A new piston Design

[Jeff Howe]

Motojunkie,

Your right on track man. That is a concern of extreme importance when you design a piston. PA did this also by raising the surfaces, leaving a larger gap between the shim and port mouths. RT accomplishes it by reducing face shim diameter. The less that flow is interupted the better.

Consider the current design of the stock KYB 28mm fork piston. That is excellent design. With smaller ports it would be the bomb in my opinion. A little work on the entry points might be nice too. But for a stock piston it is awesome.

 

[motojunkie]

Just so you guys know, I'm a suspension newbie. I'm just trying to make educated statements using what little I know about suspension theory, and what I do know about fluid dynamics.

Shocknut - I think you may have touched on the answer to Jeremy's question. Are the bridges there to create a larger gap between the shim and ports? The reason I wanted to see the better picture is because I'd like to see how the bridges are shaped, so I can determine if the are designed to channel flow. Oh, well. I guess we may get some more tomorrow. :)

 

[MACE]

Caution: Hacker stirring the pot

Guys, I couldn't see the picture until I turned the monitor brightness to 11 (you know, like on spinal tap...).

I don't know about these new pistons....

Bell shaped entrance and exits will decrease the flow resistance through a nozzle at any given flow rate, but I guess I thought that the "choking" effect was part of the very high speed damping on small port - high velocity pistons. The bells could be an attempt to make this choking come on more gradually.

I'm a bit disturbed by the lateral troughs. Wouldn't they introduce a flow component almost perpendicular to the nozzle axis? That would cause turbulence.

If the object was to provide less feed restriction in plan view (like I think Shocknut is suggesting), wouldn't you be well served by using special shims shaped like maltese crosses? The Austrians might dig that... Hey, I could even calculate the spring rate of stacks then!

Devil's advocate, signing off.:debil:

 

[Mark Hammond]

The bridges are to support the shims so that they dont bend the opposite way that they should and A/Cause a restrction through to the opposite shim stack.B/This also stops any unwanted free flow through the piston because if the shims bend the opposite way into the ports then they also lift away from the seat of the piston surface they are there to cover.

This is not a very technical explanation but at least my spellings better than yours Jer.

 

[Jeremy Wilkey]

Mark,

Because your a buddy I won't take insult over the spelling.. :) I know to the average reader this is a problem with my forum. For those who don't know I'm not a hack, but do have a problem. (its amazing what they gratduate these days is it not..)

Your right about the posts in the corners. However the fluid flow causes the shims to vibrate and chatter on the piston face. This caused erriosnons on the piston edges. You may be thinking that the bend causes the piston to leak the other direction, it dosen't. The pressure diferential cuases the shims to actually seal into rebound for instance on compression, and compression on rebound. whats amazing is how diferently the shim and piston wear patern looks with the new system..

Regards,
Jer




Anyway

 

[Jeff Howe]

Mace,

Put a shim on the face of your current shock valve and look at it. If you can't see what maybe even you might consider a "potential" flow restriction there I'll be surprised (relative to load). Your shock valve port mouths are completely covered by the shims. Now do the same with your Brass doorknob. You will see a difference there, and you will see why. What he did with this thing is good. The shim deal is a good idea, but a financial burden, at least to me it would be.

It's pretty simple really, its not a deal that we really need to twist with overengineering. Whats going to flow smoother, a straight path or one with a baffle in it? The less angle you can put on that baffle towards straight the better off your going to be plain and simple. I would hedge my bets that in simple hydraulic principles you do not really want your ports playing hide and seek with oil flow. Perhaps an application like that may exist, but I don't want it in my shock.

When I put my sled shock piston together, I went to GV type layout, but mine is on a 30/60/90 layout,in appearence it may resemble a Penske standard. It is not out of square like the GV. My compression feeds are 50% exposed beyond the rebound shim edge. I made sure I had a nice gap. I combined both the PA and RT approach to making sure the potential feed restrictions are minimized The less potential restriction I have on the "feed" the better off I am. There is better design options out there but they are expensive to machine. Most people do not want to spend a $100 on a single piston though.

If your restriction on the feed end is too great,( due to baffle type restriction) your shim deck height is too low, you run the possibility of creating a vacuum in a bad place. You can consider the consequences. Still, it can all be relative to loads placed on the damper. Sometimes though you just can't leave much to chance. I believe Jer has done well with his improvements. Very much in fact.

 

[KiwiBird]

Any of you guys ever consider a double piston with the shims in the middle, there would no restriction at all on the feed ports then.

 

[Jeff Howe]

Cool Idea Kiwi!:)

 

[Pete Payne]

Jer,
Good to hear you are friends with Martin of Elka , I love their stuff for all of the Quad guys . Elka stuff is very high qualty, and Martin is a good guy to deal with as you are . You guys are not affraid to tell about what you have and are learning . This is what is great about this and all of the DRN forums.
I love this thread !!!
Ilike the even layout of these pistons , should be good for shim wear and long shim life. The more support for them the better>>>!!!!
Pete.

 

[moto242]

Will,
sorry man, i thought you were being ****y-

anyway, i'd like to hear in more detail about kiwi's idea

 

[motojunkie]

Any of you guys ever consider a double piston with the shims in the middle, there would no restriction at all on the feed ports then.

If you do this - kind of like an oreo ;) - there will be no room for the shims to deflect. Or, am I not interpreting this properly.

 

[Jeff Howe]

You ever seen a KX style midvalve stack?;)

 

[motojunkie]

Never seen the KX midvalve, I assume it's just like Kiwi described? :o

 

[Jeff Howe]

It tapers down in one direction, then tapers back up in the other. For what Kiwi's suggesting this is what he's looking at for a stack build. If the room was there to do it, it would be cool idea to play with. I haven't put anything together in thought with it yet, so i can't comment much further on it. My first thought though was if there would be enough room on the shaft.

 

[KiwiBird]

How much room is left in the body of a stock shock at full compression?

I have a Penske shock which has a top out spacer in it which could be removed and there would be plenty of room for a double piston.

I thought about this last night and I think you could have ported paths for C and R which would be completely independant. I think the thing to contemplate would be the oil path taken during high frequency like stutter bumps as the oil flow changes direction.

 

[MX-727]

Instead of dual pistons, what about offset cross shaped shims? One stack would be rotated 45 degrees relative to the other. That would seem to accomplish the same thing as seperate pistons.

 

[marcusgunby]

KTM have special frok rebound shims(to do a simialr thing)-not many people stock/like them because of costs etc.This would also apply to 2 piston shocks-more costs in production.The bean counters usually win in these matters.

 

[Jeremy Wilkey]

Intresting stuff. I invisoin the the old KYB or WP pistons/ shims where the oil flows throught the shims from one side or the other depending..

I think the current system actually address these issues quite well, however as all your imaginations tell you there may be a better way.

The best answer comes down to who can acheive the best ballance of varibales. We gain in one area we often lose in another.

Suprising to some I'm sure, my latestest design was the result of a failure actually. I assumed I had found the most logical way to asses design fetures that I could. Even though my best scinece told me I was making a good descion, the results with my tester proved to be catosprhic failure. I'm glad it happened as It provided insight into tings I had not even begun to address, or understand completly..

That moment actually was the reason I love the feild in which I'm lucky enought to make my lifes endevor.. As far as I can tell its a never ending challange.. And thats abut the only thing in my life thats maintaind my interrest...

Regards,
Jer

 

[SoCal Steve]

It seems clear that reducing any feed restriction to the piston ports would be beneficial in reducing turbulence.... increased flow for a given orfice size, with no restrictions or sharp edges to cause a disruption or thru the orfice. We need to remember that the "low flow" or small orfice size piston still needs to move a similar amount of fluid so as not to become harsh in a high speed situation. This means that the the velocity of flow must be very high. This high velocity should create a low pressure ie. carb venturi. So what about the shim side of the piston? Is the step into the valve pocket area then beneficial in creating a higher pressure in that area? I know the pocket must be there to create a correctly sized port window at different shim deflections. It seems that there would be a large flow disruption in this area (and I mean besides the shims being a disruption!). The fluid would need to turn 90 degrees to get out to the port window, then turn again 90 at the shock body wall. Is this an issue? Would ports splayed out to direct fluid at the shim edge be more effective? But then we're back at the problem of feed side restriction. What about a venturi shaped port which flared out closer to the valve pocket edge and maintained a small inner diameter to accelerate fluid velocity. I guess what I'm getting at is whether or not flow disruptions at the shim side of the valve are significant.

Steve

 

[MACE]

Originally posted by Kiwi Bird
I thought about this last night and I think you could have ported paths for C and R which would be completely independant.

I think independant C and R paths would be a requirement.

Imagine the compression flow coming through an upward facing port then between the face shim and valve mouth. Next the flow runs into the rebound stack and has nowhere to go. So you have to add a port to the rebound piston for the comp flow to pass through and a check plate on the bottom of the rebound. Now do the same on the other side to handle rebound flow.

I think we are going the wrong way with this concept. The lack of check plates on the current shock valves is a good feature IMHO.

 

[Jeff Howe]

Sure we are, but it blows the cobwebs out of peoples minds, gets them thinking a bit. Thats energy dude, energy is good. Even when you have an idea that is "not quite there", your mind is getting exercise. I like to see that come from people.

Where this is all going is right back to where we are and making improvements to existing product is just as exciting sometimes. I have ridden on the pistons we began discussing here for 2 years now and it was very good before, now it's just got to be plain better. My potential concern with them has been addressed.

I have been informed by another source that these changes are merely cosmetic, and I publicly beg to differ.

Now, Socal Steve has brought the other end of this thing into discussion, shall we go there?

 

[SoCal Steve]

Shocknut;

Here are my thoughts:

I would think the supply or feed side would be most important to not have any disruption or restriction in flow; especially with a low flow design; because we want the orfice to be the point of most restricted flow. That way we are sure that the fluid velocity there is as high as possible.... moving the most fluid for given shaft acceleration ie. high speed compliance.

Maybe the sharp transition from piston orfice to valve pocket is an ok thing. The fluid coming thru the orfice and the resistance to opening of the shim stack should ensure that there isn't cavitation at this point. The area more likely prone to cavitation would be just after the exit at the port window as fluid is released from the valve. Pressure in the shock should take care of this for the most part.

The piston is going to flow a set amount of fluid as a function of shaft travel. This amount of fluid is going to have to also move thru the port window at the shim stack. The stack will have to deflect enough to move an equal amount as is entering the valve orfice. The only question is whether any restriction at the exit of the orfice would cause harshness..... I would think that again the most restricted point would be the valve orfice when the shims are off the seat. This is much less area than the exit path thru the valve pocket and around the shim stack at the body.

Am I missing something?

Steve

 

[Jeff Howe]

I'll be back later tonight on this....I hope.