top of the shaft to an external adjuster.

A remote reservoir (also called a canister) allows the damper designer
to add another orifice or two to the oil flow path. Only the oil
displaced by the shaft moves through these additional orifices. In this
case, a large shaft diameter is a good thing. By using one-way check
valves, it is possible to design a damper with externally adjustable
shim stack preload and externally adjustable fixed orifices for both
bump and rebound, thus producing a four way externally adjustable
damper. Most modern racing dampers are two or three way externally
adjustable.

Older Koni racing dampers have external adjustments for bump and
rebound shim stack preload. Quantum dampers have two holes in the shaft
with one way check valves and two needles to produce individual
adjustments for bump and rebound. A third adjuster moves the entire
needle/seat assembly relative to the shaft in order to change the
rebound shim stack preload. A simpler adjustment offered by Fox and
Penske consists of a drum in the canister with several holes of
progressively larger diameter drilled in the OD of the drum. Rotating
the drum aligns a different size hole with the flow path. Penske 8700
dampers also have an additional bump shim stack with adjustable preload
in the canister.

OIL FLOW CASE ONE: FIXED ORIFICE FLOW

If the damper piston moves slowly, the pressure differential across the
piston is not large enough to force the shims away from the piston
face. So, the only flow path is through the fixed orifice. Basic fluid
mechanics tells us that drag is proportional to velocity squared. In
this case, the force versus velocity relationship of the damper is
parabolic: doubling the shaft speed ****ruples the damping force. As
long as the piston speed is low enough that the shims do not open, the
shape of the force versus velocity curve is a parabola that opens
upward. Reducing the area of the orifice by moving the tapered needle
farther into the orifice or by rotating the adjuster drum to a smaller
orifice increases the damping force for a given shaft speed. That is
the only way to adjust low speed damping other than by changing the gas
pressure.

Low speed and high speed damping refer to the speed of the damper shaft
relative to the damper housing, not to car speed. Low speed damping
adjustments affect dynamic weight transfer and the motion of the sprung
mass relative to a smooth track surface. High speed damping adjustments
affect the motion of the unsprung mass (wheels and tires) relative to a
bumpy track surface. We are usually much more interested in low speed
damping adjustments than high speed.

OIL FLOW CASE TWO: VARIABLE ORIFICE FLOW

When the piston moves rapidly enough to lift the shim stack off of the
piston face, an additional flow path is created. Increasing the piston
speed forces the shims farther away from the piston, thus increasing
the orifice area. So, the shim stack is a variable orifice. Because the
orifice size changes with damping force, the damping force is no longer
proportional to velocity squared. The shims all have different
diameters, so in side view the shim stack looks like a multiple leaf
spring. Typically, the diameters of the shims are chosen to produce a
linear force versus velocity characteristic. Changing to thicker shims
increases the slope of the force versus velocity line. However, this
requires disassembling the damper. Changing the preload on the shim
stack (which can be done with an external adjustment) changes the
offset of the force versus velocity line.

The piston faces are machined with a slight cone angle ranging from 0.5
to 2.5 degrees. So, the shim stack is deflected when it is forced
toward the piston. Adjusting the clamping load on the shim stack
changes the preload, and thus the offset of the force versus velocity
line. More preload equals more damping force, but only at high shaft
speed. Changing shim stack preload has no effect on low speed damping
other than changing the force at which the shim stack opens. Most
racing dampers open the shim stack at 2 to 5 in/sec shaft speed
depending on the preload adjustment.

OIL FLOW CASE THREE: MIXED FLOW

Obviously, 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, but primarily at low
speed. On the other hand, changing shim stack preload only affects high
speed damping. Most damper manufacturers refer to fixed orifice
adjustments as low speed adjustments. Conversely, shim stack preload
adjustments are referred to as high speed adjustments.

NOW FOR THE FUN PART: GAS PRESSURE

As you go whizzing merrily around the race track, the damper piston is
continuously moving back and forth rapidly through the same small
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 the other way, causing more aeration.
Rebound travel increases the pressure on the shaft side of the piston,
increasing the rate of absorption of gas bubbles. Bump travel decreases
the pressure on the shaft side, decreasing the rate of absorption.

Assuming equal shaft speed and damping force in bump and rebound,
slightly more aeration is produced from bump 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 canister
complicate this situation further. Nevertheless, the percentage of
aeration stabilizes quickly.

Increasing the gas reservoir pressure has a more significant effect on
the shaft side of the piston because the rate of gas bubble
reabsorption is generally lower on the shaft side, so the percentage of
gas bubbles is generally higher on the shaft side. Because the working
volume of oil is less aerated during bump travel, more damping force is
produced in rebound. The effect of gas pressure is less pronounced in
bump travel because the rate of gas bubble reabsorption is generally
higher on the canister side of the piston. This is why increasing the
gas reservoir pressure increases bump damping some and increases
rebound damping more.

When examining shock dyno results, many people incorrectly conclude
that increasing the gas pressure increases bump damping and decreases
rebound damping. The error in this conclusion is that the static shaft
extension force also increases with gas pressure, thus reducing the
force applied to the shock dyno load cell during rebound travel. The
measured force has an offset due to the extension force produced by gas
pressure times shaft area. This force is almost constant and has no
effect on damping.

The fluid that you rely on to control the motion of the car is really a
mixture of hydraulic oil and gas bubbles. As the bumps get worse, the
shaft speed and therefore the rate of aeration increase, reducing the
damping forces. This is only a real problem if you run on
superspeedways or if you run very low gas pressure (less than 50 psi or
so).

When shaft speed or oscillation frequency becomes very high or gas
pressure is very low, the percentage of gas bubbles in the working
fluid becomes high enough that the damper starts acting like a spring
in addition to a damper. Koni recently released graphs of shock dyno
results from a new damper design. These graphs showed force versus
shaft position for three different peak shaft speeds. At low shaft
speed, the graph was symmetric about the vertical axis. As the shaft
speed increased, the graph distorted diagonally towards a line with
positive slope. A force versus deflection graph for a spring also has a
positive slope. These results showed that the damper was speed
sensitive, producing a spring at continuously high shaft speed. The
presence of compressible gas bubbles in the working oil volume is the
most logical explaination to produce those results.

If the canister has orifices of any sort in it, the length of flexible
hose connecting it to the damper housing should be as short as
possible. The hydraulic flexibility of the hose produces a spring
between the two stages of damping orifices, adding to the confusion.
For the last few years, the dampers that Penske made for their own
Indycars had a canister bolted directly to the damper housing with no
flex hose.

All of the damping energy that is removed from the suspension system is
converted to heat in the hydraulic oil. Air cooling is required to
prevent excessive temperature buildup which causes the oil viscosity to
decrease. Canisters significantly increase the surface area available
to dissipate that heat. However, if the engine exhaust system is close
to the dampers, or if the dampers are buried somewhere with no cooling
air flow, one can expect the handling of the car to change after a few
minutes on the track.

Keeping the suspension springs cool is also a good idea. The elastic
modulus of spring steel decreases 2.2 percent per 100 degrees
Fahrenheit temperature rise. The spring rates and ride height decrease
right along with the elastic modulus.

Of course, none of this describes the "ideal" adjustment settings to
use for any particular car. I have only attempted to describe what
happens when the damper adjusters are adjusted.

Neil Roberts
[email protected]

 

[Jeremy Wilkey]

SVI,
Very cool.. The part aboout ariation even at 450 PSI is intreging and something I came across at WP.. The new WP shocks are built in a vacum chamber and the oil is filled at a 1 bar.. This insures a complete "vaporfree" build that lowers operating temps and shock consistency etc.. However they droped a intresting tidbit of info.. They had exsperimented with lower pressures but found that minute amounts of vapor in the oil actually made the shock function better in terms of feel... This was a very intresting detail and one I found very contrary to my "upbringing".

MR. Roberts has touched on something that maybe has something to do with this. Is that e-mail still valid? I would like to e-mail him..


Here is a picture of the Vacum bleeder and a picture of one of the coolest things I've ever seen... A clear and partialy functioing shock.


Regards,
Jer

 

[Jeremy Wilkey]

Clear shock

Here is a image of the WP clear shock...

 

[Jeff Howe]

What I've been wondering all along.... The level of aeration in a KYB fork vs Showa TC. I would think the Showa would have less potential of aeration.