How do you determine what the largest clamping shim OD you would use with a shim stack?

Very difficult question, because there is no set answer.  What type of dampers are you using and what does the manufacturer recommend?  Whatever that is, it's probably a bit on the safe side.

When doing a 1-off build, I usually try to go with something kind of 'middle of the road'.  That way if I want to change it to get a little more or a little less overall damping, I can just change the clamp shim 1mm up or down and have an easy, but effective, change.

I also usually end up using a large stack of thin shims rather than a small stack of thick shims.  That's more personal preference as far as getting the curve, though.

What are you using, what are you using it on, and what is your overall goal?

One thing that I am specifically working on is a good rebound stack.

CS = Centering Shim
RS = Ring Shim

I started out with -
(1) .15 x 17    
(1) .15 x 14 CS    
(1) .20 x 17 RS    
(4) .15 x 17    
(1) .20 x 10    
    
.05 mm preload on shim stack

The fork has an external low speed adjuster that has a range of 2.5 turns. This worked good, but as the rider went faster and faster we had to turn the low speed adjuster in more and more.

So I made a change to:
(1) .15 x 17    
(1) .15 x 14 CS    
(1) .20 x 17 RS    
(5) .15 x 17    
(1) .20 x 10    
    
.05 mm preload on shim stack

While it was better - lap times continued to drop and tire wear was good, but it was not enough.
So I went with this:
(1) .15 x 17    
(1) .15 x 14 CS    
(1) .20 x 17 RS    
(7) .15 x 17    
(1) .20 x 10    
    
.05 mm preload on shim stack

At the same time I installed this stack I also changed the springs to a higher rate (from .95 to 1.0). Again lap times dropped and tire wear improved.

What I want to develop is a stack that will give the same initial blow off as the first stack (possibly slightly less even) but has the same damping at faster shaft movements as the last stack.

The reason for this is that the track we are racing at has some transitions that really upset the suspension and I want to try a "softer" initial blow off to see if that will help.

I think the only way I a going to accomplish that is with a dual stage stack.

What do you think?

On the rebound I have been thinking about trying this -
(3) .15 x 17  
(1) .10 x 12
(1) .15 x 17  
(1) .15 x 14 CS    
(1) .20 x 17 RS    
(3) .15 x 17    
(1) .20 x 10

OK, I'm back at it.  Working on the rear end now.  I will remove my Delta Valve(Gate Valves). I am pretty sure the Delta Valve is essentially a blow-off valve (i.e no progressive resistance once the valve opens).  I believe if the Delta Valve didn't cure my high speed compression problem, no change to the external valve will.  I am going back to the stock external valve.  

My shock only had 70 psi, which I suspect is due to some Delta Valve leakage.  Recently I have not been able to find a decent rebound setting, and for now I am blaming it on the low pressure.

The stock internal stack is (95 CR500):

format = (# of shims) OD x thickness

--Rebound side--
(3) 18 x 1.0 washer
(1) 21 x 3.0 washer
Shims:
(1) 25 x .31
(2) 26 x .3
(2) 28 x .3
(2) 30 x .3
(2) 32 x .3
(2) 34 x .3
(2) 36 x 3
(1) 26 x .1
(10) 36 x .15
--Piston/Valve body--
(12) 40 x .15
(1) 34 x .1
(1) 28 x .1
(3) 40 x .3
(2) 38 x .3
(1) 36 x .3
(1) 34 x .3
(1) 32 x .3
(1) 30 x .3
(1) 28 x .3
(1) 26 x .3
(1) 24 x .3
(1) 41 x 4.5 washer (thick sucker)
--Compression side--

Rebound plan:  I will move the 26 mm shim to the middle of the 36mm shim pile.  This should make the rebound more progressive.  I am very tempted to make the rebound side an orifice type by getting rid of the shim stack and spacing that very thick 41 mm washer X distance from the valve.  But I have no idea what X should be and if I take a guess I am almost certian to be re-valving after my first ride.

Compression strategy:  The dual stage stack seems to explain the high speed kick.  So I will remove the 34 and 28 OD shims to eliminate the dual stage.  I will need to remove some 40 mm shims also, but I am not sure how many.  Hmm, this is a difficult decision.  I think I'll remove two of the .3 thick 40 mm shims.

Yes, those valve bodies are steel.  I will probably drill out the compression ports in the valve a bit just to make sure they aren't contributing to my problem.

By the time any of you read this everything should be buttoned up, so I guess this post is mute.  Sure hope my guess is correct on how many 40 mm shims to remove.  I took out a lot of stiffness, but then then again the rest of the stack will be there for support (which it wasn't until the first stage bottomed out).  Something tells me I will be able to use any info you guys might be willing to share when I re-valve again!

This certianly is a brain teaser.  Is this a parallel or a series system?  Some of both I guess.

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