mid-valve description

[tmoney]

Could someone draw me a picture (word or excel document) of the mid-valve and capture how the assembly works. I ride an 03 250f and am trying to plushen my bike for HS racing. I have a few shims moved from the BV but this isn't going to give me what I want. I just don't understand the mid-valve and how it works. A picture would help me out. Also, from reading various posts it sounds like the right step is to reduce float lift and focus on the tuning via the base valve. Correct? Thanks.

 

[marcusgunby]

If you go back to a few older theads mentioned in the YZ midvlave thread you should see some photos of a midvalve.

 

[Yoken]

Hi Tmoney,

Keep reading posts again and again and use (abuse!0 the search engine. Am sure most of us on this board started that way. There are some real cool posts going on at present too.

Personally, I started to understand the basics of forks and shocks when I opened the Kayabas of my old KX... Am not sure I can describe the whole thing but let me try this:

Inside each fork tube, you've got a cartridge. It is basically a cylinder, entirely filled with oil, sealed at one end by the base valve (BV) and at the other end by a bushing and the rebound rod. On Kayabas and WPs that I know, the end of the rod outside the cartridge is attached to the fork cap and holds the rebound adjustment screw. The other end of the rod that is inside the cartridge holds a piston, sealed on the cartridge wall. That piston holds on one side the rebound shim stack and on the other side the mid-valve stack (MV). On CR's twin chamber forks, I believe it is the opposite: the rod is attached at the bottom and the cartridge on the fork cap... but the basic principal is the same.

ON COMPRESSION STROKE
The rebound rod slides into the cartridge and oil is forced through the MV stack, generating some damping. Also, as the rod moves in, it displaces oil through the BV assembly, hence generating additional damping. Some oil is also able to by-pass the BV stack through a tiny orifice obstructed by a needle (that is the external compression adjuster).

ON REBOUND STROKE
The rebound rod slides out of the cartridge and oil is forced through the rebound valving stack of the piston (opposite the MV). Some oil is also able to by-pass the piston through its center via a tiny orifice obstructed by a needle (thats your rebound external adjuster).

As the rod slides out of the cartridge, the oil previously displaced on the combression stroke through the BV is sucked back in, through a kind of one-way valve, opposite the BV compression stack.

Note that on the compression stroke, some oil is also by-passing the MV stack through the rebound needle circuit.

...well its enough for now. For those who read through, please correct me if I am wrong! Thanks.

 

[tmoney]

Marcus, I found the pictures the other day, however, I can't seem to find them now via the search for mid-valve. I can't figureout this search tool on DR. Anyway the pictures I saw were not specific enough for me to figure out how the mid-valve is assembled. any way how do you disassemble the cartridge assembly rod to get to the mid-valve on the YZF. Thanks.

 

[bclapham]

the search function is a bit broke. just do see threads from the beggining and go to it, all you need to know is still there and by going through it all you will find stuff you neever knew that you needed to know!

 

[tmoney]

folks, thanks for the info. I'll search for the pictures. I would like to know how to get to the mid-valve once the cartridge is removed from the fork assembly. I assume that the mid-valve can be tuned similar to the BV using the same shim placement logic. I would also guess that since I just tried softening via the base valve and have not noticed much improvement that the mid-valve is the flow of least resistance (at least initially) but still too stiff for have a nice plush fork...

 

[tmoney]

Marcus, changing the mv to a check plate is the opposite logic from minimizing lift correct? Seems to me like it would take forever to figure out a good BV setting if the mid-valve is converted to a check plate. If you lessen lift on the MV my understanding that this stiffens up the dampening and therefore I would need to make some big changes to the BV...does this mean the fork would be pushing more fluid through the BV?

 

[bclapham]

look for a thread "drilling the peens" you need a drill press and a 6mm mill, o got a dremel one from wallmart. dont use a drill bit, you could pierce the cartridge. drill the peens until the edge just disappears DONT DRILL THROUGH THE CYLINDER, just into the peen. then you need to make a cylinder valve holder from a big socket (to fit the four turrets) mount the CV holder in the vice.

fire up the blow torch and heat the crap out of the CV area, a few well placed wacks with the ball pein to break the threadlock. then mount the cartride in the vice/tool and then use a screwdriver throught the bottom of the cartride to unthread it. back and forth motion.

expect to make big changes on the BV when reducing the lift. you will probably need to get rid of the 0.15 for 0.1's

good luck

 

[marcusgunby]

tmoney the check plate is very good for many tuners as they can use the same(or similar) settings to what they have tested and developed for years.

Its not the best way to alter forks but can improve them over the std forks out today.

The best way IMO is to have the lowest lift that type of fork can use and still be able to dial in the correct amount of stiffness on the base valve.
Some rough numbers for you
YZ midvalve with 32mm cartridge 0.5mm of lift
KX/CR midvlave 0.3mm of lift
RM/KTM midvalve 0.2mm of lift.
These are not totally proven but will help as a guide to a good starting point for testing.
With these low lifts you will need to go very soft on the base valves.

 

[JTT]

Originally posted by tmoney
...does this mean the fork would be pushing more fluid through the BV?

tmoney, the amount of fluid pushed through the BV is not going to change, no matter what you do to the midvalve. The fluid that is pushed through the BV is the volume displaced by the rod as it is pushed into the cartridge on compression, no more, no less.

My understanding of the midvalve float is that it effects what shaft speed the midvalve shims come into play. With lots of float, and low shaft speeds, the stack works like a check plate...the lift provides sufficient flow to allow the piston to move in the cartridge, as the speed increases the shims are forced to flex to permit the required fluid velocity. Now, if you close up that gap (less float), you are going to force the shims to flex or deflect at lesser speeds to permit the same velocity of fluid to pass them. Make any sense? :think:

All that said, I still have found, from my limited experience, that (at least on Showa TC) the midvalve changes are much more subtle than basevalve changes and kind of "fine tune" things, but won't make a stiff fork plush by themselves.

 

[tmoney]

thanks to everyone. I keep learning this bit by bit.

Marcus, conceptually why do I need to mess with the MV to achieve a plush fork? Why can't I just soften the BV ALOT. Why is it a good idea to reduce float and then focus on the BV?

Also, I noticed today that when I go full soft on the compression the front end really starts to dive under breaking. I can't stand this and really like the bit to sit up in the travel but still be plush. Does this have to do with the MV/BV or both? Thanks, again.

 

[russ17]

Tmoney I think if you read your last post again you have answered your own question with the question

 

[tmoney]

Russ, I guess I did....so reducing float will help keep the front end up in the stroke and allow me to adjust the HSC of the BV to get the bike plush over the sharp edge stuff. thats my next plan of attack. Thanks.