Eric Gorr
Engine Builder
- Jun 29, 1999
- 384
- 12
Hi guys, the complete article is over the 20,000 character limit for a post so I split it up. Here is the rest of the article.
Cartridge Fork Terminology
Active and Passive Valving:
There are two valve assemblies in the fork leg. The compression valve
assembly is the passive valve because it is stationary and depends on the fork rods volume to displace oil through it. The mid-valve on the end of the cylinder rod is the active valve because it travels through the oil column when the forks compress or rebound.
Air Spring:
The air spring is a compression resistance force that works most noteably when the forks are in the final 1/3 of the travel. The air spring pressure is based on the volume of air as it is reduced when the fork compresses. As the volume is reduced, the air compressed and the resistance to that compression increases exponentially. The higher the oil volume, the greater the air spring compression ratio, and the greater the resistance to bottoming. This in turn can cause the fork to have a rapidly increasing stiffness in the lower third of the stroke and can make the fork feel excessively stiff. Conversely, if the oil volume is too low, then the fork can bottom easily. Ideally you want between these extremes.
Axle Clamp:
The axle clamp is threaded and pinned to the inner fork tube.
Bushings and Seals:
The bushings are the load-bearing surfaces of the fork tubes and cartridge cylinder. The seal and wiper are the hydraulic-sealing surfaces of the fork leg.
Cartridge Cylinder:
The main cylinder of the cartridge assembly holds the cylinder rod,
mid-valve, check valve spring, rebound valve and adjuster rod. The cylinder is topped with the compression adjuster assembly.
Check Valve Spring:
The spring seated between the bottom of the compression rod and the
valving of the midvalve. It is responsible for closing the valve as the fork finishes the compression stroke and begins the rebound stroke. If it where not for this spring, the fork could have a small delay in the full effect of the rebound valving. This is known as valve lag, and can be measured on a dyno as one type of Hysteresis. The Check valve spring is also located on the back of the passive valve and quickly closes the return check plate as the fork ends its rebound stroke, and has the same impact, only in the opposite fluid flow.
Compression Valve Assembly:
The mechanism connected to the fork cap. It contains the ICS (Internal Compensation Spring), piston, ring, and shims. It can also be known as a basevalve, passive valve, or foot valve. It meters the fluid displaced by the cylinder rod and has the most influence over lower speed ranges of the system.
Cylinder Rod:
The cylinder rod is the main shaft of the cartridge assembly. It creates the displaced volume of fluid the passive valve regulates, besides that, it also holds the active piston, and the internal rebound adjuster rod.
Fork Slider:
The outer aluminum tube connected to the triple clamps.
Fork Tube:
The inner steel tube connected to the axle clamp.
Hydraulic Stops and Cones:
An internal tapered cylinder on the bottom of the fork leg interfaces with a piston mounted on the bottom of the fork cylinder. A hydraulic stop functions when the piston and cylinder interact squeezing the oil through an increasingly small orifice, bringing the fork to smooth stop in the final inches of travel.
ICS:
The Internal Compensation Spring is part of the compression valve assembly. It seats between the ICS piston and the end cap. The ICS piston is compressed as the fork rod displaces the fluid; it thereby pressurizes the fluid in the cylinder and hydraulically adds to the overall fork spring rate.
Main Spring:
The long coil spring that fits inside the fork leg.
Mid-Valve:
The mid-valve is a series of shims mounted behind the Active piston on the cylinder rod. The mid-valve is the active compression valve. The shims bend as the valve travels through the fluid during the compression stroke; the shims provide resistance to the fluid and therefore create damping. The midspeed valve does not displace fluid; it only regulates fluid as it passes through the column of oil.
Rebound Adjuster Bolt:
The rebound adjuster bolt is fastened to the bottom of the axle clamp. This bolt holds the forks together, clamping the cartridge to the axle clamp
Rebound Adjuster Rod:
The small diameter tube that connects the clicker to the flow-control
tapered needle in the piston.
Transfer Control Valve
The transfer control valve (TCV) is a large plastic tube located inside the inner fork tube near the axle clamp. The TCV is a viscous damping system with a check-valve (this check valve will not be found on the YZ125 models), and a hydraulic bottoming cone providing resistance to the fork cylinder as it compresses. The check valve allows the fork to extend with no resistance due to a vacuum that would otherwise be created during extension.
(Sidebar)
Cartridge Fork Myths and Truths
Myth
Mid-stroke harshness is caused by excessive mid-valve compression damping.
Truth
Mid-stroke harshness is caused by excessive progressive elements (air spring and wire spring load), compounding each other against softer compression damping rates.
Myth
Reducing the mid-valve compression, or changing the spring rates of the ICS and main spring can reduce mid-stroke harshness.
Truth
Mid-stroke harshness can be improved by increasing the compression damping and reducing the air spring force with a lower oil height.
Myth
Changing the ICS results in an improvement of compression damping
Characteristics.
Truth
Changing the ICS only affects the overall spring rate and preload. Installing a softer ICS can reduce the efficiency of the fork valving, think of ICS tension as being similar to nitrogen pressure in a shock.
Myth
Switching the weight of the oil changes the damping.
Truth
Oil weight changes affect only the lower end of the damping spectrum.
(Sidebar)
FAQs with Jeremy Wilkey
Q: What are the main performance concerns of the new Kayaba cartridge fork?
A: The forks have an inconsistent feel based on position. In the first part of the stroke the forks have limited damping. Once the rod has displaced enough fluid to pressurize the cylinder, the damping increases sharply. The ICS needs more pressurized travel through the addition of a spacer between the spring and spring seat. You can reference this with the damping force graph.
The TCV provides viscous damping that contributes to the compounding of
other factors like the air and coil spring. This works for stadium racing where hard front-end landings are encountered, but it reduces the linear feel of the forks through the travel range. This might be more important to off-road and outdoor mx riders. Modifying the TCV by removing the check valve allows free flow and more linear feel throughout the fork travel.
The main spring pre-load is non-existent. Installing a spacer-ring to
provide 6 to 8-millimeters of preload is consistent with current tuning
technology, and chassis set-up parameters.
The final and most important area of concern is the task of bleeding the air from the cartridge. The new Kayaba design tends to trap air when bleeding in the upright position. By using the inclined bleeding method, excess oil can be used to force out trapped air pockets.
Q: How can a shock dyno be used on cartridge forks?
A: With special fixtures we place the forks or fork cylinders in the dyno. Buy doing this we can evaluate different aspects of the forks design. For instance, on the stock KYB forks we can actually measure the inconsistencies in damping caused by the lack of pressurized ICS travel. We can then fix the problem and document the real world results with those that we see on the dyno graphs and reports.
Getting into other aspects, we can evaluate things like seal drag and how the ICS rate adds up to the overall performance. These types of variables can be accounted for but up until you start measuring them in real numbers it’s hard to be able to reliably account for these aspects of the components.
After that you can do more mainstream things like actually looking at the damping force curves and looking for opportunity to improve performance by reducing hysteresis and inconsistencies.
Q: I’ve heard that traditional automotive shock dynos don’t account for real world mx shock speeds.
A: That’s true to some extent, without a hydraulic or electro-mechanical dyno its very difficult to match the speeds of the real world, or the accelerations; however, the data provided by the equipment is just as a valuable, you just need to learn how to use it. Interestingly, we have received reliable data by creating a test where we alter the fluid volumes to test for valve reactions at what would equate to much higher operating speeds. So like most things, creativity and good science can yield fantastic and insightful results. In sum, its really just much more detailed information, one more vital piece of the puzzle, and as a tuner, when you have more information you can make bigger changes with confidence.
Cartridge Fork Terminology
Active and Passive Valving:
There are two valve assemblies in the fork leg. The compression valve
assembly is the passive valve because it is stationary and depends on the fork rods volume to displace oil through it. The mid-valve on the end of the cylinder rod is the active valve because it travels through the oil column when the forks compress or rebound.
Air Spring:
The air spring is a compression resistance force that works most noteably when the forks are in the final 1/3 of the travel. The air spring pressure is based on the volume of air as it is reduced when the fork compresses. As the volume is reduced, the air compressed and the resistance to that compression increases exponentially. The higher the oil volume, the greater the air spring compression ratio, and the greater the resistance to bottoming. This in turn can cause the fork to have a rapidly increasing stiffness in the lower third of the stroke and can make the fork feel excessively stiff. Conversely, if the oil volume is too low, then the fork can bottom easily. Ideally you want between these extremes.
Axle Clamp:
The axle clamp is threaded and pinned to the inner fork tube.
Bushings and Seals:
The bushings are the load-bearing surfaces of the fork tubes and cartridge cylinder. The seal and wiper are the hydraulic-sealing surfaces of the fork leg.
Cartridge Cylinder:
The main cylinder of the cartridge assembly holds the cylinder rod,
mid-valve, check valve spring, rebound valve and adjuster rod. The cylinder is topped with the compression adjuster assembly.
Check Valve Spring:
The spring seated between the bottom of the compression rod and the
valving of the midvalve. It is responsible for closing the valve as the fork finishes the compression stroke and begins the rebound stroke. If it where not for this spring, the fork could have a small delay in the full effect of the rebound valving. This is known as valve lag, and can be measured on a dyno as one type of Hysteresis. The Check valve spring is also located on the back of the passive valve and quickly closes the return check plate as the fork ends its rebound stroke, and has the same impact, only in the opposite fluid flow.
Compression Valve Assembly:
The mechanism connected to the fork cap. It contains the ICS (Internal Compensation Spring), piston, ring, and shims. It can also be known as a basevalve, passive valve, or foot valve. It meters the fluid displaced by the cylinder rod and has the most influence over lower speed ranges of the system.
Cylinder Rod:
The cylinder rod is the main shaft of the cartridge assembly. It creates the displaced volume of fluid the passive valve regulates, besides that, it also holds the active piston, and the internal rebound adjuster rod.
Fork Slider:
The outer aluminum tube connected to the triple clamps.
Fork Tube:
The inner steel tube connected to the axle clamp.
Hydraulic Stops and Cones:
An internal tapered cylinder on the bottom of the fork leg interfaces with a piston mounted on the bottom of the fork cylinder. A hydraulic stop functions when the piston and cylinder interact squeezing the oil through an increasingly small orifice, bringing the fork to smooth stop in the final inches of travel.
ICS:
The Internal Compensation Spring is part of the compression valve assembly. It seats between the ICS piston and the end cap. The ICS piston is compressed as the fork rod displaces the fluid; it thereby pressurizes the fluid in the cylinder and hydraulically adds to the overall fork spring rate.
Main Spring:
The long coil spring that fits inside the fork leg.
Mid-Valve:
The mid-valve is a series of shims mounted behind the Active piston on the cylinder rod. The mid-valve is the active compression valve. The shims bend as the valve travels through the fluid during the compression stroke; the shims provide resistance to the fluid and therefore create damping. The midspeed valve does not displace fluid; it only regulates fluid as it passes through the column of oil.
Rebound Adjuster Bolt:
The rebound adjuster bolt is fastened to the bottom of the axle clamp. This bolt holds the forks together, clamping the cartridge to the axle clamp
Rebound Adjuster Rod:
The small diameter tube that connects the clicker to the flow-control
tapered needle in the piston.
Transfer Control Valve
The transfer control valve (TCV) is a large plastic tube located inside the inner fork tube near the axle clamp. The TCV is a viscous damping system with a check-valve (this check valve will not be found on the YZ125 models), and a hydraulic bottoming cone providing resistance to the fork cylinder as it compresses. The check valve allows the fork to extend with no resistance due to a vacuum that would otherwise be created during extension.
(Sidebar)
Cartridge Fork Myths and Truths
Myth
Mid-stroke harshness is caused by excessive mid-valve compression damping.
Truth
Mid-stroke harshness is caused by excessive progressive elements (air spring and wire spring load), compounding each other against softer compression damping rates.
Myth
Reducing the mid-valve compression, or changing the spring rates of the ICS and main spring can reduce mid-stroke harshness.
Truth
Mid-stroke harshness can be improved by increasing the compression damping and reducing the air spring force with a lower oil height.
Myth
Changing the ICS results in an improvement of compression damping
Characteristics.
Truth
Changing the ICS only affects the overall spring rate and preload. Installing a softer ICS can reduce the efficiency of the fork valving, think of ICS tension as being similar to nitrogen pressure in a shock.
Myth
Switching the weight of the oil changes the damping.
Truth
Oil weight changes affect only the lower end of the damping spectrum.
(Sidebar)
FAQs with Jeremy Wilkey
Q: What are the main performance concerns of the new Kayaba cartridge fork?
A: The forks have an inconsistent feel based on position. In the first part of the stroke the forks have limited damping. Once the rod has displaced enough fluid to pressurize the cylinder, the damping increases sharply. The ICS needs more pressurized travel through the addition of a spacer between the spring and spring seat. You can reference this with the damping force graph.
The TCV provides viscous damping that contributes to the compounding of
other factors like the air and coil spring. This works for stadium racing where hard front-end landings are encountered, but it reduces the linear feel of the forks through the travel range. This might be more important to off-road and outdoor mx riders. Modifying the TCV by removing the check valve allows free flow and more linear feel throughout the fork travel.
The main spring pre-load is non-existent. Installing a spacer-ring to
provide 6 to 8-millimeters of preload is consistent with current tuning
technology, and chassis set-up parameters.
The final and most important area of concern is the task of bleeding the air from the cartridge. The new Kayaba design tends to trap air when bleeding in the upright position. By using the inclined bleeding method, excess oil can be used to force out trapped air pockets.
Q: How can a shock dyno be used on cartridge forks?
A: With special fixtures we place the forks or fork cylinders in the dyno. Buy doing this we can evaluate different aspects of the forks design. For instance, on the stock KYB forks we can actually measure the inconsistencies in damping caused by the lack of pressurized ICS travel. We can then fix the problem and document the real world results with those that we see on the dyno graphs and reports.
Getting into other aspects, we can evaluate things like seal drag and how the ICS rate adds up to the overall performance. These types of variables can be accounted for but up until you start measuring them in real numbers it’s hard to be able to reliably account for these aspects of the components.
After that you can do more mainstream things like actually looking at the damping force curves and looking for opportunity to improve performance by reducing hysteresis and inconsistencies.
Q: I’ve heard that traditional automotive shock dynos don’t account for real world mx shock speeds.
A: That’s true to some extent, without a hydraulic or electro-mechanical dyno its very difficult to match the speeds of the real world, or the accelerations; however, the data provided by the equipment is just as a valuable, you just need to learn how to use it. Interestingly, we have received reliable data by creating a test where we alter the fluid volumes to test for valve reactions at what would equate to much higher operating speeds. So like most things, creativity and good science can yield fantastic and insightful results. In sum, its really just much more detailed information, one more vital piece of the puzzle, and as a tuner, when you have more information you can make bigger changes with confidence.
