Ok, here goes.
As Mr Lowe correctly states there are benefits from having the smaller interacting area at the outside edge, they don't touch so it's not friction that matters but the way in which the fluid passes the two edges (one perpendicular to the vector of travel and the other at some angle slightly less than that) and also the length it has to flow past. The length is important as the flow (yes even at the rates of movement and displacements we see in an RC shock) will transition between laminar and turbulent flow at some point along that surface depending on the fluid properties, gap between surfaces and the specifics of the motion of the piston. The interaction between this and the edge conditions will have an effect on the level of resistance of the piston to the displacement it is subjected to.
So now we have only considered the piston as a disc with no holes in and a certain amount of bypass flow around it's edge and already we see a slight (and it is slight to be honest) but still potentially important difference in resistance depending on the direction of the displacement, this difference will be more marked the faster the piston tries to accelerate and / or the heavier the damper oil weight by the way.
Now the more interesting bit, the holes in the damper piston.
Hole edge conditions first;
The simplest way to think of this, and this is a little simplified, is to just consider the outlet areas at either end of the hole when measured perpendicular to the hole itself.
Again this is one of those things where a picture is worth a 1000 words! Draw one up yourself to help with understanding (I can't upload pictures from here saddly).
If the hole in the piston is drilled perpendicular to the non tappered side the surface area will be simply the area associated with the diameter of the hole. Now where the same hole breaks out on the tappered side of the piston the area will be LARGER because on the surface it breaks out through it's form will be eliptical not round.
Does that make sense so far?
This larger area gives a higher potential flow through the piston but only for a very short distance as it soon gets throttled down to the flow associated with the normal drilling size. This is again a slight difference but not the most significant one.
The magic (or plain old orifice theory to those of us cursed with a life of fluid dynamics);
Now as we have said the hole entry flow area isn't the major factor so what is? Well it is the effect caused by the fluid flowing around the edge of the hole to enter / leave. The larger entry caused by the elipse causes, again very simplified, an effect similar to a bell mouth (often called a velocity stack in carb' / fuel injection / air filter catalogues) accelerating the fluid into the hole and stalling it on the exit in the opposite direction. This cuases the eddies in the flow path to converge in a slightly different place in the flow path than they normally would changing the position in the hole where the turbulent flow starts to recombine to become laminar (boundry conditions ignored to reduce complication).
The proportion of the flow through a short orifice that is laminar relative to that which is turbulent will dictate the flow characteristics through that hole.
The difference between the sharp edge (flat side) and the edge with the effective lead in (tappered side) is more marked than you might imagine and it only takes a small angle difference, especially if one face it perfectly perpendicular to the vector of displacement, to make a significant bias in one direction of motion!
Now there are many, many other things to consider such as the effects of the subtrate fluid (in our case silicone based oil), the pressure in the system, wether the system is pure (only contains fluid like bladdered shocks) or areated (like all RC shocks without bladders), the range of displacement speeds and crucially acceleratins in both directions .... you could go on and on and on (thankfully I won't on this occasion)
I hope that explaination made some sense to you all. The "angled hole breaking out in to a realatively large volume" trick is often used in fluid transfer systems where flow needs to be non-symetrical, say when you want to drain something slowly but be able to refill it faster in the other.
I wish I could have uploaded pictures to help explain it and I am more than happy to talk to any of you I meet at race meetings if you want further explainations, just ask
For those interested in fluid dynamics the best place to start is understanding REynolds Number
http://en.wikipedia.org/wiki/Reynolds_number and work form there. Will help if I can.
*** Not a dig @ you Rebel but more for information mate. Diesel IS compressed in modern fuel systems [i.e. it's bulk modulus changes], yes the system is pressurised which is different, in fact it's around 1400 bar this states to take place in ernest. Here is about the best link I could find quickly
http://www.hydraulicspneumatics.com/...se/70094/Issue ... hope it helps with the understanding.
I am a senior development engineer [Dr. of conceptual fuild dynamics according to my tacky business cards, typical US compnay things] for THE world technology leader in diesel fuel systems, currently working on common rail fuel systems for Euro 6 & US13 emmision regulations up to around 3000 bar and also something that will turn compression ignition fuel systems on it's head ... circa 2015 for that one though and obviously I can't talk about it on a public forum.
Sad as it is to say this sort of shizzle is my bag baby
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