Shim deflection and piston design

Hey there, i am thinking about what really happens in a shock stroke sequence,

Why do we use 4-port piston instead of 3 or 6 or maby 2 port, has it become a industri standard ?

How much influence does the port size ?

How much is the lift on the shim stack at

a impact of 3 M/s ?

What is the differens in feeling and performance if we change the lowspeed shims ex. 14x40- 0,20 to 4x40-0,30, ?

Why do all shimstacks looks the same ( Pyramid with or whitout crossover)?

Would this work ?


4 x 40-0,30

1 x 30-0,25

1 x 24-0,20

1 x 30-0,30

Stop washer 40x 4,0mm

Yea i know! i think a lot, but what are all you tuners tought about this.

Happy thinking


I did some tests about some of your questions:

I didn't see any difference with different port sizes, but am not able to say if you can feel it on the track or not.

both the little shim thickness and the pyramid are used to create a linear pressure curve.

you dont have to run a pyramid, it just gives you a finer tuning ability, i have run straight stacks in single and 2 stage(2 stage was best) they feel just like a normal shock.

shim deflection depends on stack build.

I think the 4 ports stress the shims less? more even in port growth for a given velocity.

On small pistons you may not be able to fit 4 ports.

Big ports need lots of shims, small ports need less shims, overall feel very similar.

lots of thinner shims gives more high speed compared to less thicker ones.

Mog ! Why would lots of thinner shims give moore high speed damping compared to less thicker ones ? is it because when the lift of the shim overcome it's own thicknes it become moore progressive, ex 1pc 0,20 shim need to lift 0,20mm then the progresivitet turn on compared to a 0,30 shim that need to lift 0,30mm to get to the same point of progressivitet, so the ports are 0,10 mm moore open, moore fluid flow, less dampening ?



lots of thinner shims gives more high speed compared to less thicker ones.

sorry mog, this isn't true. everyone try to achieve less high speed, that's the reason we see lots of thin shims on both forks and shocks. while you can reach a linear damping curve with more thinner shims, less thicker shims will give a progressive (more HS) damping curve.

btw. the dyno proved that..

hmm seems i have it backwards, i was told more thin shims gives a equivalent to a variable clamp, as each shim bends it kind of spreads the fulcrum to a wider point?

from seat of the pants stuff

many thinner shims feels softer ie a stack of 0.15s feels stiffer than 3.375 times the stifness of 0.1s

I reasoned that the thicker the shim the more the effective pivot width grows

seems right in practice if you make 2 different stacks from 2 different thicknesses supposedly having same overall stiffness and bend them with a feeler the many layered thinner one starts bending closer to the stem.

I get you mog more shims also increase the effective pivot width and thats important for comparing varying the pivot width vs the number of shims,

it seems though that the localised stifness of the thicker shims make the effective pivot width grow more as you move up through the stack (from the pivot then from shim to shim) than the thinner ones .

thats what it feels like anyway

could well be correct, interesting stuff

very interesting stuff, now I want to get some .15's to play with

all the forks with 0.15s tend to be less plush than ones with 0.1s so i am sure you will find many owners willing to swap :p

So thats why Showa and KYB place the thicker shims first before the thinner shims on the comp. side of their shocks.

I always thought it should be thinner shims first , o-well:bonk:

this is what I got:


how did you arrive at the design of the 0.095 stack?

how did you arrive at the design of the 0.095 stack?

it's the current stack I run. the reason for the 0.115 shims is they are even. the grind down 0.095's are not so I thought it's not bad for a face shim to be even :p

the bit i am a little confused is this, one stack has a different build to the other? so how does this show us which one has the different elements of high and low speed, we need a straight stack of equal strenghts going on the 3.3 to 1 ratio, ie a stack of 0.1s 3.3 times as many as the 0.15s.

for me it shows that both stacks have the same low speed, but different high speed pressures. to compare stacks like you said you need one-size shims. how would you build a stack with 0.15 and only a 3rd shims? the pyramid would be not the same, so does the different pyramide influence the result :p

I tried not to achieve a basic "how much 0.1 shims do I need to get the same as with 0.15 shims"

I couldn't decide if I should run 4 or 5 22mm shims, then I thought, hey, lets grind them down and prove, if thinner shims provide less HS compared to LS damping.

Made some test today, wanted to se what the difference was between 020 and 030 on the lowspeed area on a ohlins mx damper.

This is the shimstack i used (Only for comp side)

020x40x14 030x40x4
















Then i change the 14 pc of 020-40 with a shimfactor of 8 (14x8=112) to 4 pc of 030-40 with shimfactor of 27 (112/27=4,15) and run it on the dyno.

Ok, i know it's not a super dyno but i can crank it upp to almost 1 M/s and i think it would represent about 2,8 M/s on the wheel, and here is what it looks like.


The graph shows almost the same up to 0,2 m/s then the 0,30 shims are softer, I wounder if this shimfactor numbers are right ! if this is right it shows that there is less high speed damping with the 0,30 shims ?????


if this is right we wouldn't have stacks but only one shim :p

kawamaha What do you mean ? Both are ordinary stacks the only difference is 4 pc of 030x40 instead of 14 pc of 020x40 ??


Well I've always felt that the more shims of the same diameter stacked on top of each other gave a more progressive characteristic when comparing less thicker shims with the same initial stiffness.

For Example:


20.30 Clamp/Pivot



20.3 Clamp/Pivot

Deflection coefficients:

.10 = 1

.114 = 1.48

.15 = 3.38

.20 = 8

.25 = 15.63

.30 = 27

So using these numbers it would be safe to say that eight 40.10's have the same initial stiffness as one 40.20.

I've found that the 40.20 will start deflection closer to the fulcrum (pivot shim) in a linear fashion thus opening the ports earlier and allowing more flow with less deflection at the port edge. And for the 8-40.10's, multiple shims add more support around the pivot forcing the stack to deflect more uniformly across its plane by first uncovering the ports from the outer edge and eventually opening all the way to the pivot. So generally the 8-40.10's would have to deflect more at the ports outer edge than the 40.20 in order to flow the same volume.

Port area and quantity:

Typically larger ports require less deflection to flow the same volume as smaller ports. So when using a piston with lots of port area, one might build a stack with more progressive characteristics in order to achieve plush square edge compliance and adequate bottoming resistance. With smaller ports more deflection is needed in order to flow the same volume which means finer and more sensitive tuning capabilities. Sometimes this is accomplished by using a stack that is more linear.

Less ports equals less deflection distortion. If you have a piston with two ports that are located 180 degrees across from each other there exists only one pivot line. When adding ports, more lines are added resulting in more intersections. These intersections add progression to the deflection curve.

Hope this helps.

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