[Dougal]

Remember that linear elastic analysis (most FEA) can predict yeild but not ultimate failure. Because once yield starts the linear-elastic behaviour goes out the window.

In practice we see shims that are overloaded as crimped and crinkled (yielded, many cases, many causes). We see shims cracked and broken into pieces from high cycle fatigue (Fox CTD & DPS).

 

[chehlarov]

If design is made from scratch, you always want the stresses to be well within the linear range to have chance for adequate fatigue life. In a good design, a plastically deformed shims could be caused by mishandling during service.

In practice we see shims that are overloaded as crimped and crinkled (yielded, many cases, many causes). We see shims cracked and broken into pieces from high cycle fatigue (Fox CTD & DPS).

Would be intresting to see the failure modes to suggest causes.

 

[upsha]

Let's simply the question and reduce the assumptions to "What is the maximum acceptable deformation of shim 17.35x7x0.15, while keeping the stresses below 1240MPa?"

Uniform pressure is applied till Von Misses stresses reach 1240MPa:
Maximum deformation is 0.561mm,
(FYI: The resultant reaction force is 99.97N. The applied uniform pressure is 0.5MPa.)

I made a non-axisymetrical loading as well:
The achieved maximum displacement is 1.05mm. The resultant reaction force is 56N. Pressure pattern 0.3MPa and 0.25MPa.
I doubt ReStackor is using nonuniform loading.
The 3mm deflection, indicated by ReStackor, will crack the shim.

T3mppu simulated GRIP2VVC MV in another thread, although using two 0.1 shims. It seems much closer to the three port loading (6kgf @ 1mm) than uniform loading (10kgf @ 0.55mm).

https://www.mtbr.com/posts/16194065/

Really appreciate your UTS comment and I would guess Fox sometimes uses 0.1mm sometimes 0.15mm for this reason. Maybe a deformed shim is not too catastrophic as long as not adding too much free bleed comparing to the rebound needle?

 

[Dougal]

Would be intresting to see the failure modes to suggest causes.

Causes are simply terrible stack design. Stacks with multiple shims both reduce bending stress on each shim and increase curvature to reduce concentration.

Stack two shims with a sharp edged clamp plate into a dished piston and the results are kinda predictable.

 

[chehlarov]

upsha, you are correct - I match the expected shim force to its deformation. The Von Misses stresses from the same simulation are as follows:
View attachment 2096254
Spring steels, like ASTM A 228 (one of the strongest), has design stress level of 1240MPa. From the plot above, stresses as high as 1900MPa are seen. There is no material with suitable properties for such deformation. The shim cannot withstand such extreme deformations --> the actual deformations are less.

Jukis, the deformed shape you are showng has about 3 times larger displacement and 1.5^3=3.375 time stiffer. This means the stresses would be 3 * 3.375 = 10.1 times higher, compared to the once shown above. These are extremely high stresses that no spring material can withstand. I believe there is some hard error in the calcaultions or assumptions. (p.s. the restactor seems to assumed fixed inner edge, while it is pinned for the midvalve)

Uniform loading vs 3 port loading
The simulation I made is with uniform loading and it creates compression stresses around the periphery. Those stresses are seen from the vector plot of the third pricipal stresses:
View attachment 2096256

These tangential compression stresses could/will quite easily create buckling of the outer edge of the shim. In reality, the 3 port position breaks the symetry and for sure will create the edge buckling. I tried to evaluate the effect, by appling a load pattern in 3 zones while having the same reaction force.The simulation became numerically unstable, but managed to get the following results.
View attachment 2096285
As expected, the shape is 3rd order. The maximum vertical dispalcementhas increased from 0.9mm in the uniform model to 4.87mm (wow) - becoming similar to what Jukis show. However, the von Mises stresses are extrmely high:
View attachment 2096286
The last scenario (3rd mode) is impossible - if this was happening, shim will always crack/yield. I suspect during the start of load increase the buckles in much higher mode, making it stiffer and preventing developing 3rd mode. I created minor load disturbance that produced the following results:
View attachment 2096287

Peak displacement 1.06mm - little higher compared to no buckling. View attachment 2096289

Yield stresses - similar to no buckling model.
View attachment 2096290

Principal compression stresses increased on the periphery.

Final thoughts
The assumed reaction force of 487.5N is high for 0.1mm shim. The real force is lower.
The high mode buckling might be yielding the periphery of the shim, permanently changing the shape. Next service, I will measure the overall thickness for any increase.
Using 0.15mm shim (instead of 0.1mm) will reduce buckling effects as the slenderness ratio will be decreased.
Buckling might not be repeatable with each cycle - damping might not be consistent.
The buckling analysis is challenging and it is better to be handled with explicit simulation, for example with LS Dyna.

I am looking at the mid valve design of FOX's new dampers - Grip SL and Grip X have 2ports. Two-port piston will always create bending around a neutral axis. 3 port, as suggested in the quote, could create different bending modes - one beeing much stiffer than other. This unpredictability is due to the buckling at the periphery and could be percieved as "spiking" by the rider.
I would guess FOX have realized that and that's why they moved to two port desing. It is not clear to me why they keep the 3 port design in Grip X2.

p.s.
Grip SL:

Grip X:

 

[chehlarov]

I was reviewing "The Shock Absorber Handbook" and found an intresting paragraph for shims. "Even a thin shim is very stiff against compound curvature, so it prefers to bend with planar curvature" Section 6.6 elaborates further.
The messagehere is that axisymetrical bending (as I showed few posts above) is very unlikely/impossible as it has compound curviture. The shim will always bend into some other shape with 2, 3 (as I simualted) or 4 bending lines. For single shim the mode of bending is governed by the diameter of the support. If the support diameter is less than the OD / 2, 2 bending wings will form. If the support diamter is bigger, 3 bending wings will form. Intestingly, the base valve and midvalvesupprt diamter are just about half of the OD of the relevant shim. Could be a source of variation in the bending mode (2 vs 3 wing).If this happens it will cause fluctuations in the damping.

 

[Jukis]

Anyone have experience with 2025 grip mid valve tunes which seem to make float only 0.1mm?

 

[englertracing]

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