Modifying midvalves on a KYB(and others)

marcusgunby

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#1
Ive heard of this mod where you remove the float on the midvalve so it comes into play immediately-has anyone heard/tried this and what were the results,im not asking for shim suggestions just about the theory behind the mod.Also on a Ohlins fork i beleive they have no midvalve-just a checkplate like a 94 KX,if Race tech can disable a midvalve can we enable one in a Ohlins fork ie put a shim stack where the solid washer is now?I like some features on the ohlins fork like the large bush area that helps in slap down landings and it has a very adjustable bleed on the basevalve 3.75mm verses 2.5mm on a KYB.Also they are a very nice gold colour.:cool:
 

JTT

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#2
Marcus, interesting that you made this post... I just did a little experiment, with my Showa forks ('00 CR250) along similar lines. After recent discussions about midvalves and float, I decided to take a closer look.

I closed up the float gap, thinking that it would bring the midvalve into play sooner. What I expected was that the fork would likely get more "harsh", but have more "feel".

Well, to my surprise, some of the harshness actually went away...not all by any means, but it was better than with more float...or at least it seemed to be :cool:

My next experiment, I think, will be to open the float more than stock and see what happens.

Removing it completely...hummm? wouldn't that decrease the "overall flow" to a degree, I mean the only way to flow oil would be for the shims to be deflected...with float there is some "uncontrolled" flow, isn't there?
 

John Curea

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#3
This is a great topic for discussion, one of my favorite.......

Midvalve float is "speed related" , in other words, the more "float" ( the distance the midvale compression shim stack moves before it starts to deflect) a midvalve has the faster the fork must move before it is effective. In the same respect, the less float a midvalve has, it will start to be effective at a slower shaft speed.

Also, cartridge size must be taken into consideration, a larger cartridge , as on a late model Yamaha KYB which is 32mm has much more "swept volume" than on a late model Kawasaki KYB that has a 28mm cartridge. We can compare these two because they have the same piston rod size.
The midvalves in these forks are set up night and day different, the Yamaha has about 1.5mm float versus the KX which only has about .1mm float(actually it is more of a bleed, because there is a 13mm dia. x .1mm thick shim under the sealing shim). But they work very similar beacause of the different cartridge size, if you put a KX style midvalve(.1mm float) in a YZ fork, it would be very harsh. In the same respect, if you installed a YZ style midvalve (large float) in a KX fork, it would almost act as if you have merely a check plate.

JTT,
You noticed a better "feel" when you closed up the midvalve float, I would have to say that was because the midvalve started "working" at a slower shaft speed and didnt put all the load on the base valve.

About removing the midvalve compression shim stack and installing a check plate..........that would be like if you had a problem figuring out your new computer (900 mhz and 40 gig hard drive), to just go ahead and rip that modern technology out and install the OLD tried and proven 33mhz and a 5 1/4 floppy just because you understand how that works.....:eek:

Yes , you can install a midvalve shim stack in place of a check plate.......we do it on the DRZs' Showas with excellent results.........

Well, once Jeremy checks in, he'll elaborate a good bit more I'm sure, he gets a little "worked up" about the midvalve removal thing!!

Take Care, Vet
 

marcusgunby

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#4
If we removed the float on a YZ midvalve it would become harsh but what if we softened off the base valve stack to compensate.So the base valve does alot less work and the midvalve takes up the slack do to speak.:D
 

JTT

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#5
Also, cartridge size must be taken into consideration, a larger cartridge , as on a late model Yamaha KYB which is 32mm has much more "swept volume" than on a late model Kawasaki KYB that has a 28mm cartridge. We can compare these two because they have the same piston rod size.
The midvalves in these forks are set up night and day different, the Yamaha has about 1.5mm float versus the KX which only has about .1mm float(actually it is more of a bleed, because there is a 13mm dia. x .1mm thick shim under the sealing shim). But they work very similar beacause of the different cartridge size, if you put a KX style midvalve(.1mm float) in a YZ fork, it would be very harsh. In the same respect, if you installed a YZ style midvalve (large float) in a KX fork, it would almost act as if you have merely a check plate.
[/B]


So for clarity...a greater swept volume (ie: larger cartridge) will require greater float than a smaller cartridge?

OK, for two cartridges the same size (ie:same bike), closing the float will cause the midvalve to become "activated" or effective at a lessor speed, and make the base valve "work less". I guess that makes sense from my experiment, as some of the harshness went away, but overall there may have been slightly more compression overall, because I did not change the base valve to compensate?

Hope we don't get Jer "worked up" again.... :p we know how he gets about these things....:scream:
 
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#6
KXVET (or Jeremy),
what impact does damping rod size have on the midvalve? Directly or indirectly?
 

svi

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#7
Marcus,
As far as Iam aware Ohlins motocross forks do have mid-valves, only their road race forks have check valves.
 

marcusgunby

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#8
Its a shame i didnt split the catridge on the ones i was working on ,then i could talk with more authority.My comment was based on jer saying the ohlins was similar to a 94 vintage KYB.Jer would you be able to tell me if my thinking was off base?
 

John Curea

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#9
Bud-Man,
Piston rod size impacts the midvalve and base valve. A larger diameter piston rod will cause the midvalve to have less swept volume than a smaller one. The opposite will occur to the base valve, as a larger piston rod will displace MORE fluid through the base valve.

JTT
Yes, a larger cartridge (or for that matter a smaller piston rod) will have have more swept volume of fluid past the midvalve.

Even with different size cartridges a 12mm piston rod will displace the same amount of fluid through the base valve. Its the ratio of swept volume to base valve volume that changes.

Example- A late model YZ with a 32mm cartridge and a 12mm piston rod has a swept volume( amount of fluid through the midvalve) of about 6 times that of the amount of fluid that flows through the base valve.
Where as a late model KX with a 28mm cartridge and a 12mm piston rod has about a 4.5 to 1 ratio.
As you can see with the YZ, that has much more swept volume we have to have more float to get the similiar effect of the KX set up.

I hope this helps, it took me a while to grasp the concept, but it really makes total sense. I will try and "rattle" Jer out of his roost to come in here and go into much more detail and give better examples.
 

JTT

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#10
"As you can see with the YZ, that has much more swept volume we have to have more float to get the similiar effect of the KX set up."

This is assuming that the piston ports are the same dimensions on the two pistons?...or are the ports on the larger piston also larger? They must be larger...for the greater swept volume...(sorry, thinking out loud)

The face shims on the larger piston must also be larger I assume...Wouldn't the ratio of the face shim diameter to the cartridge diameter also play a role...maybe I'm stating the obvious (I do not have a 32mm cartridge to look at right now)
 

John Curea

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#11
JTT
The swept volume would be the same reguardless of piston port size.
The "active" piston will move through a given body of fluid. The port size and shim build would only cause a "resistence" through the fluid.

Lets say a piston moves 1 inch in a cartridge full of fluid, the piston would flow through the "same amount" of fluid wether it had small or large ports.

Port size and shape would be a determining factor on the damping as would shim size and stack build.

Take Care, Vet
 

JTT

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#12
Originally posted by KXVET#207
JTT
Lets say a piston moves 1 inch in a cartridge full of fluid, the piston would flow through the "same amount" of fluid wether it had small or large ports.
I guess I didn't explain what I meant very well...sorry. I was intending to say that, for example 1" of 32mm catridge movement would move a different amount of fluid that say a 28mm catridge with the same 1" of travel. With that in mind the ports on the larger piston would have to be larger to accomodate the additional volume, all other things being equal.

What I was trying to get at with the shim diameter question is that difference in area between the largest shim and the inner walls of the cartridge (combined with the float) is the area that fluid can "free flow" before having to deflect the shims. I would think that, for instance a 23mm piston, with a 20mm shim would not be capable of "free flowing" as much fluid as a 32mm piston with a 29mm shim (again, with all other factors equal).

This may have nothing to do with nothing...just trying to get it straight in my head :think
 

John Curea

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#13
JTT
This was a "bugger" for me also......

" I was intending to say that, for example 1" of 32mm catridge movement would move a different amount of fluid that say a 28mm catridge with the same 1" of travel. "
"

The size of the cartridge doesnt have any impact on how much fluid is "displaced" through the base valve

Piston rod size determines this factor,
As the piston rod is moving into the cartridge the amount of fluid being displaced through to the base valve is exactly proportionate to how much of the piston rod is in the cartridge at any given time.

The midvalve piston merely flows through the fluid, it doesnt actually push the fluid itself. Try to picture a midvalve piston in a cartridge that doesnt have a piston rod attached to it, and lets say that an imaginary force moves the piston from one end of the cartridge to the other, the piston would flow through the cartridge , not pushing or displacing any fluid.

I've been up for about 30 hours now :confused:
Let me know if I am reading your question corectly.
 

John Curea

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#14
JTT
This was a "bugger" for me also......;)

" I was intending to say that, for example 1" of 32mm catridge movement would move a different amount of fluid that say a 28mm catridge with the same 1" of travel. "
"

The size of the cartridge doesnt have any impact on how much fluid is "displaced" through to the base valve from the cartridge.

Piston rod size determines this factor,
As the piston rod is moving into the cartridge the amount of fluid being displaced through to the base valve is exactly proportionate to how much of the piston rod is in the cartridge at any given time.

The midvalve piston merely flows through the fluid, it doesnt actually push the fluid itself. Try to picture a midvalve piston in a cartridge that doesnt have a piston rod attached to it, and lets say that an imaginary force moves the piston from one end of the cartridge to the other, the piston would flow through the cartridge , not pushing or displacing any fluid.

I've been up for about 30 hours now :confused:
Let me know if I am reading your question corectly.

Vet
 

JTT

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#15
LOL I know what you feel like, my 14 month old had a bad night last night...yawn...

I understand the idea of the oil displaced by the rod, I was referring to the midvalve itself, not the base valve. For clarity sake, let's forget its even there, for the time being (at least for my question).

Thanks,