I’m going to ramble here for a bit about some testing and perhaps some conclusions that I have come to based on some work with a set of twin chamber forks, over the course of the last few days. Some of my issues, or perhaps curiosities, on this subject started back in the days of the PHASE 4 KYB USD. This, of course, is centered around the performance and ride quality associated with the designs and operations of the mid-valve as opposed to that of the base valve, and somewhere in the mix of all this, the applications of blow-off springs (ie, Dell Taco) and inner pressure springs. As many of us know, the mid-valve is a stack of shims acting against the opposing side of the rebound piston, presenting some level of resistance as the fork compresses. By comparison, in a shock, the mid-valve is effectively, the compression stack. This means that the mid-valve acts as a direct force against the rod, much like a hammer hitting the head of a nail. Sometimes I have referred to the mid-valve as a type of parachute or drag device in that it creates a conversion of energy as the fluid is met with some level of resistance as it attempts to move from one side of the valve to the other. The base valve, also implemented as a damping circuit, relieves the pressures presented by the displacement of the rod. The stiffer the base valve, the harder it is for the fluid to escape and therefore, the higher the cartridge pressures and the more resistance against the rod, (sort of like driving that same nail but with atmospheric pressure). In a shock, the base valve is the compression adjustor assembly, or more specifically, what I often call the exchange valve. To make matters more interesting, I also hold the belief (and not everyone agrees with this) that at certainly velocities, mainly those in which the pressures on the back side of the mid-valve are less than what they are on the front side, the mid-valve creates a type of pulse or wave of energy that is pushed into the base valve. I guess I shouldn’t state this as a theory as much as it’s just a matter of simple physics. It has to be this way when pressures between the two areas are indifferent. Now, what that really equates to I can’t say but obviously there can be times (given the various velocities that we deal with) that pressures will not always be equal, and since fluid will seek the path of least resistance, if it’s not at the mid then it has to be at the base. In other words, the mid-valve can create a type of short, small explosive burst of energy that can only be relieved into the base valve and out of the cartridge (open bath) or into a pressure device such as an inner chamber spring or bladder (closed bath). With that information, the properties, or shall I say - the handling characteristics that one would see (get) from damping generated at the mid-valve verses at the base valve are very different. This can be proven by shifting or regulating the designs to play less or more of a role with compression forces, then testing them. The outcome will be that the mid-valve generates a very connected and immediate damping response, whereas the base unit will tend to generate a feeling that the damping is delayed and somewhat disconnected. This may be very evident for anyone that has converted the mid-valve over to a check plate design, then attempted to absorb the inbound energy exclusively via the transfer of fluid at the base valve, (common among Race Tech approaches). In addition to these matters, we now deal with inner chamber springs (free piston springs) in much of the same manner that we deal with the bladder on a shock. That is, we no longer discard the fluid out of the chamber after it flows through the base valve, but retain it and present further pressures against the rod from the acting forces of this device as it compresses. Obviously the stiffer the ICS or the more pressure within the bladder, the harder it is for the rod to enter the enclosure. Now that aside, I’ve be dealing with the issue of how much to rely on the mid and the base to serve the need for hydraulic damping, in combination with matters associated with the proper selection of the inner chamber spring, as well as creating blow-off circuits as used with the PHASE 4 KYB USD and the current Dell Taco. If you have been following these designs, I’m sure you are well aware that this approach is unconventional in many ways as the design equates to no float with maximum lift, whereas current factory approaches (supported and used by many tunes) uses a design with some free lift, (bleed) in combination with limitations to the maximum opening. Additionally, over the last few weeks I’ve been dealing with some issues as to how stiff this controversial mid-valve blow-off spring should be. In other words, how much force should be required to get it to open, and at what rate should the progression be? On the PHASE 4 KYB USD, I learned that for many aggressive MX applications, the single Dell Taco spring was not enough, (verified by me and a few others) but for many lower speed applications, the single soft spring was more than suitable (as validated by NCMM and KillerHiller). Now...it get’s interesting. At the moment, my current blue-print involves the application of the mid-valve blow-off circuit (the Dell Taco) in combination with a lighter inner chamber spring, the 215.VM2.K5 fluid and usually a change to the fulcrum on the base stack, allowing the stack to be redirected (so to speak) as if the stack has a blow-off circuit on it (the damping curves goes flat). This design, based on the theory of “early velocity management” is the best I have at the moment, but I often speculate on some concerns as well the potential for additional improvements. This includes: 1. Are the mid-valve blow-off circuits too soft for some? 2. Does running a softer inner chamber spring cause cavitation” And lastly, 3. Could I achieve better performance from shifting the role between the mid-valve and the base valve? - In other words, less damping from the base, and more from the mid. To test for this, I started with a configuration that eliminated the base valve (yes, you read that correctly) and relied solely on the mid-valve. This is an approach that would more typical of an automotive style damper. Under this approach, I tried a host of configurations including really stiff and really soft inner chamber springs, as well as inner chamber springs that were pre-loaded. In combination with this, I tried several mid-valve designs that featured some float, no float, maximum blow-off, some blow-off, and a range of various blow-off rates as well as a host of different shim stack configurations. Although these configurations and the following tests were all done within the shop, and therefore only measured on the machines, I don’t have any seat-of-the-pants feel or feedback at this moment, but here’s what I found. 1 - The mid-valve, alone, is more than capable of producing the necessary forces required for even the most advanced rider, even when the base valve is completely removed. 2 - When the mid-valve does not have any lift limits, (such as with the Dell Taco) regardless of the velocities, the valve does not cavitate even when the softest inner chamber spring is used, with or without the base valve. 3 – When the mid-valve has no float and does not have a blow-off circuit, (and no base valve) the valve cavitates at nearly all velocities within the first 2/3 of the stroke. 4 – This caviation matter, under that configuration, was only slightly improved upon when a radically stiffer inner chamber spring was used. In other words, the first 2/3 of the stroke was still cavitating at nearly all moderate or greater velocities. 5 – Pre-loading the inner chamber spring (in this case, by 20mm) regardless of the spring rate, significantly reduced the cavitation at moderate to mid-speeds, but still did not resolve it completely at higher velocities. With that, I came to the conclusion that under a normal base and mid-valve combination, as nearly all of us are using, that cavitation at the mid-valve (if it’s even present) cannot be resolved at the low pressures presented by nearly any inner chamber spring. Additionally, I have just about concluded that limiting shim lift at the mid-valve will lead to cavitation at or above moderate velocities (such is the case with factory designs). In this case, a lighter inner chamber spring would lead to more cavitation, but the change would be insignificant. The one remaining unknown is the performance or characteristics of a fork in which more damping is shifted towards the mid-valve and away from the base valve. However, this design was the primary platform for the NCMM design, which was the first design that I had done that used the new approach of less base damping, a lot of mid-valve damping, and a very light inner chamber spring. Lastly, I should note that we are most likely going to have to have two different Dell Taco springs, or we may have to combo two different springs, mainly for those MX applications that require it, (myself included). Over the course of the next weeks or two I’ll be able to provide some feedback on this approach and how it translates to the track and an update on an optional Dell Taco spring. Till then, I guess I just wanted to open this up to the board and begin to get some input on any details, or thoughts or concerns, on things that I should test for, or areas worth mentioning. I’m sure I’m leaving something out somewhere so let me know what you guys think of this and perhaps where we should go with it. Back to the lab.