Thinking out loud about forks

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#1
I’m just trying to figure out the theory behind my forks operation, that’s it, I’m not looking for stack recommendations or anything. It’s driving me crazy, It’s like a song you can’t get out of your head. Any questions, comments, funny stories would be appreciated.

Anyway, while dissecting my 01 WP forks I made the following observations:

1. “Mid-valve” or active valving
Mid-Valve 24 mm OD * 8mm ID * .10mm (4X)
Very light mid-valve spring
Shims open approximately 5 mm

Correct me if I’m wrong, but it seems to me that the “mid-valve” or active piston has nothing or little to do with how much oil is pushed into the base valve ports. It would seem that would be a function of the ratio between the rod area vs. cartridge area.
So, as the rod moves its causes a differential pressure between the top of the active piston and the bottom of the active piston.

Example:
Rod OD= 14mm Area =154 mm2
Cartridge ID= 28 mm Area =616 mm2

So the area above the piston would be 462 mm2 vs. the area below the piston 616 mm2

So it would seem that any change in the rod dia. would have a substantial effect on the resulting pressure downstream with the same force. (Assuming the cartridge ID stays the same)
So I would guess your base valve stack would have to be based on the ratio between the rod and cartridge diameter.

Again, correct me if I’m wrong. A base valve compression stack that works so good on your friends Yamaha might not work so well on your KTM or Honda unless the rod/cartridge ratio is the same.

So this brings me to the question. Why the need for a mid-valve? The area of the active valve ports is almost as big as the area of the cartridge itself (I would guess 75%) and the shims are so easily pushed of the face of the piston (light spring) that it seems like there is no way there could be any kind of flow restriction (or very little) except at very, very, very slow rod speeds.

Would it not be more beneficial to use a substantially higher rate mid valve spring?
Wouldn’t this give a more progressive damping curve during very slow rod speeds?
It seems that if you could “shift” more of the dampening to the mid-valve for low speed stuff (landing jumps, G-outs) you could then substantially reduce the “stiffness” of your base valve stack, theoretically reducing harshness on roots, rocks, breaking bumps, etc.
OK, I know I’m starting to ramble and I’m probably way off in left field, but I’m almost done, I think.

2.“Base Valve” or passive valving

Everything I’ve read has said your valving is speed sensitive and not position sensitive, after looking at the stack I believe this to be true. Pressure can be traded off against velocity, by placing a different effective area at each side of the piston (the top shim / piston interface). The same pressure on a smaller area will move the piston at a higher speed but lower force for a given rate of fluid delivery. Translation: The more shim deflection the less resistance on the cartridge rod, thus less deflection at higher shaft speeds. So maybe this explains why if your high-speed stack is to “stiff” your forks will feel harsh and deflective in the choppy stuff.

Again correct me if I’m wrong. Now I think I understand why the race tech gold valves I used on my last bike did not work so well. All the dampening was placed on the base valve compression shim stack. The forks worked very well on the low speed stuff (jump landings, G-outs) but on the fast choppy stuff (roots, rocks, etc.) it would beat my arms to a pulp. When I reduced the HS stack so it was acceptable in the choppy stuff, I would loose low speed control. I could never find the right combination that gave me the best of both worlds.

OK, I think I’m done for now, my head is starting to hurt.
 

marcusgunby

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#2
Hmmm you have been busy.IMO most of what you say makes sense my one comment would be on the midvalve you say it only causes a restriction at very slow movements but i think its the other way round-it causes a restriction at high piston speeds.The later spec 01 forks do have a stiffer midvalve spring but drewarm says it appears to make no difference.Could you go over the base valve part again as you talk about pistons moving at high speed but the piston is fixed.
I think you are correct about the gold valves.Hard to get suppleness and control at the same time.Also it would be interesting to try a Cr valve stack in the 01 fork-i think it would feel like concrete so yes the cartridge/piston ratio is important.IMO this is why most tuners find the Wp fork hard to get right.

[This message has been edited by marcusgunby (edited 04-05-2001).]
 
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#3
Servus Degbert,

You asked for comments, you shall get them ;)

Correct me if I’m wrong, but it seems to me that the “mid-valve” or active piston has nothing or little to do with how much oil is pushed into the base valve ports.
Yes I'm of the same oppinion! Only oil which is displaced by the rod will go through the base valve. Any additional oil would have to come from outside the cartrige, i.e. passing by the piston rod seal which should not happen. And yes, the differential pressure is created behind the active piston thus working AGAINST the direction of flow! Also, did you notice that the active and passive piston valve are identical in port configuration and size! The mid valve is just a 'shimmed' version of the base rebound check valve!

A base valve compression stack that works so good on your friends Yamaha might not work so well on your KTM or Honda unless the rod/cartridge ratio is the same.
Theoretically yes, but practically I'm a little confused. The base valve shim stack of my 2001 KTM 520 14mm rod with mid valve WP fork is very, very similar to my 12mm Gold Valve without mid valve 1998 Yamaha WR stack. I like the KTM now and I loved the Yamaha back then. I'm faster now so the KTM has to be firmer, but the stacks are compareable! So what - I honestly don't know, it is just working for me.

Why the need for a mid-valve? The area of the active valve ports is almost as big as the area of the cartridge itself (I would guess 75%) and the shims are so easily pushed of the face of the piston (light spring) that it seems like there is no way there could be any kind of flow restriction (or very little) except at very, very, very slow rod speeds.
I replaced the spring with one at least two to three times stronger and didn't feel a difference. Maybe the WP mid valve configuration in general is just too weak, leaving it almost useless. But then I've seen severely bent Yamaha mid valves, so there must be something about it. The amount of oil which passes through the mid valve is 4 times bigger then the amount which goes through the base valve; ports on the base valve are substantially smaller than on the mid valve; valving is way stiffer on the base valve => what is the formula to convert mid valve damping to base valve damping to get the same overall damping? BTW, if you remove the base valve and seal the cartrige what have you got: right, a primitive shock! So, I'm still of the oppinion that active and passive valving are basically identical - you can theoretically replace each with the other to get the same overall effect. Practically though, two valves with different flow rates probably give you more possibilities of setup - I think I can accept that fact.


So maybe this explains why if your high-speed stack is to “stiff” your forks will feel harsh and deflective in the choppy stuff.
Hmmm, I don't know. Think of a stack #1 with 10 shims gradually rising OD from 10 to 20mm and stack #2 with 10 shims of 20mm OD. I think we agree that stack #2 is stiffer than #1 - BUT where, LSC or HSC? Does #2 has a HSC part and where would it start? Every stack has a CONTINUOUS DAMPING CURVE and not segments like LSC and HSC. My point is, that if the lSC part is not in the ballpark you can not blame such problems on HSC. It is like with jetting, if your main jet is too lean no matter how 'rich' your needle is, it won't solve your problems. Hope I could make my ideas clear somehow!


Michael
 

MACE

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#4
I'm the village idjut and all I've worked on are my poor KYBs and I use Visegrips on everyting but I love this kind of theory thread so here's my $.02 ($.03 CDN).

I think that the effect of the midvalve is felt when the flow through the rebound piston is sufficient to be restricted by the flow area between the bottom mv shim and the cartridge wall. When that area starts to restrict flow the restriction is modulated by the mv stack deflecting which results in a smaller shim O.D. and a larger flow area. All that coil spring does is shut the valve on rebound.

Now if this theory is true, replacing the mv with a rigid check plate (like Race Tech demands) would in fact increase the compression damping.

Ah the mysteries.....

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MACE

One night I was layin' down,
I heard mama 'n papa talkin'
I heard papa tell mama, "you let that boy MOTO,
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JTT

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#5
I too have pondered this. I think Mace is on the right track from what I have accumulated so far...

In order for the spring to have any significant effect, it would have to be extremely stiff (the pressures acting on it are considerable), but along this same thinking I am wondering why WP would upgrade to a stiffer spring (with no significant effect, as drehwurm has told us).

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JTT
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#6
Servus Mace,

flow area between the bottom mv shim and the cartridge wall
No, I don't think so. Before the oil passes through the midvalve it already passed by a 24mm shim and the cartridge wall - the closed rebound valve!!! IMHO the MV damping is only dependent by the distance the shims can move away from the valve and the shim stack itself.

Now if this theory is true, replacing the mv with a rigid check plate (like Race Tech demands) would in fact increase the compression damping.
But then the rebound check plate on the base valve would also create a significant amount of (unwanted) rebound damping - wouldn't it?


Michael



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#7
Servus JTT,

I am wondering why WP would upgrade to a stiffer spring
Me too, which leads to a mechanical question: If the spring is so stiff that the shims 'hover' between the valve and stop, isn't there a chance that the rather thin (very little axial guidance) shim stack could tilt?

Michael


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MACE

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#8
Originally posted by drehwurm:
No, I don't think so. Before the oil passes through the midvalve it already passed by a 24mm shim and the cartridge wall - the closed rebound valve!!! IMHO the MV damping is only dependent by the distance the shims can move away from the valve and the shim stack itself.
Good point about the upstream restriction. There is either more area upstream or the MV does nothing. Next time I have parts in hand, I'll try to measure them.

I guarantee you that that wimpy MV spring is not holding the MV stack closed. The MV slams into the "hat" washer and (if there is sufficient flow) bends around the "hat" washer. Any other scenario and the MV stack is nothing but a rigid check valve.


Originally posted by drehwurm:
But then the rebound check plate on the base valve would also create a significant amount of (unwanted) rebound damping - wouldn't it?
Not necessarily "unwanted". It just must work within the overall rebound requirement.
 
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#9
Servus MACE,

There is either more area upstream or the MV does nothing
Maybe it is like with electrical current: if you put two resistors in line, you get the added amount of each as overall resistance!

BTW: Did you ever notice that when bleeding your forks it is much harder to pull the rod out than to push it down! Is the rebound damping so much stronger than the comp damping?

Michael



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Jeremy Wilkey

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#10
Bravo... You guys got it!!!




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"Danger is one thing but danger combined with long periods of suffering is quite another." Sir E. Hilary
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Murf

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#11
drehwurm,
The reason (my guess) that it is so much easier to compress the fork than to extend it by hand when bleeding. The damping during extension (rebound) has to resist the spring. The damping while compressing the fork has the spring working in its favor to slow compression.

Am I right?

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John Curea

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#12
The one nagging question that gets stuck in my mind relates to the different midvalve builds in the 28mm and 32mm cartridges.

I am trying to compare midvalve build designs on my 99KX250 with 28mm cartridge and 12mm piston rods vs. a late model YZ with 32mm cartridges with 12mm piston rods I dont want to throw the late model WP into the equation just yet because it has 28mm cartridge with 14mm piston rod. I want to keep it "apples to apples" so to speak.

Anyways, I am trying to determine the performance difference between the KX and YZ midvalves. I realize the only fluid displaced through the base valve eqauls the area of the piston rod in the cartridge at any given time. Reguardless if the piston rod lives in a 32mm or 28mm cartridge, the piston rod will still only displace X amount of fluid through the base valve.

The YZ midvalve is built extremely light, including float dimensions compared to the KX midvalve.

Other than factoring in the physical size difference (the YZ being 3mm larger) of the midvalve pistons, is there any other factor that influences the design/performance of the midvalve? (I have a feeling there is, I just cant get it nailed down).
Take Care, John




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99 KX250
98 KTM50
88 LT250R
86 TRX70
 

JTT

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#13
Originally posted by KXVET#207:
Other than factoring in the physical size difference (the YZ being 3mm larger) of the midvalve pistons, is there any other factor that influences the design/performance of the midvalve?
Is there a variance in actual port size? How about the difference in leverage created by the dimension from the "pivot" location of the shims?

...just shooting ideas...

John, you also mention "lighter float dimensions"...do you mean there is a difference in how far the shim stack displaces (by compressing the spring)? I haven't seem a KX apart recently to take notice.



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JTT
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Jeremy Wilkey

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#14
Ok...

Man this kills me...


-The factors are Midvalve Float..
This impacts when the midvalve actually starts to work.... (In terms of speed.)

-Then the midvalve stiffness or stack build..

These two are directly realted to the swept volume of the clyinder. Volume is related to pie r2 x h.. So a larger cartrige is massively impacting to the total swept volume.

So lets look at to cases..

KX 28mm cartridge... Thge midvalve runs no float but does have .1mm bleed/float.. This stack is also only moderatly stiff however, because it pivots of a very small shim.. SO although in terms of speed the midvalve impacts lower speed ranges but the stack is not very progressive and does not impact high speeds as much as might be imagined..

Case 2...

YZ fork with much more float...
This set-up does not work until a slighlty faster speed, however the volume crossing the valve is much larger so apples to apples it's almost the same thing... The valving is also more progressive as it's stack is built..

A couple of more observations..

The basevalve stacks also vary greatly on these two bikes.. Why..Very simply one is stiffer one is softer. Like the circuts of a carborator the speed ranges overlap. The mid-valve of the KX has a slighly higher overall damping coeifecent and hence requires a slighly lighter valve stack and vise versa. Also the YZ has the CV which is bleeding fluid as well, also nesciating a stiffer build at the basevalve to acheive the ideal overall coifecent. Which works better? It depens on the aplication. I'll give the KX a better stadium build, and the YZ a better outdoor build.

The 28mm Midvalve trades plushness for instant damping, and hence more precise handling when riden by an expert. The YZ on the other hand is more suple at very low speeds. However in big hits is often more harsh.. (Volume of mid-valve and a very stiff midvalve just stacked up on you!)


Ok a couple of more issues I have talked about forever which no ever seems to remmber.. A KYB fork does not work effeceintly until the fork has compressed several inches.. That is often why pro's preffer the Showa forks.. Which work all the time.

This is the problem with backyard tunners.. They really don't understand how the circuits overalap..


I get imesly frustrated when I read this topic.. I feel like I've been beating this drum for literally years and noone ever gets it.... Sorry for the rant..

Also the reason compression feels so much softer is related to a few simple things.. Compression has a much smaller volume going through base-valve. (Only the rod.) If the basevalve restricted flow on return you would have chronic fork isseues such as zero damping..
The midvalve does not impact very low speeds so you don't feel it..

Rebound very stiff active valving, with a large volume.. Ofcourse you will feel it more.. And I ask you do you think you want as much compression as you have rebound?


Regards,
JEr

I know I can't spell and I rant but these are the issues. There are still some small fine points but this is good for now..




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John Curea

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#15
As the mighty light bulb above my head just illuminated (temporarily cancelling out the "blond moment")

The words "Swept Volume" gave me an Ah Ha moment!!
Take care, Grasshopper John


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99 KX250
98 KTM50
88 LT250R
86 TRX70