Some thoughts for PDS Shocks

[Jeremy Wilkey]

I've been waiting for this day! I'm ready to bring a long standing debate to the front of the line..

It is intresting to me the amount of misinformation going around about PDS systems in general. So having had the opertunity to run some simple exspereiments, exchange information with crediable sources I thought it would be finnaly a good time to bring this up. We can presnt the facts over time as you have jsutified your own reasoning.. But for those who love suspension we have plenty to talk about.. I'm going to the WP factory here in a week. We are surely going to talk about many of these things so I can bring back some perspectives form the enginers who have created this wonder of function.

So Issues we need to adress,

Nose pressure: What is the role in a PDS?

Piston? Does frinction count?

Bladder? Whats the purpose?

Nitrogen? Does volume have something to do with it?

Let the debate beging.. I'll be checking in and reporting.



Regards,
Jer


PS vote on first instict, then lets talk amoungest the group.

 

[MXP1MP]

I voted for the shock piston.

 

[Harry Reed]

Well Jer, here goes.

I think that before I, or anyone gives their theory on anything to do with suspension, we need to issue a disclaimer. I guess it should go something like this.

As we go on through the years building and tuning on these things we begin to understand one very important thing. We really do not know as much as we think ! The best thing about being a suspension geek is that almost every day I learn something that I did not know the day before.
It is still a black art that even the best engineers in the world still strugle with.
What works for one person, the next one hates.

Take for instance the debate over gas piston vs bladder. I installed a bladder in Two customers KTM SX 250's this year, one liked it, the other wanted to hit me over the head with it. The big differance was riding ability.

The guy who liked it was a better rider. I like the piston better, I can control oil volume more prisicely with the piston, I also feel that I can run lower gas pressure with out the fear of dumping & cavitation. I think that it creates a higher nose pressure, sure there is more friction, but with all that area of a bladder, do you think it would create more heat?

The fact is that they both work. I am going to do some testing in the near future on some cup cars by running a bladder against a gas piston damper. The car will have a PI system on it, so I should be able to get some very good info for you.
Hope that I did not ramble on to much.

 

[podia]

What I feel about piston is that if lack of maintenance, there will be sticktion caused and it wouldn't have a smooth movement.

I have once strip down a 98 KTM PDS shock and are shock to see that the piston are stuck! It seem to be lack of of grease, low on nitrogen pressure, and less amount of oil left.

After replacing the piston ring with grease and some polishing on the internal wall of the canister, I can hear some piston "movement" sound inside when I compress the shock on the bike. It did goes away after some riding.

I feel that a bladder would be more sensitive to smaller bumps.:D

 

[MACE]

I'm surprised if there is a noticable difference, but fundamentally I prefer the bladder because the piston has more inertia which means it will resist changes in velocity more than the bladder.

I think the bladder is a bit easier to assemble as well. My experience with the piston is very limited though (only once - my shock deflowering....).

Of course if I were designing the shock and there was little if any difference in performance, I would choose the bladder. The piston is much more expensive to produce.

 

[marcusgunby]

I fitted a bladder to make serving easier on a 00 PDS and i could not tell any differance what so ever.Im a 170lb exp mx rider.

 

[Hevster1]

Bladder vs piston

It boils down to whatever you like better and what your conditions are and what your setup is. Drew Smith of WER who is a w/p distributor and ISDE multi time medalist did mine and used a wer bladder. Drew is from North NJ as am I and knows the terrain I ride on. The shock works very well although I would like a wider range of adjustability as in more clicks on compression. That is not Drew but WP's design. It also depends on if you race MX, Enduro or just trail ride.

 

[Jeremy Wilkey]

Ok.. Good points here are some thoughts.. We need to spend some time giving reasons..!!


Ok.. Here are some intresting facts..

Bladder designs.. ALl the avlaible systems use a KYB 46mm x 103 bladder.. This is important to recognize. It has no difrence in function in terms of mechaincal area. The shape does not matter.
The Cap made by WER, and UltraMAX have significantly less nitrogen volume!

What does that do in relation to nose pressure? Increases the rate at which it rises signifactly. So the nose pressure is nearly twice that of the standard shcok at the end of the travel.



Factory connection runs a longer tube, that I beilve adds oil capacity not nitrogen volume. This may help temperature issues.


PC, and Factory R and D make a kit with a bladder and nitrogen volume exstension tank.. These actually result in a nitrogen volume that is still 25% less the standard.. Which you guesses it results in a much higher nose pressure still.

So the advanatage can't be a reducing of pressure or area.. Simply the facts don't add up. Ok which leads us to service ablity... The Bladder does make it easier to work with, for the average guy.

Friction... I think we should talk about this detail.. What is the ratio between the piston and the wheel? I've figured it out, and I want you guys to think about this point, and how it may impact shock performance.

Heat: This may be the best argument I've heard yet..How does the bladder impact the distrobution of heat, and the specific heat capacity of the shock.


Cool Keep it up! Talk with you in a few weeks...


JW

 

[marcusgunby]

A few weeks :( im really starting to think about this topic which i havnt in the past-the hole nose pressure thing -does it end up just bieng a spring which we have to take into account when lloking at the shocks intended use?im really surprised about the bladders all bieng the same.And im surprised N2 volume is reduced-this in an age where most are quoting increased N2 volumes,i will leave the friction part to others as ive read a post of your before that goes into the maths and it convinced me.

 

[pursang262]

When do we get more info on this? I'm about to get my shock revalved [98 exc 300] and the shop doing the work recomends the WER bladder kit. I like the idea because it will be easier for me to change the oil, but not if it's a performance or functional disadvantage.

 

[Sage]

Well Duh! The piston looks better!! While you there, tell them to put a schrader valve in those darn things, that charge bar is $$$

I think for everyday use, and the lack of service from the everyday rider, the bladder is better -

*it won't seize up from lack of maintaince
*its simple
*its easy to bleed air from when reassembling
*no piston depth to set

For race use I feel the piston is better -

*consistant movement in the body with little to no change when the shock temp get hot
*it dosen't have a shell that gets soft with high heat and may mush and distort
*the piston is thicker and will not transfer as much heat from the oil into the nitrogen resulting in more consistent nitrogen pressure
*piston depth is adjustable, the "bag" is not

 

[rollingp]

pds shock

PDS works great except for high speed sharp bumps and breaking bumps.
valve in more high speed comp and rebound.
Also no two progressive springs measure alike even if they are rated the same.
Watch out on any aftermarket springs the qaulity is not very good.
Look at the ends of the spring where they are ground flat.
as the spring comes from thick to thin for the ring a bad spring will
go from thick to thin and back again instead of a nice progressive
taper. This also make the spring bow and rub on the shock body more than
usual.I have sent back allot of springs because of this.
I have had pds 6 springs start out higher and end way over there advertised rating. This is one of the reasons some suspension guys recommend straight rates. It is easier to make consistant straight rates than progressive.
A pds system works ok with a straight rate but will work much better with the right progressive . KTMs turn even worse with straight rates on them because
they dont hold the back end up enough. Some people can make riding adjustments for this and are ok with it. Straight rates do work good on the high speed choppy stuff but there is a trade off.
The main thing is to get your progressive tested for its rate at 2inches of comp 3 inches and 4 inches and write these numbers down. Take any new spring break it in and also measure it on a spring tester.
Remember these guys buying these springs and selling them to you are doing it for proffit. They pay very little for the shock spring ( usually about 35 dollars) They are the same guys that tell you you need new seals on your fork when they leak but dont tell you all you have to do is wrap masking tape around the legs and cut the end at an angle work it into the fork seal and turn it to remove the trapped dirt.And they dont tell you how to tighten the springs on the seals buy unscrewing them and cutting out 1/4 inch and screwing them back together. Espeacially on 50mm coventionals.For 180 pound rider a spring that starts out a a little higher rate but ends a little softer at full compression is the ticket.I have found a good pds 8-265mm
has the best feel. But valve the shock and dont waste your money on a bunch of gimmicks like gold valve and bladders.

 

[MACE]

Re: pds shock

Originally posted by rollingp
For 180 pound rider a spring that starts out a a little higher rate but ends a little softer at full compression is the ticket.

You mean starts out a a little higher rate than stock but ends a little softer than stock at full compression, right?

I don't think a falling rate spring is possible. That's what I thought you were taking about at first.

 

[rollingp]

spring

yes say for example a pds 7 starts at7.6 and is rated at 9.5 compressed.
The better would be 8.0 ending at 9.5.
The only spring that is close to that is the pds 8 which is 8.1-9.95.
Remember though just because it is rated at 9.95 does not mean that is acurate. My stock 02 pds 7 is rated at 7.6-9.5 and when tested on a spring tester it was 7.15 at 1/2 inch of compression and jumped to 8.9 at 2inches
9.5 at 3 inches and at 4 inches of compression or equal to it being on the shock itself was at 10.3.
If a guy can get his hands on some progressive springs in varying ranges,
take a day to test them as we did the results can be pretty surprising.
Personally I like last years pds 6 spring which seems to not coil bind the first 3 coils of the shock as soon as the pds 7 and for me that works much better than getting into the stiff part of the spring so early in the shock stroke.
For every big bump there are a thousand small bumps and if the shock is riding in the stiffer part of the spring too early than the part of the spring that is supposed to absorb the small high speed stuff is not being used.
This is what myself and Kelly Smiths x Mech Mike Wiesner and I have found on this years 520sx. We also lowered the fork oil level to 140mm of air chamber
with 1.5 turns from min spring preload . The comp set at 12 out and reb at14.Shock at 35mm of unladen sag , full out on the high speed comp 18 on low and 12 on the reb. We race allot at Red Bud and we also have a very good track hereto practice on.This really helps. This setup isnt for everyone
but for me it works great.

 

[Jeremy Wilkey]

Drum Roll Plese... First Off let me say that for all the readers that have argued the bladder case with me you where in some ways correct! The info I got while at WP was very cool and very educational.. I'm going to post what I can for now, and while have a whole report comming soon.. I just need to get final approval of content so I don't say or show anything that will get me in trouble.. But the Bladder Piston Argument is dead for now.. Now we need to find out why it is what it is...

The offical exsplantion is... The bladder works better, when the nitrogen and area are equal to stock. The diffrence is slight, but the major diffrence is the cost. A bladder is much cheaper to produce. The bladder with less volume was not as good, so for all intents and purposes the kits offered now may in fact not be as good as the piston.

Here is a picture of a test bladder. This bladder actually has a reduction tank added to one side that allows the nitrogen volume to be cosinstent with Stock.

As Sage pointed out the bladder is a more reliable and less likely to fail. When a bladder fails thew nitrogen volume is lost into the oil and the shock completly goes away. If a seal fails on Piston type shock the piston wont' rupture and while the shock won't work properly it will likely be more rideable.

Future WP shocks will come with Bladders.. They will be decreaseing nose pressure by increasing diamter, and also increasing volume.

 

[Jeremy Wilkey]

Here is the top Veiw!

 

[svi]

Originally posted by Jeremy Wilkey

PC, and Factory R and D make a kit with a bladder and nitrogen volume exstension tank.. These actually result in a nitrogen volume that is still 25% less the standard.. Which you guesses it results in a much higher nose pressure still.

Friction... I think we should talk about this detail.. What is the ratio between the piston and the wheel? I've figured it out, and I want you guys to think about this point, and how it may impact shock performance.

Jer,
I understand that a reduction in the volume of the nitrogen would lead to a more significant rise in nose pressure through the stroke but I'm not sure how this would effect the performance of the shock, could you explain please?
Could you also go in to a bit more detail about the point about friction you brought up?

 

[Esp1]

I voted for the piston.
I believe it's better b.cause it has more adjustabillity and less heat-transfer to the N2. But it have to be a good quality piston, the first pistons on the Husky/Sachs shocks where crap and the shocks faded bigtime!

 

[Jeremy Wilkey]

SVI,
The rise in pressure can lead a shock to have harsh and unresponsive feel, It can also be used as a spring force. But when the volume is lowered the rise from intial to ending can be signifcant..

I would say it really has to do with how the suspension feels in midcorner..Espically hard pack..

Friction, I have argued that friction may be the only reason some have a positive veiw of the bladder, but have had a hard time understanding this point. When you consider that the wheels movement, in realtion to shock, and then factor in piston travel in realtion to shaft you have very little motion and friction almost becomes a constant..

I belive that if any real improvement actually ocured it may have to do with better heat transfeer from the oil..

Regards,
Jer

 

[John Curea]

KYB bladder in the PDS

I found that the KYB bladder (that has alot less gas volume that the stock piston) didnt seem to have the force extending the shaft as the piston. I realize that the gas pressure rises faster with less volume(as in using the KYB bladder) when the shock is compressing. On the flip side , the pressure seems to drop faster when extending.

With this large change in gas pressure as the shock is moving, I would think that it would work against the overall performance.

Take Care, John

 

[shed]

yeah it does. trick race car dampers (ie F1) now have a through rod, thus eliminating the nitrogen gas "spring", and this apparently results in a much better damper with less hysteresis.

Jeremy, have you ever run a piston vs bladder dyno test?

Shed

 

[BRush]

Originally posted by Jeremy Wilkey
…Future WP shocks will come with Bladders.. They will be decreaseing nose pressure by increasing diamter, and also increasing volume.

Just out of curiosity, did this ever happen. Are the 03 KTM shocks a bladder design?

 

[marcusgunby]

No bladder in 03-bigger N2 bottle however.

 

[DEANSFASTWAY]

I like Screw& tap Schraeder valve OK except can get close to exhaust pipe . Still looking for that PDS spring rate table , Can anyone post or Link ?

 

[Jeremy Wilkey]

Shed,
I have not.

Why waste the time? Really though I don’t see the difference technically it’s a function of rod area, and compression diameter of the reservoir.. I know this keeps coming up and in reality maybe someday, someone can go dude your wrong and here’s why.. And I go wow your right dude, that was stupid on my part, or better yet I’d be like F”” cool! But In all my travels I’ve never meet a ligament suspension engineer who could give me a reason, or could confirm any mechanical / material advantage to the bladder.. The only reasons that continue to crop up in urban Shock lore are.. Due to poor and incomplete/invalid scientific methods.. The big one being reduced relative fluid density due to the large amounts of air induced into the system.. Its about saving money, or make it easier to work on.. For the record I’m going to do some of my own testing this year on this just so I can offer some the well “you haven’t tested it-its” less fotter…

I did get a chance to test some interesting theories, and make some cool observations.. In my never ending quest to better determine the role of the damping aspects.. IE active versus passive damping. I had several of the Ohlins guys’ kind of looking at me weird but the theories where proven by observation.

I eliminated active and did passive only mapped the contribution, and then switched. The overall damping rate was merely the sum of the different values at the same rate..

Here was the wildest one of all though.. I've always argued that the passive valve and base-valve play ping pong inside the shock bouncing pressure waves off each other. (This is very relevant in open cartridge forks..) When I started to realize the pressure the shock worked normally, down to about 1 bar of pressure.. Then things got funky..

Dean-Just drop me an e-mail! I'll send it right over.. Let me see if I can copy paste..


SPRING NUMBER PART NUMBER C1 C2
PDS0-250 9121.0013. 6.5 8.3
PDS1-250 9121.0009. 7.0 9.0
PDS2-250 9121.0010. 7.5 9.7 C1 @ 20MM
PDS3-250 9121.0011. 8.0 10.3
PDS4-250 9121.0012. 8.5 11.0 C2 @ 100MM
PDS5-265 9121.0014. 6.6 8.6
PDS6-265 9121.0015. 7.15 9.05
PDS7-265 9121.0016. 7.6 9.5
PDS8-265 9121.0017. 8.1 9.95