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Basic valving/shim stack question...

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Hello, I have read all 163 pages of Doggers revalve thread (crf450r forum sticky, great thread!), but I still have a few suspension questions that maybe someone could educate me on.

I have been riding for many years, but have always used the companies for revalves, sometimes happy, sometimes not so much. I can do basic fork servicing (seals, bushings, springs and oil), and I understand the basics of the shim stack valving, but I have never messed with shim stacks. Can someone explain these terms that I have seen used but don’t fully understand what they mean, where to use them, and what they do? Start with “float”, I see that .2mm seems to be the common number, but I don’t understand where it goes, how to adjust it, or how it affects the suspension action. The next term is “crossover”, I think it means when there is a larger diameter shim placed after a smaller diameter shim in an otherwise consistently tapered (larger to smaller diameter) stack. Again, I don’t know what effect this has on suspension action, or how one would know where and what to use (what larger diameter shim and how far down the stack), or even what you might be feeling in your suspension that would let you know you needed to do something like this. I also am not sure how one would know if it was the base stack or the mid stack that they should be adjusting (or both?). Like I said, I have always let (paid lots of $$$!!!) the suspension companies to do the shim stack work, and then I experiment with spring rates, oil volume, and clickers, but I would love to learn to experiment with valving to. I am currently riding and working with a 2007 RMZ450 with showa components.

Thanks to any responses, help or information. And a big thanks to Dogger for his help in his thread, and all those with the suspension knowledge and experience that are willing to help and educate the rest of us!

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The float is what the gap in the midvalve stack is commonly referred to. I don't know too much about the functional aspect of it just yet but I can answer some stuff regarding it.

To adjust the float, shims are either added or removed to either tighten the gap (i.e. 0.10 or 0.15), or to open it up more (i.e. 0.30).

For showa forks, the float can be calculated as follows:

Collar length (usually about 2.20) - Piston recess (usually 1 mm) - total shim thickness

So, say we have (6) 20 x 0.10 and (4) 17 x 0.10...the total shim stack thickness is 1 mm

So assuming the collar length is 2.2 mm, then

Float = 2.20 - 1.00 mm - 1.00 mm = 0.20 Float.

The float can also be adjusted by using different length collars, such as a 1.8 mm collar.

So in this case you would likely have fewer shims..perhaps something like this:

(3) 20 x 0.10

(3) 17 x 0.10

So total shim stack thickness is 0.6 mm

Therefore,

Float = 1.8 mm - 1 mm - 0.6 mm = 0.20 Float

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Can someone explain these terms that I have seen used ...? Start with “float”, (and) “crossover”...?

Imagine a stack of shims against the face of a piston in a conventional manner. These are "clamped" to the piston, and the shims must be deflected (bent upward at the edges) in order for oil to pass. But now let's place the shim stack on a collar so that the piston is clamped to the collar, but the shim stack as a whole can move up or down a little bit without deflecting the shims, and then instead of clamping the stack mechanically, we hold it against the piston face with a light spring. That setup allows the oil to lift the valve stack against the spring to the extent it's allowed to without any resistance to a small amount of flow. That lift is called "float", and it's why a conventional mid valve is called that; they don't resist movement until middle and higher stroke speeds are reached. The float is adjusted by changing the amount of physical movement the valve stack has available, usually by shimming it.

For crossovers, go back to your mental picture of a conventional tapered shim stack again. Two things need to be understood: Damping valves are pressure regulating devices, and in suspension, "speed" refers to the speed of the compression or extension stroke.

Knowing that, as the pressure against the valve rises with speed, the first thing that happens is that the outer edges of the face shim lift away from the piston. As pressure continues to rise, the face shim must lift farther to allow more oil through, but the edge lift contacts the edge of the smaller shim behind it, which deflects until it contacts the next shim, and so on. So, the higher the stroke speed, the farther up the stack the deflection will take place. You can easily understand that more or thicker shims will make the whole stack stiffer, and that the reverse will make it softer. But what if we want it softer at first without giving up resistance at high speeds, or we want the initial softness of the low speed damping to last a little longer before it tightens up?

That's where a crossover comes in. Say that you have a tapered stack starting at 40mm and tapering to 12 in 2mm increments. If you place, say, a 16mm x .20 shim between the 34 and the 32, the 40 will lift until it contacts the 38, and so on, but only until the 34 begins to curl. Those 4 shims would then be able to lift .20mm beyond that point before contacting the 32 and resuming the normal progression up the stack as speed increases. The diameter of the crossover shim affects the flexibility of the stack between it and the piston, BTW, but I don't want to get too complicated here.

All that, then, separates the stack into two distinct sections, a low speed section consisting of the shims from the piston to the crossover, and the rest of the stack that doesn't become involved in the process until high speeds are reached.

That's basically it.

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Wow, great explanations, thanks for taking the time to explain it in such detail. It is slowly beginning to all make sense. It sounds like the float is only a factor on the mid valve, and the crossover basically converts a single stage stack to a dual stage? Thanks again.

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Wow, great explanations, thanks for taking the time to explain it in such detail. It is slowly beginning to all make sense. It sounds like the float is only a factor on the mid valve, and the crossover basically converts a single stage stack to a dual stage? Thanks again.

some illustrations...

http://www.supercross-online.de/Z/valving%20basics.htm

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Wow, great explanations, thanks for taking the time to explain it in such detail. It is slowly beginning to all make sense. It sounds like the float is only a factor on the mid valve, and the crossover basically converts a single stage stack to a dual stage? Thanks again.
That's fairly accurate, yes. Float can be built into any valve with enough tinkering, but it's typically only used in the mid, which again, is how it got to be named that.

Some tuners, me for instance, if I can call myself that, use a mid valve that has no float, which makes it what's commonly called a "clamped mid valve", and because of that, it's no longer really a mid speed and up valve any more, but active throughout the range.

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I just wanted to say thanks again for the great responses and info. kawamaha, the link you gave for the visuals was also a big helping with mentally grasping what is going on inside a fork or shock. Thanks again everybody!

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That's fairly accurate, yes. Float can be built into any valve with enough tinkering, but it's typically only used in the mid, which again, is how it got to be named that.

Some tuners, me for instance, if I can call myself that, use a mid valve that has no float, which makes it what's commonly called a "clamped mid valve", and because of that, it's no longer really a mid speed and up valve any more, but active throughout the range.

It worth noting that the dreaded "mid stroke harshness" comes from slamming into that mid valve stack in certain circumstance. Tuning out that mid stroke whack can be done a couple different ways.

Clamping the mid is one way for sure. Its always active so youre tuning the entire thing with the clickers, the mid stack and the base valve stack too.

I have begun to believe the OEM's know that for 80% of the guys out there that are weekend warriors the stuff is "good enough". They set the OEM stuff up with a middle of the road float that isnt going to work that great on anything, but is relatively safe, albeit not very comfortable.

Clamping the mid can aid in ease of tuning too...and could possibly benefit from a three stage base stack. The first being ultra low speed...basically a bleed but with a full face shim. Then a more normal low speed stack another cross and the high speed.

There are so many different way to go about this. Its truly endless.

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I thought the float was there to try to alleviate some harshness, allowing some oil past the shims without requiring the shims to deflect for the first .2 mm or so. How would clamping the mid valve stack (or any stack), and effectively forcing the oil to deflect the shims from the start help with mid stroke harshness?

Thanks again for the reply's.

Edited by rbreak
added info

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As one person pointed out already, the downside of a floated mid valve is that they can produce a "kick" as the float runs out and the stack starts to yield. Harshness is usually the product of a stack that doesn't open up gradually as pressure increases, causing pressures to spike a bit just prior to engaging the stack in the process. Clamping the mid and tuning it as another element of the overall compression damping across the whole speed range involves the valve stack in the damping much earlier, and takes the mid speed kick out of the picture. The setup I use has a "blow off" function (pressure limiting) built in to allow it to respond to very deep high speed hits.

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Ahhh, that makes sense. I can see where the initial plushness from the float would give way to a spike in flow resistance when the float gap ran out and the shims have to start flexing.

Thanks again.

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I would think you have to run a very soft stack or have a blow off to run zero float

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Please explain what a "blow off" is and how it works. Is it a bleed port of some sort? Thanks.

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it is where the midvalve is held closed by a big spring , when pressure build high enough the shims can't flow enough oil to stop the pressure increase , so the spring gives and allows the whole stack to move away from the piston face , it's the opposite to float in a way , float is almost free movement of shims then hitting a stop , blow off is shims pressed against the piston and then it blows open after the shims can't flow enough

what ever anyone says on here a mid is infinitly tuneable to give almost any damping you like , it's a very clever little thing

a rmz 2009 for example runs a very soft mid with 0.15 mm of float yet has zero harshness , it's actually really plush , so plush it needs the base stiffer than any crf stack to make it traceable for a aggressive rider , if a mid is harsh is because it's not tuned well, the float is almost not that important as long as you tune it for that amount of float

I have ran floats from 0.1mm to 0.35mm and when well tuned none were harsh IMO

it's all about balancing the float mid and base stacks and you can do that 100s of ways and all can work virtually as well as another , after 10 years playing with tc forks I have yet to conclude one float is actually better than another from 0 to 0.35 mm

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Thank you for the explanation. Yet another adjustment capability. Spring rates, oil weights, oil volume, base valves, mid valves, shim stacks, shim diameter, shim thickness, floats, blow offs, clamps, crossovers, bleeds, and probably a lot more that I don't know about. Wow.

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I'm new at this but is the midvalve then the shims directly under the nut and cupped washer held down by a very light spring on the opposite side of the piston from the base valve stack?

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I'm new at this but is the midvalve then the shims directly under the nut and cupped washer held down by a very light spring on the opposite side of the piston from the base valve stack?

No, the mid valve is on the compression side of the cartridge rod piston, opposite side from the nut.

The device you described above is a simple check plate that allows oil to flown back from the base compression valve without resistance.

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