WH,
One one hand I go alright a great question I can really go deep! On the other hand I go O my God such a simple question, yet one of the very most difficult questions in the world to for suspension builders as a group to simply answer. To answer this question we both need to look back into old threads and think intensensley about this topic for the average guyt to get even half a grip.. I will once again try to boil this down as simply as posible.... :)
General assumputions.. The PDS Shock moves realtively slow comparied to most Jap shocks.. It has a large swept volume of oil 50mm body tube, and the motion ratio is very close to flat.
With a stock highflow piston the tendency is to be overly stiff on the low speed so tractablity suffers, and the ride seems over stiff and somewhat unresponsive. With creative shimming these tendencies can be worked with but not maximized to an ideal realtionship..
Many people have the notion that a highflow piston offers no restriction to oil movement. They are compltely correct, however the alternative view that a low flow piston offers a certain fluid restriction or damping. This is completly untrue. (at least with current designs.)
LOW flow or high flow it's a bogus descritpion if you want to take it literally. A Pistons Job is to direct the oil flow onto the shim stack. The relationship that the piston has with it's shape of pocket, depth, angle, and actuall orifce all impact the deflection charachteristic of the shim stack. This shim stacks deflections determine what the acual size of the effective orfice is at any given speed range.
As a suspension builder it's our job to maximize the relationship of the variable orfice to shaft speed. To use a analogy to calculus, a well designed piston and shim stack are like the intergal problems where you are trying to calculate the area of an ireguarly shaped object. To get a quick guess we could take the area and draw a few squares under it's slope. This could give us a rough idea of area and it could also be simliar to a average damping coieffecent. But in racing a average coieffecent is not good ennough, and we must as designers contunie to break the aarea down more and more times so for each incroment of speed we have a corispounding unit of damping. To complete my anolgy if you integrate the eqwuation you can break the area down infinetly and solve for the exact area..
We are not there, no one is, however we have and contunie to try to further maximize or piston and shim stack realtionship in a wayt that optimizes the shock over the widest range of shaft speeds.
Some particular design challanges to the new PDS piston... Size... you have little room to work with, the Id is 16mm... The ht is very thin if you wish run dual pistons your in for trouble without getting creative.
Stablity.. You have to maintain a double bearing for stablity and integrity. For a long time we where devloping a single piston application only to ultimately realize we had gone down the wrong path ( but we learned a lot.) Materials AL won't cut it in this application. The accounting between the 2 pistons. The compression and rebound do overlap in some situations.. Now you double your varibles...
I really have grown to love the PDS suspension..I was laughing to myself (reminicing) today as i was working on one... Man in 1998 when I took the first one apart I was shocked, and somewhat skepitcal and held it in high contempt.. Today I can say i've learned more about suspension from this Shock than I have from all the others in at least in total discovery over the last few years.. It's single greatest aclaim is that it is very diffrent.. At times I find myslef comparing whatever I might be working with to what seems most comfortable, today I compare all things to the PDS.. Weird.. In a suspension builders kind of way..
Hope that helps..
Jer