Fork Basics: As you know, the forks contain a certain amount of oil who's purpose is to control the rate, or speed the forks compress and rebound, ie: go up and return to their normal level. As the forks are compressed the oil is forced through one set of holes - orifices to be precise. The size and number of holes coupled with the viscosity of the oil determine how quickly the oil will flow from one chamber (A) to another (B). When the forks extend themselves, the oil has to return from chamber B to chamber A via another set of holes. This gives us our compression and rebound damping.

The main fault with standard forks is that the only means of adjusting the compression damping is by changing the viscosity of the oil. This will also affect the rebound damping since the same oil must return to its original chamber. The upshot of all this is: change one and affect the other. It's not possible to simply change the compression damping independently of the rebound damping and vice versa. Unless of course you want to get involved in drilling the existing holes.

Another factor also needs to be taken into account at this point and that is the need for low and high speed compression damping, and NO, I'm not talking about high speed as in kph. or mph., but the speed at which the forks compress. Small bumps equate to low speed compression damping whereas pot holes, ridges and oh shit! handfulls of brake mean high speed compression damping of the front forks. The standard fork set up cannot differentiate between low and high speed compression as there is only one set control holes. That's why on one hand your forks may feel mushy on smooth roads and in normal braking situations and yet feel quite harsh when encountering potholes and ridges in the road.

What is obviously needed here is a damping system that can differentiate between low and high speed damping> This is what "Gold Valve Emulators" do. Not only do they differentiate between low and high speed bumps but you can adjust them to give you the type of ride that you want, or, to suit the type of application you require, ie, soft for cruising or firm for track days and fun motorcycling. Oh yes, did I forget to mention that these adjustments do not affect the rebound damping. Sounds too good to be true doesn't it? But they work and work bloody well. Just how they work we'll cover later in this article. Lets first see how the standard forks work.

When a bump is encountered the fork slider rises up the fork tube; at this moment the following happens (Sketch 1):

1. The ball check valve opens,

2. Rebound check valve opens,

3. The oil in chamber A flows a) into chamber B and b) through the compression damping holes at the bottom of the damper rod and exits through the top of the damper rod through two more compression damping holes into chamber C.
As you can see, the flow of oil is controlled only by the size of the holes at the top and bottom of the damper rod.
As the forks extend themselves (Sketch 2) the oil above the damper rod in chamber C flows back to chamber A and the oil in chamber B flows back to chamber A via the rebound damping hole and also in a controlled way past the clearance between the rebound check valve and the damper rod.

When the forks extend:

1. The ball check valve closes,

2. the rebound check valve closes stopping the free flow of oil through the valve, and forcing it through the small rebound hole in the damper rod.
As you can see there are 2 different circuits. One for compression damping, and one for rebound.
I guess it could be simplified by saying that chamber A provides the compression damping and chamber B the rebound damping.

Low speed compression damping (Sketch 4)

Here we see that the oil is free to leave chamber A, flow through the damper rod and exit the emulator though a small hole in the emulator plate.

This small hole gives us our low speed compression damping.

The theory being low speed movement only requires a small hole to control the small amount of oil passing through.

High speed compression damping (Sketch 5)

We've hit a humungus pot hole and the forks are sliding up the stanchions fast.

The speed and pressure of the oil has lifted the emulator plate off of it's seat opening up larger ports for the oil to flow through quickly.

The faster the forks move the more the emulator plate lifts off its seat thus flowing a greater amount of oil.

You'll notice that the spring on top of the emulator plate can be adjusted to increase or decrease the pressure on the plate. This is how we adjust the emulator.

Rebound compression damping (Sketch 6)

With the forks now extending back to their normal position the check plate in the emulator has fully opened allowing the oil in chamber C to return unrestricted to chamber A.

The rebound compression damping is controlled by the small rebound damping hole, the seepage of oil past the rebound check valve and the weight of the oil.

We now have fully adjustable front forks.

The adjustment of the emulator is quite infinite. One turn of the adjusting screw will give a plush ride. Three turns is quite firm. The choice is yours.

As I mentioned before, the rebound is adjusted by changing the viscosity of the oil in the forks.

The emulators can be quickly removed from the forks by undoing the fork cap, pulling out the spring and then hooking out the emulator with a bent piece of wire

Fitting the Emulators:

Parts required:

2 off FEGV 3801S RaceTech Gold Valve cartridge emulators.

2 off Mounting Spigots (Marzzocchi forks only) - see below

2 off Emulator spacers (Marzzocchi forks only) - see below.

Where to get the Emulators:

RaceTech's webpage http://www.racetech.com/index.asp will lead to a dialog box with local distributor's contact details in your area or contact Ash Vaughan at distsales@racetech.com and he'll point you the right way.

Tools required:

Long 8mm Allen key socket.
Air Impact gun
8mm drillbit Drill Centre punch Hammer Ruler.
Hacksaw
1L of either 5, 10 or 15W Ultra Slick, Motul , Bel-Ray or Ohlins Fork Fluid

Dismantling:

Remove the forks from the bike and take the springs and the damping rod bolts out, turn the fork upside down and the damping rods will fall out.

An air impact and a long Allen key socket helps a lot with stubborn damping rod bolts.

Flush out all the rubbish and clean them carefully.

While you have the forks open - check the bores, floating check valves, sliders and damper rods for wear. Replace as required and put in new seals.

To fit emulators in Marzocchi forks:

You'll have to machine up the two pieces in the sketch above and drill out the damper rod like this.
After drilling, chamfer and deburr the compression holes, inside and out.
Do not add or enlarge the rebound holes and leave their edges sharp.

Remove the ball check valve and fit the mounting spigot over the top of the damper rod to seal off the two compression damping holes and locate the emulator.
The spacer holds the spring off the emulator because the springs are too small in diameter to allow the emulator to open.
For those with Ceriani forks and springs only the holes need drilling as per the above sketch.
No spigot or spacer is required, although it may be necessary to fill in one or two of the rebound holes, depending on how many are drilled into the damper rod.

Checks: Check the emulator fits into the fork tube - if not; you have the wrong model.
Check the fit of the emulator by placing it on the top of the damping rod.
The step on the emulator must sit into the top of the damping rod and must completely cover the end of the rod.
The inner diameter of the fork spring must be at least 4mm larger than the emulator plate itself (small OD plate with the bleed hole) for proper flow - that is why the Marzocchi's need a spacer.

Assembly: Drop the emulator and damper rod down the tube.
The emulator sits on top of the damping rod with its valve spring and adjuster nut facing up.
Do not put it in upside down - it wont work.
Put in the main spring and screw down the top nut.
Now you can put in the damper rod bolt. Tighten this up - do not forget this - the consequences are not plesant especially it you like doing wheelies. Remove the springs and fit both stanchions into the bottom triple clamp.
Completely compress the fork.
Now fill them with dedicated fork fluid. Do not use oil or ATF! They aerate badly under severe use. Synthetic fork fluids will last longer at high temps; and it will get hot on bumpy roads, and so they will keep their designed viscosity - read damping for far longer.
Some good fork fluids are: Motul 10W-15W, Bel-Ray HVI 10W or Ohlins #10.
The weight of oil used will depend on the wear in the fork, clearances and style of riding.
Pump the forks a few times during filling to ensure there are no air gaps to mislead you.
The top of the fluid should be between 110-140mm from the top of the stanchion measure on the rear edge.
This air gap depends on type of riding, style etc etc.
Make both legs the same.
Fit the top triple clamp.
Install the spring and any preload spacers you are using.
Fit the nut. Done.

Tuning: To adjust the emulator, remove the springs and using a thin welding rod with 6mm of both ends bent over 90 degrees into a hook - fish them out. Adjustment of the emulator is by screwing down the nut and increasing the valve spring preload. This will make the fork stiffer.