Understanding....midvalves...

[JTT]

OK...stepping back, I look at the valving arrangement in my fork (twin chamber Showa) and in my shock (Showa) and overall they appear fundamentally the same.

That being said, it would appear to me that the "midvalve" (or active valving) in the fork would be the area that one would begin their tuning quest...as this seems to be the primary area (active valve) tuned on shocks.

Here's what my stock midvalve looks like:

10x.2(x3)
17x.1(x4)
20x.1(x3)

I would like to take some of the "spike" out of action in situations such as braking bumps. Would this not be a "midvalve" situation?

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JTT
Logic Over Hype Coalition

 

[Jeremy Wilkey]

JTT,
The difference is areas.. They are fundementaly sillimar in purpouse however. IT is a function of speed and volume that make them different..

There is more than enough info on this board to identify what must be done..

Regards,
Jer

 

[JTT]

I used my stack as an example only. I am just trying to understand how and when the midvalve works.

Could anyone elaborate on the "speed and volume" differences? Why the spring arrangement supporting midvalve valving? Why would it not be set up in a similar manner to shock piston (rigid)?

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JTT
Logic Over Hype Coalition

 

[Vester]

JTT,
First off, thanks for starting this thread. I feel this is going to be another one to get the grey matter a thinkin.

It seems I heard somewhere that stiffening the active valving (midvalve) will take some of the strain off the compression valving allowing you to soften the LSC for greater tunability over slow speed stuff like braking/chatter bumps. In other words a setup thats super plus when slow but firm enough to resist bottoming.

Ever thought about stiffening it up just to see what the ride was like? Then maybe going full soft on the clickers and recording the effects?

Trying to get some ideas flowin.

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Vester
96 RM250
"Let's see if an "A" rider tries it first.."

 

[drehwurm]

Servus JTT,

Today, while driving

on the highway I started thinking about this midvalve thing on the FORKS. Where is the difference between active and passive damping? Isn't it the same, does it matter if the oil moves through the valve or the valve through the oil? Couldn't you just swap the midvalve and base comp valve without making any difference?

To my understanding each valve has it own damping curve and the overall daming is just an overlay of both, meaning that if you want to alter damping it is sufficient to just alter one. Maybe I'm off the track here, but even though I was pounding this for almost an hour I couldn't think of a reason why.

Michael

 

[Jeremy Wilkey]

Guys,
I've been beeting this drum for years.. Espically one this forum...
Everything you asked has been said dozens of times.. Vester does have the right idea..

What volumes and speeds?, well think about it. 23mm Cartridge vs 50mm verses 2.25 linkage to 1:1 hum... Thinkabout it!!!

I guess I'm getting annouyed with all here's my valve stack stuff. What do I do.. I like to help people invisoin what is going on but it just seems like many just want handouts lately..

Regards,
Jer

 

[Jeremy Wilkey]

drehwurm,

Ok I like where you are going... Major differences..(again covered at least a dozen times). Think about it somemore, but your on the right track.. Think about what active compresson really is and vise versesa.. Think about how changing different elements of the valves and or shim stacks impacts performance.. Then your going to be getting somewhere..

Jer

 

[JTT]

Originally posted by Jeremy Wilkey:
I guess I'm getting annouyed with all here's my valve stack stuff. What do I do.. I like to help people invisoin what is going on but it just seems like many just want handouts lately..

Regards,
Jer

I apologize Jer, I did not intend for a "handout", in fact, just the opposite. You'll see in drehwurm's post, I actually defended the idea of learning, not getting "handouts". I merely posted a sample stack for comparison sake...maybe my wording was misleading of my intension.

I realize about the linkage ratios and the differences in volumes in the two dampers, but am just trying to get a handle on how they work.

The linkage ratio should produce significantly slower piston speeds, that I can understand. Am I on the right track?

The volume issue still has me a little puzzled. Although the two are different size chambers, the shaft sizes are also different, and at a glance, the ratios seem at least close (shaft to chamber)?

Again Jer, I did not intend to add to the annoying "tell me how to do it, for free, so I don't have to pay some guy, who has to make his living from this" posts. I would just like to understand better, as I think many others would.

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JTT
Logic Over Hype Coalition

 

[drehwurm]

Servus Jer,

Think about what active compresson really is and vise versesa..

Probably it is too little background knowledge what is holding me back here, but I didn't come to another conclusion. If you have a chamber filled with fluid which has two exits the pressure on either of the exits will be the same regardless what you do. Even if you raise the pressure behind one of the exits (i.e. oil displaced by the rod) absolute pressure will stay equal on both exits. The above example is static and again I don't know very much about fluid dynamics, but even with the exits being spring loaded doors and one exit moving I don't see a difference in pressure or, and that's my point, flow on both of the doors. Now if the doors are build different, flow would be different BUT overall pressure would still be the same. Ok, I want to say that IMHO passive and active valving is just a term describing a technical solution (valve moving or oil moving relative to cartrige), but with no functional difference!

Nevertheless, this leads me to a much more interesting fact - terminology. Is it really right to talk about speeds (shaft speed) here? Wouldn't this mean we want to set the valaving for a certain speed, which I consider wrong. If your fork is bottoming at a certain obstacle and you measure a speed of 8m/sec what does this mean - nothing relevant. 8m/sec is just the max. speed, important is the acceleration, because this finally tells you the load on the valving. Where is the point? Well a high acceleration over a short period (sharp edge) of time results in the same 'max. speed' as a low acceleration over a long period (g-out)!

I think, I for myself have a better understanding on this if I visualize it as low, mid and high acceleration vs. speed.

Michael

 

[JTT]

Originally posted by drehwurm:
Well a high acceleration over a short period (sharp edge) of time results in the same 'max. speed' as a low acceleration over a long period (g-out)!

Michael, I think the speed will be faster for a "sharp edge". Remember, speed is distance divided by time. Although the distance traveled in a sharp edge bump is small the time of travel is very short. In a G-out situation, the distance is great but the time is equally long, relatively. Using the example of 8m/s it would take .4 secs for the piston to move 20mm, if the speed remainded the same for the g-out situation, the time to complete the shock movement would only be .04 secs. We know that it takes considerably longer to compress the shock in a g-out than that, therefore the speed must be slower.

Maybe you are right about the "overlay of both"....and maybe this is why "some" tuners tend to work mostly with base valve (and in some cases, eliminate midvalve completely). It has a greater range of shims and could be more easily "fine tuned"...hmmm...

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JTT
Logic Over Hype Coalition

 

[JTT]

Originally posted by Jeremy Wilkey:
What volumes and speeds?, well think about it. 23mm Cartridge vs 50mm verses 2.25 linkage to 1:1 hum... Thinkabout it!!!

I just did a "very rough" calc on the displacement of oil from the two shafts, at full compression and they were surprisingly close. If this is true (and I'm not totally convinced...my math skills have suffered over time

), then perhaps there is a relationship to the surface area of the pistons, and the two different base valves? ...just "spouting" off top of head, throwing ideas out there.



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JTT
Logic Over Hype Coalition

 

[drehwurm]

Servus JTT,

Uhmm, I'd say that at 8m/s it takes the piston only 0.0025s to move 20mm. But apart from that I think you unintentionally proved my point. The piston is NOT travelling at 8m/s for 0.0025 seconds, it just reaches a peak of 8m/s. Speed in the beginning is 0 or maybe even negative (when still expanding from a previous compression). So if you have only 0.5s to go from 0 to 8m/s acceleration is way different than if you have 2s; max. speed is identical though.

Michael

 

[JTT]

DOOOHHH! sorry, I royally screwed up that calc! That's what I get for trying to do several things at once...oh, well.

I see your point though...so, if piston is accelerating to speed over a relatively long period of time, then I would imagine the low speed portion of the stack being effected in the initial travel, with the HSC only coming in once piston attains a given speed?

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JTT
Logic Over Hype Coalition

 

[drehwurm]

Servus JTT,

Well, I guess this doesn't lead to anything. Somebody in another thread mentioned a book about the more scientific aspects of suspension (maybe even for cars) - anybody know the title, author or ISSN number? Back then I didn't think I'll ever go that far

Sorry, I tend to forget what 'search' means:
http://dirtrider.net/ubb2/Forum24/HTML/001067.html

Michael



[This message has been edited by drehwurm (edited 02-28-2001).]

 

[marcusgunby]

This thread has got me thinking do we need a base valve at all-the only reason i can think of is maybe the cartidge size is small compared to a shock body so we need 2 pistons to get the required amount of damping.If we didnt have a base valve the crud would go to the bottom and do less harm-no shims to stick in-not really an advantage but im not sure where my brain is going at this point.Im thinking of fork speeds being alot higher as no linkage and smaller cartidges and more volume is what i suspect but im not sure how much volume an active fork piston sees.I suspect its similar overall because although the fork volume is higher than a shock ,the midvalve passes through a percentage of the total volume.So maybe its even.Then the cartridge is smaller but it travels 2* as fast.I wonder if this is why some fork valve stacks are just like mini shock valve stacks(in shape)-or am i talking out of my uranus??

 

[JTT]

...is it possible that the midvalve does work in the same manner as a shock piston?... but because of the greater speeds it is exposed to, it uses the "spring seat", acts as a sort of relief valve? Also with the smaller orifices, the shims resistance to bending must be much lighter...if they must be lighter, then the base valve would have to take up "the slack"? Could this be why the base valve exists?

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JTT
Logic Over Hype Coalition

 

[MIKE]

I have been reading along for some time now on these valving threads, and am learning and learning... I wonder if the base valve is there to give you the ability for some external adjustment on compression. Otherwise I can't see any reason the midvalve wouldn't do all we need as long as it had the ability to flow the volume of oil.
...Just a thought.

MIKE 2000 KTM 300MXC eastern wash.

 

[JTT]

Mike, the external adjustment (clicker) is actually a needle and seat arrangement that effects bleed through an orifice (that is located in same area as base valve). The actual base valve is a shim stack in itself.

But basically that was my line of thinking...by changing base valve shims, you could make more accurate or fine tuning adjustments. The midvalve shims are very light and few in number, as you can see from my original post, and any changes to them, I would think, would make a fairly large change to overall characteristics...but I'm only guessing.

The more heads the better...maybe

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JTT
Logic Over Hype Coalition

 

[marcusgunby]

Im thinking the base valve is useful to adjust comp clicks like Mike said-didnt think of that.Also i think the midvalve could have been used to work in the same range as the base valve but what would be gained?The engineers must have throught to have another stack that operates on a higher speed range than the base valve so thats why its spring loaded-so it operates at a different speed range.Im thinking the order of damping ranges are as follows:
1 bleed circuit with adjustable needle
2 initial shim stack(LSC)
3 secondary shims on base valve(HSC)
4 midvalve-this must overlap with HSC on base valve.
3 and 4 should probably be reversed?
All the damping curves probably overlap but we have 4 different areas to tune the way we need to.
Am i close Jer??

 

[JTT]

Marcus, I would think that (4) midvalve would operate at nearly all ranges, just as in a shock body, only at the extreme slow end of range it would have little effect due to the spring arrangement allowing shims to lift and allow "almost" free flow (like a relief valve)...and perhaps at extreme HS when (very light) shims are fully flexed?

 

[drehwurm]

Servus,

Another thought: How much oil actually passes through the valves? If there would pass more oil through the base valve than is displaced by the rod where would the additional oil come from? If not, it would mean that actually only a very small amount of oil goes through the base valve, while the whole cartrige volume would pass through the midvalve on a full stroke. Could you say then, that you need the base valve to be stiffer by the the same percentage as the volumes differ to archive the same damping?

BTW, the update kit for the KTM forks also includes a new spring for the mid valve - so we not only need shims but also springs to tune the valves

Michael