From: "Eric Gorr " 
Newsgroups: rec.motorcycles.dirt
Subject: Suspension jive wanted!
Date: Fri, 16 Jul 1999 23:57:37 GMT
Organization: Forward Motion

Hi this is Eric Gorr and I'm working on a Y2K compliant version of my MX
Performance Handbook. A lot of people asked me to expand the suspension
section, especially about revalving. So here is a sample chapter. I invite your
criticism, contructive comments, harrassment, suggestions or what ever else you
want to throw at me. 

Sometimes the obvious is oblivious to the guy who is surrounded by revving dirt
bikes all day! So don't think that your thoughts aren't of interest to me. I
may not be able to answer questions quickly by email because I'm working on two
books that are 150,000 words each.  One thing is for sure, the boyz at Dirt
Rider won't be posting any phototorials here and asking for your opinions!
Thanks

Video Suspension Tuning
Suspension tuning can be a mystery for both the rider and his mechanic. As a
rider on race day, you go out for your practice session and your suspension
nearly kills you! You come back to the pits and your mechanic asks you if the
high-speed rebound feels too fast? You haven't got a clue because for the last
20 minutes you tried to keep your motorcycle on two wheels! After riding and
tuning motorcycles for years, I still cannot diagnose suspension problems by
riding or watching the bike on a racetrack.  The best suspension tuners in the
world have a well-developed sense of high-speed vision. They can watch a bike
and rider on various sections of the track to determine how well the four
different suspension circuits are working. You can acquire that same sense of
high-speed vision with the help of a video camera. After video-taping the rider
attacking various sections of the track, you can replay the tape one frame at a
time and see exactly how the four different suspension circuits damp the
impacts of jumps, whoops, and other track irregularities. 

This section tells you how to use videotape to tune your suspension. First,
I'll explain the four suspension circuits and the track sections that help
isolate each circuit. At the end of this section is a troubleshooting chart
that will help you to identify problems with each circuit. A suspension-data
log sheet is also provided so you can record all the pertinent information on
your bike and have the data reviewed by a suspension-tuning expert. The data
log will help you to develop a mental framework for setting up your suspension
properly. Finally, I will explain what changes suspension tuners make during
revalving. A warning though: Do not try to revalve your own suspension! One
small mistake can put you over the bars! First, set up your suspension with the
proper springs, settings, oil heights, and so on. If you still need revalving
performed, at least then you will know exactly what your suspension needs. Too
many people have their suspension parts revalved without first trying to set up
the bike properly.

Tuning with the Damping Circuits
As previously mentioned, the four suspension circuits of the forks and shock
are the high-speed compression (HSC), high-speed rebound (HSR), low-speed
compression (LSC), and low-speed rebound (LSR). Your main objective in video
suspension tuning is to make video samples of the rider on sections of the
track that best isolate two of these circuits at a time.  Before you start
riding and taping, change the suspension fluids, grease the linkage, and have
the proper spring rates and sag settings on the shock and forks.

Low-Speed Compression (LSC) and Low-Speed Rebound (LSR) Tuning
The low-speed circuits work in two common track sections, braking for tight
turns and accelerating on a straight with far-spaced, shallow whoops. When
taping a rider, be sure to have the whole bike and part of the ground in the
film frame. Stand far enough back from the track section and pan with the rider
for at least 25 yards. Replay the tape one frame at a time and pay attention to
how the wheel follows the ground as the bike hits the bumps. The wheel
shouldn't compress quickly or rebound abruptly. All Japanese dirt bikes have
suspension adjusting screws that affect the low-speed circuits only. Turning
the adjusting screws clockwise will increase the damping and slow/stiffen the
low-speed circuit. Turning the screws counterclockwise will decrease damping
and speed-up/soften the low-speed circuit.

High-Speed Compression (HSC) and High-Speed Rebound (HSR) Tuning
The high-speed circuits work in two common track sections, landing from big
jumps and accelerating on a straight with tightly spaced, sharp-edged whoops.
Video a rider as he lands from a big jump, and for about 15 yards after he
lands. That is important because there are usually many small bumps in the
landing path after a big jump. Replay the tape one frame at a time and watch to
see how equally both the front and rear suspension compress and rebound. If the
rear shock rebounds too fast, the rear end may spring up so fast that it loads
the forks. If both ends rebound too fast, the whole bike may spring up off the
ground. That can be hazardous if there is a turn after the jump.  When taping
in whoop sections, try to pan the rider in as much of the section as possible.
Watch how the suspension reacts to the sharp-edged whoops at speed. The rear
wheel shouldn't pack-up. Packing is caused when the HSC and HSR are too slow to
react to the terrain. The wheel will stay compressed as it hits the next whoop.
Eventually the rider loses control and must slow down. Taping in whoops also
helps the rider; if the bike is reacting properly, he may gain enough
confidence to go faster through the section.

If the video tape indicates that you need to change the high-speed circuits,
you must take the suspension to an expert in revalving because there are no
external adjustments that you can make to the high-speed circuits. Some
suspension tuners are starting to encourage riders to make video samples for
review.


Suspension Revalving - How Damping Works
Suspension fluid (oil) flows through the ports of the piston and up against the
shims. The shims pose a resistance to the oil flow, which provides a damping
affect. The damping affect is directly related to the diameter and the
thickness of the shim. The shims act as a series of tiny springs, flexing to
increase the flow area for the oil. The greater the flow area, the greater the
oil flow and less the damping affect. The first shims that the oil encounters
are the ones that affect the low-speed damping. These shims are large in
diameter and thin in thickness. The oil deflects these shims easily because of
their large surface area and the relatively thin steel poses low spring
tension. The shim stack or valving is arranged in a taper shape. The
large-diameter low-speed shims are positioned closest to the piston and the
small-diameter high-speed shims are positioned farthest away from the piston.

Why Revalving
The term revalving is often tossed around in the dirt bike magazines, but have
you ever wondered what suspension tuners do to re-valve a set of forks or a
shock? The answer ranges from not much to a whole lot. Some unscrupulous tuners
just power wash the outside of the components, turn the clickers, and charge
you a lot of money. Other tuners replace the pistons and valve stacks,
carefully crafting the arrangement of the valve shims to suit your riding
demands and compensate for the idiosyncrasies of your model bike. Tuners need
information about you and the way you ride in order to revalve your suspension.
If they don't give you a survey form to complete or interview you, then be
suspicious about the work they are asking to be paid to perform! Revalving can
be defined as the removal, reposition, or replacement of shims in the valve
stacks of the compression and rebound pistons of a cartridge fork or rear
shock. Revalving should be performed when you've exhausted the basics like
setting the sag, making sure your bike has the right springs, and the forks and
shock have fresh oil, seals, and bushings. Only then can you make a
determination whether your bike needs revalving in order to make it handle
better. The main reasons why you need good handling suspension on a dirt bike
is:

 	To keep the wheels in contact with the ground to provide traction and
drive for the rear wheel and steering for the front wheel.
	To minimize the impacts and vibration on the motorcycle.
	To minimize the stress loads on the rider and prevent fatigue and injuries.

The rear wheel must stay in contact with the ground in order to provide driving
force. The front wheel needs to stay in contact with the ground in order to
provide steering control. Impacts on the motorcycle can cause all sorts of
problems like loose bolts, foaming of the fuel in the carb's float bowl,
long-term damage to the bearings that support the suspension components, and
long term damage to the electrical components. The chronic problems to a rider
from a poor handling bike are much more obvious.  Forearm pump-up is probably
the most common. Long term damage to the a riders neck and spine may take years
to manifest but some people might be immediately sensitive pain. Having a
professional suspension tuner re-valve your suspension might seem expensive
(Average cost of total rework with parts $600) but what price do you put on
pain?

The main things that a suspension system is affected by are:

	Changes in the sprung mass from moving up and down.
	Changes in motion like acceleration, braking, and turning.

The sprung mass of a moving dirt bike can be hard to define because the entire
motorcycle leaves the ground! Technically the sprung mass includes everything
except the wheels, swing arm, lower fork tubes, and the rear shock. Those parts
are considered unsprung mass. Because dirt bikes are capable of jumping,
gravity and the weight of the rider affect the sprung mass. The movement of a
motorcycle's suspension going up is termed rebound and the movement down is
compression. 

Changes in the motion of a motorcycle can cause it to roll, pitch, yaw or any
combination thereof. When a motorcycle accelerates the bike pitches backward.
The driving chain forces try to wrap the swing arm underneath the bike. Of
course that cannot occur because the shock is a finite length and connects the
swing arm to the frame, but it causes a transfer of force. The rear wheel
pushes down into the ground, transferring force up the swing arm and causing
the front end to lift. The natural tendency of the rear wheel is to hop because
the damping isn't enough to compensate for the spring force. When a motorcycle
is braked for a turn the bike pitches forward, shifting the weight to the
front. The rear end tends to kick because of the torque reaction of the brake
caliper on the swing arm and the weight shift.

When a motorcycle is turned it rolls, pitches, and yaws at the apex of the
turn. A complicated motion! The front end is forced to either compress or
change the fork angle or extend and plow out of the turn. Meanwhile the rear
end tries to make a radial motion without loosing traction and spinning out. 

Internal and External Adjustments
Suspension dampers can be adjusted two ways, internally and externally.
External adjustments are limited to the riding circumstances and the adjustment
range on the compression and rebound clickers. Internal adjustments are
virtually unlimited because it encompasses revalving and re-porting of the
damper piston and valve shim stack.  The external adjusters, low speed
compression and rebound, can only affect minor changes in handling. Typical low
speed compression or rebound riding situations might include far-spaced shallow
whoops, tabletop jumps, braking and accelerating around tight turns. All
compression and rebound clicker adjusters are marked S and H, meaning soft and
hard. That can also be interpreted as soft fast and hard stiff. 

The focus of a professional suspension tuner's work revolves around internal
adjustments. When a suspension component is revalved it is also rebuilt,
meaning that the bushings and seals are checked for replacement and the oil is
changed. Revalving is the discipline or repositioning, removing, or replacing
valve shims in such an order as to affect a change in the damper's performance. 

Aftermarket Piston Kits
The latest trend is to combine a piston design with a valve shim pack so as to
affect a greater change. There are two main types of piston/shim systems. The
manufacturers are Race Tech (Gold Valve) and Pro-Action (3-Stage Incremental).
The main difference between the two is the port design of the piston. The Race
Tech set up relies on a high flow piston with a large series of shims that can
be rearranged in set patterns to adapt to the needs of set number of rider
profiles. The Pro-Action set up relies on a piston with smaller ports and a
multi-stage shim arrangement that separates the circuits of passive and active
to give the damper a wider tuning range. The piston works at the edge of the
spectrum and provides a hydraulic lock capability during riding situations
where all the suspension travel is used quickly. 

From a marketing standpoint, the Race Tech Gold Valve is simple and can be
installed by inexperienced technicians. The support provided by Race Tech is
excellent. The kits come with detailed instructions, a video is optional, as
well as training seminars geared towards amateur race tuners and home-based
mechanics. The Pro-Action 3-Stage Incremental valve isn't available over the
counter. It can only be installed by a Pro-Action franchise. The reason is that
the valving must be set-up for the individual, and there are a wider variety of
valving patterns to suit virtually any rider profile. The Pro-Action approach
also relies on matching the proper spring to the valving. Pro-Action's set-up
is more expensive than a typical revalving job, but its more comprehensive and
produces a truly custom result.

How Incremental Valving Works
The rear shock valve stack is comprised on a series of steel washers with a
variety of outer diameters and thickness, mounted on two sides of a piston.
This is called a bi-directional valve. One side handles the compression damping
and the other handles the rebound damping. The valve shim stacks have different
arrangement patterns because the compression stack aids the spring and the
rebound stack controls the stored energy release of the spring. With regards to
the sizes of the shims, the larger the diameter and the thinner the thickness,
the more easily the shim will bend and increase oil flow through the piston.
The faster the oil flow the less the damping.  Stock Japanese dampers use high
flow pistons with a complicated series of shims that aren't very sensitive at
slow shaft speeds. The shims don't open at slow shaft speeds and mostly the
clickers control the damping. However that can cause some potential handling
problems when accelerating out of turns. The bike is riding at a point on the
rear spring where the clickers don't provide enough damping and the piston
valving isn't in the response range, so the bike chatters. The Pro-Action
incremental valving concept separates the three main damping phases of low,
mid, and high. They do this by using a special piston and a valve stack with
transition shims to separate the three circuits. The incremental valve stack is
more sensitive at low shaft speeds so the clickers don't have to carry the
damping load.  The mid speed valve helps make the transition from low to high
speed damping modes to give a plush ride especially under an acceleration load.
The piston has smaller ports, which provide a hydraulic lock affect at high
shaft speeds. That reduces the load on the nitrogen charged gas bladder and the
elastomer foam bumper.

On the front forks, the evolution of design has changed at a greater rate than
rear shocks because riding techniques have changed over the years.  Riders tend
to do more front-end landings so the manufacturers have concentrated on
cartridge fork design. Modern forks contain a bi-directional valve on the
rebound rod and a mono-directional valve for the passive compression.
Pro-Action is the only company that uses a two-stage passive compression
circuit with a low flow piston that handles a greater share of the active
compression damping. In some cases they even remove the active valve stack from
the rebound rod. 

 The Shock Dyno
A shock dyno is a computer controlled electro-hydraulic machine that simulates
and measures the damping characteristics of dampers (rear shock or front fork).
A shock dyno is comprised of an electric motor, a hydraulic ram, a mounting
guide, and a load cell (pressure transducer). A shock dyno quantifies how much
resistance (force in lbs.) the damper produces at different shaft speeds
(velocity in inches/second) and stroke lengths (displacement/travel in inches).

The load cell is connected to a PC program that plots the damping of the
compression and rebound over a range of shaft speeds. The two basic types of
graphic plots that a shock dyno provides is force vs. velocity and force vs.
displacement. There is an optimum profile for these plots, so a suspension
technician can use the results of the plots to see if there is an obvious
problem with a shock or fork. The force vs. displacement plots can show how
smoothly valve shims are opening, if there is air trapped in the shock, the
condition of the seals and bushings, and the condition of the oil with regards
to fading over time. The shock dyno can also test the condition of the
adjusters, the gas bladder, and the bearing on the top mount.  In autosports
it's routine to test shocks before and after servicing. It enables the
professional suspension technician to test and verify his work.

Suspension Tuning for Uniquely Sized People
The one thing that most frustrates me about dirt bikes is that they're all the
same size. Us riders are made in all different sizes but dirt bikes are made
for skinny guys who are 5'10". Over the years my web site has been posed
questions from every shape of dirt biker imaginable. These questions can be
divided into two categories; too big, or not tall enough to touch the ground.
Here is some set-up tips from some experimentation on real dirt bikers.

Shortening Suspension for Rider Comfort
Ever wonder why dirt bikes are built with 12 inches of travel when 4 inches of
it is sag? Why don't the manufacturers offer an option of providing a kit to
give a bike 8 good inches of travel? Perhaps they will after they read this
book! Suspension tuners have the capability of modifying forks and shocks for
shortened lengths. FMF's contractive suspension was basically a suspension
whereby springs were fitted to the rebound side of the shock shaft or fork
damper rods. When the bike topped out its suspension it would rely on the
springs to contract to the point where the bike normally sags to (8 inches).
The magazine tested some prototype bikes are raved about the handling through
turns but criticized the bike for it's vulnerability at getting grounded in
deep ruts.

A suspension tuner outside Chicago, Jeremy Wilkey of MX-Tech (www.mx-tech.com)
specializes in all sorts of suspension tuning but especially shortening
suspension for DTX (dirt track) or other off-road applications. When performing
such modifications many factors have to be considered. When you make one change
to a suspension component it affects several other things. A suspension
component can be shortened a few different ways. For a rear shock a spacer can
be turned on a lathe and fitted between the rebound stop washer and the seal
pack. A one-inch long spacer will shorten the rear travel by four inches
because of the linkage system. Most shocks have enough threads on the shock
body to accommodate adjustment of the spring. If they don't then the spring
must be shortened.  When a spring is shortened it becomes effectively stiffer.
When the spring is stiffer the rebound damping must be increased to compensate
for the additional potential stored energy of the stiffer spring. Generally
speaking if you are a heavy person, shortening the travel will adjust the
spring to your weight but the shock will still need to be revalved. If you are
a lightweight person you may need to switch to a progressive shock spring.
Obviously by changing the ride height of the bike the rising rate of the
linkage system is going to be narrowed. One product that is available to adjust
the linkage ratio is the Devol Link. The mounts of this product are fitted with
adjustable lugs. For modifications to the front forks, it may be possible to
just shorten the spring and place the cut-off section of spring on the rebound
rod in place of the top-out spring. I did that on my XR600 to make contractive
suspension. Some types of cartridge forks don't have the space for a spring but
a plastic or elastomer foam rubber spacer can be made to shorten the fork
travel. Like the rear shock the spring rate is the biggest factor. You don't
have to shorten the front travel as much as the rear because you can still
adjust the forks at the triple clamps. Normally a bike with shortened travel
will be better suited for low speed riding. Consideration will need to be given
when jumping or riding through deep ruts because the lowered ground clearance
will make the bike more prone to grounding-out.

Suspension Tuning for Big Guys
Big guys face the same problems as the vertically challenged. Dirt bikes just
aren't designed for them either. Some of the things that big guys do to adjust
their bikes is revalve the suspension and install a stiffer set of springs.
Tall guys have the added problem of leverage. When they stand up and lean
forward or backward they can easily change the pitch of the bike and
drastically affect the handling. That's why stiff springs are important. Some
popular mods that big guys perform to customize dirt bikes is a taller seat
using special seat foam and covers, ATV handlebars with a high rise, forward
off-set handlebar clamps, extended shift and brake levers, and wider foot pegs.
These mods are done to adjust the ergonomics of the bike but may compromise the
handling.

The Engineering and Mathematical Aspects of Suspension Tuning
Engineering students and suspension tuners email all the time with questions
about other books on suspension engineering. The two best books that I've found
focus on the shock absorber and roadracing motorcycles. Of course the material
can be applied to any type of suspension engineering.  My favorite
motorcycle-engineering book is "The Racing Motorcycle" By John Bradley.
Available from Motorbooks (1-800-826-6600) for $65.00 In England this book is
available from the author at Broadland Publications (01904 414763) The Society
of Automotive Engineers publishes an excellent book on shocks, "The Shock
Absorber Handbook" By John Dixon sells for $55.00 Call the SAE at
(1-724-776-4970) Both of these books contain material geared towards university
level engineering studies. These books contain a wide range of mathematical
formulas, and an understanding of algebra is required.