Cylinder Valves

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#1
How about a different topic for discussion. The cylinder valves have been with us since 98 when they were first introduced by Yamaha in their 46mm USD fork. They have been tried and abandoned by both Honda and Kawasaki yet still Yamaha persist with them. They allow oil to bleed out of the cartridge in a rather uncontrolled manner on both rebound and compression. This has resulted in less oil being pushed through both the rebound and the base valve. In order to compensate for the lower oil volume being pushed though the valves Yamaha has increased the overall stifness of the shimstack dramatically. This has resulted in a rather harsh feel, yet, on severe hits the fork still bottoms easily. In 04 model bikes Yamaha has dropped the elastomer bottom out in favour of the more conventional hydraulic unit. Was the elastomer system to blame? Or are Yamaha simply trying to save face for using the cylinder valve in the first place? To me the fault was not so much with the bottom out system. The fault was in the fact that the fork reached the bottom of the stroke in the first place. I mean if a car runs off the road and crashes through a brick wall. Do we blame the wall for being poorly constructed? To me I see little value in the cylinder valve and prefer to replace it completely. RaceTech have a cylinder valve seal which simply stops oil from passing through the valve. These seals worked fine when yamaha were using mild steel damper rods. But since the introduction of alloy rods in 2000 the added seal drag produced excessive stiction and we had a whole new range of problems to deal with. Here in Australia we have a cylinder valve replacement kit which consists of a billet alloy piece in the same shape and dimensions as the original unit. The difference is it contains only a simple shaft bushing, the same as other fork designs. This kit allows the fork to work as any other fork. The shimstack can be valved much lighter for a plusher, more controlled ride. Stiction is reduced and bottoming resistance is dramatically increased. I have yet to be convinced of the positives of the cylinder valve but I am willing to listen. Anyone...?
 
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#2
I'm really looking forward to you sticking this out, Terry. We NEED to hear the LOGIC behind RT's way of doing things......I haven't had any Yamaha forks apart so no info coming from me......I'm here to learn.

It's my understanding that the cyl-valve helps to reduce cavitation at the rebound/mid-valve and promote cartridge bleeding........
 
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#3
In 04 it seems that alls KYB did is find away to press fit the bottoming cones and still use the CV from what I have seen, and this is there way of contolling the bottoming issue.
I believe I read somewhere the initial concept of this was to eliminate air
pressure in the fork cartraidge. (I might be wrong on this though) as it turned out to be another passive valving so to speak and consisted of high speed
linier shims that float a touch allowing bleed at low speed. The pressure in the fork is suppose to seat the valve which intern prevents leakage.
I feel that this bleed ( which is never consistant ) is what brings the inconsistant feel in the forks.weather its a worn bushing or some sort of contamination that causes this

Russ
 
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#4
Terry
Welcome! I am very interested to hear the different views in suspension theory and products.

Russ
 

Jeremy Wilkey

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#5
I think we have several issues at stake hear...
We can clearly all see most of the short coming and issues with the design, but in many ways I think they are somewhat over played upon.. That being said I'm going to logic this out as best I can give me a few paragraphs to weave it together...

When cartridge size was smaller or equal too say 25mm, and often used a circultaing design meaning oil went out the rod at the beginning of compression cycle the check valve closed and then freely on rebound we had an opportunity to evacuate trapped air in the course of normal fork action. Now this in and of it self was a good future, but as cartridge size increased at the same time midvalves where implemented for obvious reasons. Someone realized that the midvalve could do more if it given more oil flow.. So cylinder diameters grew from 25 to 28 and then 2 problems slapped the engineers in the face...

First some more background.. No CV, or oil lock, or bushing or seal should ever seal perfectly.. It must leak in a non-circulating system or the fork would have to be filled from the bottom, it would only fill to the cylinder volume minus rod if it sealed firmly.. So whatever the system it is what I call a "virtual seal" or rather a buffer seal. Meaning it seals relative to the demands of the pressure waves inside the fork but not to slow currents of air or long term internal pressure.. One more tid bit.. If a fork has a seal that leaks slowly to air it wont leak relative to oil.. Its the difference in the the thickness of the fluid.. Emulsified (air oil mix) oil will travel through faster than "non-emulsified" oil.. ((That's very relative BTW)) No back to the feature presentation..

The engineering Quandary:
As cartridge diameter grows the amount of fluid volume in the cartridge increases rapidly with even small changes in diameter.. If we have 1-2mm Tall bubble at the top of cylinder that's not much for the rod to compress to a state of greater pressure than the valving and the system will work normally,(The trapped air has to be compressed to a point where its pressure is higher than that of the pressure of valving restricting oil) but as the diameter grows so does the rapidly increasing volume of air and since we have not changed rod diameter we are going to be using more and more of the stroke in an effort to build enough pressure where we can initiate proper fluid flow dynamics... Ok so simple why did we not continue with the circulating system..

Well the fork uses pressure inside the overall to exerted force on the fluid inside the fork.. Now what happens at the top of the stroke? We get decreasing pressure in the rebound direction and that means that the fork tends to undefiled the fork at the top because with out significant internal pressure little more than cohesive forces actually fills the thing. (this problem is still relevant BTW more on this latter) With the circulating system we dump a significant amount of oil (rebound clicker value) out the top of the fork where it is emulsified further and thats also means more oil has to be refiled into the cartridge. This is one reason why I tell people not bleed air out of there open cartridge forks every ride.. By running a small internal pressure your helping your fork work better (more consistently), and that's a small price to pay relevant to the small added air spring..

Enter the KYB 32mm cartridge. I currently believe that the fork has way to big a cylinder proportion to Midvale post diameter, unless you run it in super cross, where you can run no midvalve float.. But for out door riding the cartridge is two big to juggle float with initial contribution speed and overall contribution even with our "solid clamp Midvale" which pivots on a 11mm shim.. But at 32mm 1-2mm of trapped air almost equates to 1/2 of the total fork stoke.. Clearly unacceptable. And what's worse I've seen guys put some of these hydraulic stops in them with no leak and the forks have to be built upside down or they won't fill. If you notice the RT seals have a small little bleed.. for this reason...

Now the engineers being the cleaver fellows they are, figured up a way to make a "better leak" . The unofficial, official design protocol of the KYB fork is to create a fork that functions as good as a twin chamber with out being a twin chamber and as many of you have seen this is a interesting subject, with the donut fork, and many other cool ideas, presented as patents or searches found by our bright forum readership.. What does function as well as mean? With perfect efficiency, meaning no internal loss of efficiency due to the need for pressure rise, etc.. In simplest terms get the air "out the valing" in ghetto slang.... If you can do that in Supercross the Twin chamber is at a disadvantage because the cartridge size is smaller than the KYB, and the KYB can in theory make better low and midspeed compression damping with less high due to the ratio of pivot shim to outer diameter and of course more fluid flow.. Given more efficiency.. hum....

Here is more interesting design protocol.. If you categorize the circuits in the fork over a speed range form lowest to highest speeds here you go...

1.)CV Well think about it this thing bleeds oil when the fork sits still, most importantly it bleeds air very easily this allows the oil that comes out of suspension in the oil to escape through its thin gill like valves easily and quickly.. But offers significant restriction to oil by comparison.
2.)Bleed
3.)Midvalve
4.)Base..

That's what they want.... Now that's a little messed up I agree, but that's KYB's idea, the very lowest speeds are function of the peritectite between CV, and Bleed.. Weird.. But very interesting.. And the only thing I see as weird is that the CV needs to be more tunable if it is going to have more control than the base-valve at least in the range of lower speeds. Over the years the CV has gotten stiffer and that's a good thing.. With simple revalving the ranges can be reset and made to work much better. Bottoming? The bumpers where being tested long before the 32mm cartridge and they have a better low speed bottoming feel than the oil locks just not quite as good in high-speed Bottom.

I'm waiting for a true hybrid, it would be easy make the spring guide like the WP fork, only sliding on a bearing, and then have it bump into the oil lock, and the bumper, in low speed bottoms the the oil lock will go right through and give you the cushioning form the bummper , if its high speed you'll get cushion and then oil lock.. That would be cool....

Anyway back to this whole deal.. Bottoming control, and plushness while not as good as say the 28mm fork is quite good on a 32mm KYB with a midvalve that is configured properly..And its not harsh maybe not as good as a 28 but quite good, 2 years ago I thought it was much better but as midvalve technology has increased, namely figuring out how to run lower floats and smaller pivot diameters, the 32 has been surpassed.. I would be inclined to believe that for MX the 12.5/28 and 14/28 are very nice numbers. I'd like to try a 14/30 (with a CV) and in supercross the 12.5/32 is the bomb.

So what else is going on, Ross Maeda and I have a small disagreement over the CV, I also think it limits the high-speed characteristics of the rod and I believe this to be a very good thing as it gives a noticeable decrease in the steepness of the tail of the graph.. But since its not easily revalved we can't tune it as much as might be nice. I keep waiting for them to just make it a tunable valve like the ones in the factory forks.. But for those of you who wish you can rebuild the CV quite easily, and make it work nice again if its been contaminated, which is its big issue.. Just machine off the rolled edge press it all apart, and then clean and use a washer to fill in the space and hold it all together tight on reassembly..

In my view Technology is always the answer given time.. Its true that often what worked in the past will work now, yet given continued development, technology will always rise past the old way as long as its complexity is not traded in the guise of making things easier.. I'll like the complexity, if you can't already tell. :)
 

marcusgunby

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#6
Well i have my own little theory on CVs-i built up my 03 KX fork to have low lifts, soft mid and base valves.I didnt make the stacks any different to compensate for the CV-and it worked well.Very well infact.

Now the reason i think is because the stacks are soft enough to not promote oil passing past the cv in a way it might with a typical YZ250f build.
 

bclapham

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#7
its funny how all the mags rave about yz suspension, but i always find it spoils my day somewhat since they are always too stifff on rough outdoor MX, yet the rm125 showas are supposed to be overly soft yet with only a small tweak i can get them working well for my 220lbs, i know i am slow, but not that slow- the rm is cushy plush yet even with the wrong (light) springs its harder to bottom than the king of stiff fork yz i own.

so my question, if we replace the CV with an after market bushing setup, what would be considered as light valving? i have redone mine with a lift of 1mm and around 7-8 0.1mm shims on both the mid and base valves (stock base has a big lift but lots of 0.15s on the BV)?

and my next question, is why do the yz forks have sooooo much midvalve lift in the first place when we are constantly being told lower lifts are the key to suspension nirvana? is this tied in with the CV bleed issue and is this why i am not happy with the fork, do i return them to a larger lift/float????
 
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#8
Back in 98 when the cylinder valve was first introduced we found ourselves addressing the problems associated with the valve by reducing the base valve shimming to promote a plusher ride and increasing the stiffness of the midvalve to assist with bottoming. We found this worked rather effectively at the time. Problem was, midvalve shims and cylinder valve shims don't appear to have the greatest durability. Our original setup began to be less adequate as the shims began to fatigue. Obviously we could simply replace the shims on a regular basis to maintain consistancy but that would rely on having customers with a religious service ethic. Does such an animal exsist? We had to find a better way. Jeremy has clearly delved deeper into the concept behind the valve than I have. I guess we look at things differently. I tend to ask, "Does it work?" If not, why not, and how can we fix it. If it does work, how can I improve it. He clearly analises to a far greater degree. As for the cylinder valve itself, we have seen it stiffened up from the original example yet we still get the same complaints. Feels harsh. Bottoms easily! To answer bclapham; When you replace the valve you can lighten the shimstack dramatically. For instance, for a guy your size I could replace all the tapering shims with 0.1s and maintain only 3 of the 24x0.15s. This stack is is only indicative of the reduction in stiffness and by no means a recommendation.
 

bclapham

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#9
terry, i assume you are talking of swapping out the base valve 0.15's with 0.1s and then retaining the stock midvalve lift with that CV swap out? the only reason i say this is that my BV is already quite light on shims since ive gone to the lower midvalve of around 1.0mm.

if you have any photos of the replacement parts you can email them to me at bclapham@scripps.edu i will host them onto the thread just so every knows what we are talking about looks like!:)

thanks
 
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#10
bclapham
A typical YZ shimstack may have the following shims all at 0.15
11,12,14,16,18,20,22,24x5 (base valve)
You could replace 11 through to 22 with 0.1 and maintain 3 of the 24x0.15s.
You may even find this a little firm.
As I said also in the previous comment I find the midvalve shims tend to fatigue after a while. Surely by reducing the amount of lift you reduce the durability even further.
 
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#11
Sorry
I didn't say everything.
The cylinder valve replacement kit is an exact replica of the cylinder valve itself except with none of the internal workings. Just a Teflon Coated shaft bushing.
 

Jeremy Wilkey

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#12
BC,
Midvalve lift is high because volume of swept fluid is large.. Lift determines the speed at which the valve starts to contribute. If you had too little lift the fork would be too stiff intialy. The greater the lift the less it can contribute overall and the stiffer they must be made to impact valve area, this in turn causes issues with minor speed changes causing big changes in damping coefficient.. (or valve area)

Lift has no connection whatsoever to CV's.. CV's only rod volume and rebound never Active compression.. CV's do however add or subtract to the passive coef. and thereby do impact the overall..
At one speed consider the damping coefficient to be the sum of all individual contributors.. Passive+ Active+CV=compression Coefficient. However that is miss leading as really the Passive or CV (another form of it) could actually buffer the total at some speeds.. Think of it as two stiff springs in a series.. Not in parallel.. Two stiffs make a much softer spring that 1 stiffer spring.. Valving is the same way because we are reducing the volume of flow at a given speed thereby reducing the damping coefficient..

The trick to dealing with lifts is Pivot diameter.. Like I've said I have something very unique that in general allows me to reduce the high speed impact of the midvalve by an average of 66% thereby allowing me to run very "stiff" midspeed valving and very low lifts, yet still making it "plush". I only use Stiff as a absolute value of shims and there common description.. Not as a descriptor of damping coefficient.. Read more about assumptions below..

My friend Jeff Howe really was the first person I know to address the midvalve wear stuff directly.. The early perspective was each shim is bending too much and like a spring overstressed it will eventually break.. So we needed to distribute force, more equally, and we need to provide way to prevent over deflection without the the limiter preventing the valve from limiting area. (which would produce compression in the worst case at the largest deflections which means highest speeds) What we've discovered though as this notion is not correct inside the scope of our use.. The shims can control area beyond their maximum speed range yet still not be overstressed, so the real trick is designing a stop that provides that. In other words momentum bends them farther than there point of limitation. You must allow them to create sufficient area, yet not over deflect from momentum that will produce wear. (I've never seen a CV wear out only get contaminated never fatigued...) Its tricky in a big cart. like the Yam...

When I read a post I some times squirm in my chair and it reminds me of when I was teaching High school math and science..You guys are so close, yet you still don't see it.Your somewhat stuck by the limitations of your own testing and time, and by the problem that I only give theory and not answers..For someone who is bad a HAADD as me its hard. I've got a business to run and somehow have a life.. I could do DRN 24/7....... I should not expect you to, its just if you only knew.. You'll get it over time, I was arguing this stuff with people 4 years ago on this forum about what was in a cartridge. How it worked and being called closed minded radical for saying that a piston did not push oil in the fork etc etc.. I was a radical then, and still am but finally no one argues with me about that anymore.. So I guess progress is being made..:)

Anyway a couple more notions to think your way around.. Don't presume .15 means stiff, its just a shim stack, what else is working with it or against it.. .15's in a system that flows little oil could be soft relative to the fork with .1's and more flow, or less shims with more deflection can store more energy and be stiffer than .25's deflected a little.. Taking note of what Terry exsplained if you remove the CV you can ^1/3 the shims in the base valve, yet still have someting stiffer and potentially harsher. Beacuse we have doubled the fluid flow.. (Roughly and assumptive in nature.. I for one don't think the CV really decreases the rod charge by half, but you can see the point)

I think the first mistake we all make a tuners as to look at something and make conclusion without detailing the system, and determine what is going on...Add it all up and then make assumptions but not before..
If you look at the changes made to the yamaha fork in 02 they designed a midvalve psiton that would produce less deflection and thereby would lend itslef to smaller flaots etc. It was an improvment.

BR,
Jer
 
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#13
Jeremy
BCs bike is a 2001 which came out before the revamped midvalve pistons. I understand what you are saying about the improved durability. Yamaha increased the cross sectional area of the ports thereby distributing the load over a larger area, reducing deflection and and increasing longevity. Hmmmm, High flow, high efficiency pistons. Why hasn't someone thought of that before? I applaud your faith in technology over time but when a guy brings his bike in for a performance upgrade he generally wants results straight away. Given the problems associated with the CV from the start it was hard to see it as a giant leap forward. After 2000 when the YZs came out with an alloy damper rod and stiction became an issue. We began looking to get rid of the CV completely. Since then we have never looked back. Granted Yamaha may be closer to the mark and someday may perfect the system. But at least we stuck it out for three years which is two more than Kawasaki and one more than Honda.
 
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#14
Servus Terry,

High flow, high efficiency pistons
In my understanding a so called "high flow"/low restriction/low damping coefficient piston would put more stress on the shims. The overall damping created by the piston AND shimstack has to be the same as with a "low flow" piston setup, so if you let the piston do less work the shims would have to take more - correct?

What do you mean by "high efficieny piston"? I guess I'm having troubles understanding the bigger area/less deflection/same damping paradigm. Also, does "bigger area" automatically means larger ports or is just the contact area between fluid flow and shim that counts. So if you use small ports with a large outlet (like MX-Tech uses on its pistons), shim stress would be minimal in a given scenario?

I have to admit, this goes a little over my understanding of this matter, but it is nice to have such discussions back here. DRN/Suspension has been a rather quiet place lately. Welcome Terry and welcome back Jeremy!

Michael
 
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#15
I guess the easiest way to understand the "High Flow, Less Stress" concept is to revert back to the basics of hydraulics. Pressure over area equals force. If we look at the way a car jack works, we have a small piston which we apply pressure to via a lever. This piston displaces fluid which in turn acts upon another larger piston in the jack itself to create lift. The amount of lift that is created is proportionate to the surface area of the pistons. If the piston activated by the lever has a surface area one tenth that of the piston of the jack, then for every 10mm we move the lever operated piston we will get 1mm of lift at the jack. Also the velocity is reduced, slowing the acceleration of the jack piston to one tenth that of the plunger piston. If we were to reduce the size of the piston in the jack to one tenth the size of the plunger piston we would then see ten times the movement and ten times the velocity. Now think of the port area of the midvalve as the piston in the jack and the cross sectional area of the cartridge minus the area of the damper rod as the plunger piston. There are also some other factors at play within the cartridge but we are simplifying for the purpose of explaination. When it comes to the base valve, the same is true only this time the port area is only influenced by the cross sectional area of the damper rod itself. Once again simplified for explanation. So, high flow = low velocity, less deflection. Reduced flow = high velocity, more deflection. I guess I could have said it easier by using an example of a garden hose with no nozzle attached, flowing water. Now reduce the diameter by placing your thumb across most of the hole. Which one would bend a shim more?
Regards
Terry Hay