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Posted (edited)

I've been playing around with re-valving my own forks.  Looking for some feedback about how the actual valve stacks work and how changes to different parts of the stack affect what part of the travel. Given this mid-valve compression stack:

14 x .1, 20 x .1, 18 x .1,16 x.1, 14 x.1, 13 x.1, 12 x.1, 8 x.3

1 - What changes should be made to soften the later part of the stroke?

2 - Does one change have a major impact on all of the shims above them or can you fine tune just one part of the stack without drastically affecting the characteristics of the entire stack.

3 - My goal is to allow the fork to travel further down into the stroke.  I've exhausted all of the combinations on the base valve with no luck.  Any changes I made to make it softer, affected the ride height and made the fork feel harsh.

4 - Do changes to the mid-valve rebound stack affect the performance of the compression stack or does each circuit work independently of each other? 

Trying to learn here so any input is appreciated.

 Thanks,

Rick

Edited by Ktmshorty

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The mid valve has the greatest effect at the top of the stroke where compression speed is greatest.  Softening the mid will bleed off less energy on a high speed stroke and will require more travel to soak up any given impact, until the spring force and base valve damping finish off the stroke and the fork rebounds. 

Rebound and compression on the mid are independent until cavitation occurs, which is more or less likely depending on how well balanced the mid and base valve are.  If the cartridge is cavitating that means the cartridge is pushing oil out through the base instead of through the mid valving you will lose rebound damping until the fork has rebounded enough to build pressure behind the mid valve in the rebound chamber.  Other than that though, compression and rebound have no relation to each other. 

For that mid you could pull the 13x.1 shim to soften the stack and increase the float:

14 x .1, 20 x .1, 18 x .1,16 x.1, 14 x.1, 12 x.1, 8 x.3

The float is the biggest consideration on the mid valve.  More float allows for higher compression speeds before the mid comes on.  Changes to the stack itself have much less distinctive effects.  You want to look at maximum stack deflection, how far the shims can bend before they hit the backing plate.  Most forks allow for a lot of shuffling and configuration here, you can usually run a taller stack for more deflection without necessarily closing up the float.

You may want to go back to stock or near stock on the base valve.  I think your issue is the mid and going too soft on the base will just make the fork work poorly.

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Great post turbo.

1 hour ago, turbo dan said:

You want to look at maximum stack deflection, how far the shims can bend before they hit the backing plate.

This could very well be his problem and solution. In my experiments, I've found that tuning the max deflection makes a huge difference.

1 hour ago, turbo dan said:

I think your issue is the mid and going too soft on the base will just make the fork work poorly.

Agreed.  When in doubt always lean toward stiffer base and softer mid.  The right combo of comp damping is key.  As is fine tuning the mid comp for low speed float bleed, mid speed stack stiffness and high speed flow allowed during max deflection.

 

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Great post turbo.
Agreed.  When in doubt always lean toward stiffer base and softer mid.  The right combo of comp damping is key.  As is fine tuning the mid comp for low speed float bleed, mid speed stack stiffness and high speed flow allowed during max deflection.
 
The shim stack itself on the mid has little effect ,try pulling it with your finger nail? Not hard to bend it at all , and the mid moves very fast with a large swept volume , so the shim stack mostly sets the total port opening

Eg is you had a shim stack with a large taper of 0.15mm shims ,and one with 0.1

Say the 0.1mm had a total shim lift ( till it hits the backer ) of 0.6 mm

And the 0.15mm shims give a total of 0.9mm I bet the 0.9mm mid is softer

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3 hours ago, Ktmshorty said:

1 - What changes should be made to soften the later part of the stroke?

Remember the base and mid valves don't respond or activate to any position in the stroke. The valves produce damping based on stroke speed.  It's the relationship between stroke position and speed and spring rate that gives people the impression that valving is position dependent.

3 hours ago, Ktmshorty said:

2 - Does one change have a major impact on all of the shims above them or can you fine tune just one part of the stack without drastically affecting the characteristics of the entire stack.

The answer depends on what shim you change. It's easy to tune low speed without impacting high speed, but not so much the other way around.  Buy and read the Race Tech Susp Bible. The edu chapters at the beginning are short reads and well worth it.

3 hours ago, Ktmshorty said:

3 - My goal is to allow the fork to travel further down into the stroke.  I've exhausted all of the combinations on the base valve with no luck.  Any changes I made to make it softer, affected the ride height and made the fork feel harsh.

It's great that you are having a go!   What fork is it?    Don't deviate far from a sane base setup adjusted to match your riding. Tune the mid. Try to feel your mid comp damping on various impacts. Feel the very early part of the comp stroke.  Float bleed being too tight can be felt as too much feedback from the front tire. Float too wide and the fork wont hold up enough so you ride on the stiffer range of the spring.  Medium speed mid-comp damping is what helps to hold up the front and share the workload that the base valve cannot do on its own.  Very high speed damping is key and this could be your problem.  Too much very high speed damping (not enough max deflection) is felt as a spike. You should be able to ride into sharp objects at pace and (assuming you don't crash) never feel anything harsh.  But too soft on very high speed and you can also bottom out. Always ask yourself if your damping is sharing the workload adequately.

Beware that softer comp damping is often harsh because damping is late and you feel too much spring rebound.  Plush damping is about spreading the the force over time of stroke/impact. Spring matching damping is also key.  Soft comp damping usually requires less bleed (clickers in) so you don't get enough very low speed travel which makes the bike too difficult to ride.  If your comp clickers are not middle of range (allowing designed bleed) then chances are high that your suspension could be better.

3 hours ago, Ktmshorty said:

4 - Do changes to the mid-valve rebound stack affect the performance of the compression stack or does each circuit work independently of each other? 

They are dependent, unfortunately. For starters you only have the single clicker for the mid valve. So reb and comp feel is impacted by that clicker. Also rebound speed (due to damping, spring rate and spring preload) determines how fast the tire re-contacts the ground, which in turn impacts the ability for the comp damping to do it's thing on the next bump.

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The mid valve has the greatest effect at the top of the stroke where compression speed is greatest.  Softening the mid will bleed off less energy on a high speed stroke and will require more travel to soak up any given impact, until the spring force and base valve damping finish off the stroke and the fork rebounds. 
Rebound and compression on the mid are independent until cavitation occurs, which is more or less likely depending on how well balanced the mid and base valve are.  If the cartridge is cavitating that means the cartridge is pushing oil out through the base instead of through the mid valving you will lose rebound damping until the fork has rebounded enough to build pressure behind the mid valve in the rebound chamber.  Other than that though, compression and rebound have no relation to each other. 
For that mid you could pull the 13x.1 shim to soften the stack and increase the float:
14 x .1, 20 x .1, 18 x .1,16 x.1, 14 x.1, 12 x.1, 8 x.3
The float is the biggest consideration on the mid valve.  More float allows for higher compression speeds before the mid comes on.  Changes to the stack itself have much less distinctive effects.  You want to look at maximum stack deflection, how far the shims can bend before they hit the backing plate.  Most forks allow for a lot of shuffling and configuration here, you can usually run a taller stack for more deflection without necessarily closing up the float.
You may want to go back to stock or near stock on the base valve.  I think your issue is the mid and going too soft on the base will just make the fork work poorly.


Thanks Turbo. I totally get it. Great explanation. So, if I still don’t get the last three inches of travel, you’re suggesting eliminating the 13 as my next change correct? Not disputing it in anyway, just trying to learn, but can you explain why you would remove the 13 vs the 12? Just want to understand the different affect it would have.
Thanks!

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Posted (edited)
9 hours ago, mog said:

The shim stack itself on the mid has little effect ,try pulling it with your finger nail? Not hard to bend it at all , and the mid moves very fast with a large swept volume , so the shim stack mostly sets the total port opening

In my experience with the KYB and Showa CC forks, tuning the mid stack shim stiffness makes an easily noticeable difference to the feel on initial comp movement. After a few experiments I found that each fork likes a certain stiffness range.  In the KYB OC forks mid stack stiffness changes makes a large difference.

Agreed completely regarding total valve opening. When tuning my mids I try to calculate and control changes I make to max open. Different mid designs offering different options to do it.  Last week in my 47mm CC Showas all I changed was allowing 0.1mm of extra max open and the improvement was huge.

Edited by numroe

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They are dependent, unfortunately. For starters you only have the single clicker for the mid valve. So reb and comp feel is impacted by that clicker. Also rebound speed (due to damping, spring rate and spring preload) determines how fast the tire re-contacts the ground, which in turn impacts the ability for the comp damping to do it's thing on the next bump.


Thanks Numroe, they are Xplors with the DS kit. So far I changed the 16x .15 to a 16 x .1 in both base valves. Changed the 9x .3 to 8 x .3 on the compression side mid valve and the 9 x .3 to a 10.3 on the rebound side rebound stack on the mid valve. Oil height is 130 and preload is 4 mm. Both mid valve changes and reducing the preload from 8 mm to 4 mm. Didn’t ride it yet, just trying to plot my next move if this doesn’t solve the problem (not using the last 3” of travel).

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1 hour ago, Ktmshorty said:

 


Thanks Turbo. I totally get it. Great explanation. So, if I still don’t get the last three inches of travel, you’re suggesting eliminating the 13 as my next change correct? Not disputing it in anyway, just trying to learn, but can you explain why you would remove the 13 vs the 12? Just want to understand the different affect it would have.
Thanks!

The 13 stiffens the stack somewhat under high deflection/high speed and forces the shims to bend further out from their center which requires more force.  Pulling the 12 may put undue stress on the 13 with an 8mm clamp and could cause the 13mm shim fatigue over time.  Tapered stacks are designed to deflect in more of an arc which makes them more resilient.  Going from a 13 to an 8 will try to kink the 13mm shim at the edge of the 8mm shim.

In rear shocks, for example, the tapered portion of the compression stack often goes from 2mm steps down to 1mm steps toward the clamp.  Having only 1mm steps makes the stack stiffer because the angle of the stack is steeper and the shims under the taper can't deflect as far without the next shim in the stack providing support.

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8 hours ago, Ktmshorty said:

Thanks Numroe, they are Xplors with the DS kit. So far I changed the 16x .15 to a 16 x .1 in both base valves. Changed the 9x .3 to 8 x .3 on the compression side mid valve and the 9 x .3 to a 10.3 on the rebound side rebound stack on the mid valve. Oil height is 130 and preload is 4 mm. Both mid valve changes and reducing the preload from 8 mm to 4 mm. Didn’t ride it yet, just trying to plot my next move if this doesn’t solve the problem (not using the last 3” of travel).

 

You're welcome.   I have no experience with those forks.  I was curious because I didn't recognise your stack.   I read some TT threads on them and cannot remember much detail. I do remember them being somewhat like a SFF type fork. So you might never get full travel without some major compromise. 

If I had some, I'd be looking to convert cheaply to good old OC forks with adjustable base clickers. Surely some good kits exist.  Clickers on the bottom is no big deal if the fork works well enough after you find a setting where you rarely need to touch them.  Bigger dampers or more softer dampers (2x base + 2x mid) sharing the compressing damping allowing lighter oil pressure and softer shims allows for easier shim lift for the same total damping. This helps achieve that plush yet firm feeling. I know that closed/twin chamber designs are better in terms of consistent damping, but for trail riding I think OC is just fine for 95% of riders.

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The 13 stiffens the stack somewhat under high deflection/high speed and forces the shims to bend further out from their center which requires more force.  Pulling the 12 may put undue stress on the 13 with an 8mm clamp and could cause the 13mm shim fatigue over time.  Tapered stacks are designed to deflect in more of an arc which makes them more resilient.  Going from a 13 to an 8 will try to kink the 13mm shim at the edge of the 8mm shim.
In rear shocks, for example, the tapered portion of the compression stack often goes from 2mm steps down to 1mm steps toward the clamp.  Having only 1mm steps makes the stack stiffer because the angle of the stack is steeper and the shims under the taper can't deflect as far without the next shim in the stack providing support.


Got it! Thanks for the detailed explanation!

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The 13 stiffens the stack somewhat under high deflection/high speed and forces the shims to bend further out from their center which requires more force.  Pulling the 12 may put undue stress on the 13 with an 8mm clamp and could cause the 13mm shim fatigue over time.  Tapered stacks are designed to deflect in more of an arc which makes them more resilient.  Going from a 13 to an 8 will try to kink the 13mm shim at the edge of the 8mm shim.
In rear shocks, for example, the tapered portion of the compression stack often goes from 2mm steps down to 1mm steps toward the clamp.  Having only 1mm steps makes the stack stiffer because the angle of the stack is steeper and the shims under the taper can't deflect as far without the next shim in the stack providing support.


Dan,

The mid-valve compression stack in the rebound leg is: 20 x.1 (3), 18x .1, 16 x .15, 14 x .1, 12 x .1, 16 x .15, 11 x .2

Would you change anything on this in conjunction with removing the 13 from the compression leg? Would you do it one step at a time or together?

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He has the D's kits in them

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2 hours ago, Ktmshorty said:

 


Dan,

The mid-valve compression stack in the rebound leg is: 20 x.1 (3), 18x .1, 16 x .15, 14 x .1, 12 x .1, 16 x .15, 11 x .2

Would you change anything on this in conjunction with removing the 13 from the compression leg? Would you do it one step at a time or together?

 

I would pull one of the 20mm shims.

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Why not make both mid comp  stacks the same like a normal OC instead of adjusting 2 different stacks ,seems  more complicated, like both 3-20.1,18.1,16.1,14.1.12.1-10 or 8,oem uses 16.15

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6 hours ago, numroe said:

You're welcome.   I have no experience with those forks.  I was curious because I didn't recognise your stack.   I read some TT threads on them and cannot remember much detail. I do remember them being somewhat like a SFF type fork. So you might never get full travel without some major compromise. 

If I had some, I'd be looking to convert cheaply to good old OC forks with adjustable base clickers. Surely some good kits exist.  Clickers on the bottom is no big deal if the fork works well enough after you find a setting where you rarely need to touch them.  Bigger dampers or more softer dampers (2x base + 2x mid) sharing the compressing damping allowing lighter oil pressure and softer shims allows for easier shim lift for the same total damping. This helps achieve that plush yet firm feeling. I know that closed/twin chamber designs are better in terms of consistent damping, but for trail riding I think OC is just fine for 95% of riders.

I didn't see that.....that's what I did to mine/xplor

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3 hours ago, turbo dan said:

I would pull one of the 20mm shims.

Should I make both changes at the same time or one at a time?

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Do them both.  Pulling the 13mm from one stack is a minor change other than its effect on the float.  Pulling a 20 from the other side will increase the float on that side by the same amount and soften that stack as well.

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Do them both.  Pulling the 13mm from one stack is a minor change other than its effect on the float.  Pulling a 20 from the other side will increase the float on that side by the same amount and soften that stack as well.


Thanks, I’ll give it a try.

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