- Jue Oct 27, 2005 22:44
#124487
Este documento esta en ingles, pero es la biblia del funcionamiento de por lo menos las suspensiones de las motos japonesas, si alguien se ofreciese a traducirlo nos haria un gran regalo.
Eric Gorr es un guru de la mecanica muy conocido y respetado.
Video Suspension Tuning
By Eric Gorr
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. Some suspension tuners are starting to encourage riders to make video samples for review.
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.
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 effect. The damping effect 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 effect. 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. The low, mid, and high-speed circuit shims are separated by transition shims. Think of the valve stack as gears in a transmission, and the transition shims as shift forks. The more tapered the valve stacks, and the thinner the transition shim, the suspension becomes plusher in it's handling. Less plush suspension is typically too stiff to absorb the small bumps on acceleration, and too soft for square-edged bumps at speed. Much of the problem has to do with a mismatch between the piston's port arrangement and the overall valve stack.
Why Revalve?
The term revalving is often tossed around in the dirt bike magazines, but have you ever wondered what suspension tuners do to revalve 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 a rider's 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 and 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 www.motorbooks.com 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) www.sae.org 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.
Eric Gorr es un guru de la mecanica muy conocido y respetado.
Video Suspension Tuning
By Eric Gorr
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. Some suspension tuners are starting to encourage riders to make video samples for review.
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.
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 effect. The damping effect 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 effect. 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. The low, mid, and high-speed circuit shims are separated by transition shims. Think of the valve stack as gears in a transmission, and the transition shims as shift forks. The more tapered the valve stacks, and the thinner the transition shim, the suspension becomes plusher in it's handling. Less plush suspension is typically too stiff to absorb the small bumps on acceleration, and too soft for square-edged bumps at speed. Much of the problem has to do with a mismatch between the piston's port arrangement and the overall valve stack.
Why Revalve?
The term revalving is often tossed around in the dirt bike magazines, but have you ever wondered what suspension tuners do to revalve 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 a rider's 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 and 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 www.motorbooks.com 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) www.sae.org 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.
KTM 450 EXC
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XTZ660 tenere
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XTZ660 tenere
