I’m new to the forum, but not to motorcycles. I realize this thread has been quiet for a while, but I found it a good read (all of it) and wanted to respectfully throw in my 2 cents. Forgive the length.
Full disclosure, I am with RPP Moto, a newly launched division of Ranger Point Precision, and we make products that compete with those mentioned in this thread. I’m not here to try to sell you my products though. I’d rather try to give you some additional information regarding your NC forks, which may help you tune what you have. Anyone curious about my company and its products can track us down easily enough.
As most of you are well aware, the NC employs some very old technology called damping rods, and this design is fraught with compromise. As you probably also know, the current industry benchmark (though not the cutting edge) is a late 20th century technology known as “cartridge” forks.
Cartridge dampers work pretty darn good, and the reason for this is that they employ two separate valves to control oil flow during compression and extension (rebound) of the fork. Both damping curves are tune-able via a stack of flexible shims, which allow variations in flow volume according to pressure levels (varied wheel velocities).
In addition to the shim stack, most—but not all—cartridge valves have a low speed “bleed” orifice that can be regulated by means of an external “clicker.” The primary purpose of these low speed circuits is to regulate chassis movement, eg, brake dive, wallowing in dips, etc. Actual bumps that initiate wheel movement will force the shim stacks open to varying degrees.
Damping rods, on the other hand, have no sophisticated variable valves. They rely on fixed orifices to regulate oil flow. Trouble is, only so much oil can be forced through a hole of a given size before an exponential increase in pressure is required to flow more. So the holes in damping rods are necessarily a compromise between a low speed circuit (for chassis stability) and a high speed circuit for bump absorption.
Those hand slaps you feel as your stock NC rolls over bumps is the unusually small compression orifices reaching their flow limit, whereupon the forks transmit that sharp spike in pressure directly to the rider. A few companies, like the ones mentioned in this thread, have attempted to address this compression compromise by introducing a compression damping valve that sits atop the damping rod, and then de-restricting the factory compression orifices or recommending a lighter viscosity oil. These piggyback compression valves will usually result in a subjective improvement in ride quality, but there is more than one fly in that ointment.
I’m not going to name all the flies, but one that begs examination is the rebound circuit. Be aware that in damping rods, rebound oil flow is controlled through a completely separate chamber and circuit than compression. Therefore, any company that claims to improve rebound damping characteristics simply by means of a piggyback compression valve, is being disingenuous.
This separate rebound circuit is controlled by a single, small fixed orifice near the “cap” of the damping rod. Generally this hole is biased way toward the low speed end of the damping curve in order to achieve a respectable level of chassis stability. Introduce a lighter oil, and you will get the dreaded pogo effect because the orifice can’t damp well enough to achieve rapid stasis in the fork spring.
Less understood (because it isn’t talked about) is what happens when you introduce a thicker than specified oil. Your chassis stability will improve marginally, but you will be selling off ride comfort, traction, and a degree of safety in return. The reason is simple. Now your wheel, when upset by a sizeable bump, returns very slowly to earth because rebound oil flow is more restricted than ever. Most tire makers agree that their products work best when in contact with the ground. Ride comfort too, is higher when the tire returns rapidly to contact, rather than the whole chassis falling to earth.
Rebound damping is one area where cartridge forks have it all over damping rods, and conventional wisdom has it that damping rods can’t be improved in this regard. I happen to disagree, but I promised not to get salesy, so I’ll leave it at that.
For now, as you fiddle with your forks, just be aware of the compromises you make.
I'm pleased to be part of this community, and happy to engage in a more detailed discussion.