Explanation on how Showa TC forks work


14 replies to this topic
  • punkrock MX

Posted October 16, 2006 - 12:03 PM

#1

Does anyone care to offer an explanation on how the Showa Twin Chamber forks work? or an article about it that can be read by someone that isnt a PHD in physics/engineering?

thanks

  • 450exc115

Posted October 16, 2006 - 05:16 PM

#2

You want to know how they work compared to open bath forks or you just want to know how forks work period?

  • punkrock MX

Posted October 16, 2006 - 05:26 PM

#3

ehh both wouldnt hurt, but i need to know how they work period before i can compare them to a open bath type fork such as the kyb ..

  • John_Curea

Posted October 16, 2006 - 08:40 PM

#4

Good question...here goes.....

Twin chamber operation:

As first seen on mid to late 90's Suzuki and Hondas, these have now become the industry standard on all late model MXers. The industry has recognized the benefits of the advantages of the twin chamber. Either that or the main players evolved in a classic game of "one upmanship". Either way, I'm glad to see all the major brands offering a version of this fork. It'll be interesting to see who offers this style of fork first on their mini line.

Benefits:
1. Separation of the fork bushing lube fluid and the valving fluid. This is important because all of the debris that gets past the fork seals and the by product of the springs wearing on the inside of the chrome tube is isolated from the valving circuit. Of course the bottoming cone is still exposed in this style of fork versus the open cartridge fork, but this disadvantage is far out weighed by the advantages.

2. The valving fluid under pressure from the inner chamber springs or from a bladder as we find in the Marzocchi 50mm and the WP '06 SXS and WP '07 production twin chambers. KTM still hasn't surrendered to this style of forks for the offroad models.
Applying pressure to the fluid creates a much more efficient fork. By doing this, cavitation is reduced and controlled. More on this later.....

How is fluid displaced through the base valve?
As you look at the internals of a twin chamber, we see a base valve (which we will refer to as the passive valving) and the midvalve piston (as we will refer to the active valving). The compression damping is handled by the combination of both the base valve and the midvalve. The rebound damping is controlled only by the rebound side of the midvalve.

When the chamber is extended, it is full of fluid, which at this point is held under a small amount of pressure from the inner chamber spring (which some refer to as the pressure spring) or a bladder filled with nitrogen pressure. As the cartridge rod (sometimes referred to as the piston rod) is pushed into the chamber, depending on speed of the cartridge rod, a host of hydraulic fundamentals are taking place.

One quick note: The midvalve piston doesn’t actually “push” fluid through the base valve. Fluid is displaced by the area of the cartridge rod entering the cartridge. Analogy: Inserting your finger into a full glass of water causes the water to overflow from the glass.

As the rod is entering the cartridge, the midvalve compression shim stack (which usually has a certain amount of float) opens up letting unrestricted fluid pass. As the speed of the fork compresses, this amount of float is no longer capable of handling the fluid flow requirements and will start to deflect the compression side shim stack, this causes resistance which is better known as damping.

At this point cavitation of some measure starts. Cavitation is caused by a differential in fluid pressure on either side of the active valving. Cavitation is reduced in a twin chamber environment because there is positive pressure on the fluid from the pressure spring or bladder. This is where the twin chamber design shines in comparison to the open cartridge design. In an open cartridge design (’04 and prior KYB design) there isn’t any additional pressure to help reduce cavitation until the fork compresses enough for the air spring to apply pressure on the active valving. Unfortunately, depending on oil height, isn’t until the fork is at least half way through the stroke.

Earlier we discussed that the amount fluid displaced through the base valve is equal to the area of the cartridge rod that has entered the cartridge. The variable in this equation is the amount of cavitation present. Whatever the amount of cavitation behind the active valving is directly equal to the additional fluid being displaced through the base valve in addition to what the cartridge rod has displaced.

If you do the basic hydraulic mathematics, depending on actual size of the base valve, the cartridge, and the cartridge rod. 3 to 4 times the amount of fluid is passing through the midvalve in comparison to what is being passed through the base valve. This perspective magnifies the importance of the midvalve compression contribution to the overall damping equation.

It sounds like the twin chamber has it all over the open chamber. But as with everything, there is a trade off of this increase in performance. Since the twin chamber is a much more precise fork compared to the open chamber models, this can come to the rider with an increased amount of feedback to the handlebars, which can be perceived as harshness. Especially to riders who are in the saddle for 3 plus hours at a GNCC race. This is where we as suspension tuners are constantly chasing the holy grail of the correct combination of feedback and comfort for our riders.

I’ll let this end here for now, but we’ll continue to add content as the interest grows….

Take Care, John

  • trailpimp33

Posted October 16, 2006 - 09:31 PM

#5

John, great explanation!!
One thing you did not also include is the additional tuning options with the twin chamber type forks, such as spring seats, oil lock collars, and different weight pressure springs, and the different external oil weights and heights.

But that was one of the best LAYMAN explainations I have yet heard, can I steal it??

p.s. on that mini thing, I am almost finished with retro-fit kit for several forks.

  • theDogger

Posted October 17, 2006 - 11:47 AM

#6

John I know that you hate typing but keep preaching......great info.


theDogger

  • freds4

Posted October 17, 2006 - 12:14 PM

#7

FINALLY! An explanation of why my '06 KX-250F forks feel so harsh on their initial stroke. Now if I just knew how to adjust that harshness out.......

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  • chrismxer

Posted October 18, 2006 - 04:30 PM

#8

Great post John!!

A few points to add about the pluses of the design of the twin chamber over the open bath system...

Unsprung weight- Since the cartridge is now fixed to the top of the fork it makes it sprung weight, also a good amount of oil stays within this cartridge also making a portion of the total weight of oil sprung weight. This makes for quicker reaction times to stroke movements giving less feedback to the rider.

Cavitation- I am assuming what little cavitation happens on the bottom side of the mid-valve has to come from the seal on the cartridge rod. Having that seal facing down and submerged in oil more so than the open bath system makes it even harder for the possability of cavitation. I am definatly not a physisist but seems it logical to me??

LCG-This could be a wash but with the spring being on the bottom of the fork, you have one of the heavier steel parts of the fork as low as possible. On the flip side you have the cartridge filled with oil and a small steel spring on the top of the fork sooo....

Question for you John, How does precise = harshness?? I have found that TC's can handle a wider range of situations than the older open bath's could. I could never get the performance out of the old KYB's that I can with the TC's. I Through a set of TC's on my 03 RM250 that came stock with an old 46mm KYB. It only took one revalve / setup to make it better than the old forks. That was my experience.:devil:

And what the F happened to the spell check:smashpc:

  • DROB

Posted October 19, 2006 - 06:07 AM

#9

This John Curea guy seems pretty sharp.:devil: :thumbsdn:

  • thillsam

Posted October 19, 2006 - 08:08 AM

#10

...The compression damping is handled by the combination of both the base valve and the midvalve...


...One quick note: The midvalve piston doesn’t actually “push” fluid through the base valve. Fluid is displaced by the area of the cartridge rod entering the cartridge. Analogy: Inserting your finger into a full glass of water causes the water to overflow from the glass.

...As the rod is entering the cartridge, the midvalve compression shim stack (which usually has a certain amount of float) opens up letting unrestricted fluid pass. As the speed of the fork compresses, this amount of float is no longer capable of handling the fluid flow requirements and will start to deflect the compression side shim stack, this causes resistance which is better known as damping...

...If you do the basic hydraulic mathematics, depending on actual size of the base valve, the cartridge, and the cartridge rod. 3 to 4 times the amount of fluid is passing through the midvalve in comparison to what is being passed through the base valve. This perspective magnifies the importance of the midvalve compression contribution to the overall damping equation...



All of the above statements that I've clipped from your excellent article lead me to ask you for alittle more explanation for all of us.

Could you describe what types of motions each part of the valving controls? For example, the when the midvalve is moving fast enough to float, but not to deflect the midvalve shims, the fork is relying on the base valving ONLY for compression damping, no? What sort of velicities/movements does this phenomenon occur in? What part of a given(short and fast, long and slow, long and fast...etc.) compression movement is controlled by what elements of the system?

If you had to tune a TC for solely for trail junk and babyhead rockpiles, for example, what part of the valving would you change and what would you do? How about solely for sharp, square edged potholes like occurr on a flattrack? And how about a version solely for whoops?

I'm not asking for shimstacks here, just general concepts that we can draw conclusions from, like:

Add more midvalve float for less compression damping during quick, short movements. This works because the midvalve compression stack floats farther before it's forced to deflect, raising the velocity OR distance that the fork can move before the midvalve stack begins to restrict flow...

Add more lowspeed shims in the base valve for more slowspeed, large movement control. This works because

Removing face shims from the midvalve has the following efect(s)....


...and so on.
Enough info so that we can see the IF > THEN causation from a given change...followed by theory, the most important part!


Also, any discussion on the effects of stiffer or softer ICS springs/bladder pressures in the case of a fork would be most appreciated!!!

You have a better handle on a lot of this stuff than some very experienced tuners I've spoken at length with... You have analyzed WHY something works/doesnt work, not simply just experimented with it until it "worked".

  • 450exc115

Posted October 19, 2006 - 09:39 AM

#11

I'm sure JC will do a better job explaining this but I can at least answer generally to some of it.

trail junk and rock heads are mostly MV with a little BV. Pot holes are nearly all bv. Whoops are a combo of both MV and BV.

ICS springs are a double edge sword. First off they do add a litte linear spring force to the whole fork setup, second they change the pressure rise and fall in the cartridge based on spring force, and lastly they can change the effectiveness of the valving.

Not owning a dyno I would love to see how float changes LS damping. Use the same valve stacks and just change the float type garphs.

  • punkrock MX

Posted October 19, 2006 - 11:41 AM

#12

thanks john, ive been reading what you posted alot (it takes me awhile for things to sink in) so i appreciate it

would you say another benefit of the TC forks is that its easier to make trackside adjustments since the valving is on top underneath the caps? it seems like it would be easier for a suspension tech to pull the caps off between motos and make adjustments as needed...

  • 450exc115

Posted October 19, 2006 - 01:29 PM

#13

Yes make BV adjustments is very easy on TCs.. Well at least ones without bladders.

  • thillsam

Posted October 19, 2006 - 02:16 PM

#14

Not owning a dyno I would love to see how float changes LS damping. Use the same valve stacks and just change the float type garphs.


Me too. I would also like to play with the spring rate and/or preload of the spring that holds the MV stack against the face of the piston - this HAS to have a drastic effect on when and how the MV opens...JC, do you have any insight here?


Also, how much effect does the inertia of the MV stack have on how it reacts? I hae a hard time beleiving that inertia does not play into the equation...somethign like the "inertia active system" that Edelbrock automotive shocks use for opening the valving during very quick movements of the suspension and keeping it closed during slower movements?

  • 450exc115

Posted October 19, 2006 - 07:27 PM

#15

thillsam, most mv springs on the market at the mv will do little compared to the force of the oil moving through the valve that open the face shims.





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