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Do YZ SSS forks have a cupped washer in the base valve ?

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That is what's known as a bleed stack

 

How does it work, why is it called that and is it present in the YZ stacks too ?

 

http://www.thumpertalk.com/topic/957655-kyb-fork-bleedstack/

http://www.thumpertalk.com/topic/482655-bleed-stack/

http://www.thumpertalk.com/topic/747388-bleed-stack/

Edited by MidlifeCrisisGuy

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It's a secondary stack to control the flow from the adjuster. You will notice the holes under the first shim. I believe the intent is to control dive from bypass bleed during heavy braking, mainly on the 4strokes. I valved a set of 250F forks for a two stroke and removed it.

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It's a secondary stack to control the flow from the adjuster. You will notice the holes under the first shim. I believe the intent is to control dive from bypass bleed during heavy braking, mainly on the 4strokes. I valved a set of 250F forks for a two stroke and removed it.

 

So its just further restriction on the compression clicker circuit ?  Instead of going straight to through the clicker, it goes through the clicker circuit and then to the shim pack mounted on the "stem" ?   And its only really in play when a regular clicker circuit is in play, ie always but the orficed flow is only applicable at slow velocities, after that the base valve and then midvalve open and takes over ????

 

So one can basically ignore the bleed stack because its only going to have an effect at very low velocities ?

 

Thanks for the reply, btw.

Edited by MidlifeCrisisGuy

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How exactly does one physically remove the bleed stack ?  Reverse the shims so the clamp shim doesn't block any of the ports ?   Replace the stem with the YZ part that doesn't have the "stem" ? 

 

How does one compensate for removing the bleed stack ?   Won't the clicker circuit be way more free flowing without the shims in place ?

 

I would love to have some spare shims.   Could the bleed stack shims to removed entirely and used elsewhere in other stacks ?

 

Thanks

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Reverse the bleed stack or use a few washers to space it , you don't have to compensate for it

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You can remove it entirely but you need to space the main stack out to not occlude the ports with the big thick washer-shim. You could probably just remove the big shims and leave a couple of the small ones in place to space it up a bit.

It's there so that the clicker circuit isn't just an open hole, it allows you to control how the bleed/bypass feels. I have removed it on my KYBs but it wasn't a significant change. I may experiment with putting it back in... Like any other shim stack, it can be tuned. For example, removing one of the face shims and the medium shim behind it will make it softer.

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You can remove it entirely but you need to space the main stack out to not occlude the ports with the big thick washer-shim. You could probably just remove the big shims and leave a couple of the small ones in place to space it up a bit.

It's there so that the clicker circuit isn't just an open hole, it allows you to control how the bleed/bypass feels. I have removed it on my KYBs but it wasn't a significant change. I may experiment with putting it back in... Like any other shim stack, it can be tuned. For example, removing one of the face shims and the medium shim behind it will make it softer.

 

Being its a shim stack after an orifice, its mainly about low speed damping, right ?   Yamaha didn't make the clicker orifice larger than normal and then put a stack on it to allow adjustment of damping outside of low speed ?

 

Here is the thing... a couple of us 2012+ WR450F owners are getting jolts through the bars on roots, rocks and ruts.  Opening up the compression clicker helps on this stuff, way more than I would have imagined.  This has side effects like making the fork a bit (or a lot) soft overall, but its not really bad.

 

Do you think that the clicker orifice has enough flow such that one could modify the bleed stack to react to high speed events, thus giving the bike an adjustable high speed damper ?  Or should we stick to handling high speed damping in the base or mid valve ?

 

Thanks for your reply, btw.

Edited by MidlifeCrisisGuy

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No. I don't remember the exact orifice size for those forks, but it's a small size just like every other compression clicker, so it's primarily a low speed adjustment, although obviously it affects the whole range. The bleed stack is just there, presumably, to give you the ability to choke it down a bit at very low pressures (speeds) to help with chassis pitch control.

With regard to WRs, you just need to get in there and revalve the darn thing. It'll respond just like the YZs that are used for woods riding, with perhaps a slight subtle change due to a bit more weight. If the stacks you posted in that other thread are right, it's got a single stage base and the same midvalve stack (no float and no sleeve measurement, so it's hard to say). Just like a YZ, a bit softer base and/or going to 2-stage and a bit more float on the mid will probably fix it right up.

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It's a secondary stack to control the flow from the adjuster. You will notice the holes under the first shim. I believe the intent is to control dive from bypass bleed during heavy braking, mainly on the 4strokes. I valved a set of 250F forks for a two stroke and removed it.

 

Being this is on a WR450F, I like the fact its there.  Diving on heavy front braking, particularly going down hill, is one of the things I would like to improve on my bike.

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No. I don't remember the exact orifice size for those forks, but it's a small size just like every other compression clicker, so it's primarily a low speed adjustment, although obviously it affects the whole range. The bleed stack is just there, presumably, to give you the ability to choke it down a bit at very low pressures (speeds) to help with chassis pitch control.

With regard to WRs, you just need to get in there and revalve the darn thing. It'll respond just like the YZs that are used for woods riding, with perhaps a slight subtle change due to a bit more weight. If the stacks you posted in that other thread are right, it's got a single stage base and the same midvalve stack (no float and no sleeve measurement, so it's hard to say). Just like a YZ, a bit softer base and/or going to 2-stage and a bit more float on the mid will probably fix it right up.

 

 

Excellent reply, thanks.

 

Would you 2 stage the base valve or the mid valve ?   I'm unclear on which valve should be set up to handle high speed stuff.   The mid valve will have a lot more flow going through it and because we are mainly interested in small stuff, it would be more appropriate ?  

 

Is it better to handle small jolts with more float or with a softer stack ?  I guess this is where Restackor could help out ?

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Short answer is you need to try things and see what works, there's no formula. However, lots of people have posted stacks that work in the same forks on a similar bike in the same conditions, so those ought to get you close.

I would 2-stage the base valve. I'm not a huge believer in 2-stage mids, I don't think they hurt anything but I also don't think that they help that much. Midvalve stacks are already super soft and they have a soft spring behind them, so I don't see lift at low pressures to be the primary problem. On the mid, I would adjust the float (depending on what it is now).

I'm not sure about your comment about the midvalve having more flow and "because we're mainly interested in the small stuff" it would be more appropriate.

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Good advice.

 

I'm not sure about your comment about the midvalve having more flow and "because we're mainly interested in the small stuff" it would be more appropriate.

 

The flow through the base valve is the area of the damper rod.    The flow through the mid is the cartridge piston less the damper rod, usually about 3x the flow through the base.  <-- I'm sure you already knew that.  

 

Page 51 of the Suspension Bible talks about a damping lag, "particularly on very small movements", when the base valve is used for damping.  He seems to suggest that the mid valve is more appropriate for high speed small movements, which is essentially what we are after.

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I agree with the flow difference and the possibility for "lag" (hysteresis?) of the base valve due to fluid compressibility on small fast movements.

I don't necessarily agree that therefore the mid is the source of harshness. For one thing, "small movements" that don't push through the base are SMALL. Over 1 inch of stroke (for example) you're pushing oil through the base, I really doubt you're compressing that much volume. For another thing, even if the mid is a big player in this regime, the base valve can very well be the source of harshness. Those forks are generally valved so that the base and the mid share the damping load pretty evenly (depends a lot on speed, but still), so they are both significant contributors.

It's extremely likely that base AND midvalve changes will be required to achieve the effects you want.

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Thanks !

 

One more question while I have your attention... we are trying to lighten the damping on small, fast hits.  Landing jumps also creates fast damping events.   If we lighten the damping for trail trash, aren't we going to cause more bottoming on jumps ?   Is there a way to achieve one without the other ?

 

How do we tune the damping so that the trail trash jolts get reduce while maintaining firmness for fast sections (mostly low speed damping) and jumps ?  I think a 2 stage stack would maintain firmness for fast sections, but how do we keep firmness for jumps ?  On the rear with a linkage system (which the WR has) damping increases as travel increases.   Is there a way to accomplish this in a fork ?  Some of the Restackor demos discuss having the oil in a cavitated state when the fork is at the top of its travel ??? 

 

Any thoughts on using a Huck Valve at the bottom of the fork to at least soften the bottoming event ?

Edited by MidlifeCrisisGuy

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That's the classic damper tuning problem. There are a few ways to handle it but there isn't any magical solution.

For one thing, you can use the air spring (oil level) and the fork springs themselves to control travel and bottoming, and then use the damping tuning to reduce harshness.

Single-stage damping tends to be linear or digressive, which can mean the worst of both worlds (harsh at low speed on trail trash, soft and bottoming on big hits). Midvalves and 2-stage stacks can help with this, but trail trash (and jump landings) have a lot of overlap in damper velocity so you can't just tune them independently.

With regard to cavitation, I would be very hesitant to try to use that as a tuning tool. That's like NASA-level stuff. I think it would be very, very hard to predict that behavior exactly and get repeatable results.

I have no experience with the Huck Valve. Seems like a very sound idea, but the cost/benefit isn't worth it for my application. I don't huck it and when I do bottom it's not the end of the world.

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I agree with your comments about the Huck Valve.

 

I'm very new to all this, but I have some thoughts.

 

1) When landing from jumps the forks are going to be fully extended.   This gives the suspension over 11 inches to damp out the energy from the landing.  So even if the damping is soft, at least it has lots of travel to work with.

 

2) Clickers are orifice damping.  If they are opened up enough to make a change on HS stuff, they will be flowing enough oil to make low speed stuff mushy.  If the clicker circuit is tightened up the rest of the fork can run stiffer shim damping for the same LS result.   Shim damping is linear or even digressive on HS events, so it can be firmer too.  

 

3) Float in the mid valve, especially big floats, is essentially orifice damping, especially on small movements.  One poster in another thread pointed out that his best suspension was a tight mid valve float (0.1mm) with very soft midvalve shim pack.

 

4) Orifice damping, unless its really high flow, is still going give a jolt on HS stuff.  If you can get rid of the orifice jolt and replace it with slightly less linear damping you can probably run a stiffer spring and/or a higher oil level for the same or less jolt feel on a bump or jump.  A stronger spring means the fork will run higher in the stroke too, which is a side benefit.   

 

Comments ? 

Edited by MidlifeCrisisGuy

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