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Discussion Starter · #1 ·
My title might say it all, but I just can't figure this out. The valve/piston/thingthatdoesthework is submerged in oil at the beginning of the stroke - how does the air pressure and volume above the oil have any bearing on anything prior to the valve leaving the oil (as liquids do not compress)? Is it suppossed to leave the oil? Manitou's tech specs for a Skareb 100 w/ SPV states that oil level should be in the 75mm height range. With 100 MM of travel.....

What gives? Am I missing something or am I as think as you stupid I am?
 

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bikeguy73 said:
My title might say it all, but I just can't figure this out. The valve/piston/thingthatdoesthework is submerged in oil at the beginning of the stroke - how does the air pressure and volume above the oil have any bearing on anything prior to the valve leaving the oil (as liquids do not compress)? Is it suppossed to leave the oil? Manitou's tech specs for a Skareb 100 w/ SPV states that oil level should be in the 75mm height range. With 100 MM of travel.....

What gives? Am I missing something or am I as think as you stupid I am?
No, you're not think am as stupid as I. :confused: ;)

You're missing a few things.

First, the SPV valve itself is never exposed to air (or above the fliud line) inside the leg. There's a step that's machined inside the leg. The SPV valve cannot travel above this step - the valve would be destroyed. On my Minute 1:00 this step measures about 147 mm from the top of the leg. When filled properly the fliud should be at 84 mm. Big difference, huh?

Also, displacement. As the valve moves up (remember, the valve is essentially attached to the bottom of the outer) it not only passes fluid through itself, it also pushes fluid above it. It can only pass so much fluid at a time (this is a minor part of damping - not all!). Again, the valve is never "above the fluid".

The air that's inside the leg is used to control two things - one is the SPV pressure and the other is bottom-out control.

The bottom-out control is simple - as the fluid is trying to be compressed (note the word TRYING, i'm not saying it is) by the valve it's continuously being given less room to exist. If the volume adjuster is cranked down then you are giving it less and less room. The fluid can't compress so stoke is terminated earlier than if you were to have the cap in it's top-most position. Less room for compression = less room for travel. This is commonly referred to as the "ramp up rate", also.

Lastly, the all-might SPV. This has to do with the spring inside the top cap of the SPV valve itself (not to be confused with the top cap of the leg!). The air pressure is used to keep a certain amount of pressure on the assembly (via user input - pumping air into the leg). When the impact pressure has enough force to open the valve (surpassing the ability of the little spring) then and only then do you have travel. It's based on the pressure differential between the two forces.

Above all else - while your last sentence makes sense - it doesn't (sorry). The fork oil height is measured from the TOP of the leg. There's more than 75 or even 100 mm of oil beneath the top of the fluid level. ;) You saying "75mm height range. With 100 MM of travel" - you're looking at it from "the wrong direction". It's not like there's a 25 mm air gap at full compression. There IS an air gap, but it's not 25 mm - it's 75mm (the "height range"). Actually, less - the air does get compressed a little bit.

If you need pics you can find some here. If you're having problems with the "new and improved" :rolleyes: search let me know and I can take pics of an '04 valve if it will help you to better understand.
 

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The SPV (same as CV/t, except maybe for the spv snap valve, haven't seen one of those yet) is a pressure sensing valve. It is essentially an air piston, and the internal pressure inside the fork or shock applies pressure to it which generates a force that closes off the compression ports (in it's simplest form). When you strike a bump, the oil pressure / force acting on the face of the control valve must exceed the force generated by the air pressure pushing the valve closed, and when it does, oil flows and the fork compresses. Now as the fork or shock compress, the internal air pressure increases due to oil being displaced by the damper shaft entering the damper. As the pressure rises, the resulting force of the control valve trying to close rises, further restricting oil flow, increasing damping as you go through the storke. This is what gives your bottoming resistance / position sensitive damping. Again, in its simplest form comressoin resistance in the first third or so of the travel is very NOT speed sensitive, I mean that the resistance to compression at 1 in/sec is not really any different than the resistance at 30 in/sec. As you travel trough the stroke, deeper in the travel, the damper starts the create more velocity dependant spread. The compression resistance at the bottom of the travel can be twice what it is at 30 in/sec than it is at 1 in/sec. Now there are many refinements that can be made beyond the simple control valve. Ideally, some velocity dependant damping is desirable at lower speeds and earlier in the travel, the 5th coil uses a shim stack and compressoin adjusters in the reservior of thier dampers, and there are other ways to get about it too.

I hope I made sense.

BM
 

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Discussion Starter · #4 ·
:eek: Oh, wow - sounds like I have made a fundamental error! So the oil level is suppossed to be measured as the top of the oil column being approx. 75 mm from the top of the leg!? That would make a bit of a difference, eh?! :eek:

I THINK I understand what you guys are telling me, but let me summarize my understanding to this point to see if I am on the right track:
The air above the oil column is exerting pressure on the oil column - both by virtue of being pressurized and by virtue of it being compressed by displaced oil upon compression of the fork. This pressure on the oil column increases the "hydraulic pressure" exerted on the SPV valve by the oil - more air pressure above the column translates into more pressure exerted by the oil column which translates into more upward pressure necessary to open the SPV valve and allow oil to pass through (thereby allowing travel).
 

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bikeguy73 said:
:eek: Oh, wow - sounds like I have made a fundamental error! So the oil level is suppossed to be measured as the top of the oil column being approx. 75 mm from the top of the leg!? That would make a bit of a difference, eh?! :eek:

I THINK I understand what you guys are telling me, but let me summarize my understanding to this point to see if I am on the right track:
The air above the oil column is exerting pressure on the oil column - both by virtue of being pressurized and by virtue of it being compressed by displaced oil upon compression of the fork. This pressure on the oil column increases the "hydraulic pressure" exerted on the SPV valve by the oil - more air pressure above the column translates into more pressure exerted by the oil column which translates into more upward pressure necessary to open the SPV valve and allow oil to pass through (thereby allowing travel).
Yes... and deep into travel, the increasing pressure tries to close the valve again, producing the anti-bottom out feature.

If you take a look at the different damping systems around, all try to give a rather high compression damping at low compression speeds and when position sensitive at early travel, then they open wide to provide little damping thru the middle of the stroke and/or medium speeds and from there they either increase hydraulic pressure to close the compression circuit or increase spring rate (mechanically or pneumatically) to prevent bottoming.
 
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