which stack is stiffer

I think it's pretty much assumed when you do a comparison like this that you try to keep almost everything else identical?

I got ahead of myself on with these stacks.

The one on the left is just a made up stack for comparion sake. The one on the right is the stock stack for the 07 RMZ 450. I had dyno tested both stacks using an 07 RMZ 450 shock, and was ready to whip out some numbers after everyone took a guess.

But after looking more closely, I realized I had dyno tested the first stack on 4-29-10, but had tested the second stack back in 2007. Because these stacks were tested so far apart, I don't want to use the numbers I have for comparions. That's because of two things:

1. The tests were performed on two different shocks, and back in 2007 I didn't pay much attention to seal drag. So I can't remove the seal drag force from the second test's damping force numbers. One shock could have had 10 lbs seal drag, and the other could have had 20 lbs. I just don't know.

2. Back in 2007 we ran the dyno tests at slightly different stroking speeds, and that will have an influence on the numbers.

I will have to grab another 07 RMZ 450 shock and test both stacks again, then post the results.

I find it interesting that only NastyR1 considered what bike the shock was from. If I did the tests on two different shocks, the differences in the cadj would/could be enough to negate any comparions.

but this got to do with the question I start?

I think it's pretty much assumed when you do a comparison like this that you try to keep almost everything else identical?

http://en.wikipedia....ientific_method

And my SWAG is left stack is stiffer.

Edited by Markopolo400

IMO.

more lo-SPEED less Hi and overall stiffer for the left.

But not so much difference in overall stiffness.

What do you want to show Kevin?

I started this post to show that each person intreprets shim stacks different. That is why some say the left is stiffer, and some say the right is stiffer. Then I thought I would show how the stack actually tests on the shock dyno. That don't mean I'm trying to get anyone to change their opinion about how stacks work, and my guess is that most won't. It's just for fun.

I started this post to show that each person intreprets shim stacks different. That is why some say the left is stiffer, and some say the right is stiffer. Then I thought I would show how the stack actually tests on the shock dyno. That don't mean I'm trying to get anyone to change their opinion about how stacks work, and my guess is that most won't. It's just for fun.

ok, but a comparison with bench, must have different stack but shock equal, otherwise the dynamometer can not give a definite answer of hardness on the stack, but only show the difference of force that is not necessarily all due to the stack.

it seems to me a contradiction in what you ask for

I think he's going to get another 07 RMZ 450 shock so he can re-test both stacks on the same platform :excuseme:

Based on the late learnings of mine distance to backplate also has a huge impact to hs damping. Even in shock.

Based on the late learnings of mine distance to backplate also has a huge impact to hs damping. Even in shock.

That's interesting. What are some of the distances you have noticed that affect the hs compression? And I guess the taper/shape of the stack comes into play.

Also, I'm half done with the stack comparions. It is taking much longer than normal because we have our new pressure sensors and are taking some pressure readings as well. The hard thing about getting new equipment (e.g. the pressure sensors) is you have to spend a tremendous** amount of time figuring out the best tests to run and how you want to intepret the results. Unfortunately, you can't just call someone up and get the info.

** We're talking some serious hours. Way beyond what mear mortals can comprehend.

Edited by kevinstillwell

That's interesting. What are some of the distances you have noticed that affect the hs compression? And I guess the taper/shape of the stack comes into play.

This is mostly based on discussion with Russ from DGS suspension and playing with Restackor. When playing with WP's, he has been pointing this out to me and I'm starting to see the light. I think that KTM has been using this feature a lot for bottoming control (both front and rear). It seems that if your bigger shims are really close to backer in faster oil flows (for example WP PDS 28 x .2 to 21 x .3 gap), it will really restrict the HS oil flow a lot. I have also noticed this same impact in front fork testing when altering just the back plate size in BV. And when then increasing the HS taper and keeping everything else the same, it will make the actual stack softer in HS damping to certain point. After getting enough distance it starts to stiffen again. This limit depends fully from stack setup and I have not really simulated these two examples. I'm not sure how much this can be really seen in dyno, since it will need a fast flow.

The yamaha 46mm piston shocks use this back plate limiting concept for their overall damping scheme...

We have played with shim limiting in mv comp stacks in some older open bath forks. The concept is shown in the penske shock manual.

I've also been testing the approach in twin chamber bv's....with Russ of course.

Edited by GDI70

It's easy to play with in a mid valve too..

I've tested mids with no shim stacks - just one thick shim, and just adjusted the total lift available.

def right stack. thicker shims is huge..2-.10mm shims is less than half the damping of 1 .20mm shim

It's going to come down to what velocities are tested - at relatively low shaft speeds where bleed plays a large influence - there will be a very small difference (of course). In early stack deflection/low shaft velocities the left stack probably edges the right stack for damping force due to cross over thickness - as well as the 42.2 placed in the right stack flexing despite the 44.25's not.

Something to wrap your head around for a second - shim bending occurs at the weakest link and onwards to the next weakest link.

So if we imagined a shim stack like a stack of coil springs placed on top of each other.....it paints the picture a little more clearly...

One (extreme) example purely for illustration purposes would be a stack like this

10 x 44.30

38.15

30.15

44.25

42.30

40.30

38.30

36.30

34.20

32.20

30.20

28.20

26.20

24.30

In that stack, the 34.20 poses as a weaker link than say the 44.30 and shims above it...and in that stack you would get flex occurring in a different location than say a reverse thickness taper of as shown (more conventional - thicker as you get smaller)

Edited by harrperf

i bet kevin's going to tell us were all wrong and in fact...

they are both very similar on the dyno lol

i bet kevin's going to tell us were all wrong and in fact...

they are both very similar on the dyno lol

At what tested velocity....is the big question..

Here are the results. The right left stack is stiffer. (Opps, a typo, I initially said right stack, but looking at the numbers it's obvious the left stack is stiffer)

At 1 and 2 ips, the forces are the same. Those velocities show the affect of the bleed (in this case, the bleed is = reb 10 out). At 3 ips, the shims are starting to open, and the right stack starts to get stiffer.

The forces show a steady increase, and there is no sharp increase when the low speed stack hits the hs stack. Ya'll are putting too much emphasis on the crossover shims as their effects are much more gradual on the damping curve than one thinks. In other words, the low speed stack does not necessarily produce soft damping and then amp up when it hits the hs stack.

One thing to notice is the difference in force is not linear. It starts at 96% at the low velocities and goes to 93% at the midspeed velocities. If it was linear, it would be 96% all the way.

1414_vs_1415.png

Edited by kevinstillwell

Great info Kevin. Perhaps I'm not reading the graph correctly but the left shows more damping force -

As stated before in a previous post when I used Restackor on this it showed the right stack to be stiffer - if you removed the 42 x .10 in the stack above the crossover on the right (I hate calling it low speed because it contributes so much more that just damping at low shaft velocities) both stacks were about the same in overall stiffness. In reality I don't know if removing that 42 x .10 shim would cause as much of a change that you found running both setups on the dyno but it would be interesting to try it to see.

Liked your observation about the crossover too - with recent testing in the last couple of weeks I'm finding that I can't really tell that much of a difference changing it a lot so I'm just going to start using a .15 thickness shim at 75 - 80% of the face shim diameter (so on a 44 face shim I would use one 34 or 36 x .15 crossover). I've never tried a shock stack without a crossover but it would be interesting to see the effects on the dyno.

At the end of it all would it be fair to say that 2 different shim stacks can take different forms but provide the same amount of damping force and that they're just one big straight-rate spring - there's really no "low" and "high" speed section..

Edited by NastyR1

And would you say kevin thats a reasonable amount of change most perceptive riders would notice? ie you need that big a difference to make it worth changing?

also could the adjusters be used enough to make them almost the same? ie crank in the hs of the other one , would it be close?

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