Installing Race Tech Cartridge Emulators in a Motorcycle Fork

The Great Pretenders: Make your motorcycle's fork think it's a cartridge type. From the April 1997 issue of Motorcycle Cruiser magazine. By Evans Brasfield.

If you read magazines that feature sporting motorcycles, you probably know that cartridge forks have been all the rage for a while. By allowing adjustable damping rates for compression and rebound, cartridge forks give riders more control in their search for the ideal suspension set-up. Until now, folks with damping-rod forks could only adjust the spring preload, change the spring rate, or vary oil viscosity and hope things went the way they wanted. If the suspension was firm enough, the ride became harsh; if the right level of plushness was found, the bike would wallow, beginning to feel like a mid-70's Cadillac. Paul Thede, mechanical engineer and president of Race Tech, Inc., recognized the problems inherent in damping rod forks and designed the Gold Valve Cartridge Emulator to give damping-rod units the plush yet firm ride offered by cartridge forks.

Why should we want a plush but firm ride? Plushness keeps the rider from getting hammered by large road irregularities. Firmness lets the rider continue to feel the road, reduces excess suspension and controls the bike.

A fork's actions are affected by three forces: friction, spring resistance, and damping. Since most people don't toss around the big bucks that factory race teams use to put low-friction coatings on fork sliders, the static friction between the stanchion (inner fork tube) and slider remains constant and is not affected by other suspension changes. Springs, one of the most common modifications people make, know only one thing: Position, how much the spring is compressed. The more compression, the more pressure the spring pushes back with. If a bike wallows or bottoms frequently, increasing the spring's preload (i.e. increasing the spring's initial pressure) or putting a heavier spring in your fork will result in a firmer ride, possibly eliminating the problems. However, since the spring's increased pressure acts on all bumps, whether their shape and size require the increased pressure or not, the firmer ride may suffer from excessive stiffness. Variable-rate springs attempt to remedy this problem with a softer spring rate in their initial travel and more resistance as they compress. Thede feels progressively wound springs address the problem in a backwards way. The spring becomes its stiffest when he believes you need the most flexibility -- absorbing large bumps.

Damping responds to variations in road surface in a way that is different from but complimentary to a spring's response. Since small bumps produce different forces on the front wheel than big bumps, damping responds to the velocity of the fork's compression or rebound. Traditional damping rods control the velocity of a fork's travel by pumping oil through holes in the damping rod. Since these holes are a fixed size, a compromise needs to be made to accommodate the slower fork movement of rounder road irregularities and the fast movement of square-edged bumps. As with any compromise, neither situation's requirements are completely met. What was needed was a way to make the fork firmer during low-speed compression and softer during high-speed compression.

By separating the low-speed and high-speed damping portions of a fork's duties, cartridge forks address the compromise inherent in the less sophisticated damping rods. Using shims and washers to restrict the oil flow when the slider is compressing slowly, the bike has a firm ride that doesn't feel mushy and will be less prone to squatting or wallowing in corners. When the tire encounters a large or sharp-edged bump, the washers yield to allow larger volumes of oil to move by quickly, giving a plush ride that doesn't hammer the rider. The thickness and thereby the stiffness of the shims and washers can be varied to tune when and how the fork reacts to compression speed. Nirvana!

Cruiser riders (or any riders with bikes using damping-rod forks) could only watch from the sidelines as this suspension advance moved through the sport-bike world. Fortunately, Thede's deceptively simple invention, the Gold Valve Cartridge Emulator, gives a conventional damping rod fork a cartridge-like ride by providing tunable compression damping. Measuring only 14mm in thickness, the Cartridge Emulator sits atop the damping rod and conducts its business through a flapper valve (called the Emulator plate) and a spring. The Emulator plate has a small hole to control the low-speed compression damping, and the high-speed compression damping is tuned by varying the Emulator spring's preload. Crank in more preload with an allen key, and more pressure is required to lift the Emulator plate, resulting in a firmer ride. With the compression damping control separate from the damping rod, the rebound damping can be controlled through oil viscosity.

Anyone with $125 and a little mechanical know-how can install a set of Cartridge Emulators, or you can always use one of Race Tech's authorized shops or any shop proficient enough to rebuild forks. The required tools -- a small flat head screw driver, an allen key, a long allen socket, pliers, a power drill, a hack saw, and a tape measure -- will be found in most people's tool boxes or can be bought relatively cheaply. You will also need some way to support your bike while the forks have been removed. A scissors jack with wood across the lower frame rails might suffice.

To demonstrate the Cartridge Emulator's installation process, we, once again, enlisted our long-term Kawasaki Vulcan 1500 Classic to be our patient. Race Tech's Technical Shop Supervisor, Todd Davis, handled the surgery.

With the front wheel off the ground, remove the caliper (but don't leave it hanging by the brake hose), fender and wheel. Pop out the plastic fork caps and loosen the two allen pinch bolts for each fork tube. If you've already installed a light bar on your Classic, you do not need to remove the top of the triple clamp or the fork covers to remove the forks. If not, you will need to remove the 27mm bolt in the center of the triple clamp, lift off the handle bar and triple clamp, remove the upper fork cover and loosen the bolt holding the turn signal mounts to the fork tubes. Using a metal punch and/or plastic mallet, tap the fork cap (the top of the fork, not the plastic cover) to slip it loose from the triple clamp.

Although not required, most of the remaining steps will be easier if the fork tube is placed in a vice, but be sure not to over-tighten the vice, which can distort the tube. Remove the fork cap by pushing it down and removing the circlip. Lift out the spring. Drain the fork oil by inverting the fork over a suitable container and pumping the tubes until no more oil comes out. Remove the allen bolt holding the damping rod in the bottom of the lower fork tube. If the damping rod spins inside the fork, preventing its removal, try the time honored method of inserting a wooden broom handle or dowel to hold the damper rod in place. An impact driver may help too. Once the bolt is removed the damper rod should fall out of the inverted fork. You do not need to separate the fork tubes on most bikes to install the Cartridge Emulator.

Before proceeding any further, check to see that the Cartridge Emulator fits on the large end of the damping rod, completely covering the opening. Make sure the inner diameter of the fork spring is at least 4mm larger than the Emulator plate.

The Cartridge Emulator requires an unrestricted oil flow in order to work its magic. To meet the necessary minimum flow-through, the damping rods need at least 6 (3 sets of 2) 5/16-inch compression holes. Since most fork's damping rods are more restrictive than the Cartridge Emulator, the existing compression holes need to be enlarged to the correct size and extra holes drilled if necessary. The Classic's existing compression holes are already large enough for the Emulator but two new holes need to be drilled. Make sure the new holes are drilled perpendicular to and at least 10mm from center to center to the next-closest holes to keep from weakening the damping rod. Mark the damping rod and make an indentation with a punch to keep the bit from walking as you start to drill. Continue drilling through both sides of the damping rod. Chamfer and deburr the new holes, leaving a smooth surface on both the inside and outside of the rod. Do not enlarge or make any modifications to the rebound holes.

With the most difficult part of the installation complete, start reassembling the fork by sliding the thoroughly cleaned damping rod back into the inner fork tube. Install and torque the allen bolt in the bottom of the fork tube. If the Cartridge Emulator is being installed in a fork with a preload spacer, the spacer will need to be shortened 12 to 14mm, depending on the model of the Emulator, to maintain the correct preload. Since the Classic does not use a spacer but uses the fork cap to give the spring 30mm of preload, Davis felt the additional 14mm (to a total of 44mm) of preload would be excessive and recommends replacing the stock spring with a Race Tech spring kit. The Race Tech spring is slightly firmer (0.95 kg/mm versus 0.82 kg/mm), and the extra firmness is compensated for by shortening the preload to 20mm (including the 14mm of the Emulator). Although not required, Davis said Race Tech's spring kits are available for all Emulator models.

The Cartridge Emulator comes from Race Tech with 4 turns of preload on the Emulator spring and should be adjusted before final installation. Consider 4 turns to be a race setting and set the Emulator's preload to cruise by loosening the lock nut on the bottom and loosening the allen bolt 2 turns. If you're the type of person who likes to make sure everything is set correctly or you just like to fiddle with things, the number of turns of preload is counted from the Emulator spring being loose on the bolt (i.e. touching but no tension). Increasing the preload results in a firmer ride, and according to Davis, 1/4 turn increments of adjustment can be felt by properly calibrated riders. Don't forget to retighten the lock nut and use a drop of Loctite.

Fill the fork with the viscosity of oil recommended by its manufacturer and pump the slider a few times to get any air out of the system. Drop the Cartridge Emulator, with the tensioning spring pointing up, into the fork tube. Measure the oil level with the fork completely compressed and add or remove oil as necessary. Do not measure the oil level with the fork spring installed. Davis set up our Classic with Race Tech's US2 fluid, an equivalent to 10w oil, with an oil level of 160mm from the top of the tube.

Replace the fork cap and circlip. Pump the fork a few times to check for binding caused by an improperly seated Emulator. Reinstall the fork on your bike and go have some fun. Try hitting some bumps.

We found our Classic's emulated fork to be much improved in all riding situations. Gone were the two criticisms we had of the stock fork, the sweeping turn wobble and the highway expansion joint rocking. Bumps of virtually any size pass beneath us without upsetting the chassis. The bike's stability and control in cornering increased our confidence at any speed. The front wheel felt more planted yet more compliant than it previously had. By reducing the squatting in corners, the Classic's ground clearance also improved noticeably. The only thing we'd like to change about the fork now is not connected to the Cartridge Emulator at all: We'd like a bit more rebound damping since the fork extends a bit quickly after being compressed (most noticeably after braking), but a slightly higher viscosity oil should lick the problem. The Race Tech Gold Valve Cartridge Emulator works so well we're trying to decide which of our personal bikes we are going to add them to first.

_Evans Brasfield is a former staff editor and current contributor to Motorcycle Cruiser magazine. You can learn more about him at his website:

Race Tech
1501 Pomona Rd.
Corona, CA 92880
(909) 279-6655

For more articles on how to maintain and modify your motorcycle, see the Tech section of

Small but effective, the Race Tech Emulator simply slips inside your fork tube. The spring holding down the emulator plate controls high-speed damping. Photography by Lynn McCredy.
_A few tools and a place to work on your fork tubes is all that's required, or a motorcycle dealer can install the Emulators. _
In the damper retaining bolt doesn't want to come loose (often the case when it is first removed), try shocking the threads. Here an aluminum dowel is used to apply a couple of good whacks.
When removing the fork cap, remember that there is a large spring waiting beneath to launch it. Wear eye protection. You might also want to put a rag over the fork to slow down the cap's escape. On our Kawasaki, you depress the cap with a punch or screwing device while removing the circlip.
Even if you don't have a drill press, a steady hand will suffice when drilling the damper rods.
The top damper rod has been modified. The top holes have been drilled, and all burrs have been removed with a belt sander.
This is one fork leg with the Cartridge Emulator installed. From the bottom up: damping rod, top-out spring, Cartridge Emulator, and main fork spring.