Kayaba Cartridge Forks - Dirt Rider Magazine Online

Kayaba introduced a new design of cartridge-style fork that comes standard on the 2005 Yamaha YZ125, YZ250, YZ250F and YZ450F and Kawasaki KX250 called the Air/Oil Separate fork. It is similar to the Showa Twin Chamber design that has been in production since 1994 on Suzuki RM and since '97 on Honda CR dirt bikes, but there are some differences. So we asked Jeremy Wilkey of MX-Tech to highlight them and provide a service guide for changing oil, seals and bushings and to tune the new fork for better performance. For this article, we've selected the '05 YZ250F fork for demonstration.The generic term for this type of fork 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. The fork's performance also 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 a slurry of finely ground metal particles that serves to break down oil (through oxidation) and clog the tiny passages of the piston and valve shims. This process slowly degrades the action of the fork by altering the damping characteristics and increasing friction.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 Kayaba fork and works as 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 make the slider tube stiffer.The Kayaba's reservoir piston for the compression-adjuster assembly is closed off and operates independently of the air spring. The Showa is pressurized by the Internal Compensation Spring (ICS) and the air spring. The benefit is the ICS isn't affected by the exponential character of the air spring, as when a fork is in the final third of travel.The Kayaba fork has no preload force on either the main coil spring or the ICS. When the fork is extended, the springs have as much as 5mm of free play. All previous Kayaba cartridge fork designs used some measure of preload on the springs.CleaningThe best method for cleaning the oily components of the fork is a high-flash point (120 degrees Fahrenheit) mineral-spirits solvent. The solvent breaks up the mixture of grease, oil and metal debris that contaminates the fork. We used PB Blaster. Nonchlorinated brake cleaner is the best choice for final cleaning. Allow the parts to drip-dry.Tools Of The TradeIn order to service the Kayaba fork, you'll need some special tools and some common sockets and wrenches. Starting from the top of the fork, an 8-point 50mm box wrench is used for the fork cap and a hexagonal slug for the ICS cap. For routine servicing, a split-collar seal and bushing driver will be needed 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 17mm 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 fork. Race Tech makes inexpensive universal sets of aluminum shaft blocks that will ensure the sensitive tubes don't get damaged when clamped in a vise.Some other assorted tools will be needed, such as small straight-blade screwdrivers, a 6-inch vise, a pointed scribe, a 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 such as PB Blaster, available from Pep Boys and other auto parts stores. Mail-order companies such as Harbor Freight (805/388-3000; www.harborfrieght.com) sell covered parts washers with a pump and brush plus a 5-gallon container of solvent for about $150 delivered.Maintenance IntervalsModern two-stage 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:Seal and Wiper Replacement: High-performance seals with relatively low drag need to be changed about twice a season or approximately every 40 hours. You can buy durable seals that require less maintenance, but the stiction will be so great that it will affect the damping performance.Bushing Inspection and/or Replacement: The bushings are the load bearing surfaces of the fork. There are three bushings in each fork leg 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 expensive high-quality oil, just one with good lubrication properties. The service interval for cleaning and oil changing is the same as for the seals and wipers.Cartridge Service: The cartridge needs to be cleaned about once per season or after 80 to 100 hours of usage. High-quality oil such as Yamaha Suspension 01 should be used because its formulization best matches the cartridge bushings and bleed setups.