1 Standard damping rod fork:...
1 Standard damping rod fork: compression stroke
2 Standard damping rod fork:...
2 Standard damping rod fork: rebound stroke
3 Fork Gold Valve Cartridge...
3 Fork Gold Valve Cartridge Emulator: low-speed compression stroke
4 Fork Gold Valve Cartridge...
4 Fork Gold Valve Cartridge Emulator: high-speed compression stroke
5 Fork Gold Valve Cartridge...
5 Fork Gold Valve Cartridge Emulator: rebound stroke
In the August '94 "Technicalities" we discussed the question, "Why cartridge forks?" We answered that question, but in doing so we generated a barrage of new ones like, "What can be done with damping rod-type forks?" If you haven't read the aforementioned article, now would be a good time because we're going to take off from there.
Let's begin by talking about the "ideal" ride. Most riders prefer a ride that is firm yet not harsh, plush yet not mushy, with good resistance to bottoming. How can suspension be all those things? One answer is bending shim-style valving in the form of the cartridge fork. But what if you currently own a damping rod fork? For this application Race Tech's Gold Valve Cartridge Emulator is another solution.
Let's first look at how standard damping rod forks work by looking at the fluid path through the fork. On the compression stroke (figure 1), chamber A is getting smaller as the inner fork tube displaces fluid. This fluid is virtually incompressible and has to go somewhere. The bulk goes through the compression orifices at the bottom of the fork then heads up through the center of the damping rod and shoots out like a geyser into chamber C. All the compression damping is created by shoving oil through these fixed orifices. This is the damping rod's Achilles' heel. The type of damping this creates is called "velocity squared" damping and is extremely progressive-in fact, too progressive (see "Technicalities," SR, Aug. '94). A small portion of the fluid goes past the open floating check valve into chamber B, as chamber B is getting larger.
On the rebound stroke (figure 2), chamber B is getting smaller and the floating check valve is closed. The fluid escapes from this chamber two ways: through the rebound hole in the damping rod and through controlled leakage between the damping rod and the check valve itself. The restriction in flow out of chamber B creates rebound damping.
During the rebound stroke, chamber A is getting larger. This creates a negative pressure differential (the pressure is greater in chamber C than in chamber A) and causes fluid to flow down the center of the damping rod, through the compression holes refilling chamber A. A problem that emerges during the rebound stroke is cavitation. If the compression holes are too small, the fork has a hard time refilling the chamber fast enough and an air pocket or vacuum is created. The smaller the holes or the thicker the oil, the bigger the problem.
Emulators work in an entirely different way. On compression, chamber A is getting smaller and fluid is going through the compression orifices and up the center of the damping rod. Emulators take advantage of this fluid path. They're tunable valves that sit on top of the damping rods and are held in place by the fork spring. By enlarging the compression holes in the damping rod, their effect is virtually eliminated from the damping picture. This transfers the job of creating damping to the Emulator itself and allows much greater tunability and a radically improved damping curve.
During low-speed damping (low suspension movement speed, not necessarily low bike speed), the fluid goes up the center of the damping rod and runs into the Emulator (figure 3). Until enough pressure builds to lift the spring-loaded check plate off its seat, the fluid goes through the bleed hole. This is a relatively small hole that creates damping at low velocities.
When suspension velocities increase (figure 4), enough pressure builds behind the plate to overcome the Emulator spring preload and the plate lifts off the seat. The stiffness and preload on this spring determine high-speed compression damping.
On the rebound stroke, damping is created in the same way as without the Emulator (figure 5). The Emulator's benefit on rebound occurs because the compression orifices have been radically enlarged and the Emulator is designed to be very "free flowing." The pressure differential between chamber C and chamber A causes the Emulator check plate to open easily and refill chamber A. This means the problem of cavitation is drastically reduced or entirely eliminated.
There are four tuning variables for damping: oil viscosity, which is selected to properly control rebound damping; the preload on the Emulator spring, which controls low- and mid-speed damping; the Emulator's spring stiffness itself, which controls high-speed damping; and the bleed-hole size, which affects low-speed damping. This gives the tuner tremendous control of the damping curve and the Emulator is simple to install and adjust. The net effect is a damping curve that emulates or copies the damping curve of a cartridge fork, hence the name Gold Valve Cartridge Emulator.