Fork Fluid Question

By the way, I'm beginning to become a little suspicious as to what is causing the mid-valve to "wear-out".

I possibly could argue that the mid-valve itself is not the problem, but that it's just extremely sensitive to the quality-state of the oil. As is the seal head assembly. Mainly that when I have lost sizable amounts of compression, that fresh fork oil seemed to resolve it.

I've done the flip-the-shims approach as well, but never without new oil.

This has also contributed to my bias towards the longevity of certain lower grade oils.

Any additions to this theory would be helpful.


Okay – I’ll shut up now.

DaveJ, thanks for the feedback. Suspension Science is Chinese to me. You have a way of explaining things very clear to me regarding this post & your past "nirvana" post. Is there a way to strengthen or beef up the midvalve w/o affecting oil flow performance in a negative manner? Would a "strengthen plate" work ie..larger or thicker shim? Also, who do you recommend for bottoming cones?

DaveJ, on the quality of fresh oil issue. I had just changed fork oil less than 2 mo. ago. After completely cleaning tubes, all internals & removing Compression valve to clean, I tried Silkolene 5w for a change (probably never again). I have about 6 rides, including 2 races on these forks. Would heat affect oil flow - as temp. here go from 70's in am to 90's in afternoon?


i have the bottoming cones in my forks. they are SUPER!!! my friend john (emaracing) had them in his forks. he loved them as well. they really do allow you to control the bike as it bottoms. worth every penny in my book.

scott f,

i don't worry about fluid. my '98 had the forks done by Factory Connection in july of '99. in july of '00 i sent them back. they wanted to know if i had even ridden the bike. yes, it had 18 races (all harescrambles) on them. they were shocked at how clean the fork oil was.

fixing the mid-valve can be done by any of the 3 guys i mentioned above as well as putting in the cones.

DaveJ - another quest. I see racetech & others have a midvalve replacement part to change to a checkvalve. What are your thoughts of this. Just remove it all together,replace with checkvalve....

The bottoming cones are worth every penny, and they are easy to install if/when you go in deep enough to fix the midvalve.

When I say midvalve, I am talking about the comp shims on top of the rebound piston. These shims flow a lot of oil, and can be damaged or wear out. They can be removed/disabled, which will give a lot of plushness for off road riding, but for jumping it is better to leave them in. These shims can be replaced, or the stack can be changed for more or less stiffness.

Steve, as long as the oil is clean, that's the main thing. We've all seen smelly gray oil come out of poorly maintained suspension. That stuff is like lapping compound. Fortunately, our modern suspension has anodizing on all aluminum parts, so oil stays cleaner much longer.

Ga426, heat and viscosity breakdown are not much of an issue in forks like it is in shocks. A high VI or viscosity index means that the viscosity is stable over a wide range of temperatures. Hi-rollers can use shock oil in forks for the highest VI.

Guys, who sells these bottoming cones???How much $???

You just read more good stuff from DaveJ.

As for the base valve, it flows about as much oil as the cartridge rod displaces when it plunges down into the cartridge. So, some oil gets pushed out of the cartridge through the base valve, and some flows through the midvalve.

Scott F if I understand correctly, you prefer keeping the "midvalve / top valve" in place, especially for Jumping.

DaveJ you prefer replacing it with a "check plate" type of device which makes the forks behave like older style 2-3yrs older style used to be w/o the midvalve.

Scott F -- MXTuner - Mark Klein wants to remove the top valve because he feels they are defective or relocate them farther down the rod. What are your thoughts on this? By the way - Mark told me to tell you hello.

Do either of you see any advantages to leaving this valve in place or removing it all together. This is where I get a little confused.

DaveS - You're spot on. And yes, the restrictor plate makes up (or blocks) the additional flow into or around the seal head. That additional flow of course is caused by removing the mid-valve.

So everyone should now conclude two things from all of this.

One, every manufacture is only designing their own little piece of what you take and make into the big picture.

Secondly, as ScottF has implied, there is more than one right way of doing something "right". His methods work wonders, and so do many others.

The bottom line, if you're a mad man SuperCrosser jumping machine (need I mention names), then I would stick with a version of a mid-valve installed mod that works for you. Bottoming cones a must have.

Crazy trail riders that love to hit a few good jumps and fly high, pull the mid-valve and replace with a check plate. Bottoming cones a must have.

Those traveling at a "let's just have fun speed", leave the bike alone and save yourself the headache.

If you like everything but hate the hoarseness, need I mention ultra-adjusters (that allow external adjustment of high and low speed compression.) See C-Cycles again. Not a bad quick fix for a few bills.


Dave S, you're getting warm! But I think you are confusing the seal head and the rebound piston. The seal head is the top cap of the cartridge tube. It is screwed in, locktited, and dimpled so as to never come apart. However, it is possible to break into that chamber, but it requires careful technique. You have to take apart the cartridge to work on the midvalve or rebound piston.

'Removing the midvalve' means replacing the shims with a check plate.

AFAIK, most or all of the oil that leaves the cartridge goes out through the base valve. I don't think the seal head flows much oil, and when you install bottoming cones, it leaks little to none. I would say they leak more under rebound than compression because there would be more pressure then.

As for the base valve, it flows about as much oil as the cartridge rod displaces when it plunges down into the cartridge. So, some oil gets pushed out of the cartridge through the base valve, and some flows through the midvalve.

I'm not claiming to know everything about suspension, I only know what I have done, and I learn more as I do more. These are only my opinions based on my experience.

Sez Dave: (I sure it would be much more clear if I could just see inside one!)

So tear your forks apart! :)

DaveJ summed it up nicely and I agree with his recommendations.


You are right, I was thinking that the seal head was the rebound piston. That helps alot. Thanks.

Suddenly your statement 'the base valve, it flows about as much oil as the cartridge rod displaces' makes obvious sense. since the top of the cartridge is sealed (seal head! :) ) the only oil that would want to leave the tube would be that that is displaced by the rod. Unless... Is there also a shim stack/valve under the seal head that allows oil into the tube?

I'm sure I will open the compression tubes some day soon but whenever spare time comes my way I seem to be ridin' more than wrenchin' !

BTW, Thanks for the info/replies it is ver helpful!

Dave S

I dismantled a sealhead once to check out the inner workings, and I did see some shims in there, but I don't recall what the function or oil path looked like. I don't remember if the oil can bypass the seal through the shims.

Well, it's sounds like the fork-chat thread is coming to a close.

What should we talk about now?

Any gossip out there?

It ain't over yet. :) After further thought, I remember that the seal head does flow some oil, I just don't know how much. I do know that if the shims in the seal head become damaged or fatigued, it will flow way too much oil, causing a lack of damping and a wrecked fork. The only fix is the Race Tech "block off plate" or the bottoming cones. Since the bottoming cones are "solid", the only leakage path is past the bushing, which would be negligible.

Sometimes these seal head shims get mistaken for the midvalve shim, as in "blown midvalve", but "blown seal head" would sound more appropriate. Neither one is a good thing.

Sound right Dave?

DaveJ,Dave S, Scott F --- thanks for the lession in YZF Fork Physics. You have enlightened several facts that I never considered for setting up these forks, as well as realistic "internal specific tuning" ideas, thanks. I believe the "magazines definition of Blown MidValve" is far too vague & possibly wrong. Now it is time to rebuild & test.


Not so fast, I'm a slow learner :)

I know you have been all the way through these forks. Is there a shim stack on the bottom of the seal head? It seems that if there was, it would have a significant role in balancing the flow through the rebound piston/mid-valve path and the base valve path during compression. Restricted oil flow into the cartridge tube from the seal head (top) would force more oil through the mid-valve because if oil cant come in above the rebound piston, it cannot displace oil out the bottom. Is the seal head merely a check valve or does it also have valving shims?

I guess this is a good a time as any to admit my stupidity... I have been checking the oil level with the compression rod extended. I just looked in the manual and it says to check in with the rod fully compressed. I wonder how much extra oil I have been running! That explains why they are so much stiffer than all my friends bikes! DOH!

Dave S

[This message has been edited by Dave S (edited August 31, 2001).]

Ok, maybe a little light is going on now.

BTW, this is a question, not an answer... :)

The base valve is the 'thingie' that you remove from the bottom of the fork to get the cartridge out, right? It of course has a valve stack that controls compression damping and a clicker for some adjustability of that damping.

The midvalve is inside the cartridge tube and ALSO has a valve stack that controls compression damping. So these valves work together (in parallel for those electrically minded types) to provide the total compression damping.

'Removing the midvalve' means to take the shim stack off the midvalve (seal head?) so that oil flows past it freely? or at least as freely as the valve design allows it to. Soft of like a calibrated leak. The check plate is required because the seal head is also the rebound damper and leaving the holes open would obviously allow oil to bypass the rebound stack and render it useless.

Is this even close to being right?

If it is, I would think the ports (holes) in the seal head would need to be pretty restrictive so as to allow for some control of compression damping at the base valve. What has me wondering if any of this is correct is that if someone is complaining of a 'blown midvalve' I am thinking that the midvalve is not doing its job of regulating oil flow therefor all the oil is going freely through the midvalve at the seal head and very little/none is forced out the base valve. This makes me think that removing the midvalve would never allow adaquate compressions damping because all the oil will simply go freely through the seal head and the base valve cant to its job.

What am I missing. Is a restrictor plate required to limit the oil flow by the seal head and I would guess the stack on the base valve NEEDS to be adjusted to compensate for the differnce in its 'compression damping partner', the seal head/mid-valve.

(I sure it would be much more clear if I could just see inside one!)

Dave S

Yes, I prefer to leave it in for jumping large jumps. You need all the compression damping you can get when you squash it like you did it that photo. It's hard to describe these things in great detail unless you are quite familiar with how it looks, works and goes together. I don't know what is meant by "relocate" the shims, but you can move the "open" position slightly closer or farther from the piston surface by changing the shim stack height or the collar height. The midvalve works like both a check valve and a shim stack. The shims are backed up by a spring, like a check valve. So when there is positive pressure on the bottom of the shims, they pop open a small distance, say .100", then they flex farther depending on the pressure.

When Mark says remove it, I think he means convert it to a check plate. I would do this for plushness, not bottoming resistance or jumpability. Mark knows what he is doing. He may have some setting that will work great for you. You could turn him loose on it, and if you don't like the result, you'll know which way to go from there. If you do disable the midvalve, the bottoming cones would be even more valuable/important for jumping. The cones also allow you to run a lower oil height for a smoother more linear stroke, while still providing bottoming protection with a big "slowdown" the last two inches.

Watch him do it if you can. It will be a good learning experience.

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