Marzocchi 45mm mid valve check plate

[stay_upright]

The midvalve check plate appears to allow a LOT of oil through. On my forks the plate can lift 2mm. That gives an area of about 120mm^2 past the plate.

By comparison the base valve has 3-4mm holes for the oil to flow so that's a maximum 50mm^2 (if holes are 4mm) but since it's shimmed it's probably a lot less.

Does the compression clicker adjust the flow through the hollow bolt that holds the shims? If so that's only about 3-4mm so about 12mm^2 max

So the base valve and the compression clickers seems to have very little influence on the compression damping unless I have got something wrong??

My personal experience would indicate the base valve does have some effect as greatly reducing the shim sack (similar to Berghag) does smooth things quite noticeably.

Further I cannot imagine marzocchi designed the fork with 100mm^2 of free bleed effectively and then made the range of adjustment on the compression clickers 0-12mm^2... though I've never noticed any difference moving the compression clickers.....

[stay_upright]

Bump...


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[nick790]

The way I understand it is that the mid valve is more a control valve rather than a restrictive like the comp valve.

It's the one area I haven't touched in my forks as I can not find much info on them.

Hopefully someone will clarify for us both.

[twowheels]

There are several things going on during the compression stroke that need to be considered.

On the check-plate/midvalve, basically you're moving oil from one side of the rebound piston to another. Motocross and supercross bikes often have a shim stack instead of the check-plate, but that arrangement can lead to compression harshness. The check-plate really serves to create effective rebound damping while having minimal effect on compression.

What is important is the oil displaced by the rod entering the cartridge as the fork compresses. That same oil has to move through the base valve, either across the piston (by deflecting the shims) or through the low speed circuit (the passage in the compression tap with the needle adjuster in it).

At low compression speeds (rolling terrain etc) the tendency of the oil is to go through this metered low speed circuit, since it is easier than deflecting the shim stack. As the compression speed increases (landing from jumps etc) the flow of oil through the adjuster-regulated circuit reaches its hydraulic limit, and the alternate pathway across the base (compression) valve piston is utilized to a greater extent.

Stiffening up the compression adjuster changes the point at which the oil starts to move across the base valve piston, or in other words the transition from low to high speed compression damping. You effect low speed damping by changing the size of the orifice (the needle nosed adjuster), and mid and high speed by changing your base valve stack.

The compression adjuster becomes less effective (per click) the farther out it is turned, but a useful experiment is to set up a short course and then ride it repeatedly, starting with the adjuster set in its normal position, then progressing to full stiffness, and then working your way back out. That will define the useful range of the adjuster for you, and give you an indication as to whether you need to revalve the base valve or if the clickers offer enough adjustment to satisfy your needs.

[nick790]

Originally Posted by twowheels

There are several things going on during the compression stroke that need to be considered.

On the check-plate/midvalve, basically you're moving oil from one side of the rebound piston to another. Motocross and supercross bikes often have a shim stack instead of the check-plate, but that arrangement can lead to compression harshness. The check-plate really serves to create effective rebound damping while having minimal effect on compression.

What is important is the oil displaced by the rod entering the cartridge as the fork compresses. That same oil has to move through the base valve, either across the piston (by deflecting the shims) or through the low speed circuit (the passage in the compression tap with the needle adjuster in it).

At low compression speeds (rolling terrain etc) the tendency of the oil is to go through this metered low speed circuit, since it is easier than deflecting the shim stack. As the compression speed increases (landing from jumps etc) the flow of oil through the adjuster-regulated circuit reaches its hydraulic limit, and the alternate pathway across the base (compression) valve piston is utilized to a greater extent.

Stiffening up the compression adjuster changes the point at which the oil starts to move across the base valve piston, or in other words the transition from low to high speed compression damping. You effect low speed damping by changing the size of the orifice (the needle nosed adjuster), and mid and high speed by changing your base valve stack.

The compression adjuster becomes less effective (per click) the farther out it is turned, but a useful experiment is to set up a short course and then ride it repeatedly, starting with the adjuster set in its normal position, then progressing to full stiffness, and then working your way back out. That will define the useful range of the adjuster for you, and give you an indication as to whether you need to revalve the base valve or if the clickers offer enough adjustment to satisfy your needs.

So by changing the midvalve (reducing the number of shims) it would have little or no effect on the compression feel?

My compression stack currently looks like this:-
1, 22x1.0
2, 12x1.0
3, 20x1.0
4, 12x1.0
5, 17x1.0
6, 16x1.0
7, 14x1.0
8, 13x1.0
9, 12x1.0

Which I'm told is quite soft, but I still have my clicker set nearly all the way out?

[stay_upright]

As two wheels says that's how your suspension (should) work.

Let me sanity check here though. In compression the oil under the piston can go 3 places -
1 - Flow through the piston (check plate) oil doesn't move,
2 -Oil gets pushed down and goes through the base valve (shimmed)
3 - Oil gets pushed down and goes through the low speed circuit hole - adjusted by clickers.

I measured how much area each option (1,2,3) as per the first post and the results were 120mm^2, 50mm^2 and 12mm^2.

What I struggle with is the free flow past the checkplate is so large the clickers will do practically nothing (120mm^2 vs 12mm^2 both 'free flowing' no shims. The base valve will have a moderate effect during fast movement of the piston (50mm^2 shimmed vs 120mm^2 free flowing).

The results are so 'odd' I question whether I'm missing something - hopefully other people can comment. Perhaps a few questions can help me find any errors..

e.g. - What should the orientation of the piston be - my forks were such that the large holes are used for flow during compression strokes and the small holes in rebound. - hence I assume that the large holes do not restrict the flow only the height the checkplate lifts to restricts the flow.

also - is the spring on the checkplate designed to use the checkplate to restrict the flow - on my forks is it weak enough to just look like it keeps the checkplate in place for rebound and offers little/no resistance to oil pushing the checkplate open.

and a simple one - the checkplate allows free flow through the piston right (in compression) - and this is the same as the low speed compression circuit?

[twowheels]

I'll try to be a little clearer, keeping in mind that fluid dynamics will turn a man's head to mush if he's not careful (don't ask ...)

First off, you may be right in the area of the ports on the piston and around checkplate, but there's more to it than that.

The checkplate on the back of the rebound piston is appropriate for this application precisely because it takes almost no pressure to lift (weak spring). It's primary job is to direct oil through the correct ports on the rebound stroke and prevent cavitation. You can change the checkplate and spring out for shim stack in order to provide additional compression damping, but that's not what you've got right now. The checkplate lifts to allow oil from one side of the rebound piston to the other - that's all.

At low compression speeds oil will move through the center of the compression tap past the adjuster regulated needle. Consider the low speed circuit as a two concentric circles, with oil flowing in the area between them. The outer circle is the diameter of the hole. As you turn the adjuster in (harder) the inner circle grows, so the area for the oil to flow is reduced, and flow is restricted. At some speed and flow area you end up with choked flow, meaning you can't pass any more fluid volume through that circuit and it has to find another path.

The other path is through the compression piston. The pressure in the oil has become great enough to work the shim stack, and so it deflects to a lesser or greater amount, depending on compression speed. As the low speed adjuster approaches fully closed initial damping is stiffer and you get into the compression tap stack sooner.

Total flow volume through the low-speed circuit and across the compression piston/valve stack is equal to the volume of the damper rod entering the cartridge. No oil escapes out the top unless you have a bad seal, in which case your damping would be horribly inconsistent.

There is also an air spring in play, and it becomes progressively stiffer as the forks stroke into compression. Raising oil level in the outer tube essentially makes the spring a higher rate.

I know this is awfully theoretical, but I hope it helps.

[stay_upright]

I still don't understand how oil does not flow past the checkplate in compression. - The plate can just move out of the way (easily) by lifting 2mm and allow a huge amount of oil through the piston.

I also remember another member (pobit?) doing tests showing the difference of altering the checkplate lift so oil must be flowing through there...

Maybe I'm still missing something?

[twowheels]

Oil does flow past the checkplate on compression - it's supposed to, and with as little restriction as possible, hence the weak spring and high lift. Once you introduce resistance you've created another damping component (ie midvalve)

Consider the cartridge as a cylinder of ever decreasing volume (because the damper rod is being introduced to greater and greater lengths). Since we have a seal at the top oil can't go out there, but it doesn't matter what side of the rebound piston the oil is on if the cylinder volume is getting smaller. We have a low speed circuit and compression piston on the bottom of the cylinder. The oil is incompressible, so it will flow out the path of least resistance (the compression circuit), providing damping along the way.

If you limit the lift on the checkplate it begins to resemble a very stiff compression stack, up until the point that no oil would flow through it, and all of the oil would be forced by the rebound piston out the compression circuits. However, on rebound there would be no oil on the top side of the rebound piston to provide damping, and we don't want that

Hopefully this sheds more light - if not, keep refining your questions and I'll try to answer them.

Originally Posted by stay_upright

I still don't understand how oil does not flow past the checkplate in compression. - The plate can just move out of the way (easily) by lifting 2mm and allow a huge amount of oil through the piston.

I also remember another member (pobit?) doing tests showing the difference of altering the checkplate lift so oil must be flowing through there...

Maybe I'm still missing something?

[nick790]

It's all now crystal, thanks twowheels