Question of the month..!! Revisited

Jeremy Wilkey

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#1
Ok.. Having gone throught the midvalve post I came to a intresting point, often asked in my training sesions.

I ask my forum readers why does a high flow piston work beter in a Midvalve and a low flow work better in a passsive situation? The best answer gets a reward yet to be determined... It is fundemental to understanding suspension...

Best Regards,
Jer
 

marcusgunby

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#2
The base valve only sees the oil volume displaced by the cartridge- not much on a 12mm rod.The midvalve goes through a column of oil whose volume is much larger(ie it sees alot more oil) so if you had a low flow midvalve the shim stack would be super sensitive and most likely would almost lock the fork solid under high speed hits.Its a speed volume problem.

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MACE

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#3
...and besides that, then the fork would be a shock with a 1:1 leverage ratio - that stings - and with no N2 pressure we would get Mobil 1 milkshakes.

Of course if we had N2 pressure......

(This is really odd, just this morning I was thinking about Jer's "Let's have some fun" quiz post from about a year and a half ago. This is good stuff.)
 

John Curea

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#4
OK, Here we go......

The one factor I had to figure out was approximately how big the difference of the midvalve swept volume versus how much fluid is actually passing through the base valve. The answer I kept hearing is "allot more", "big difference". I believe that those generalizations are true, but I need numbers!

A couple quick notes;
1.I am using a 1999 KX fork for reference (28mm cartridge)for reason being it has the same size piston diameter as the base valve.
2.My calculations are the total volume difference after the fork has traveled the entire stroke (manufacturer specs this at 12.2in.) (309.9mm)
3.I also am not trying to calculate any fluid "leaking" by the bushing at the top of the cartridge.

1. I took 3.14(14x14)309.9 to find the volume of the empty 28mm cylinder. (190725)
2. I figured 3.14(6x6)309.9 to find the volume of the piston rod (35031)
3. 190725 - 35031 = 155694 (m.v. swept volume)
4. 155694/35031 = 4.44

Result - almost 4 1/2 times as much oil flows past the midvalve as compared to the volume that flows through the base valve.

Ok, back to the original question of why a high flow piston works better for a midvalve. I can see how the midvalve that flows 4.44 times the amount of fluid might cause a restriction if we use a low flow piston. But actually, with really light valving who knows?? I guess you have to figure out how much fluid can flow through a low flow piston at max shaft speed without causing a resistance.
Anyone ever try a low flow piston for a midvalve??

How about tossing this " bone into the soup".....
A late model WP fork with a 28mm cartridge and a 14mm piston rod. This set up comes out to about 3 times the difference of the amount of oil passing through the midvalve as compared to the base valve.
Now, since the difference isnt as large (as compared to the KX) Would a low flow piston design work for a midvalve????

Yeah, I know, I answered a question with a question......

Allright , I am tossing this to you guys........whaddaya think??

Take Care, John




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JTT

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#5
What about differences in fluid pressures under the two different circumstances?

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Jeremy Wilkey

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#6
JTT, KXVET,

It might be better to think of it in terms of velocity through the valve. Flow is not just dependent on volume but also how fast we are pushing it throught the orfice.. Valving is after all nothing other than a varable orfice.. We could drain lake havisu with a Goldvalve given time..

I've done lowflow pistons in the midvalve and they don't work well.. (Case in point MArzoccchi forks) I tried quite alot with this my 93 KX :eek: back in the day..

As "Valve Pressure" increaese due to velocity the valve stack works less effceintly. When we can bleed from a large orfice we decrease deflection (all thing being equal) This lets us modulate flow easier with a shim stack than if we have a huge deflection. The flow patern is like a jet, But the more we deflect the harder it is to modulate the flow... That is my wholw point in my piston design.. Not restricting flow but braking the relative deflections into increments that can be more preciuse based on displacement of fluid.. (a function of speed..)

Hope this sheeds some light.. There is much more to talk about here.. I see what you guys do and I will come back..

Regards,
Jer
 

Sage

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#7
Originally posted by Jeremy Wilkey:
passsive situation
Define please, do you mean the bleed jet on some pistons?



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Sage Wilkinson
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Jeremy Wilkey

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#8
Sage,
Passive and active are terms I created about 5 years ago to acuretly describe the relationship of the basevalve and midvalve. To say passive we are talking about a displaced volume.. (IE basevalve) and active describes the act of a piston traveling through a colum of fluid..

These terms have become very mainstrem... something I'm quite proud of..


Regards,
Jer
 
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#9
Servus Jer,

Passive and active are terms I created about 5 years ago to acuretly describe the relationship of the basevalve and midvalve.
Sorry, I'm probably a little slow in understanding here, but do I see this right: Active and passive in this context are terms related to fork (suspension) design and not hydrodynamic principles.

Michael




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#10
good day Jer. let me have a go at this.

please correct me if there is any mistakes.

in order to have a smooth entire stroke movement thru the cartidge, the oil have to be able to flow thru the orifice without much "blockage". But at a controlled speed ( by the different shim setting).

So either the oil will passes thru the midvalve more or thru the base valve. If both end are set up stiff, the oil ain't going any where smooth and fast enough and will causes harshness and tired the rider faster.

the main objective I think Jer is putting across(I hope I'm correct) the velocity the oil is controlled by the deflection of various shim stack set up, with a large orifices, oil flow thru at a certain pressure , higher pressure will causes the hi-speed shim to deflect and thus allowing more oil to flow true, at controlled velocity of cause.

This thread really rack up my brain juice, and it is really cool. :)

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Jeremy Wilkey

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#11
Michael,
You are not a slow learner at all...You are quite right... These terms won't be found in a Fluid Dynamics text book. I think they work much better for laymens terms than the other options..
Regards,
Jer
 

JTT

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#12
Originally posted by Jeremy Wilkey:
JTT, KXVET,

It might be better to think of it in terms of velocity through the valve. Flow is not just dependent on volume but also how fast we are pushing it throught the orfice.. Valving is after all nothing other than a varable orfice.. We could drain lake havisu with a Goldvalve given time..
It's making much more sense...Funny, last night I was sitting eating supper and thinking that exact thing. Maybe a more accurate term would be "rate of flow" (that being a volume in a given period of time, such as gallons per minute)? The time period is what is effected by the "speed".

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JTT
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#13
Ok probly wrong but i will take a shot

High flow works better in the midvalve because you are able to push a higher volume of oil which will increase the efficency of the valve stack without causing harshness at higher speed.Speed being the fork speed.

Because the midvalve has a higher flow rate the low flow in the passive situation will decrease the possibality of oil cavitaion.

Welp interested to find out the real reason but there you go Jer let me know if im even close.

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MACE

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#14
We are answering half of the question. The second half is why a low flow valve is better for the base valve. This will be a basic difference in philosophy between Bourbonais and Pomona.
 
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#15
OK here is my crack at it in a over simple way.

mid (active) valves need to be high flow because what it does is move fluid from the bottom chamber to the top chamber minus the rod Dia. which gets pushed through the passave valve. the goal is to do this without resistance. The high flow valve does this by not accelerating the fluid through the valve as much as the low flow valve (less fluid acceleration = less resistance)

On the other hand the passave valve might work beter with a lower flow to accelerate the fluid displaced by the rod (which is not much) through the valve onto the compression stack which inables the stack to better control the fluid to a finer degree which lets the tuner to dial a stack that will be plusher and still not blow through the stroke.

Just my 2 Cents worth

Ken

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