I'm very pleased that Jeff Hoffman has allowed me to reproduce his 'in-depth' look at front suspension, with a particular view to cartridge emulators. Anyone wanting to improve their front suspension set-up, but really doesn't want to go to all that trouble of transplanting a front end from another model, or indeed wants to retain a completely OEM appearance, this one's for you.

Jeff's text appears in mauve below. He has kindly included his e-mail address, so if you have any questions you can contact the man direct !


Damper Rod Forks vs. Cartridge Forks vs. Cartridge Fork Emulators

By Jeff Hoffman


When talking about front suspension on motorcycles, we hear terms like "Damper Rods" and "Cartridges." We know they refer to different types of forks, but do we really understand how they work and how they are different? You may also hear the term "Emulator" being thrown around when talking about damper rod forks, but have no idea what these are or what they do. That is OK, because I will attempt to answer these questions, along with a few others:

· What are the pros and cons of each type of fork?

· Is a cartridge fork really that much better than a damper rod fork, and why?

· What does a Race Tech Emulator do and why is it so good?

· How do I install Emulators?

· How much do RT Emulators cost and where can I get them?

· Are there any FJ specific installation requirements?


Before I continue, let me explain a few terms and acronyms:

CF - Cartridge Fork(s)

DRF - Damper Rod Fork(s)

Damping - Controls the oscillations of the fork spring. Increasing damping increases the forks resistance to movement. Used to reduce a forks ability to compress and extend.

Velocity - Refers to the velocity of the fork, and hence oil, movement and not the velocity of the bike. Think of it as vertical wheel speed and not the MPH shown on the speedometer.

Force - The force that is transferring through the forks.

Low Speed Damping (LSD) - Damping taking effect during slow fork movement (i.e. braking). Increasing LSD will make the forks feel stiffer, while decreasing LSD will make the forks feel "mushy".

High Speed Damping (HSD) - Damping taking effect during fast fork movement (i.e. Riding over a sharp bump in the road like railroad tracks or expansion joints). Increasing HSD will make the forks harsher over bumps, while decreasing HSD will make the forks softer over bumps.


Damper Rod Forks



Most bikes prior to 1994 come with a Damper Rod fork. A DRF (Damper Rod fork) uses a rod with two sets of orifices (holes) in it to control flow. The bottom set of orifices control the compression damping, and the top set of orifices controls the rebound damping. As the forks are compressed, the oil is forced through, and slowed by, the lower set of orifices. When the fork is rebounding, oil is again forced through, and slowed by, the upper set of orifices. As the orifices get smaller or fewer in number the damping rates rise. Likewise, as the orifices get larger or greater in number, the damping rates fall. DRFs are great because they are easy to manufacture (Re. cheap), and easy to disassemble and reassemble.

You are wondering how oil is forced through two different sets of orifices aren't you? For the answer read the following paragraph. You will have to look at the picture to the left so you can see the 3 chambers (labeled A, B, and C) and the floating check valve I will be discussing. This picture show exactly what is inside of the 91-93 model years and does not show the adjustable rebound valve of the '84 to '90 model year FJs. The differences between the adjustable rebound valve and a "regular" damper rod are negligible, as they serve the same purpose. They only differ in the placement of the rebound orifices.

During the compression stroke, chamber A is getting smaller. At the same time the stanchion tube (inner chromed tube) and the fork internals are displacing fluid. Since fluid is virtually incompressible, most of it goes through the compression hole on the bottom of the damper rod and shoots up the inside of the damper rod into chamber C. A small amount of fluid goes past the floating check valve into chamber B, which is getting larger. The restriction of the fluid through the compression holes is what creates the compression damping.

During the rebound stroke the floating check valve closes, and chamber B is getting smaller. The fluid escapes chamber B in two places. The first is through the rebound holes on the top of the damper rod. The second place for the fluid to escape is by controlled leakage between the damper rod, and the check valve. The restriction of the fluid leaving chamber B is what creates rebound damping. Chamber A is also getting larger when the forks are rebounding. Since the pressure in chamber C is greater is than the pressure in chamber A a negative pressure differential is created. The pressure differential causes fluid to flow down the center of the damper rod, through the compression holes and refills chamber A.


Without resorting to complicated features such as check balls or position sensitive orifices, if you want to adjust the damping rates of these old fashioned forks, you need to switch to different weight fork oil or change the size or number of orifices. Changing the oil weight is relatively easy. If you want more damping, you change to heavier weight oil, and if you want less damping, change to thinner weight oil. Be aware that changing oil weight has an affect on both rebound and compression damping. Changing the orifices is a little more complicated as it usually involves brazing or soldering up the old orifices and drilling new ones. This is time-consuming, and it is difficult to make minor changes to the damping characteristics of the forks. Aside from the trouble of adjusting DRFs, the biggest problem is that they inherently generate a progressive damping curve. They have very little low-speed damping (LSD) and a lot of high-speed damping (HSD). With little LSD, the forks feel mushy and dive excessively under braking, and with lots of HSD you get a harsh spike over large bumps-the worst of both worlds.


Figure A

These problems result because oil can flow through an orifice up to a certain velocity that is determined by the orifice size. Once the oil reaches this velocity, the system will experience hydraulic lock. Hydraulic lock causes the forks to lock up and transfer all energy up though the forks, and chassis and finally into your body. In other words, at some point you hit a bump sharp enough to overwhelm the forks' damping system, and you feel a jolt in the handlebars. You can see this in action by looking at the damping curves of a DRF. Damping curves are plotted on a graph by comparing velocity and force.

Figure A shows the damping curve of a DRF. Ideally, the forks will be smooth over bumps both large and small. Unfortunately, DRFs have a progressive curve because, as with any orifice-controlled device, the force increases exponentially with each unit of velocity. If we double the velocity, we will increase the force four times. As mentioned earlier, you could change the rate of damping by changing oil weight or the number and size of the orifices, but you cannot change the shape of this curve. The orifices are either going to be too large to provide adequate low-speed damping, or not large enough to eliminate hydraulic lock. If you try to fix one, the other will get worse. There is no way to get around this simple fact. That is why cartridge forks were invented.


Cartridge Forks


Cartridge Forks (CF) are named so because the entire damping system comes out as one assembly. The damping assembly consists of two separate assemblies of pistons with shims. The lower piston and shim stack control the compression damping, and the upper set control the rebound damping. As the forks are compressed and extended, oil passes through the designated piston and shims stacked on it. The oil flow causes the shims to deflect. The amount of deflection depends on the oil velocity. When the velocity is low, the shims don't open as much and therefore provide more damping. As the velocity increases, the oil forces the shims open, allowing more oil to flow through the piston and around the shims providing less damping. The shim's ability to flow more or less oil, depending on velocity, keeps the oil flowing, the damping curve linear, and is what allows cartridge forks to absorb a variety of bumps with minimal fuss. You can see two possible damping curves of a Cartridge forks in Figure B.

Figure B

One of CF's advantages is that they are fully adjustable. Adjustments in damping can be made by either changing the piston or, more commonly, changing the shim stack. If you put in thicker shims, you increase the amount of damping force, and thinner shims will decrease the amount of damping force. By changing the pistons and the shims, you can make an almost infinite number of adjustments. You can change both the LSD and the HSD independently of each other. This allows total freedom to tune the fork to your requirements and preferences.

The only disadvantage of CFs is its expense when compared to DRF.

Which type of fork is better and why?

 I am sure you have already figured out the answer to this one, but let's do a comparison.


Damper Rod Fork

Advantages: Easy to manufacture, cheap.

Disadvantages: Limited adjustments, progressive damping curve, hard to tune


Cartridge Forks

Advantages: (Almost) Infinite tuning capabilities, ease of adjustment.

Disadvantages: Expensive.

The key thing to remember is that the primary job of suspension is to absorb bumps; because of their tuning capabilities, Cartridge forks absorb more bumps with less trouble. This bump-absorbing ability combined with their ease of adjustment is why Cartridge forks are far superior to Damper Rod forks.


Race Tech Emulators

How does a Race Tech Emulator work and why is it so good?

Some people have asked if Emulators really emulate something. Well, the full name of these wonderful devices, brought to the market in 1993 by Race Tech's owner/founder Paul Thede, is Race Tech Cartridge Emulators. So yes, they do emulate something: Cartridge forks.

RT (Race Tech) Emulators are little devices that use a single shim to control the compression damping of the fork. The shim sits on top of a machined brass housing that is placed between the damper rod and the fork spring. When the forks are compressed, oil is forced up the damper rod and toward the Emulator. Oil is then forced through a small orifice on top of the Emulator. This orifice provides the LSD. When the velocity increases, the shim opens to let oil pass through. This shim controls HSD. As the forks are rebounding, a check valve underneath the body of the Emulator opens to let oil pass back thought the Damper Rod. The Emulator does nothing to affect the rebound damping characteristics of the forks.

Changes to the compression damping are made with changing the preload on the valve spring. As you increase the preload you increase both the LSD and HSD although, it will have more of an effect on LSD. Decreasing the preload on the valve spring will decrease both the LSD and HSD and again, it will have more of an effect on the LSD. Changes to rebound damping are made with a change in fork oil weight. If you desire more rebound damping, increase the fork oil weight. If reduced rebound damping is desired, decrease the fork oil weight. Be aware that changing the oil weight will also have an effect on compression damping.

The single greatest advantage to the addition of the RT Emulator is the ability of it to provide a linear damping curve. Put simply, your damping rod forks will now work like cartridge forks. What does this mean to you? Your damper rod forks can be tuned to provide increased LSD and decreased HSD. Your bike will dive less under braking, and it will soak up the nastiest of bumps and expansion joints. You will be riding down the highway with more comfort, and riding your favorite back roads with greater speed and a greater margin of safety. Are they really that good you ask? Yes they are! As many have said, "the best $140 you can spend on your bike." 


Figure C

How do I install Emulators?

The instructions supplied with the Emulators are well written and easy to read, so I won't elaborate with the details. The forks will need to be removed from the bike but they do not need to be completely disassembled. You need to remove the Damper Rods and disable your existing compression damping system by drilling six (6) 5/16" holes in them. The enlarged holes allow sufficient oil flow so the main controlling factor is the Emulator, and not the Damper Rod orifices. The Emulator will add effective length to the damper rod, so you will need to reduce the existing spring spacers by the same amount. The process will take approximately 2 hours from start to finish. You can see the completed assembly in Figure C.


How much do RT Emulators cost and where can I get them?

RT Emulators are currently selling for $140usd. You can order them directly from Race Tech online at www.race-tech.com or by phone at 909-594-7755. I purchased mine from another great suspension shop, Traxxion Dynamics. They can be reached online at www.traxxion.com or by phone at 770-592-3823. If you don't want the hassle of ordering Emulators yourself, your local dealer should be able to get you a set.


Are there any FJ specific installation requirements?

 The RT Emulator that fits all FJs is part number FEGV S4101. The FJ forks can be broken down into two basic designs. The 84-90 model years with adjustable rebound damping, and the 91+ model years without adjustable rebound damping.

The 91+ years only require removal of the damper rods to drill the aforementioned holes in them. Once the damper rod is re-installed, the Emulators can be placed in the forks and you are ready to ride.

On the 84-90 model years, the top of the damper rod may need to be slightly modified to allow the Emulator to fit flush on top . I will allow Rich Baker to explain. Rich wrote these instructions after installing Emulators in his '90 FJ, and they were originally posted on the Yahoo FJ club.

"There is a rotating valve, with a D-shaped hole in the center of it, in the top of the damper rod. The rebound damping is adjustable by rotating this valve. The valve is inside the damper rod, and has 2 holes (one larger than the other) in it's side that line up with a larger hole in the side of the damper rod. Minimum rebound damping = large hole, medium rebound damping = smaller hole, and maximum rebound damping = no hole. These holes are 120degrees and 240degrees apart. A D-shaped rod extends down from the rebound adjuster in the fork cap, through the spring, to the rotating valve. You need to cut this rod about an inch from the fork cap's 'spring seat.' The rebound valve is held in the damper rod with a circlip similar to the fork's seals. I had to take my Dremel to the top of the rebound valve and remove some, but not all of the 'D' to clear the bolt/nut on the bottom of the Emulator (the nut clears, but the bolt sticking through, hits the flat of the D). By 'hogging out' the top of the 'D' in the rebound valve, you can now use the D-shaped rod and pliers to adjust the rebound damping, after removing the fork spring and Emulator." Thanks Rich, for allowing me to use these instructions here. If the Emulators fit flush on the top of your damper rod no modifications to the damper rod are required.

I should add that installing Emulators will disable the anti-dive system of the '84-'87 model years. On the anti-dive model forks your existing compression orifices are under the tapered spindle (AKA oil lock piece). The new compression holes you drill should be above the tapered spindle but as low as possible. This will mean you will have to ignore Race Tech's instructions and not drill out your existing compression holes, but drill 6 completely new ones. With new holes in place above the spindle, oil will now flow up the inside of the damper rod before being forced through the anti-dive assembly. Once the Emulators are properly installed you can either leave your anti-dive assembly in place or remove it and install a block off plate. Normally, when installing a block-off plate, you will need to machine a passage for oil to flow from one orifice to another. However, with Emulators installed you can install a flat block-off plate. The oil passage is not needed, as oil no longer flows through that part of the fork. This procedure is not as difficult as it sounds. Just remember to drill the 6 - 5/16" holes in the damper rod as low as possible, above the tapered spindle. If you have any questions about this send me an e-mail (address below).

The Emulators that fit the FJ will add 12mm of effective length to the damper rod, so you will need to reduce your spring spacer length by the same amount. Race Tech recommends 15W fork oil, filled to 130mm below the top of the fork tube. This level is measured when the forks are fully compressed, and neither the springs nor the emulators are in the forks. Race Tech also recommends 2.5 turns of preload on the valve spring be used as the staring point. Final settings on the fork oil weight, level and spring preload may vary according to personal preferences. As of this writing (12/00) the cheapest price I could find on RT Emulators is $111.95 from Zanotti Motor Co. They can be reached by phone at 724-283-2777 or by e-mail at zanmot@aol.com


If you have any questions or comments on this subject please feel free to contact me at:- jefman@mindspring.com


Please accept a big 'thank you' Jeff, for a very thorough and detailed article. Now FJer's from around the world may benefit from your experience and ideas.