Low Flow piston from MX-Tech for PDS-Advantage?

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#1
Jer,

I think I understand the advantage to shim changes and how that can customize a fork or shock for a given rider in a given application. That said, I would think that unless there was a problem with materials or a fundamental design flaw you wouldn't need to change the piston/valves.... What is the reasoning/advantage behind changing to an aftermarket piston or valves over just reshiming? (and adjusting oil type, amount, Nitrogen pressure.... ).

I'm really curious about your new piston for the KTM, being as I just bought a 520:confused:

Trying to be an educated consumer:cool:

WH
 

Jeremy Wilkey

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#2
WH,
One one hand I go alright a great question I can really go deep! On the other hand I go O my God such a simple question, yet one of the very most difficult questions in the world to for suspension builders as a group to simply answer. To answer this question we both need to look back into old threads and think intensensley about this topic for the average guyt to get even half a grip.. I will once again try to boil this down as simply as posible.... :)


General assumputions.. The PDS Shock moves realtively slow comparied to most Jap shocks.. It has a large swept volume of oil 50mm body tube, and the motion ratio is very close to flat.

With a stock highflow piston the tendency is to be overly stiff on the low speed so tractablity suffers, and the ride seems over stiff and somewhat unresponsive. With creative shimming these tendencies can be worked with but not maximized to an ideal realtionship..

Many people have the notion that a highflow piston offers no restriction to oil movement. They are compltely correct, however the alternative view that a low flow piston offers a certain fluid restriction or damping. This is completly untrue. (at least with current designs.)

LOW flow or high flow it's a bogus descritpion if you want to take it literally. A Pistons Job is to direct the oil flow onto the shim stack. The relationship that the piston has with it's shape of pocket, depth, angle, and actuall orifce all impact the deflection charachteristic of the shim stack. This shim stacks deflections determine what the acual size of the effective orfice is at any given speed range.

As a suspension builder it's our job to maximize the relationship of the variable orfice to shaft speed. To use a analogy to calculus, a well designed piston and shim stack are like the intergal problems where you are trying to calculate the area of an ireguarly shaped object. To get a quick guess we could take the area and draw a few squares under it's slope. This could give us a rough idea of area and it could also be simliar to a average damping coieffecent. But in racing a average coieffecent is not good ennough, and we must as designers contunie to break the aarea down more and more times so for each incroment of speed we have a corispounding unit of damping. To complete my anolgy if you integrate the eqwuation you can break the area down infinetly and solve for the exact area..

We are not there, no one is, however we have and contunie to try to further maximize or piston and shim stack realtionship in a wayt that optimizes the shock over the widest range of shaft speeds.

Some particular design challanges to the new PDS piston... Size... you have little room to work with, the Id is 16mm... The ht is very thin if you wish run dual pistons your in for trouble without getting creative.
Stablity.. You have to maintain a double bearing for stablity and integrity. For a long time we where devloping a single piston application only to ultimately realize we had gone down the wrong path ( but we learned a lot.) Materials AL won't cut it in this application. The accounting between the 2 pistons. The compression and rebound do overlap in some situations.. Now you double your varibles...

I really have grown to love the PDS suspension..I was laughing to myself (reminicing) today as i was working on one... Man in 1998 when I took the first one apart I was shocked, and somewhat skepitcal and held it in high contempt.. Today I can say i've learned more about suspension from this Shock than I have from all the others in at least in total discovery over the last few years.. It's single greatest aclaim is that it is very diffrent.. At times I find myslef comparing whatever I might be working with to what seems most comfortable, today I compare all things to the PDS.. Weird.. In a suspension builders kind of way..

Hope that helps..

Jer
 
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#3
Jeremy, kind of off topic, but just by the way you explain things and go into such detail, I can't help but appreceate what the quality of your work must be. You or one of your associates will definetely be getting the suspension of whatever bike I get next!
 
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#4
Okay Jer, I read that about 6 times and I'm getting a fuzzy picture of what a you're talking about and what a daunting task suspension design is from an engineering point of view.:D

I started ask questions but realized I've homework to do first!:eek:

On a practical level..... How on earth do you convey this information to your Associate Shops? The issues with the PDS system make me reluctant to send it to anybody but you....

Now where did I put that shipping box!?:cool:
 
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Jeremy Wilkey

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#5
WH,
My operations manager,( Barry Wilkey), my other staff, heck my parents even becoime involved.. (You might ever know it but MXT is a faimly business.. :) ) (thats another story....)

What I'm saying is this.. We spend massive amounts of time being selected and ultimatley being very selective ourselves. We don't bring people on board who don't have what it takes to deiver high qulaity work and good customer service. We do everything we can to contuinely educate and train SAs and stay on top of any issues. We have formed a solid team, we all have similar goals, and work together on a day to day basis to make the todays happen so tommrow will be here for us.

I'm getting a little out of charachter here so I need to be careful, I don't like to plug MXT directly so I'll wrap this up.. The SA's contribute in there own way honestly I can say that the very information I post on this forum is a direct result of teaching others, and secondly the imput of the SA's as they have critqued me and added good info along the way..

Have no worries... I stand with the rank and file of our team...

Jer
 
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#6
We spend massive amounts of time being selected and ultimatley being very selective ourselves. We don't bring people on board who don't have what it takes to deiver high qulaity work and good customer service. We do everything we can to contuinely educate and train SAs and stay on top of any issues. We have formed a solid team, we all have similar goals, and work together on a day to day basis to make the todays happen so tommrow will be here for us.
Jer, it shows in your work and service. I will stop commenting off topic here now.
 
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#7
Jer,

.) Materials AL won't cut it in this application. The accounting between the 2 pistons. The compression and rebound do overlap in some situations.. Now you double your varibles...
What exactly did you mean by the rebound and compression overlaping in certain situations? I'm a little confused. Also what problems did you encounter useing aluminum? Hope I'm not prying, but I'm rather curious!
 

Jeremy Wilkey

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#8
Bud-man,
The steal shock body of the PDS uses a composite metal piston ring. This ring does not wear well on AL pistons, even harder grades. Secondly the shock piston set-up needs lots of suport so a small Garlock band does not work well either.

The overlap I'm reffering to is the fact that the primary and secondary compression and rebound stacks overlap at times regarldless of needle shaft postion.


Regards,
Jer
 
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#9
Jer,
Thanks! I figured that's what you ment regarding the damping and I also suspected that this was occuring.