Backer shims - what if...

Ok, to demystify some of the information I have been gathering up over the last few months, I want to try to clarify backer shims.  I have read that :

 

*Backers limit the travel of shims above them to some extent

 

*Backers on the other hand allow more travel of the shims above them

 

OK with that being mentioned, lets look at two similar fork stacks:

 

Stack A:

24x.1  (3)

12x.1

22x.1

20x.1

18x.1

16x.1

14x.1

12x.1

8x.30

base plate

 

Stack B: 

24x.1 (3)

12x.1

22x.1

20x.1

18x.1

16x.1

14x.1

12x.1

8x.30

12x.1

16x.1

base plate

 

With the addition of the two "backer" shims in stack B, would the face shims in stack B open farther before touching the top out plate under the 16x.1? 

 

Reason I am asking is because I have seen different set ups listed where some tuners add in backer shims and say that this will have a different effect on damping charachteristics.   

 

I'd love to hear what you guys are going to say!  BTW, Restackor will not work on my computer. 

 

What is the diameter of the base plate?  If anything I would say the base plate is your backer "shim".  The additional shims above the 8.3 shim (12.1 & 16.1) just space the shimstack away from the base plate.  So without doing any sims I'd say stack B will allow for more shim deflection.

 

Now whether that matters and its effects I would need to see in a program like restackor.

The 12 and 16 mm shims after the clamp would have no effect on this stack. Typically a backer on a base valve is .25 or so thick. The thickness of the clamping shim and the diameter of the backer determines how far the stack can flex prior to contacting the backer. The thickness of the backer determines the extent it limits shim edge lift after contact.

The 12 and 16 mm shims after the clamp would have no effect on this stack. Typically a backer on a base valve is .25 or so thick. The thickness of the clamping shim and the diameter of the backer determines how far the stack can flex prior to contacting the backer. The thickness of the backer determines the extent it limits shim edge lift after contact.

 

Exactly, that's why I asked about the diameter of his "base plate"... because it'll act as a backer shim is its diameter and thickness are large enough.  Not unlike the base plate on the compression side of a rear shock.

I have been experimenting with valving and have not removed or added any backer shims other than what was there from the factory.   It is a WP open chamber '07 fork.   The backer that is there under the clamp shim is 18x.25.  I just put that shim on the base plate first then starting with the clamp shim build the stack up from there.  I believe the base plate is flat all the way across, so it would be in the 24mm diameter range.    

 

I guess looking at it from this perspective, it would be:

 

...

14

12

8x.30

18x.25

24 solid base plate

 

My thoughts reading your replies are:

 

Shim deflection would increase if the clamp were thicker, and decrease if the clamp were thinner?   Say 8x.30, vs. 8x.10.... Would I be on track to say that by using an 8x.10 clamp it would limit the HS shims above it from hitting the baseplate?

 

For example:

 20, 16, 12, 8....  I could see the 8x.10 limiting the above shims from bending too far.. Or if they bend easily, to limit how far they  bend adding to damping by restricting how far the face shim comes off of the piston?     Maybe in this situation, running a softer stack lets the face shims move off of the piston sooner, but restricts how far they can bend on hard hits, thus keeping the bike from blowing through on big hits? 

 

 

Another example:

 

24, 23, 22, 21, 20, 19, 18, 17, 16, 15, 14, 13, 12, 10, 9, 8x.10.... I could see that no matter what, the shims above the clamp probably won't ever touch a 24mm base plate due to the gradual taper here.  The edge of the 24 shim would have to bend 1.4mm to touch the base plate.   

 

Knowing that shims deflect to let oil pass is the basis of how this stuff works.  Could or should a face shim ever bend 1.4mm?  

I'd love to hear what you guys are going to say!  BTW, Restackor will not work on my computer. 

Why is that?

I don't know... I was running it in Microsoft office...some of the program worked for a day or so, then it stopped working all together.  I'm not the greatest computer wiz either...

I don't know... I was running it in Microsoft office...some of the program worked for a day or so, then it stopped working all together.  I'm not the greatest computer wiz either...

What OS are you running? Have you checked Restackor website? Because the question you asked is best answered by pictures and graphs generated by restackor. It would allow you to see exactly how backer shims work and what is what. I'm not sure how restackor compares to real world data, but even if it has oversimplified models you still can learn a lot.

Both of your stacks will dampen the same way but stack B has a little more range due to the extra 12x.1 and 16x.1 shims. Both stack are really soft due to the 8x.30 clamp shim. Going to an 10x.30 clamp will significantly stiffen up the entire stack. Remember, the whole stack pivots off of the clamp shim O/D. The BACKER (Base plate) is solid and is there to limit the travel of the stack and generally has an O/D =/< the O/D of the face shims.

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