2002 Ktm

[marcusgunby]

Right then the externals first;
It is a largeheavy shock-it runs a 50mm body-only showa as far as i know use a 50mm.It is a steel machined construction rather than a forged alloy.
It has a 18mm shaft-so it displaces alot of oil through the comp adjuster.
It has a comp adjuster that goes backwards to most normal shocks-the 02 has a normal adjuster.
It uses a progressive spring.
The springs are of 2 types 250mm long ie PDS1-250
or a 265mm long spring ie PDS 6-265.
The 2 springs have a different point where they progress.
The rates on both set of springs are similar.
I will do the internals and history of the pds later.
If anyone has any specific questions on the shock please ask so i can direct the thread.

 

[MikeS]

This may be off thread some but maybe you can elaborate on the PDS function.

As a newbie to suspension & only haven taken a Ohlins shock (97) apart to change oil and seal..with dealers help. I have little knowledge of function. I understand the shim principal.

My Question is with all the talk about compression dampening and mid valves, HS/LS valving, do you need to pay the same attention to the rebound side?

Or is rebound a very linear thing, not progressive and doesn't make a difference to the rider. I have a mental block about this concept, thinking at certain shaft speeds per a certain terrain we have compression valving working in stages...so rebound should react the same????

May be just a dumb question...

Thanks

 

[Bud-Man]

On the internal stuff:
-Overlap between the main and secondary (bottoming) valving
-Added drag of two pistons in contact with the shock body
-16mm ID shims
-Limited space to work with when building valve stacks on the main piston.
-labor intensive to service/revalve

Jtt,
If you look at the picture of the shock shafts you'll notice the two pistons. The lower piston is the main valve body and the top one is the secondary or bottoming valve body. The main piston and it's valving work throughout the shock's entire stroke but the secondary piston and valving only work when the shock is close to the end of it's travel. If you look closely you will see a spacer between the pistons that has holes drilled in it. Oil passed through the end of the shock shaft and out these holes allowing it to bypass the top piston/valving. When the shock is near the end of it's stroke, it engages a needle mounted on the top of the shock. The needle goes into the end of the shaft and closes off the passage way, which makes the secondary piston and valving active. It works much like a needle jet in a carb and over the past couple of years WP has changed the profile of the needle to alter the damping characteristics of the shock.

The oil flow through the shaft and out the holes is the principle stumbling block. It requires a large diameter shaft to allow adequate oil flow which in turn requires shims with larger than normal I.D.s (16mm vs. 12mm on a KYB/Showa/Ohlins). The larger shaft diameter also displaces more oil which can create problems with the passive valving on the compression adjuster. Passing the oil through the shaft also creates another problem as it limits the amount of space you have to work with when building the main valving. The holes in the spacer have to line up with the holes in the shaft for the oil to bypass the secondary piston/valving. Stock, there isn't much room so you have to get creative with the valving. As Jeremy has said before, there are issues with the secondary valving overlapping the main valving, and there is debate about this occuring even when the holes are properly aligned.

Passing the oil through the shock shaft makes servicing/revalving the shock more labor intesive as well. When the shock is reassembled you have to take care to align the holes in the spacer to those in the shock shaft. In addition to this you need to make sure the pistons are in line with each other and the seal head. Because both pistons seal against the walls of the shock body, it's very difficult to properly bleed this shock. Even if you specialize in working with this particular shock it will be more difficult and time consuming (i.e. $$$) than a typical shock.

Each year WP makes improvements to the design and the '02 shocks have a couple of big improvements as Jeremy pointed out earlier. Unfortunately you still need a lot of special tools to work on it and it's still takes more time. I'll post in about some of the differences in the Ohlins PDS later as I'm retyping this for the second time and my fingers are killing me! :D

 

[cp380sx]

Originally posted by marcusgunby
SVI you got the idea-its about generating discussion.
Im not an expert but i was under the impression alloys dissapate heat very well and better than steel.The KTM PDS seems to get very hot during use and stays hot for a long time afterwards(ive not done any real tests-this is just from feeling a KYB and a WP after the same race)

Marcus, there is a huge advantage by using a steel shock body over an aluminum one. The steel body can go much longer intervals without the oil contamination that you'll see with an aluminum body. It will also last many times longer. The downsides are extra heat and it's heavier. With the 20 minute motos we run around here shock fade does not come into play.

 

[Bud-Man]

cp380sx,
True the alum. will contaminate the oil faster and not last as long, but you could hard coat the surface which would raise it's hardness, reduces it's coef. of friction. Wear and contamination would be dramatically reduced. Although you missed one good point of the steel body: it expands at the same rate as the steel piston ;) . Also don't forget your offraod brothers! I run 3 hour GNCCs so shock fade can easily rear it's ugly head!

 

[cp380sx]

Yea Bud-Man, a steel body/piston combo will expand at similar rates although I wasn't sure whether or not the WP shock piston is steel or not. I have yet to have mine apart.

A 3 hour race would give me a serious case of monkey butt.;)

 

[marcusgunby]

Ahh bud man my saviour:)
Things i would like to add.
History of the PDS:

98 the very 1st pds i believe was a limited edition ohlins version(very few around)ive heard it was very restrictive and peformed badly.Ohlins wasnt willing to make the shock for the price KTM wanted to pay so WP was comissioned to build one.

The 1st 98 WP pds was not good-very heavliy damped.It was modded by KTM foc i seem to remember.This shock and the 99/00 had the oil passing down the centre of the shaft assembly right down to the rebound adjuster and back up.WP altered the shape of the rebound rod in the centre of the shaft over the years but ive never seen one to confirm.This design wasnt a good idea as really fast riders got the shock oil so hot the threadlock melted that locks the shaft to the clevis at the bottom(they then come loose).Over the years WP have altered the height of the piston rings slightly.The 01 was like a half SXS as it had the conventional design of the rebound needle(no oil down to the rebound adjuster)it has a normal rebound rod with a tapered end that closes off a hole.The smaller the hole the more oil has to pass through the rebound shim stack so its stiffer.The 02 is a copy of the 01 sxs with a normal comp adjuster with low and high speed and it has a clevis with a locking nut that jams against the clevis a bit like a KYB.One big difference to the sxs is the piston rings on the std shock are teflon coated metal and the sxs is a flexable teflon band like on the wp forks midvalve.
IMO i think back now and i seem to remember my 99 shock was the best out of the 99/00/01. Ive heard KTM and ohlins still have issues over the patent on the pds system but i seem to remeber the design was done originally by an englishman-not much else to do on the cold winter months:D

 

[marcusgunby]

Mike s -good question. The rebound side of things is alot more important than most people think.My Cr was done by a local tuner and the rebound stack is more modified than the compression.It has alot less damping than std(CRs are valved for SX straight out the crate)Its very important to get the wheel back on the ground after hitting a bump.This is where traction comes from.If the rebound stack is very stiff you can get packing where the if bumps are consecutive the rear end doesnt have time to get back up in the stroke so it get lower and harder on each bump.Alot of UK tuners are using 2.5wt oil in KYBs and showas to help the shock feel more active as the valving seems to be for US SX tastes:( this IMO is stupid as very few riders can really do SX well and if they can they could probably get a sponsor to valve the shock any way they wanted.Bike manufactures should valve bikes for real world rider and screw the mags/supercross wannabes.:p

 

[Jeremy Wilkey]

Marcus,
Great write up.. thanks for taking the time.. A few quick addtional points at the cost of your leg-work..

The 98 was really wild as the pistons I belive overlaped almost half the time. The needle was VERY long and the center hole was not a nut but a bolt.. So the valving and so on was held in place by bolt!! This is intrestng because the head of the bolt was very small as was the exit ports at mid shaft.. The primary piston was very thin and had a 4mm piston ring... The secondary piston had a "normal" wp ring however taller than say a current one..

I have never had to rework a 98, only service thank good because I think the can would be the best option.. :eek:

The 18mm shaft is fine actually great I think. It makes the shock more adjustable. I have some stuff I'm waiting to talk about here but I'll wait a little longer.. The new 2002 SXS has a larger resivor tube and I'm sure this is to reduce nose pressure.... I'm working on something for this issue and it with the bladder question have much in comon.. I can't wait to elaborate.. I hope this helps get this going..

Regards,
Jer

 

[Greg in Oz]

Well,
I am one of the Öhlins PDS owners. Here's my take on things. When I went to a PDS4 spring on my Husaberg cause I'm 275 lbs, I found that the progression of the spring didn't match the shock damping so there was a lot of pogoing going on at the back. I couldn't be bothered trying all sorts of straight rate spring/damping experiments so I went to the Öhlins distributor and plonked down the aussie dollar equivalent of US$800 and got one. Comes with a straight rate spring, 10.1 for me. A few clicker adjustments and I've had no issues. They use a heavy spring because of the smaller shaft dia and therefore lower pressure from the gas. The shock has done 2 seasons of light use and will get a service when I have the body switched over to suit the KTM I'm getting. I love the way it works and I'm going to closely compare the Öhlins unit with the SXS WP when the 540 arrives in Jan. Will report back.

 

[marcusgunby]

Jer would you mind expanding on the nose pressure-what is it?is too much bad etc

 

[MikeS]

RT Graph

Just curious about this. I am starting to follow you guys here but stumbed across these. If referencing these are inappropiate then please delete this post. I am not trying to stir up anything but just absorbing info.

Thanks

 

[marcusgunby]

Is this graph showing just the theoretical spring strength?if so it doesnt take into account the pds massive increase in damping due to the 2nd piston.A light spring with massive damping will resist bottoming very well-jer can you help us interpret the graphs?

 

[MikeS]

Marcus not sure if theory or fact...

This is the review from Rt opinion on riding the 02 KTM:

The stock 2002 KTM 250 is the leader of the bad suspension group. The front end is harsh throughout and even harsher on "slap down" landings. The bike developed headshake when accelerating and the rear-end deflected and skated around when hitting the small, square-edge bumps. The problems escalated much worse with speed. Test riders didn't feel that the suspension bottomed-out, indicating the harshness is felt through the entire travel-curve. This condition made the KTM display very inconsistent behavior when riding on the track, as the bike did not respond the same from obstacle to obstacle.

The rear shock spring needs to be much more progressive contrary to the popular aftermarket "straight rate theory" (see rear wheel force curves). The KTM's leverage ratio curve is the flattest of the 2002 models, indicating that the action has very little progression. Unfortunately for the bike, the firm seat takes away from the ride, particularly in corners and turns, where the hard seat makes the suspension feel even harsher than it really is.

RT has developed A "NEW" Spring that has a better progressive feel in lieu of different valving to solve problems....I think Jer has pointed to this progressive spring theory before as being tough to work with since true control of crossover is difficult, unlike Works Performance approach with a properly designed shock body and pre load system for dual and triple rate progression.

I would like to read Jer's comments on these graphs and more on the workings of the PDS.

Thanks Guys

 

[cp380sx]

In my earlier post I came to the same conclusion as Race-Tech. I know going to an even more progressive spring will result in the need for revalving just as it does when utilizing a straight rate spring.

 

[Strick]

Mike S -

I just wanted to post my thoughts about the '02 suspension and the mags.

One of the biggest reasons I like the internet, and the dirt bike sites is real world experience. I have used this only once before on this site I beleve, but my Dad used to say: 'believe none of what you hear and half of what you see.' I take everything I read in the magazines with a grain of salt (a large grain)!

Here's is something I find interesting about the mags. They take 4 or 5 test riders, and turn them loose on these bikes. They post in the article the clicker settings front and rear that they have chosen for the test bike(s). What about young test rider - Johnny who weighs a buck 40, vs. HunK Herman who goes 260? Where is the reasonablitly?

I have an '02 SX, race desert, and love the suspension! Supercross is coming up. Watch Grant Langstons bike track the whoops, then watch some of the other 125's track. The KTM's look pretty impressive! I know they are factory set, but the basics are the same as my bike.

I have not seen many complaints (yet) about the '02 suspension. Jeremy was actually quite complementary about the suspension when this thread started.

There's my .02

 

[MikeS]

Strick

I agree with you about the mags 100% and know your are still grinning ear to ear about your bike. :)

My post was to stimulate more thought about the theory behind the PDS, Knowledge I am lacking :o and enjoy learning.

The article I quoted was from a racetech news letter on their site. Part of one of Jers posts mentioned that even their gold valves set up properly will achieve good results and they get a bad rap at times due to the do it yoursef guys that has no clue, so I assumed they are a reputable source for information. That is my reason to get feedback on the graphs and the approach they have taken into a New progressive spring vs a straight rate and revalve.

Strick thanks for your 2 cents it is always appreciated.

Mike

 

[Bud-Man]

Man I love how this thread is progressing!!

Marcus,
I looked at the graphs and didn't see anything that would imply damping force on the graphs so it appears to be entirely spring force. Unfortunately they didn't list the rate of the "straight rate" spring. I think that would have been useful but I guess the more energetic of us can approximate it from the graph. I'm not sure about the light spring/heavy damping question but I would suspect that you could have excessive damping in certain situations were you weren't near bottoming.

MikeS,
I agree with what cp380sx said and have to wonder if RT really is implying that this spring is a cure all, or merely a component of a larger solution. I'd bet you would need to adjust the valving to get the full benefit of the spring. Looking at that graph, I'd think that you would have some serious rebound damping issues if you just installed the new RT progressive spring.

Is anyone else surprised at how little progression in force there is on the KTM even with the PDS spring? I'd often wondered if the ratings that were published were applicable in use. In other words, can you compress the spring enough within the limits of the shocks travel to achieve the higher end of the spring's rating? The PDS-2 spring I run on my 400sx starts at 7.5Kg/mm and ends at 9.7Kg/mm which seems pretty progressive. Yet the gragh just doesn't fit the mental image I had considering the range of rates it would cover. (Granted, the latest PDS springs aren't rated as progressively as my older PDS-2.)

 

[marcusgunby]

Mt take on this is Race tech have managed to mimac the spring rate throughout the travel of the CR250-but the PDS has different damping towards the end of the stroke that does not increase with the travel the same as the cr-the crs will increase at the rate defined by the linkage the KTM will do so as well, at a lot lower rate until the 2nd piston hits the needle and then we get a big increae in damping-my guess would be more than the cr but i might be wrong.What we need is a shock dyno that can use the linkage curve-do they do that??Then we could see the total force required to bottom both shocks at various speeds-doesnt sound too hard to get a rig to do that:p

 

[cp380sx]

Race-Tech has a shock dyno so I imagine they doing some serious testing with it if they are going to the trouble of developing their own line of progressive springs just for the KTM.

 

[JTT]

Although my experience with PDS (or suspension in general ;) ) is limited, I would have to agree with Marcus on this one. It appears that the graph isolates one variable (spring rate), but fails to take into account other variables in the equation (such as the unique PDS damping system).

If there is one thing I have learned so far in my suspension education, it's that you cannot look at items independant of each other, even though it seems to make it easier to understand :D .

 

[robmann]

Last I knew the RT shock dyno was a crank/Sinusoidal type which operates at one speed at a time, which for real word MX applications is useless, you need to be able to vary the speed of the shock at different parts of the stroke, also these types of dynos are low speed so they never get the shock up to a high speed situation that you would see on the track.
We are working on a dyno that we can take our collected data acquisition information and play back here in the shop on the dyno, the only problem that we are having is getting the dyno up to the high speeds that we see, once this is solve we will have the dyno in the shop. We are also working on making it so that we can test both shocks and forks.

Rob Mann

 

[Onore GT]

This is a great thread, it is really interesting reading everyone’s comments.

When you think of it, all of the manufacturers have been heading down the same path with rear suspension for a long time, a real long time. KTM is the only manufacturer in the recent past to split off and bring to production a somewhat different rear suspension system for off road bikes. Many people are ready to throw it out but I think that the PDS biggest flaw is that it falls out of the standard suspension tuning guidelines and it gives people unfamiliar with the shock fits when trying to tune it.

The PDS has had some design flaws and has gone through several generations to improve the action of the shock. One of the biggest problems of the shock is really a problem with the chassis. The shaft travel of the shock is very short which seems to create problems associated with high nose pressure (I’ll let Jer explain that one). It seems to make the metering of the compression adjuster much more significant (in the case of the PDS, it makes a somewhat adequate compression adjuster seem pretty poor) and valving the shock much trickier. You can’t just add some shims against the piston, or change the bender and get the same affect you would in a non PDS shock.

For KTM to change the length of the shock it would basically require them to re-engineer the entire motorcycle. New mainframe, swingarm, subframe airbox, plastic etc. This is obviously very expensive thus the emphasis so far with KTM has been to improve the shock. KTM faces a bigger problem when developing new products than other manufacturers because they share so many parts between models. For them to make the change they need in the rear suspension geometry, they would have to change every SX, MXC, and EXC model, 2 stroke and 4 stroke.

Therefore I do not think the PDS concept can be thrown out. There are way too many factors that have not been explored, or brought to production anyway. WP sells a single piston shock with the same body/shaft diameters as the PDS as replacement for the standard Showa or KYB on other motocross bikes. Typically the shaft travel of linkage type motocross bikes is longer. I have heard real good things about these WP shocks. I am not sure if that means that the two piston system is the source of the problems or the amount of shaft travel, but I am willing to bet it is the latter.

 

[cp380sx]

I was curious so I gave Race-Tech a call to get the low down on the spring that was tested for their Force/travel graph. Interestingly enough the very progressive spring that they plotted was none other than a PDS-4.

According to them the PDS-4 is the hot set-up.

I have been running a PDS-4 all year and I've been very happy with the rear suspension on my '01 380.

 

[marcusgunby]

The works 125s run PDS 1s so i guess race tech know something they dont:confused: