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-   -   Understanding Midvalve (http://www.dirtbikeworld.net/forum/showthread.php?t=33585)

dwb79 16-07-2007 10:20 AM

Understanding Midvalve
Hey all,

The following all relates to my 48mm WP forks.

I'm trying to get an understanding of the relationship between the midvalve and the basevalve. What are the pro's and con's of running a tapered stack midvalve in a KTM?

Correct me if I'm wrong but I understand that the midvalve flows/controls the oil into the chamber behide the midvalve in the cartridge during compression. The basevalve flows/controls the oil displaced by the cartirdge rod, which is a much smaller amount of oil then the midvalve sees.

If for instance the basevalve was sealed, as in no oil flow, then the fork would not compress. If the midvalve was sealed the fork would still compress and could be controlled by the basevalve although the rebound circuit would cavitate and be rendered useless.

Where does the balance lie between functioning base and midvalves? How firm can a midvalve get before it starts to cavitate inside the cartridge?

In a standard KTM setup the midvalve doesn't seem to do much in the way of damping therfore relies heavily on the basevalve to control compression.

When trying to tune a fork what is more important shim configuration or float? Should you set the float, then leave it and work out a shim stack? Or setup a stack and then play with the float?


walshy 16-07-2007 10:44 AM

Struth! ^tng

Terry Hay 18-07-2007 07:50 PM

Oil must flow behind the midvalve as a priority function. The midvalve must always present less pressure than the base valve in order to control cavitation. Whether you choose to use a tapered stack or a straight stack would depend on the function you require from the valve itself. A tapered stack that deflected the same distance as a straight stack would present greater progression. The relationship between float and shimstack stiffness is paramount. Low floats generally require a more supple shimstack that is allowed to flex over a greater distance. High float MVs can utilise stiffer stacks with less chance of cavitation. The relationship between float and shimstack stiffness is one of transition. The greater the amount of float the later the shimstack is called into play and vice versa. Of course you can throw spring stiffness into the equation and you now have a whole new set of parameters.

walshy 18-07-2007 10:07 PM


mickv 18-07-2007 10:14 PM


Originally Posted by walshy (Post 429613)

yeah struth alright

dwb79 19-07-2007 09:01 PM

More Questions

Originally Posted by Terry Hay (Post 429523)
Of course you can throw spring stiffness into the equation and you now have a whole new set of parameters.

While I'm letting the rest of the information filter into my head, do you change the midvalve spring often? What applications?

At the suspension night you gave in Brissie you asked for more technical questions on this forum, so here goes!

Terry, what methods do you use for destaking (for what of a better word) the stakes on the compression holder on the Base Valve and the rebound tap? Do you simply loctite the nut back on or do you restake?

A couple of years ago I had a very strong reccomendation to get my suspension tuned by someone else. At first I thought I was very happy, I thought that was as good as it gets. After a year of riding this setup, I was stripping my forks down to replace the seals, upon removing the basevalve I noticed that there was a piece of swarf against one of the pistons holding the shims open. Knowing this was not right I removed this piece of metal to allow the shims to seal back onto the piston.

Of course doing this has stiffened up the stroke and I have lost the plushness that I had and liked. This leads me to ask about bleed shims against the face of the piston. Do you consider this a good tuning technique? Do you use them and in what situation?

I now know that there is much much better suspension around and would not use this clown again. I have ridden a few bikes that you have tuned and have been impressed. This has lead me to try and improve my bikes handling. After trying a tapered stack midvalve in my bike with a lighter basevalve, I can say that it is far better than what the other clown had done which was only reshim the BV.

The questions will continue!!!!!

threadersmal 19-07-2007 09:08 PM


Originally Posted by walshy (Post 428004)
Struth! ^tng

Ditto ^BGHD

ktm400 19-07-2007 09:11 PM

strooth !!

threadersmal 19-07-2007 09:15 PM

obviously a Thinker :D

Suspension does my head in ^BGHD

Terry Hay 19-07-2007 10:22 PM

Wherever possible I prefer to replace the original nut with a nylock unit. Otherwise I use loctite and torque correctly.
I often use bleed shims when the need arises. The beauty of the bleed shim is that it negates any breakaway force required to move the shimstack away from the piston face, making the transition from adjusting circuit to shimstack hardly noticable. Unfortunately it reduces the effectiveness from the midvalve meaning the lion's share of height control is now handled by the main spring.
The midvalve spring is a tuning variable that can be utilised to great effect, but very difficult to quantify. I prefer to use the spring merely as a way to reseal the valve and simply rely on lift and shimstack configuration as tuning mediums.
Most tuners avoid the midvalve which is probably best. Unless you are aware of where the actual problems exist in a fork you can easily take a backward step. By simply adjusting the compression you can achieve a result that can be appreciated by the customer. Generally not the best result.....but one that can be noticed.

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