Question of the month..!! Revisited

John Curea · Apr 17, 2001

Mace
I am looking for a second set of suspension for my KX for just the reason you stated. Unfortunately, I lost the bid on a complete set about a year ago.

That hindsight thing again.......

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01 KTM 50LC (juniors new rocket)
99 KX250
98 KTM50 (for sale)
88 LT250R
86 TRX70

svi · Apr 17, 2001

If a low flow rebound piston is used wouldn't the shims just blow open at high velocity and the piston orifices then be the damping medium which if they are not of sufficient size lock the forks ?
With a high flow piston it would take the piston well outside of the speed range where piston orifice damping is likely to occur.

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Sage · Apr 17, 2001

uuggg, big hole better for high speed, won't hydro lock, little hole dosent flow a lot of oil so when fork goes up/down real fast you don't get a "flat spot" in the damping due to shim float, shim not blown 10 feet of seat so it returns home faster and does its job. hehehe, this is fun, yahyah, fun

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Sage Wilkinson
sage@sagesdirtandstreet.com

[This message has been edited by Sage (edited 04-17-2001).]

Jeremy Wilkey · Apr 18, 2001

Hey ALl... Keep Going.. I'm hoping to help some UOLC (London U) design some car suspension.. I'm kind of a in the feild type expert advisor since my BS is "ONLY" in "GEOLOGY" and all those years of Math and Science don't apply to the point where I can be called an "ENGINEER".. (Sorry just had to dish it out..) :)

Anyway.... Hopefully If I get on this program I can Glene some info to share.. These guys are doing a really cool things and it includes lots of dyno work..

Regards,
Jer

DEGBERT · Apr 18, 2001

Ok, I think every one contributing to this thread has got a pretty good handle on what’s going on. Is it just me, or did anybody else notice that you keep seeing the same 5 or 6 names over and over. I don’t know about you guys, but my wife thinks I need to go to suspension junkie’s anonymous (SJA). I spend more time tinkering on my bikes than I do riding them.

Ok, back to the subject, now that we have established that a high flow piston works better in the midvalve and a low flow piston works better in passive piston or base valve. But now lets see if we can look at quantifiable ratios or values instead of just saying “low flow” and “high flow” piston.

If we have a 14-mm rod displacing fluid through a 28-mm base valve (00 & 01 WP forks), how do we determine the optimum area of the receiving orifices in the base valve? How about it Jeremy, throw us a bone, can you give us at least a ratio between rod area and optimal total orifice area.

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Vic · Apr 18, 2001

Excellent question ! :)

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John Curea · Apr 18, 2001

DEGBERT,
SJA....???....I can quite anytime I want :confused:

James Dean · Apr 19, 2001

The "optimal" base valve area has to depend on what peak velocity is desired by the fork.

Evaluate the conditions - SX, MX, desert, woods, rocks;
Rider level - play rider, novice, intermediate, expert, pro;
Fork design - CV or no CV, bleeds or bypass, charged system, downstream restrictions outside the cylinder...

:eek:

Optimum base valve almost begins to sound like optimum spring stiffness. :confused:

These considerations and the midvalves physical limitations to flow are critical in deciding the base valve's port size. The base valve must prevent the mid-valve from cavitating. The base valve and restrictions in series provide the maximum pressure in the system. The midvalve is in parallel and cannot have a pressure differential above the basevalve limits, determined by the factors listed above. There are ratios of port size between the base and midvalve which need to be adhered to in accordance with the relative flows between the two described in this thread above (3:1 to 5:1 volume ratio) in order to maintain precise valve shim control of the damping. Oversize the basevalve and lose valving control, undersize and limit peak compression speed. Generalizations are hard to come by.

Don't you wish these things could be a little simpler? :)

James

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[This message has been edited by James Dean (edited 04-19-2001).]

Sage · Apr 28, 2001

Just thought I'd put this back on top & get it going again.

Jeremy Wilkey · May 1, 2001

JD,
You summed that up very well. Could you elaborate on the optimum spring comment. I'm not clear on wether your referring to the shim or the physical spring. I would imagine the valving as it would be more along the lines of the speed sensitive side of things..

I've heard that Enzo is working on some radical new shim material that will make the old 1095 obsolete. ?? ****** I think******* that this was a loaded question by a certian induvidual as a check of sincerity on my part but I don't actually know.. Whith fiberglass shock sporings becoming more comon, this does notnesicarly surprise me..

Regards,
Jer

James Dean · May 2, 2001

Back to the original question,

The idea of using the mid-valve to add and tailor damping characteristics has limitations. It will always need the basevalve to produce high enough pressure to prevent cavitation. The midvalve must take a secondary role, so may as well have large ports. Putting a large port compression valve at the base doesn't gain much when the midvalve still needs that pressure to flow fluid through it. Also, there are still restrictions between the basevalve and outer reservoirs to limit flow. The compression valve port size is best designed for the system, looking at all the passages and required flow rate for the intended use.

The heart of the cartridge system, which too often seems to be ignored, is the simple rod diameter. Changing from a 12 or 12.5 to a 14mm rod is an enormous jump. The fluid flow rate thru the compression valve and the reacted pressure on the rod area has a multiplying effect. The potential for improved damping control is available with the 14mm WP fork rods, but keeping the level of plushness of a smaller rod will be difficult to match. Add some innovative design work and new materials for shims could take the damping control to a higher level.

James

KawieKX125 · May 2, 2001

Vould someone please clear this up for me. What is a midvalve? If it is like a basevalve, ho can it become, "blown" as some say?
Thanks:think :)

JTT · May 16, 2001

Kawie, I was just re-reading this post when I realized you never got and answer...sorry...

Inside the fork is a shaft attached to the fork lower and a cartridge attached to the upper. Inside this cartridge, and attached to the shaft (rod) is a piston with valve stacks on both rebound and compression sides (at least unitll RT gets hold of it :D ). This is your "midvalve", or active valving.

If this cartridge was sealed off at the top with no air space the oil displaced by the shaft (like putting your hand in a glass of water) as it is compressed into the cartridge would hydraulically lock the whole affair, or explode the cartridge....so a second valve is installed at the top to meter this oil displaced by the shaft. This is the "base valve" of passive valving.

Although I have not experienced it..yet, I believe what people are referring to when the say "blown" is that the valve stack on the compression side gets permanently bent, thereby no longer sealing over the orifices and allowing unmetered (uncontrolled) oil flow in the rebound part of the stroke...boingy, boingy.

Hope this helps.

JTT · May 18, 2001

I am wondering about the relationship of the ratio of flow between the midvalve and basevalve. Ratios like 5:1 and 3:1 were mentioned as examples. I noticed on my CR Showas that with a 12mm shaft and 23mm cartridge that the ratio on this fork is significantly closer (in the ballpark of 2.5:1). I would presume that because of this closer ralationship, that the midvalve would play a lessor role, as less fluid (relative to larger ratios) is flowing through it? This seem contrary to "rumours and speculation" I have heard in the past....maybe it's backward?...the lessor the ratio, the greater the effect?

KawieKX125 · May 19, 2001

Okay, thanks. But, if there is compression and rebound valving on the midvalve, why is there a need for a base valve? Maybe you explianed this in you glass of water analigy, but I was very cofused by it. Maybe I need to buy a used set of forks off of ebay for a low price and experiment taking them apart.

JTT · May 20, 2001

Kawie, that water that was displaced when you put your hand in the water?...that is effectively the same thing that is happening when the shaft is being pushed into the catridge on compression. The volume of the shaft (ie: is crosssectional area x length of travel) is displaced adn must go somewhere. The cartridge is sealed at one end and the other end has the base valve on it. Since the fluid cannot displace into the solid end, it is forced through a piston and shim stack affair on the base valve and into a "holding chamber". When the fork rebounds, a vacuum is created as teh shaft is removed and that fluid from the "holding chamber" is re-released, through another series of valving (rebound) back to the area below (at least in the case of UPS forks) the midvalve piston, that the shaft just left from.

To go back to our glass of water...try to hold back the water from overflowing the glass with your other hand. If you could get a good enough seal with your hands you would see the the water would be forced through your fingers, no matter how hard you try to seal it...your second hand if the base valve.

...making any sense?

mxneagle · May 21, 2001

JTT,
I'd have to agree that on the showa's the more even ratio would make the contribution of the mid valve stack less of a player. Look at it from the other point of view...like say a rear shock. The ratio on a shock must be 10:1 or higher and all of the valving is typically done on the piston stack (ie equivelent to the mid valve on a fork) and usually none is done on the stationary valve (the compression adjuster stack)

Just to throw another nut into the pot, The YZ's are using a stack in the top of the cartridge tube. So now oil is flowing out both thru the base valve and the cartridge cap.

Question of the month..!! Revisited

John Curea

svi

Sage

dirtbike riding roadracer

Jeremy Wilkey

Owner, MX-Tech

DEGBERT

Vic

***** freak.

John Curea

James Dean

Sage

dirtbike riding roadracer

Jeremy Wilkey

Owner, MX-Tech

James Dean

KawieKX125

JTT

JTT

KawieKX125

JTT

mxneagle

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Question of the month..!! Revisited

dirtbike riding roadracer

Owner, MX-Tech

***** freak.

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