Crossover thickness and diameter

I have been tinkering in the garage again with the shock valving on my KTM with PDS. 

 

Previous primary rebound stack I had was this:

 

36x.25

32x.20

28x.20

26x.20

24x.20

20x.30

 

This worked well in the single bump range, but in stutter bumps it was too slow and packed.  I have since revalve and came up with this stack:

 

36x.20 (3)

28x.20------crossover

34x.2

32x.2

30x.2

28x.2

26x.2

24x.2

20x.30

 

It could still use a bit faster rebound on the small chop..  I am 30 click out.  In the big stuff the bike feels super controlled and steady. 

 

I wanted to maybe get rid of one of the 36x.2, but I am indecisive on what shim to run for a crossover here.. I have thought of using a 30x.15, 28x.15, 26x.15, or a 30x.2.  

 

Which one would allow the face shims to bend easier so I can run the clicker in more towards the 20 range? 

 

2 face shims and the 26 .15.

stick with conventional I reckon.

or you could just change one thing at a time to see the result and remove a face shim only.

What size spring?

Going to a bigger crossover will stiffen up the action of the LSC, similar to adding face shims. More force will be required to have them flex. Thats the wrong direction..Maybe try a 26x.2?

I was thinking that the .20 would allow the face shims to bend farther before hitting the 34.... maybe the .20 is too stiff? 

 

I think a smaller shim like say a 26x.15 would work.  

 

It's thinner, so the face shims wont have as far to bend before reaching the 34, but it would be easier for it to bend.  

Guys, I am 170 in street clothing, 8.0 straight rate spring.. 40/120 sag, RT bladder, 150 psi.   I have a PDS2 7.5-9.7 sitting on the bench that I have used before, but not with this particular valving.  Being that the rebound is pretty stiff, I would guess that the progressive spring would work fine in there with the stiffer stack controlling the extra spring energy. 

 

The rear rides great, I just wanted to get the clicker back to the middle of the range.  However by having the rebound clicker out farther I'm betting that is adding to some of the plushness I am feeling as well. My compression clicker is at 10 out with the current stack I have in now. 

 

What's funny is that whenever I change either the front or rear valving wise, I seem to need to make changes at the other end to compensate. 

Edited by Bailey28

I was thinking that the .20 would allow the face shims to bend farther before hitting the 34.... maybe the .20 is too stiff? 

 

I think a smaller shim like say a 26x.15 would work.  

 

It's thinner, so the face shims wont have as far to bend before reaching the 34, but it would be easier for it to bend.  

 

Yes, which should make it faster initially, but once it reaches the 34 it'll be crossing over and activating the high speed stack and firming up sooner too.

I just came in from the garage and had a thought... What if, say, I wanted to increase the gap between the face shims and the tapered stack but I didn't want to use a single .25 or .30 shim as a crossover as it might be too stiff? 

 

So I thought of the shim comparison chart.  If one .1 shim is equal to three .15 shims in stiffness, and say eight .1 shims is equal to one .20 shim-- hmm..

 

I might be able to get rid of the single 28x.20 shim and stack the rebound this way: 

 

36x.20

36x.20

36x.20

28x.10

28x.10

28x.10

34x.20

32x.20

etc...

 

So.. 28x.10 (3) = the thickness of a .30 shim but with the stiffness of only a .15 shim total.  I think this would give the face shims even more bend initially, keeping the rebound fast for trail trash and such but slow it down on larger whoop type movements after they contacted the 34 below the crossover.   

 

 Great, I gave myself something more to think about..  

I could be wrong, but I know that the stiffness of the clamp makes very little difference to the rigidity of the stack, and being that the crossover is basically just a clamp for that stage, I'm not sure if the stiffness of it makes much difference. In saying that, if the crossover is bigger than the clamp, it must flex and add to total stiffness at some point in engaging the HS as well which would make a difference.

 

Give it a go Bailey!

Get restackor as otherwise your guessing , it often doesn't pan out the way we think it will , crossover thickness plays a huge role as does actual stack height , if you use 2stacks the same and space one 3mm away from the top out plate , the 2 shocks will have different damping , the spaced one has less hs , same stacks !

That is a very interessting, complex and sometimes strange topic.

 

The biggest mistake would be to generalize. It depends on a lot of things how a XO or a clamp shim works.

 

There are LOTS of possibilities how to use a XO shim or a XO shims arrangement.

Some have been named here already.

 

Clamp shim thickness plays a role as mog said.

You will find stacks in modern suspension where clamp thickness is used to create certain hs speed restrictions.

The newer style WP linkage shock rebs work that way.

 

But one cant state it as a fact that e.g. a .1mm clamp shim will cause the flexshims to hit the base plate creating more hs damping.

It depends on the taper been used and the shims before the clamp shim too.

So while crossover thickness indeed plays an effect, what effect does the stiffness of the crossover make? Any chance anyone with restackor could have a play?

Mog, by spacing the stack away from or closer to the top out plate it can either limit or allow the shims to travel further?

 

Example..

 

14

12

10

8

plate

 

 

would the following equal more or less shim travel?

 

14

12

10

8

12

14

16

18

 

Or

 

14

12

10

8

10

14

18

20

 

I guess in another sense, some recommended building a "proper" midvalve like the following:

 

24x.1 (2)

20

18

16

14

10

14

16

18

20

spring. 

          Instead of

 

24x.1 (2)

22

20

18

 

I can see the mirrored taper letting the midvave face shims bend more to a point up to the backer shims.   How much different would the above two examples be?

Edited by Bailey28

Its not just that the plate limits travel , it also acts as a restriction to a small degree , the 2 mid valves are hugely different you shown in your example

Its not just that the plate limits travel , it also acts as a restriction to a small degree , the 2 mid valves are hugely different you shown in your example

 

+1

 

Regardless of float, diameter of the backer shim or cup washer, one MV has a 18 mm clamp and the other a 10 mm.  Just that is a big difference... I don't understand why you would reverse christmas tree shims after the clamp on the MV.  If you wanted to space the stack away from the backing shim, to allow the stack to deflect, I would just install the same diameter shims as the clamp.  I guess if you want to avoid buying shims.

For example this is a MV stack I plan on running in my DRZ

 

24.1 (3x)

22.1

20.1

18.1

16.1

14.1

12.1

10.2 (clamp)

10.2 (shim to space stack)

10.2 (shim to space stack)

18.25 (backer)

OK.. Lots of schools of thought getting to the same product at the end..  I appreciate you guys clearing up the MV shim backer as well as the backer stuff in other posts.  (Trying not to cross contaminate!)

 

MV as current is:

 

24x.1 (2)

22

20

18

spring.

1.10mm float   

 

WP open chamber forks 07 12mm rod.

 

I will try to build the stack farther out with a 16, 14, 12 10.. I think it would close up the float a little.  However I did lathe down the  8mm tap where the MV shims ride .028" and have a float of 1.10mm with the current stack. It rides good but after stiffening the rear up some, the front now bears extra load.  I will try to tune with the base valve first and if that doesn't work I will work on the mid. 

Edited by Bailey28

A two stage rebound stack is good but you said it is still to slow with the (3)36x20 LSR face shims. The stack looks close so my suggestion to you is a bleed hole in the piston to get the clickers where you need it, but you might have to adjust your LSC stack.

Edited by WR 911

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