Jan 30, 2000
Not that this explains it all but it's interesting anyway.

Penske Shock Manual page 23 shows the pistons they offer. Interesting in that they offer different pistons ergo they think that certain pistons work better in certain conditions.

The $64,000 question is which works for what and why?


Nov 13, 1999
I think that "low flow" / "high flow" is a misnomer. For any given displacement of the wheel there will be a given displacement of fluid through piston port (and bleeds and leakage). In other words they all flow the same for any given wheel velocity.

How many pounds of damping resistance you get for that given wheel velocity is a function of port design and shim stack.

The issue (I believe) is how much the shim stack must deflect to provide sufficient port area (area between port mouth and shim face) for that given volume of fluid to pass through. I don't fully understand the significance of shim deflection but I intuitively believe that more is generally better for tailoring the resistance curves.

If I were to set up a a "high flow" valve and a "low flow" valve to give identical damping resistance at one wheel velocity, I would be very surprised if they still gave the same damping resistance at double that baseline velocity. Generally, I would expect the "low flow" valve to give increased damping resistance at high wheel velocities because the ports themselves are restricting (modulating) the flow.

The Penske manuals are very good as Kiwi pointed out.

It is so interesting to see the pro shops choose sides on big ports/small ports. I think the big port design is easier to market because "bigger is always better". While that may be true of handguns, hemis and hooters, I don't believe it is true of suspension valves.

(The author used the 3H analogy to illustrate a point. A person's worth is measured by their character, not a tape measure.)


Dec 9, 2000
Servus Mace,

This was the best explanation I've read so far on this topic. Hi vs low is very misleading in this case! I wish I could express my thoughts equally good.


Jeremy Wilkey

Owner, MX-Tech
Jan 28, 2000
Very well done.. Your clarity is awesome..

To elaborate further.. The shim stack is used to create the damping. (DUH) This shim stack and piston relationship is critical. The port of the piston determines what the effective area of the opening is as a function of deflection. Larger areas do flow more oil (at specific speeds) however they tend to flow too little at very low speeds, too much at mid speeds.

The valve and shim relationship is simmilar to a concept in CALC where you need to find the area inside of a iregular shaped object. You can make a few small rectangles and aproximate it's area. On the average this is halfway corect.. This would be simmilar to a "damping coefficent" listed in a manual. This damping coifecent is the same for a awesome shock and a very poor working shock on the average.

Continueing with our allogy what if we could divide that wavy surface into smaller "chuncks" This would give use a more acurate test of area. Now we are talking about a shim stack and piston that is still the same "average coeficent" however it has more variations to speed and restaiance.

Using calculus we couldfind the exact area of the object, in suspension I still can't find the ideal damping, but with contunied fine tunning we have tried to idealize both the realtionship of the shim stack and piston design to optimize the deflection characteristic and the piston design as the two, thow not biological have a "symobitic" existance (For lack of better word.)

Another critical point. Many times in exsplantions to customers they look at my stack and go man that shim stack is so soft.. Or the stock stack is so stiff that is why my suspension stinks.. The problem with this is that the valve is again not understood in relationship to its stack. A higher flowing valve also produces less deflection which actualy strains the shims less and produces a softer feel.

A low flow pistonmight use 50% less shims yet strains them 3 times as much and actually at some speed range produce a stiffer coefficent.



Sponsoring Member
Dec 6, 2000
I have to admit that for the first time I actually have started to understand a small (and I do mean small) part of how my suspension works. Do we get certificates for graduating from suspension kindergarten?
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