Well,.. I did months of research so the things I tried and the information I learned is not simple to express. Mostly because the more variables you have, the more the combinations multiply. I'll try to summarize what I have come to believe. Bear in mind that I am not an expert, just a very curious person who has an obsession with mechanical stuff...
A stock damper has only two adjustments without resorting to drilling extra holes in the forks or modifying stock parts. One adjustment is fork oil viscosity and the other is spring rate. With the dampers I modified there's a few more adjustments and the valves are dual circuit valves so they work differently on smooth and rough roads. These modern dampers have separate adjustments for compression damping and rebound. It also has preload adjustments to set the rider sag. Those extra variables add to the complexity of setting up the suspension.
Still with all this newer design and adjustable mechanisms you can't have both ends of the stick at the same time in this case because the spring rate isn't adjustable in the same way that the valves are. So you can dial greater or lesser damping or a combination of compression and rebound that feels best for you, but the spring rate you chose sort of tells you which end of the stick you are aiming at... By that I mean, if you want a high performance suspension that maintains maximum traction between the tire and the road, you can't have a soft spring which gives a softer ride. The length of time that your tire's traction is reduced by an impact with a bump will be longer with a soft spring than a stiffer spring. This kind of means that if you want the softest ride possible you give up the quicker responding spring and settings and lose some traction, or at the very least you compromise one for the other... There's no combination of modern adjustable dampers where you get a soft ride and high traction performance. Maybe the magnetic computer controlled suspension of the William's F1 suspension can do that, but that suspension doesn't have a spring. Spring choice characterizes your suspension to one end, the other, or it compromises one for the other and you get neither the softest ride nor the best performance.
The greatest advantage of modern suspension comes into play where you only care about higher performance, like in a sport bike being ridden aggressively. If all you wanted was a soft ride you don't need anything more than the stock commando suspension and maybe Jim Schmidt's damper parts upgrade kit to make the stock parts a little less sloppy (sloppy parts makes their response less consistent)
If you want to ride more aggressively, you don't want soft suspension because the amount of time that your traction is reduced from any surface irregularities is increased which makes your bike feel like it's floating along with less grip. I originally thought that the dual circuit valves in the damper might give the best of both worlds, but the spring rate is integral in the response time of the suspension so it characterizes the response where it can't be "soft" some of the time and be "stiff" when it needs to deliver higher performance.
So, you have to make a spring choice, a viscosity choice, and chose your settings. You still get the benefit of easier tuning of your suspension with externally adjustable damper controls, but you can't turn a "racetrack" spring and settings into the soft suspension of a cruiser without changing the spring rate. (and maybe fork oil viscosity too, but adjustable valves give a much wider range of response to any chosen oil viscosity)
WTBS,..... my bike is set up with a .67kg/mm spring, which is a compromise spring from what the spring rate calculator on Racetech's website says I should use. Their calculator said I should use a .93kg/mm spring. I called them on the phone and they stuck to their guns on their calculator, but I bought a .80kg/mm spring anyway because I knew that the stock commando spring is about .63kg/mm and I've never bottomed it out at 210lbs. I also bought springs off ebay. I bought .75kg/mm, .67kg/mm, and a .80-55kg/mm progressive spring. The .80kg/mm was approaching a racetrack bike feel IMO. I think Racetech's calculator is for race bikes in spite of them saying it's not. I can't imagine how stiff the .95kg/mm spring would have been. I tried them all and the .67kg/mm spring was aggressive enough to take away any floaty feel to my handling without being harsh. I've made a few sets, so I can swap in a different spring/set and continue to test at some point...
Through my conversations with a few different people I was trying to go to the thinnest fork oil possible and the smallest valve openings. I didn't like that much since I felt like the adjustments didn't do much at all. I went as low as 2.5wt silkolene RSF. I ran 5wt silkolene RSF for almost the whole summer, but now have switched to 7.5wt silkolene RSF. Fork oil changes viscosity with temperature, so I buy an expensive brand because it's viscosity index is high, so it changes viscosity less than cheap oil which leads to more consistent response across the heat range. I'm trying the 7.5wt now again with larger damper valve openings. I think it delays the secondary circuit from kicking in too early and the damper response of the second circuit is greater. I haven't experimented with the secondary valves yet, but aftermarket valves are available for sale from Racetech or I could just dremel the existing valve openings in a set to make a more responsive secondary circuit... Honestly, I'm not that interested in it because I try to drive around the pot holes as much as possible and the secondary circuit works fine over the expansion joints of the highway as it's set up right now anyway. Shim stack adjustment is one of the least known fields of expertise in motorcycling. I wish I had access to someone who was an expert in that field. I'm sure anyone who is an expert has become one in pursuit of racing on a track, not working for a dealership...
For me, this project was eye opening. I thought I would get buttery soft riding and race track handling in the same set up. I did not. I got to chose where on the spectrum of handling that I wanted to be in order to fit the kind of riding I do. Moderately aggressive sport riding. I gave up the buttery soft stock suspension with my choice. I think if a person wants to go down the higher performance handling route this modification (or the cosentino kit) is a good choice. You should get good sticky tires too, light alloy rims, and good adjustable rear suspension too. You also want good brakes if you're going to ride aggressively. Just like any modification, the entire package yields the most noticeable change.
If you look at my modification, the only external difference you see is the fork cap with the exposed controls for rebound damping and preload. I also drill a pair of holes in the front axle to adjust the compression valve with a hex key but that can't be seen so easily. The sad part of this modification is that it's a handling performance modification, but it's outlawed in many vintage race classes because it wasn't available during the era of these vintage bikes. I guess that's the way it has to be or guys would be mounting their commando engine in ducati frames or some other crazy stuff and the vintage classes would be one where the winners wouldn't be racing legitimate vintage machines... So it's a vanity project to improve the handling of an old sport bike...
below you see the fork caps with controls, the drilled front axle for compression adjustment, and a few sets with different springs and one longer damper with an internal top out spring.
I've been looking for something like it for a while.
Thank you all for your help!
