Valve stack


The complete upper stage totals out to 2.3mm, or .090 of an inch.

Approximately, the amount of room for this stack to defect from the piston is about .75mm or .029 of an inch before it will make contact with those three 18mm shims. That, to me, is not enough for that shim stack to present a progression damping rate, let along flow enough oil for the cycle.

I would buy this story if you were telling me that there's more responsibility for a progressive oil flow taking place up at the mid valve.

In addition to this, when I stack three .5mm thick shims on top of each other, there are stiff, but they don't seem so stiff as to not deflect with a force equal to that of when a fork collapses at the rate that it does.

And if that last 24 mm shim is not backed by the valve assembly, meaning it too can deflect, I have to continue to defend that there are three stages of movement in this stack.

Lastly, I can almost buy the anti-dive chopped throttle thing, but you're going to have to describe a very special base valve assembly (with a bleed circuit exclusive to the rate that which oil would flow when the bike dives from such an action) before I get confident enough to send user Tex off to remove shims from the upper stack.

But, perhaps I'm just embarrassing myself.



As the old saying goes....a pic is worth an hour of me trying to type!! :)

Anyways...on the subject of shim deflection, first we must ask, what determines how much fluid passes through the base valve shim stack?

The cartridge rod entering the cartridge displaces fluid. The YZ has a 32mm(I.D.) cartridge, when the fork is fully extended, the cartridge is full of oil. AS the fork starts to compress the cartridge rod (or sometimes called the piston rod)starts to enter the cartridge which displaces oil, forcing it in to three different paths:

1. Base valve

2. Bleed circuit (clickers)

3. Cylinder valve (there is an inward deflecting shim stack inside the C.V.)(this is a "blow off" stack, designed to dampen out high speed hits).

The total amount of fluid displaced equals the total area of the piston rod that has entered the cartridge. The YZ KYBs use a 12mm piston rod. On complete compression of the fork, there is only a few ounces of fluid that actually passes through the base valve.

The midvalve piston doesnt actually "push" any fluid, it merely flows through the fluid. This is what we refer to as "swept volume". A misconception is that the midvalve piston actually "pushes" fluid. If this actually happened, what would be behind the piston? A vacuum would be present because there is no way for the fluid to fill benind the piston. If we had a void of fluid behind the piston, what would happen on the rebound stroke? (remember, all of the rebound damping is done through the midvalve piston also). There would be no fluid flowing through the piston to create any damping (resistence).

Think about the shim stack build on the base valve versus the compression shim stack build on the midvalve. The midvalve shim stack is very soft compared to the base valve (taking into consideration the float and the shim stack build). Now, if the midvalve compression shim stack build was built stiffer (and eliminated the float) than the base valve shim stack, then the midvalve piston would actually "push" fluid. This would be result in no rebound damping.

Now, talking about shim stack deflection. After understanding that there is only a couple of ounces of fluid flowing through the base valve, we can understand that shim deflection is very minor. Actually, the larger the piston ports on the base valve, the less the shim stack needs to deflect to flow the fluid.

Again, I need a pic of a late model compression adjuster to explain better how the passive bleed shimstack deal works. Basically, the fluid that travel through the clicker (bypass circuit) has to deflect the 24.1 shims to permit fluid flow. It is a different design than that of a '00 model.

Take Care, John

I have been away for a while. I may be able to clear up the questions about the 01 stack. John is correct the compression bleed works like the other base valves in that the oil flows through the middle of the base valve and is adjusted with the clicker. After the clicker though, it feeds into a recessed area at the bottom of the base valve(usually it is just a hole that bleeds in behind the valve). This recessed area is covered with the last 24mm shim then the next shim to create a "reversed" shim stack for the compression bleed. These shims flex upwards opposite the direction that base valve shims flex. The .5mm thick shims do not flex and act as clamping shims and spacers. The previous and new base valves have done away with this system. Also my friends 01 250F does not have it either.

John, do you know why?

Dave, Let me know if you want to get together to ride some time. I will be riding this weekend, but I may race at Marrysville Saturday night, not sure yet.


[ March 12, 2002: Message edited by: John Curea ]

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