Eric Gorr
Engine Builder
- Jun 29, 1999
- 384
- 12
A message from Eric Gorr…..
This article was partially published in the November 2005 issue of Dirt Rider. DR had room for the most important part of servicing the new style forks but not enough room for the sidebars. I encourage you to buy the magazine for the photos in the step-by-step guide. Jeremy and I though you guys might like to read the rest of the original article plus the sidebars on a cartridge fork dictionary, myths exploded, and Q&A shock dynos.
KAYABA AIR/OIL SEPARATED FRONT FORK SERVICE GUIDE
By Eric Gorr and Jeremy Wilkey
Kayaba introduced a new design of cartridge style forks that come standard on the 2005 Yamaha YZ125T, YZ250T, YZ250F, YZ450F, and Kawasaki KX250. The new Kayaba fork is called the Air Oil Separated fork and is similar to the Showa Twin Chamber Design that has been in production since 1994 on Suzuki RM and 1997 on the Honda CR dirt bikes. There are some differences in the Showa and Kayaba forks and we asked Jeremy Wilkey of MX-Tech to highlight the differences and give you a supplementary service guide to changing oil, seals and bushings, and tune the new forks for better performance. For this article we’ve selected the 2005 YZ250F for demonstration.
The generic term for these types of forks is “Twin Chamber”. The main advantage is that the fork produces more consistent damping because it does not rely on the progressive air spring to pressurize the fluid. Secondly, the fork performance doesn’t degrade as quickly because the high wear components are separated from the valving components. In the open cartridge fork, the coil springs, tube and slider, and bushings are constantly rubbing and wearing, producing slurry of finely ground metal particles that serves to breakdown oil (through oxidation) and clog the tiny passages of the piston and shim valves. This process slowly degrades the action of the fork by altering the damping characteristics and increasing friction.
Differences between the Showa and Kayaba designs
The main difference between the Showa and Kayaba designs is the Transfer Control Valve (TCV). It is a large plastic oil lock located in the bottom of the fork. It works as a viscous damping system with a traditional hydraulic stop and includes a check valve in the form of a sliding seal band. The only way to access the TCV is to remove the axle clamp and slide it out the bottom of the fork tube. The TCV doesn’t require maintenance but tuners will likely modify this valve.
The Kayaba has less internal volume and air space than the Showa. As a
result the Kayaba has a higher air spring compression ratio. Much of the internal volume is occupied with plastic spacers and tubes like the TCV. The slider tube has a unique bell-bottomed shape to allow for volume; it may also makes the slider tube stiffer.
The Kayaba’s reservoir piston for the compression adjuster assembly is
closed-off and operates independent of the air spring. The Showa is
pressurized by the Internal Compensation Spring (ICS) and the air spring. The pros are the ICS isn’t affected by the exponential character of the air spring, as when the forks are in the final third of travel.
The Kayaba forks have no pre-load force on either the main coil spring or the ICS. When the forks are extended, the springs have as much as
5-millimeters of free-play. There appears to be no logical reason for this. All previous Kayaba cartridge fork designs use some measure of pre-load on the springs.
Tools of the Trade
In order to service the Kayaba, forks you’ll need some special tools as well as some common sockets and wrenches. Starting from the top of the fork, an 8-point 50-millimeter box wrench is used for the fork cap and a hexagonal slug for the ICS cap. A split-collar seal and bushing driver will be needed for routine servicing, as well as a seal-bullet to protect the seal from tearing upon installation on the fork tube. To remove the cartridge from the fork tubes, a straight flat-wrench is required. A 6-point 17-millimeter socket and ratchet fits the rebound adjuster bolt.
A set of shaft blocks will be needed to clamp any of the round tubes of the forks. Race Tech makes inexpensive universal sets of aluminum shaft blocks that will insure that the sensitive tubes don’t get damaged when clamped in a vise.
Some other assorted tools will be needed, like small straight-blade
screwdrivers, a 6-inch vise, a pointed scribe, light duty locking agent, and a tube of seal grease.
Cleaning the parts and capturing the waste oil are important considerations. Most automotive garages and express oil stations accept small sealed containers of waste oil. The parts are best cleaned with a high flash point mineral spirits solvent like PB Parts Blaster, available from auto parts stores like Pep Boys. Mail-order companies such as Harbor Freight sell covered parts washers with a pump and brush plus a 5-gallon container of solvent for about $150 delivered.
Maintenance Intervals
Modern two-staged sealed cartridge forks don’t require as much maintenance as the older style cartridge forks because the cartridge is sealed from the contamination produced from the fork tubes and springs. Here is a list of the typical tasks associated with servicing modern forks:
Seals and Wipers
High performance seals with relatively low drag need to be changed about twice a season or approximately every forty hours. You can buy
durable seals that require less maintenance but the stiction will be so
great, that it will affect the damping performance.
Bushings
The bushings are the load bearing surfaces of the fork. There are three
bushings in each fork leg. The bushings are located in the slider tube, on the fork tube, and inside the fork cylinder. The bushings are very durable and don’t wear quickly unless they become damaged during a seal and wiper change.
Outer Tube Oil Change
The outer fork tubes contain the oil that lubricates the seals and bushings as well as provide a means of tuning the air spring. There is no need to use high quality expensive oil, just one with good lubrication properties. The service interval for cleaning and oil changing is the same as the seals and wipers.
Cartridge Service
The cartridge needs to be cleaned about once per season or 80-100 hours of usage. High quality oil like Yamaha Suspension 01 should be used because its formulization best matches the cartridge bushings and bleed set-ups.
(STEP-BY-STEP PHOTO GUIDE)
Disassembling the Forks
Use a Race Tech 8-point 50-millimeter box wrench to remove the outer fork cap.
Depress the slider tube and drain the oil from the outer chamber. Capture the oil and recycle it at an automotive garage or oil express.
Unthread the rebound adjuster bolt with a 17-millimeter socket while holding the axle clamp in a vise. The rebound adjuster bolt will separate from the fork tube, but it still needs to be unthreaded from the cylinder rod.
Extend the cylinder rod by pushing on the top of the cartridge to expose the damper rod jam nut. Place a Race Tech flat-wrench on the cylinder rod and loosen the jam nut and the rebound adjuster bolt.
Remove the cartridge assembly and the spring from the top of the fork leg.
Clamp the upper part of cylinder in a shaft-block and vise, then use a
hexagonal socket slug to remove the compression valve assembly. Pull to
separate the compression valve from the cartridge cylinder.
Disassemble the cartridge for cleaning. Start by clamping the cylinder rod in a shaft-block and unthread the jam nut. Now slide the cylinder rod out of the top of the cylinder.
The best method for cleaning the oily components of the fork is a high flash point (120F) mineral spirits solvent. Solvent breaks up the mixture of grease, oil, and metal debris that contaminate the forks. Commercially available products include PB Blaster available from most auto parts stores. Non-chlorinated brake cleaner is the best choice for final cleaning. Allow the parts to drip dry.
Changing Seals, Wipers, and Bushings
The fork tube and slider must be separated to change the seals and bushings. Use a small straight-blade screwdriver to pry off the wiper; now remove the circlip and slide the parts down the tube.
Heat the lower part of the fork slider with a propane torch to expand the fork slider away from the outer bushing; this will make it easier to separate the tube and fork slider. This will also prevent the outer bushing from being pushed over the inner bushing, which will avoid scrubbing off the Teflon coating. Grasp the fork tubes with your hands and quickly extend them several times until the tubes separate.
The bushings, seals, circlip, and wiper will be retained on the inner fork tube.
Remove the fork tube bushing by spreading it with a screwdriver and
slide it off the end of the tube. Discard the old oil and dust seals and inspect the bushings for wear.
Put a dab of grease under the lip of the oil seal and dust wiper.
Install a new wiper and bushing on the fork tube. Before installing the seal, slide a seal-bullet over the end of the fork tube to prevent damage to the new seal. Make sure to install seals, bushings, aluminum spacer and circlip in exact order.
Use a seal driver on the aluminum spacer to seat the outer bushing in the fork slider, and then use the driver on the seal. Install the circlip and use a plastic mallet to tap the wiper evenly into the slider.
Set the fork tubes aside and get ready to assemble the inner cartridge.
Assembling the Cartridge
Insert the piston rod assembly into the cylinder using a 12-millimeter T-handle to manipulate the piston rod into place.
When installing the jam nut on the end of the piston rod, ensure that the jam nut is threaded all the way up on the piston rod.
Clamp the cylinder in the shaft-block and tighten the vise so you can hold the cartridge upright for oil filling. Overfill the cartridge to an oil height of 130-millimeters from the top edge of the cylinder with Yamaha Suspension Oil 01. You’ll need a total of two cans to refurbish both fork tubes.
Perform non-pressurized bleeding of the cartridge by moving the rod a total of one inch up and down to help displace trapped air to the top of the cylinder. Stroke the rod about thirty times to bleed the cartridge, and then wait ten minutes for the air to bubble upwards.
Wipe a dab of grease on the o-rings prior to inserting it into the top of the cartridge. Tighten the fork cap to factory specs.
Perform pressurized bleeding by stroking the cartridge no more than two
inches for a total of thirty times. This will compress the remaining air bubbles and position them near the top for final bleeding.
Clamp the bottom of the cartridge in the vise and set at about a
15-degree angle so the air travels up to the top. Position the holes of the cartridge at 6 and 12 0’clock so the air purges out the top hole and the excess oil purges out the bottom. Compress the cartridge slowly through its full travel.
You should hear air bubbles trickle out of the top hole while oil drips out the bottom hole. This procedure will dramatically improve the consistency of the fork action.
Clamp the axle mount into the vise and slide the white plastic tube spacer and the spring into the tube with the metal ring of the spacer facing the spring.
Install the metal spring spacer on to the cartridge with the ring facing the spring.
Insert the cartridge into the tube and compress the cartridge until the rod extends far enough to install the flat wrench. Install the long aluminum rebound adjuster rod into the piston rod then thread on the rebound adjuster until it bottoms out on the rod not the jam nut. There is the danger that the forks will be unequal lengths if you get this procedure wrong.
Tighten the jam nut against the rebound adjuster by holding a wrench on the jam nut and a socket on the bolt. Take care when removing the flat-wrench because the spring pressure will cause the rebound adjuster to slam into place. Put a dab of grease on the threads of the rebound adjuster and tighten it into the fork tube to factory torque specs.
The next step is to fill the outer fork tubes with a measured volume of oil. The oil volume and height generates the air spring’s progression. The lower the oil volume the less progressive the effects of the air spring. The higher the volume the more progressive your air spring will be. Consult your owner’s manual for suggestions on oil volume for your riding demands. There are no bleeding considerations for the outer fork tubes.
The last step is to tighten the fork cap hand-tight because the triple clamp provides the extra torque to prevent the cap from unthreading.
This article was partially published in the November 2005 issue of Dirt Rider. DR had room for the most important part of servicing the new style forks but not enough room for the sidebars. I encourage you to buy the magazine for the photos in the step-by-step guide. Jeremy and I though you guys might like to read the rest of the original article plus the sidebars on a cartridge fork dictionary, myths exploded, and Q&A shock dynos.
KAYABA AIR/OIL SEPARATED FRONT FORK SERVICE GUIDE
By Eric Gorr and Jeremy Wilkey
Kayaba introduced a new design of cartridge style forks that come standard on the 2005 Yamaha YZ125T, YZ250T, YZ250F, YZ450F, and Kawasaki KX250. The new Kayaba fork is called the Air Oil Separated fork and is similar to the Showa Twin Chamber Design that has been in production since 1994 on Suzuki RM and 1997 on the Honda CR dirt bikes. There are some differences in the Showa and Kayaba forks and we asked Jeremy Wilkey of MX-Tech to highlight the differences and give you a supplementary service guide to changing oil, seals and bushings, and tune the new forks for better performance. For this article we’ve selected the 2005 YZ250F for demonstration.
The generic term for these types of forks is “Twin Chamber”. The main advantage is that the fork produces more consistent damping because it does not rely on the progressive air spring to pressurize the fluid. Secondly, the fork performance doesn’t degrade as quickly because the high wear components are separated from the valving components. In the open cartridge fork, the coil springs, tube and slider, and bushings are constantly rubbing and wearing, producing slurry of finely ground metal particles that serves to breakdown oil (through oxidation) and clog the tiny passages of the piston and shim valves. This process slowly degrades the action of the fork by altering the damping characteristics and increasing friction.
Differences between the Showa and Kayaba designs
The main difference between the Showa and Kayaba designs is the Transfer Control Valve (TCV). It is a large plastic oil lock located in the bottom of the fork. It works as a viscous damping system with a traditional hydraulic stop and includes a check valve in the form of a sliding seal band. The only way to access the TCV is to remove the axle clamp and slide it out the bottom of the fork tube. The TCV doesn’t require maintenance but tuners will likely modify this valve.
The Kayaba has less internal volume and air space than the Showa. As a
result the Kayaba has a higher air spring compression ratio. Much of the internal volume is occupied with plastic spacers and tubes like the TCV. The slider tube has a unique bell-bottomed shape to allow for volume; it may also makes the slider tube stiffer.
The Kayaba’s reservoir piston for the compression adjuster assembly is
closed-off and operates independent of the air spring. The Showa is
pressurized by the Internal Compensation Spring (ICS) and the air spring. The pros are the ICS isn’t affected by the exponential character of the air spring, as when the forks are in the final third of travel.
The Kayaba forks have no pre-load force on either the main coil spring or the ICS. When the forks are extended, the springs have as much as
5-millimeters of free-play. There appears to be no logical reason for this. All previous Kayaba cartridge fork designs use some measure of pre-load on the springs.
Tools of the Trade
In order to service the Kayaba, forks you’ll need some special tools as well as some common sockets and wrenches. Starting from the top of the fork, an 8-point 50-millimeter box wrench is used for the fork cap and a hexagonal slug for the ICS cap. A split-collar seal and bushing driver will be needed for routine servicing, as well as a seal-bullet to protect the seal from tearing upon installation on the fork tube. To remove the cartridge from the fork tubes, a straight flat-wrench is required. A 6-point 17-millimeter socket and ratchet fits the rebound adjuster bolt.
A set of shaft blocks will be needed to clamp any of the round tubes of the forks. Race Tech makes inexpensive universal sets of aluminum shaft blocks that will insure that the sensitive tubes don’t get damaged when clamped in a vise.
Some other assorted tools will be needed, like small straight-blade
screwdrivers, a 6-inch vise, a pointed scribe, light duty locking agent, and a tube of seal grease.
Cleaning the parts and capturing the waste oil are important considerations. Most automotive garages and express oil stations accept small sealed containers of waste oil. The parts are best cleaned with a high flash point mineral spirits solvent like PB Parts Blaster, available from auto parts stores like Pep Boys. Mail-order companies such as Harbor Freight sell covered parts washers with a pump and brush plus a 5-gallon container of solvent for about $150 delivered.
Maintenance Intervals
Modern two-staged sealed cartridge forks don’t require as much maintenance as the older style cartridge forks because the cartridge is sealed from the contamination produced from the fork tubes and springs. Here is a list of the typical tasks associated with servicing modern forks:
Seals and Wipers
High performance seals with relatively low drag need to be changed about twice a season or approximately every forty hours. You can buy
durable seals that require less maintenance but the stiction will be so
great, that it will affect the damping performance.
Bushings
The bushings are the load bearing surfaces of the fork. There are three
bushings in each fork leg. The bushings are located in the slider tube, on the fork tube, and inside the fork cylinder. The bushings are very durable and don’t wear quickly unless they become damaged during a seal and wiper change.
Outer Tube Oil Change
The outer fork tubes contain the oil that lubricates the seals and bushings as well as provide a means of tuning the air spring. There is no need to use high quality expensive oil, just one with good lubrication properties. The service interval for cleaning and oil changing is the same as the seals and wipers.
Cartridge Service
The cartridge needs to be cleaned about once per season or 80-100 hours of usage. High quality oil like Yamaha Suspension 01 should be used because its formulization best matches the cartridge bushings and bleed set-ups.
(STEP-BY-STEP PHOTO GUIDE)
Disassembling the Forks
Use a Race Tech 8-point 50-millimeter box wrench to remove the outer fork cap.
Depress the slider tube and drain the oil from the outer chamber. Capture the oil and recycle it at an automotive garage or oil express.
Unthread the rebound adjuster bolt with a 17-millimeter socket while holding the axle clamp in a vise. The rebound adjuster bolt will separate from the fork tube, but it still needs to be unthreaded from the cylinder rod.
Extend the cylinder rod by pushing on the top of the cartridge to expose the damper rod jam nut. Place a Race Tech flat-wrench on the cylinder rod and loosen the jam nut and the rebound adjuster bolt.
Remove the cartridge assembly and the spring from the top of the fork leg.
Clamp the upper part of cylinder in a shaft-block and vise, then use a
hexagonal socket slug to remove the compression valve assembly. Pull to
separate the compression valve from the cartridge cylinder.
Disassemble the cartridge for cleaning. Start by clamping the cylinder rod in a shaft-block and unthread the jam nut. Now slide the cylinder rod out of the top of the cylinder.
The best method for cleaning the oily components of the fork is a high flash point (120F) mineral spirits solvent. Solvent breaks up the mixture of grease, oil, and metal debris that contaminate the forks. Commercially available products include PB Blaster available from most auto parts stores. Non-chlorinated brake cleaner is the best choice for final cleaning. Allow the parts to drip dry.
Changing Seals, Wipers, and Bushings
The fork tube and slider must be separated to change the seals and bushings. Use a small straight-blade screwdriver to pry off the wiper; now remove the circlip and slide the parts down the tube.
Heat the lower part of the fork slider with a propane torch to expand the fork slider away from the outer bushing; this will make it easier to separate the tube and fork slider. This will also prevent the outer bushing from being pushed over the inner bushing, which will avoid scrubbing off the Teflon coating. Grasp the fork tubes with your hands and quickly extend them several times until the tubes separate.
The bushings, seals, circlip, and wiper will be retained on the inner fork tube.
Remove the fork tube bushing by spreading it with a screwdriver and
slide it off the end of the tube. Discard the old oil and dust seals and inspect the bushings for wear.
Put a dab of grease under the lip of the oil seal and dust wiper.
Install a new wiper and bushing on the fork tube. Before installing the seal, slide a seal-bullet over the end of the fork tube to prevent damage to the new seal. Make sure to install seals, bushings, aluminum spacer and circlip in exact order.
Use a seal driver on the aluminum spacer to seat the outer bushing in the fork slider, and then use the driver on the seal. Install the circlip and use a plastic mallet to tap the wiper evenly into the slider.
Set the fork tubes aside and get ready to assemble the inner cartridge.
Assembling the Cartridge
Insert the piston rod assembly into the cylinder using a 12-millimeter T-handle to manipulate the piston rod into place.
When installing the jam nut on the end of the piston rod, ensure that the jam nut is threaded all the way up on the piston rod.
Clamp the cylinder in the shaft-block and tighten the vise so you can hold the cartridge upright for oil filling. Overfill the cartridge to an oil height of 130-millimeters from the top edge of the cylinder with Yamaha Suspension Oil 01. You’ll need a total of two cans to refurbish both fork tubes.
Perform non-pressurized bleeding of the cartridge by moving the rod a total of one inch up and down to help displace trapped air to the top of the cylinder. Stroke the rod about thirty times to bleed the cartridge, and then wait ten minutes for the air to bubble upwards.
Wipe a dab of grease on the o-rings prior to inserting it into the top of the cartridge. Tighten the fork cap to factory specs.
Perform pressurized bleeding by stroking the cartridge no more than two
inches for a total of thirty times. This will compress the remaining air bubbles and position them near the top for final bleeding.
Clamp the bottom of the cartridge in the vise and set at about a
15-degree angle so the air travels up to the top. Position the holes of the cartridge at 6 and 12 0’clock so the air purges out the top hole and the excess oil purges out the bottom. Compress the cartridge slowly through its full travel.
You should hear air bubbles trickle out of the top hole while oil drips out the bottom hole. This procedure will dramatically improve the consistency of the fork action.
Clamp the axle mount into the vise and slide the white plastic tube spacer and the spring into the tube with the metal ring of the spacer facing the spring.
Install the metal spring spacer on to the cartridge with the ring facing the spring.
Insert the cartridge into the tube and compress the cartridge until the rod extends far enough to install the flat wrench. Install the long aluminum rebound adjuster rod into the piston rod then thread on the rebound adjuster until it bottoms out on the rod not the jam nut. There is the danger that the forks will be unequal lengths if you get this procedure wrong.
Tighten the jam nut against the rebound adjuster by holding a wrench on the jam nut and a socket on the bolt. Take care when removing the flat-wrench because the spring pressure will cause the rebound adjuster to slam into place. Put a dab of grease on the threads of the rebound adjuster and tighten it into the fork tube to factory torque specs.
The next step is to fill the outer fork tubes with a measured volume of oil. The oil volume and height generates the air spring’s progression. The lower the oil volume the less progressive the effects of the air spring. The higher the volume the more progressive your air spring will be. Consult your owner’s manual for suggestions on oil volume for your riding demands. There are no bleeding considerations for the outer fork tubes.
The last step is to tighten the fork cap hand-tight because the triple clamp provides the extra torque to prevent the cap from unthreading.
