This article was originally printed in CA Bike 11/94.
All the new sportbikes and dirtbikes are coming out with cartridge style forks. This is all fine and well, but it doesn't do much for the older motorcycle which needs to get the front end dialed in. Paul Thede of Race Tech has developed a little gizmo he calls a "Cartridge Emulator" which allows a standard dampen rod to behave more like a cartridge fork. Race Tech is the number one fork service in the US for motocross racers, who as you may imagine are a particularly brutal crowd when it comes to suspension work. Randal Vancil, proprietor of California Sportbike Racing, has teamed up with Race Tech to bring this technology to the street crowd. My job is to let you readers find out about it all. But first, we need to talk about conventional damping forks.
If you want to alter the damping rates of an old fashioned fork, you can switch to a different fork oil or change the size of the oil flow orifices in the fork. As the oil gets thicker and the holes get smaller, the damping rates rise. Unfortunately, you can't change the rates for compression and rebound separately, and there's no way to reshape the damping curve. At the limits of high speed damping, the system becomes unable to respond to some situations, such as the hydraulic "spike" in fork velocity you experience when you hit a square edged bump, like an expansion joint or a pothole.
The oil has a tendency to flow through orifices up to a maximum terminal speed, and when it goes beyond that limit, the system experiences what is known as hydraulic lock. At this point forces get transmitted directly up the fork to the rest of the bike. In other words, if you hit a sharp enough bump the fork damping mechanism gets overwhelmed and you feel a jolt through the handlebars. A similar phenomenon also occurs in the rebound mode. If the forks move too quickly during rebound, the system creates vacuums and develops pockets of air in places where you want to have oil. This is called cavitation, and when it happens, you essentially have no damping at all during some sections of the fork travel.
A cartridge style fork works by having a separate valving system for rebound and compression. Without getting too technical, the fork oil goes through valving shim stacks mounted in opposite directions for compression and rebound. These shim stacks act like little tiny cylinder head valves, offering flow rates which increase with the lift of the appropriate shim stack. As the speed of the fork oil increases, lift increases, and the size of the orifice increases.
At this point, it is important to define what we mean when we are talking about oil velocity. The oil moves faster when the fork is moving up and down fast. It doesn't necessarily have any relationship to how fast you are actually going. When you are whipping down a smooth road, the forks aren't moving up and down very much at all. You feed in some front brake, and the forks are compressing at a relatively low velocity as the nose dives. When you hit a sharp edged pothole, the fork velocity is very high. Now let's think about how you want your forks to react to these situations. When you hit the brakes or bank into a turn, you want the forks to offer a large amount of compression resistance so that the bike doesn't dive excessively. But when you hit an irregularity in the road, like a rut or the odd bit of unexpected debris, you want to have very low compression damping, so that the fork can diffuse the force over a longer section of its travel. This allows the suspension to absorb more of the force, instead of directing it up into the frame and your wrists. And when you are just tooling down the roadway, you want your forks to offer very low damping resistance to the numerous small, high frequency irregularities of the street surface. The ideal fork should offer high resistance to low speed vertical movement, and relatively low resistance to high speed movement. People have for years talked about having progressive spring rates, when in fact what they have been looking for is less progressive damping rates.
A well set up cartridge fork offers just such an arrangement. It is more plush as you ride over the slings and arrows of the outrageous misfortune of our "modern" roadways, yet offers more resistance to the suspension effects caused by accelerating, braking, and turning. If you plot the damping resistance offered by a standard fork and a cartridge fork against oil flow velocities, you will see that the standard fork's resistance rises faster, since its damping curve is a function of velocity squared. The rising flow rates offered by a cartridge fork give it a flatter curve because it's orifices grow as velocity increases.
So now we can discuss the Cartridge Emulator. It's a little doohickey that you pop on the top of a traditional damping rod. It has a little disc which regulates compression damping flow. The disc is held down by a little spring which is loaded down by a threaded screw which you can of course adjust with an Allen wrench. Installation requires drilling out the compression holes so that they basically offer no flow resistance. This leaves the job of restricting oil flow under compression to the Emulator. Rebound damping remains a function of bleed between the different components of the damping system. This remains unchanged. Actual installation can be a little bit more complicated than that, due to the tremendous variation in dampening rod systems in circulation. On some forks this means eliminating the anti-dive circuit (which actually makes the forks unresponsive to bumps while braking in some systems), and on many it eliminates the external adjustment capabilities of the forks. But on all forks the basics remain the same.
Once the system is in place, you can adjust the rebound damping by adjusting the oil viscosity, and can then adjust the compression damping by turning the screw on the Cartridge Emulator or changing the spring on the Emulator itself. This is easy enough, because you can just remove the fork cap, pull out the spring, and fish out the Emulator with a hook on the end of a stiff wire. Just take care that you keep everything extra clean, since dirt will kill the sensitive components of your fork.
I just so happened to race a little Suzuki GS500, which seemed to be the perfect test bed for this device. The racetrack magnifies any problems a bike might have on the street. The forks came off and got the whole works, different springs (because the stock ones looked a little light for racing purposes), new fork oil, and a pair of emulators. While it was apart, I also switched to the earlier clip on style handlebars which would allow me to adjust the forks higher in their clamps. Then I took it to Willow Springs.
In the parking lot, the bike felt a lot firmer, no doubt due to the stiffer springs. I dropped it in the triple clamps an inch to maintain the old ride height, since it had lost about an inch of sag. The first thing I noticed was that I had forgotten how to race, but that went away after the first practice session. During subsequent practices, I realized that the ride was, indeed, smoother over the rougher sections of the track. This allowed me to take slightly deeper lines on some parts of the track, which I had previously avoided because they are a bit bumpy. But the one area where the new forks really shined was their ability to keep the front end cool and stable under the brakes.
Since the GS doesn't have big sportbike power, the only place I brake hard is turn three. Turn three has a section of patched pavement before the turn, where I used to avoid hard braking. I used to roll off the throttle before the patch and ease onto the brakes in the patch to keep the bike from getting upset. Now I can brake hard while in the patch, and the job at hand is to muster the determination to brake later and harder. That's racing. I can also slam harder into three, since the bike feels more stable as it compresses for the banking up the hill. I can hit full throttle sooner. I don't brake much harder coming down the steep downhill right/left transition into turn five, but when I do, it feels much more stable. I dropped almost a second off my admittedly novice lap time, but more importantly, the fork tuning has opened up areas where I can now start shaving fractions off my lap times with more confidence. Now I need more track time so I can start fiddling with the Emulator.
Race Tech can be reached at (909) 594-7755, and California Sportbike Racing can be reached at (714) 642-0104.
By Paul Peczon
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