marcusgunby

LIFETIME SPONSOR
Jan 9, 2000
6,450
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Right its been a bit quiet here so im going to give a run down of the pds shock and its history.

 

In 1997 KTM were making quirky bikes many enduro riders loved-in mx they were less liked and rode by strange people.

Late 1997 KTM released a totally new  limited edition 125.It had a new engine and chassis and a link less rear suspension.They called the shock a PDS , progressive damping system i think it stands for-couldnt be less true in reality but we will talk about that later.

The late 1997 KTM 125sx had a ohlins designed shock, for 1998 KTM wanted ohlins to porduce the shock in larger numbers but they declined as they felt it wasnt cost effective for the price KTM wanted to pay.So KTM who owns WP got them to produce it.

Now the original design was British and had a patent on it, ohlins wouldnt let WP build it the same, so WP built a slightly different version. (BTW this is all from memory so it maybe not perfect)

 

So WP designed a shock which attached straight to the swingarm, it has no linkage but does have progression as its angled and this gives a small amount of progression.

To stop the heavy bottoming that would come form a soft progression WP needed to slow up the shock movement on harder hits-they decided to build a 2 stage damping setup.

This is done internally by having 2 pistons and shim stacks.The first piston is used all the time as in a conventional shock.The 2nd piston moves with the first piston but doesnt create any damping till the last few inches of travel(like a linkage bike has way more damping in the last few inches of travel)how does the second piston not have damping for most of the travel but then suddenly comes into play you ask??? well it has a great big hole in the centre and all the oil just flows through it, UNTIL near the end of the travel, the shock body has a great big needle(at the top of the area where the piston move up and down) that plugs the hole.So when the hole it plugged(last few inched of travel) the oil has to go through 2 pistons and 2 shim stacks.This causes a massive increase in damping.

That is basically the design,

How has it changed over the years??

In 1998 the PDS had a straight rate spring and a very long internal bottoming needle.Another strange design was the bleed for the rebound actually ran all the way down the chrome shock shaft and through the adjuster clevis(big bottom part that attaches to the swingarm)also the compression adjuster worked backwards-in was softer. 

In 1999 they made the needle shorter so the second piston only came into play at a later stage.Also a progressive rear spring was used.

In 2000 they made some changes but i think they were only small.Progressive spring used.

In 2001 they changed the rebound bleed to not go down inside the shaft and through the clevis and it was more like a conventional shock.(sorry if this part is confusing but its very hard to explain this part unless you know how a conventional shock works)progressive spring used.

2002 was a step forward, they used basically a semi factory SXS shock in production, this had a totally new adjuster with adjustable  high speed.This adjuster works the same sirection as a KYB/Showa , in gives stiffer damping, I havnt seen inside a post 2001 so this is all i know.Still progressive spring.

2003 refined the 2002 shock and used softer progressive spring.Also used a copied idea of a top-out spring-this is a small spring that sits inside the shock seal head and gives a bit of movement when the shock is fully extended like when you nearly high side. I believe this idea comes from road bikes. 

2004 they have gone totally full circle and are uising a stright rate spring and a long damping needle so the second piston comes into play ealier in the stroke.

 

We will talk about design flaws once any questions have been cleared up.

 

So fire away:)

 
 
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Zenith

Member
Jan 11, 2001
483
0
Very interesting, thanks!

Why don't more bike manufacturers go for a link-less setup? Surely it is lighter, cheaper, simpler (well the shock itself might not be but from an end-user perspective it would be) and most importantly it gets us away from having to grease linkage bearings every other week! I'd nearly buy a KTM for the last reason alone :) !
 

marcusgunby

LIFETIME SPONSOR
Jan 9, 2000
6,450
2
Philip the answer lies in the design flaws of the pds-can you have a guess at any of them?Look ate your questions one by one and see if you can think of your own answers and i will give you my opinion;)
 

russ17

Member
Aug 27, 2002
301
0
I had a 2001 Pds shock come in a while back when I took the shaft assembly out of the body there was alot of metal pieces stuck in the valving. Well to make a long story short, the cause of this was when the piston bans wears out it allowed movement of the piston shaft essembly which in turn when the shaft entered the needle ( at the bottom of the body or top depending on how you look at it) there was a misalignment between the too, causing the inner part of the top of the shaft to break. Kind of a chipping away action. I dont know if you would say this could be concidered a disign flaw or the fact that if you dont have these shocks serviced frequently and checked for wear it could end up causing you a shaft. This is just my opinion.
Russ
 

marcusgunby

LIFETIME SPONSOR
Jan 9, 2000
6,450
2
Not seen that one before russ, i guess its not a design flaw as such ,just the fact that it will do that when worn, a bit like when a normal shock wears the bush in the seal head, it can loose pressure due to sidewards movement of the shaft.
 

dthoms

Member
Oct 6, 2002
175
0
Marcus,
I have a 2002, and wonder how the WP units differ from the Ohlins?
I rode a 02 Cannondale and couldn't believe how well the rear shock worked in different terrain. The Cannondale also had no linkage but the it felt better in almost all areas, and it was heavier too.
All in all I have a bigger problem with the forks on my KTM than with the shock. I feel they have to much flex and a harsh mid stroke.
The mid stroke might be able to be massaged out but I'm afraid the flex is there to stay.
I played with my suspension all summer last year and never seemed to quite get is but this year I added more pre-load, and higher oil level to the fork, and stiffened up the spring on the rear and it has helped a lot on the fast stuff and it is a lot better in the big sand whoops. But I now get a lot more feed back from roots and rocks in the slower sections.
I'm curious why they went back to the straight rate spring, I have changed out more fork springs than shock springs but it has been my experience that the progressive springs work better for my riding style and area.
The two piston system would seemed to be flawed from the get go. I can't see how you could every get away from having a glitch in the stroke right at the point where the system changes from one piston too the other. I would think that any non-linear action at the deep end of the stroke could cause some odd side to side kicks if you hit something at the wrong angle.

I am not against the linkage systems and I don't doubt my next bike will go back to that direction. Not having linkage is nice, if you don't like to maintain your bike, but other than that I'm not seeing any advantages yet.
But I would like to see what one of those Ohlins would act like on my KTM. I had one on a CR250 and it was one of the best acting rear ends I ever rode.
Well I got to go take my girls hiking so I'll check back later.
DT
 

DEANSFASTWAY

LIFETIME SPONSOR
May 16, 2002
1,192
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Russ theres a piston alignment tool that is used to align the two pistons before torquing them down on the shaft . It helps keep the piston bands from wearing prematurely. Its like a 50mm Id sleeve that slides down over both pistons . Maybe the last fellow who worked it didnt use it or actually the whole time those shocks are in use they atre trying to bend themselves , way more than on a linkage bike Id wager to bet. CHEERS DEAN
 

russ17

Member
Aug 27, 2002
301
0
Yes Dean I am a where on the piston alignment tool. I ended up taking it to JC because he had the tools to do the job. plus he just got that bleed pump in, cool piece of machine there!
Marcus I get very few PDS shocks in and when I do I usually send them to JC so I have little knowledge of these design flaws.( but definetly interested to learn of them)
I would have to agree with dthoms on the issue of the no linkage. From talking with people the only responce from them is it's just nice not to have to grease them other than that no real advantage.
Russ
 

marcusgunby

LIFETIME SPONSOR
Jan 9, 2000
6,450
2
dthoms asks a good question-whats ohlins do differently?
well on the ohlins the 2 stage damping is produced by 2 pistons liek a Wp-however the ohlins uses a smaller bottoming piston -so the oil runs around the outside during normal travel, then as the shock gets deeper into the stroke its smaller piston goes into a recess at the top of the shock body-this then brings the second(bottoming piston)into play.Its very like a bottoming cone idea on a fork.

So the difference is one has the oil flow around the second piston and the wp has the flowing through the centre of the piston.

In thoery the WP design should be better as the needle is very tunable-Wp can alter the angle of the end of the needle and the lengths.So if Wp had a long tapered needle it should mimac a rising rate shock as it will bring the bottoming piston into play in a progressive manner.The Ohlins system cannot really do this and will always have a set point where the damping inceases dramatically.

Not sure why they went back to straight rate springs-not sure they know either.However when i had my 01 i liked the 7.5 straight rate for mx as it gave better steering.

Dthoms do you have the 48mm Wp forks? im guessing you must have the 43s?? as i can only see flex bieng a problem on the 43s?As for the performance of those -its down to the midvlave-the Wp forks are setup with lots of lift and so have a stiff base valve-not a good setup.
 

marcusgunby

LIFETIME SPONSOR
Jan 9, 2000
6,450
2
Ive remembered a good reason for using straight rate springs-the progressives are very hard to get consistancy in, if you have 3 different PDS1s they will all be different.Also i forgot the progressives have 2 different ranges, a 250mm and a 265mm, this was a odd move as the ranges are not very different in terms of stiffness-just the cross over point of the stiffness was a bit different..
 

drehwurm

Member
Dec 9, 2000
129
0
Servus Marcus,

What's up - isn't little Ross giving you enough excitement in your life to start such a thread ;-)

I'm on KTM 4-strokes since 2001 and almost as long I'm using an Öhlins PDS - never came to terms with the WP PDS, even a MX-Tech revalved shock couldn't convince me. The WP PDS system may be theoretically advanced, but obviously in reality it is not. So take a shock with a sometimes "problematic" performance and make it as hard to service as possible. Voila, you've got a WP PDS shock!

On the plus side, removing a PDS shock from the bike is as easy as removing two bolts. Some say, a necessity on those bikes :->

I can't comment much on KTMs move back to straight rate springs, but I think you are right on with production problems being a reason. BTW, the Öhlins PDS has always used straight rate springs.

Some time ago (before the specs of the 2004 bikes were know) Jeremy made a comment about KTM going back to straight rate springs because of marketing reasons - maybe he can shed some light on this without becoming a "persona non grata" at KTM.

Michael
 

marcusgunby

LIFETIME SPONSOR
Jan 9, 2000
6,450
2
drehwurm so nice to have you back-i knew it would just take the right thread to bring you out the closet so to speak.Ross is keeping me plenty busy:) but i wanted to spark up the forum with a good debate.

You notice i said in theory the Wp has more tuning options with the needle set up, than the ohlins.In reality i know many riders/tuners have found the ohlins with its more predictable nature is the one that ultimatly works better for them.

I can see the point jer makes about marketing but im not convinced-marketing is ok but if it rides worse than a 03 it will not sell bikes-no bike mag has ever placed a KTM much off the bottom of the pack(not that we believe them much, in this case they are correct)

So what i want to know from the readers of the thread is why do you think the 2 stage setup of the Wp works badly compared to the ohlins?? there are a few clues in the design differences, the ohlins works well and has oil flowing around the bottoming piston , so ........

and michael you are excluded from answering this part:))
 

russ17

Member
Aug 27, 2002
301
0
Haven't studied the WP design. But from what has been said I woud say there is more friction or stiction than the Ohlins do to the fact you have two pistions that are constantly sealing against the shock body compared to the ohlins that has oil flowing around the piston (until it reaches the recess area.) I would also think that the oil going through the piston would have some what more restriction than the Ohlins
Russ
 

marcusgunby

LIFETIME SPONSOR
Jan 9, 2000
6,450
2
Yes russ correct on both:0
so we know the oil is restricted on the flow through the bottoming piston, what happens as an effect of this?? i know im pushing on this but its no fun if i just give up the answers it took me 3 years to find;)
 

marcusgunby

LIFETIME SPONSOR
Jan 9, 2000
6,450
2
Close but no cigar:)
the main problem tuners have found is when the shock works at very high shaft speeds the oil cannot bypass the bottoming piston , so the oil actually opens up the shim stack on the bottoming piston during the part of the travel where it should play no part.

So on low speed movements we are dealing with one piston, on high speed it actually has both pistons working all the time.This does cause a spike feel and a big kick under breaking bumps, so you valve it softer and now it feels all mushy on slow shaft speeds.Its like a moving target.I proved this by removing the shims on the bottoming piston-it felt very different to ride even when going fairly slow.

Now i forgot to say before, the shock also uses a 18mm shaft-the ohlins uses a 16mm so that makes a difference in how they feel, as the WP displaces more fluid through the compression adjuster.I know not just jer has been modifying the olders compression adjusters by drilling out the ports slightly.The new comp adjuster from what ive heard is a better design and doesnt need modifying.

We could talk about the nitrogen bladder vs piston setup next-i know KTM talk loves that topic:)
 

marcusgunby

LIFETIME SPONSOR
Jan 9, 2000
6,450
2
LOL not ridden a 04 yet-i didnt say it was impossible to get the pds to work-just that upto 04 they hadnt achieved it.Simple facts.

 

KTM riders are such a defensive bunch, i had a CR125 and it had slow gutless engine, the KTM has poor suspension so far.The KX has poor suspension out the create but it easily fixable.The YZ is tall and horrid to ride IMO, the RM has a super soft seat and crampred riding position.Husky's dont even get a mention.There you go i have slagged all the brands:)
 
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