shim shuffle - KTM WP forks Part 2

[drehwurm]

Servus,

This is the sequel to the thread 'shim shuffle - KTM WP forks' with new experiences,ideas and questions. For reference and starting points see part #1 under
http://dirtrider.net/ubb2/Forum24/HTML/001153.html

DISCLAIMER: Do not attempt to use any of the following as suggestion or even advice to setup your own suspension. This could be dangerous and lead to serious unhappiness with your forks. Just take this as what it is: the irrelevant babble of a curious Austrian.

While waiting on my shims to arrive an Öhlins PDS shock from MH-Racing (thanks Mark) found the way onto my KTM – an investment worth any penny! But while the Öhlins is a great improvement over the 'not so bad' stock WP shock, the problems of the fork still persisted. Finally my shims arrived and in a last minute decision I decided to use the following stack # 1:

24x0.1 3x (plus one)
12x0.15
22x0.1
20x0.1
19x0.1
17x0.1
14x0.1
13x0.1 new
12x0.1 new
11x0.1 new
18x0.3 base plate

and an oil level of 140mm. Note, that the overall height of the stack remains unchanged (13,12,11x0.1 replace stock 10x0.3 clamping shim)

The difference was amazing, but not in the way I expected it too be.

.) Bottoming from large jumps was reduced, but not as I had hoped. Still, this is not surprising as it has been stated before on this forum, that this type of bottoming is more a midspeed thing. Do I have to modify the midvalve now?

.) Headshake was unaffected and still there so I just installed my Scotts and forgot about it. This thing (Scotts steering damper) should be considered as 'live saving' equipment on KTMs !!!

I was able to reduce headshake to a minimum with very fast rebound settings or lots of static sag, but both of this actions caused other way more negative effects.

.) On the positive side, the bike was much more stable now. I could go straight through the worst acceleration and braking bumps or little whoops where before I was fighting the bike to go straight. There was no deflecting, but the ride was tiring. Great setup for plain MX-use and a probably slightly heavier rider.

All in all I liked the way the bike handled now, just a tad too much HSC for my general purpose use. Nevertheless, I was in 'experiment' mode and wanted to know if the increase in diameter on the clamping shim from 10mm to 11mm made such a difference or if it was the increased HSC. Easy enough, I left the stack like it was and just added the 'old' 10x0.3 clamping shim on the bottom.

This is the stack #2 I tried the next day on the same course, almost same conditions:

24x0.1 3x
12x0.15
22x0.1
20x0.1
19x0.1
17x0.1
14x0.1
13x0.1
12x0.1
11x0.1
10x0.3 new
18x0.3 base plate

Holy s**t, what a difference. The bike almost felt like at the start. Pleasant and nice, but unstable and hard bottoming. I had to go in on the comp clickers from 12 before (stack #1) to 6 come at least into the range of the lap times with stack #1.

Well, I need some explanations! This 10x0.3 shim made so much difference, but why? Is the increase in overall height (the shims can bend more till they hit the base plate) or the reduced diameter (from 11mm to 10mm) responsible – or is it a combination of both??? Is this actually a 3-stage valving with the base plate being a SHSC (super high speed comp) 1-shim shim stack?

At least I know now WHERE I want to be – somewhere between stack #1 and #2. Ideas include a thinner (0.1 or 0.15) 10mm clamping shim or a thicker (0.15 or 0.20) 11mm clamping shim. As I only have a 11x0.15 at hand right now stack #3 will look like this:

24x0.1 3x
14x0.15 a little more LSC still
22x0.1
20x0.1
18x0.1 a little less LSC to compensate for the 11mm clamping diameter
16x0.1 ditto
14x0.1
13x0.1
12x0.1
11x0.1
11x0.1 makes for 11x0.2
18x0.3 base plate


Michael



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[marcusgunby]

Do you ned the base plate shim on the RFS?On the 125 there is no lead on the bottom of the tap so there is no need for a bottom plate shim.Dont forget the last shim sets up the shim stack strength for the whole stack.So a 10mm shim will reduce high speed and low speed compared to a 11mm.Your suggestion 3sounds good to me.i would consider using 4 24.1 initial comp shims and leave the 12.15 crossover

 

[drehwurm]

Servus Marcus,

How is the CR doing ;-)

Do you ned the base plate shim on the RFS

I don't think you actually need it, but it obviously has a positive effect on the valving. If you'd remove the base plate, the seating area on the valvebody itself has a diameter somewhere between 12 and 16mm (I measured it, but forgot how much exactly). When I raised the overall height of the stack by inserting the 10x0.3 the influence of the base plate was seriously effected - and as it showed was not to my liking.

Dont forget the last shim sets up the shim stack strength for the whole stack.So a 10mm shim will reduce high speed and low speed compared to a 11mm.

I'm aware of that. It would be interesting to know if adding 1mm to the clamping shim would be the same as adding 1mm to the OD of all the shims - anybody?

Michael

 

[marcusgunby]

The Cr is going well the front forks are the best ive ridden ive changed the oil only and put 1 click on comp.The rear we are testing a Jer kit and its going well so far.The engine needs more development than the KTM but we are making good progress.BTW the CR steering is something else-seems fairly stable too.

 

[James Dean]

drehwurm,

This is a good question, what is the more significant factor, the clamping shim diameter or the thickness (gap) at the base plate?

Your stack #3 won't give us a complete answer unfortunately. Seems hard to imagine that going from 10mm to 11mm on the clamping shim would be so noticeable. It does have an affect on the whole stack though.

Stack #3 looks to be a good middle choice either way.

BTW-Nicely detailed posting.

James

 

[PT564]

Hmmm --- This is interesting...

Notice how Jer doesn't comment in threads where your discussing shim stacks... Seems he would be able to help if he was interested in doing so...

How about it Jer -- why don't you jump in a help this young man with his stack....

 

[drehwurm]

Servus James,

This is a good question, what is the more significant factor, the clamping shim diameter or the thickness (gap) at the base plate?

Thats an easy one! Just see it as a crossover shim like between LSC and HSC. And where is the answer now? Right there is none. If the gap is so big that the shims never hit the next stage, it doesn't matter if you make it even bigger. On the other hand, if you make it too small you actually go from a two to a one stage stack! IMHO this is like the apples and oranges thing ;-)

I'd consider the question if adding 1mm to the clamping shim diameter is equal to adding 1mm to all the shims above?

Michael

 

[drehwurm]

Servus PT564,

Seems he would be able to help if he was interested in doing so...

I think you are a little unfair against Jeremy. First it is his business and nobody should ask for his knowledge for free. Second this is not a 'finding THE one and only great shim stack thing', because it doesn't exist. Third, I think Jeremys advice couldn't compensate for the info which is gained through learning here. And finally fourth, I'd say that if we go in the absoulutely wrong direction somebody would step in and point that out.

Michael, on a quest again :-)



[This message has been edited by drehwurm (edited 02-19-2001).]

 

[PT564]

But isn't the objective of a moderator to "guide and teach"? If so Jer could simply state some guide lines for what this gent is tring to accomplish.... Simple -- doesn't bit into his bottom line one bit (and will actually have the inverse affect)..

Just my 2 cents...

 

[JTT]

Rest assured PT564, Jer is surely following this thread. drehwurm is right that Jer telling him how to do it, will accomplish little in teaching him about how things work. Letting him (and us) figure it out on our own is the best way to true understanding. And,I too, am sure he will step in if the topic gets too far off track. He is already WAY ahead of most suspension guys, who won't give out the information to their "associates" (franchisees), that Jer provides to all at DRN.

I, personally,am enjoying this thread, thanks drehwurm! Keeps the "gray matter" firing.

------------------
JTT
Logic Over Hype Coalition

 

[PT564]

I agree this is a GREAT thread!

Infact, this is what we should see more of on this board. It will give us all a chance to learn more about the different affects of taking away and adding different shims to a stack...

Thus, shim on.......and on....

BTW: I recently rode a bike with a great rear suspension. And, the compression stack was like nothing I've ever sceen before. It was made up of shims all with the same diameter! There was cross over in the stack, but other then that it was all 38mm OD shims (square shaped stack if you will)! The thing rocked.

 

[Vester]

Yeah..great thread. I've been following this closely too. I've got most of it highlighted and written in a notebook. I really want to learn this stuff and this is about the only place that discusses it. I love this forum.

------------------

Vester
96 RM250
"Let's see if an "A" rider tries it first.."

 

[drehwurm]

Servus,

Great that everybody likes this thread!

Still I'd like to get some input on the 'shim mechanics' like clamping shim dia vs. thickness vs. ... - nobody?

BTW stack #3 is waiting in the garage to be tested, but it seems like winter is giving his comeback here in Austria. Do I need a different valving if I put a ski on the front?

Michael

 

[Vester]

I found some stuff on Eric Gorr's site last night that has me puzzled. This is a quote from his article on suspension tuning.

"With regards to the sizes of the shims, the larger the diameter and the thinner the thickness, the more easily the shim will bend and increase oil flow through the piston. The faster the oil flow the less the damping."

Which this leaves me confused now. Like on your #3 stack when you opted to change the 12x0.15 shim for the 14x0.15 for more LSC, you in turn lightened the damping because of the larger shim diameter. Same goes for the 19x0.1 shim that was switched for the 18x0.1 for less LSC. Because of the thicker diameter you in turn stiffened it?

Could someone clear this up for me?



------------------

Vester
96 RM250
"Let's see if an "A" rider tries it first.."

 

[marcusgunby]

I think i get what you are asking.The normal shims(ie the ones that sit against the piston or comp adjuster)are the ones the get less damping effect when they are made lager/thinner.The crossover/bendover shim does the opposite as the 1st stack(against the piston)bends over it so a larger bendover shim causes the stack above it to become stiffer.It takes a while to get your head around it.Its easier if you have a stack in front of you.

 

[James Dean]

Vester,

Adding to marcusgunby's comments-

The 12 to 14 was basically intended as an intermediate "clamping" shim for the low speed stack, stiffening it. Bigger clamping shims stiffen the whole stack from the base up.

The switch from 19 to 18 (and 17 to 16) were on the HSC stack (labeled LSC by mistake). Smaller shims are stiffer at their edge, but don't confuse this with the idea that in this case it is allowing more flex at the outer edge of the shims above. This should be an offsetting (softening) factor to the bigger 11mm clamping shim below.

James

 

[drehwurm]

Servus Vester,

I'm sure there will someone come up with a formula, but that's how I did it: take two long and flat pieces of wood and clamp them in a vise to simulate your shims. Now try to bend the wood - the results will speak for themselves.

Another way would be to imagine what happens if I replace all the shims in my stack with 24x0.10 shims. Do you really think this will be softer?

Michael

 

[Vester]

OK I think I understand it now. A larger crossover shim will stiffen the shims above it. Not allowing(those shims) to flex more.

But.....(I know I'm an idiot) to reduce HSC, why the change in the middle of the stack? Why does it need to be on the 19 and 17 shim. Why not the 14 and 13? For decreased HSC couldn't you just start with bigger diameter shims "after" the crossover and taper it from there. Patience O' great ones....patience.

------------------

Vester
96 RM250
"Let's see if an "A" rider tries it first.."

 

[marcusgunby]

I think im right in saying this- that if you alter the HSC shims near the clamping shim you run the risk of altering the high speed stack too much.The ones near the top of the HS stack will have the right affect for what we want ie a small change in stiffness.Also you people adgree that in general to make 2 changes to a stack at once is asking to confuse and already complex problem?Please note im on a very steep learning curve here so i dont claim to be right or an expert.

 

[James Dean]

Vester,

There are many options to do the same thing here. There is no right or wrong way and the results can be the same. Instead of 17 to 16 it might be 14 to 13 and you would never know the difference. Some stacks just look more elegant, with a smooth progression.

The benefit of 2 changes at once is covering all the possibilities. It may (or may not) save another tear down. Unfortunately we won't know which change had the greater effect. A suspension tuner doing this for a living would try both at some point to get an indication.

This is their livelyhood, I would not expect anyone to share this for free.

James

 

[MACE]

Do any of youse guys pull your base valves in the field? I've been planning a testing ride where I'll use my N2 tank to run my impact wrench to pull the valves in the parking lot. I'm thinking plug a few fluid vent lines and lay the bike over on it's side with the front wheel elevated on a milk crate and Brrrrrt out comes the base valve. With a little care for dirt I think it should work fine.

Any thoughts?

------------------
MACE

One night I was layin' down,
I heard mama 'n papa talkin'
I heard papa tell mama, "you let that boy MOTO,
it's in him, and it got to come out..."

 

[Vester]

Thanks guys for the info. You're helping me to see this alot better now.

drehwurm,
It looks like you had fairly good results with stack #1 with the exception of some bottoming and headshake. Your proposed stack #3 looks like headshake will be reduced even more with the increased LSC (thicker clamping shim)
Things got unstable in stack #2 with the addition of the 10x0.3 shim. Why not just pull it and go back to the: 13x0.1, 12x0.1, 11x0.1, baseplate: combo like you used in stack #1? Then maybe increase the oil height a little to fight bottoming.

Just tossin around some ideas here.

------------------

Vester
96 RM250
"Let's see if an "A" rider tries it first.."

[This message has been edited by Vester (edited 02-20-2001).]

 

[drehwurm]

Servus marcus,

The normal shims(ie the ones that sit against the piston or comp adjuster)are the ones the get less damping effect when they are made lager

No, I'm not of your oppinion. If you have a 24mm piston and install a shim with a 50mm OD it won't be softer than a shim with a 24mm OD. BUT a 50mm piston/50mm shim combo would be softer the a 24/24 combo - so IMHO larger shims are only softer if you also move the point where the force attacks.

Maybe that is what you meant anyway?

Michael

 

[drehwurm]

Servus Vester,

why the change in the middle of the stack? Why does it need to be on the 19 and 17

This is partly explained in part #1. The original stack had the 16 and 18 shim which I changed to 17 and 19 to open a larger gap (14=>17) to fight a deflection problem. Now I'm stiffening the whole stack with the 11 vs 10 clamping shim and therefor figured I try the little softer 16/18 combo again.

Regarding the #1 stack. Riding just MX I'd be very tempted to leave it. The fork action was very controlled but also stiff over the whole range. It felt like what you would expect when you turn in the 'high speed comp' adjuster on some of the new shocks (just to give you a feeling what I mean). Nevertheless, I know from experience that you can have this controlled feeling without the stiffness and that is what I'm aiming at with the changes in stack #3!

Michael

 

[marcusgunby]

Yes drewarm that is waht i meant we cant apply this thoery to the shim that sits on the piston only the ones after that as the one that sits against the piston is indeed acted upon by the distance between the ports.Every shim after if it is larger(but not larger than the 1st shim)or thinner will create a lesser damping effect.
As a comparison i checked the shim stack from a 2000CR125 here it is
24.1(*10)
22.1(*2)
20.1(*2)
18.1(*2)
16.1
14.1
11.2
3.5mm holes in base valve and a 12.5mm damper rod.
This may not be exactly right but its close.
So you can see the Kayaba fork does things radically different to the WP-why i have been asking myself does the Kayaba work so well -without any answers.