Clamping Shim OD
Clamping Shim OD
(OP)
How do you determine what the largest clamping shim OD you would use with a shim stack?
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS Come Join Us!Are you an
Engineering professional? Join Eng-Tips Forums!
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail. Posting GuidelinesJobs |
|
RE: Clamping Shim OD
When doing a 1-off build, I usually try to go with something kind of 'middle of the road'. That way if I want to change it to get a little more or a little less overall damping, I can just change the clamp shim 1mm up or down and have an easy, but effective, change.
I also usually end up using a large stack of thin shims rather than a small stack of thick shims. That's more personal preference as far as getting the curve, though.
What are you using, what are you using it on, and what is your overall goal?
RE: Clamping Shim OD
I am looking to increase overall damping on a compression stack that currently uses a 9 mm clamping shim with a 6 mm ID. I know I could stiffen up the stack by increasing the shims, but I also know that I can do it by increasing the clamping shim size.
I really have not played around too much with different clamping shim OD's and would like to find out what their effect is vs increasing the shim count.
RE: Clamping Shim OD
If you just want to increase overall damping, you could put heavier weight oil in and not touch the shim stack. It would effect both compression and rebound, though.
If you just want to do one side of the curve (in your case, bump), then a larger clamp shim would be a good enough approach. Keep in mind, that by going up with the clamp shim, you will be affecting the whole curve something close to proportional, low and high speed. I would make small steps, like 1mm at a time. Even though it's a small change, it should feel considerably different.
What is the entire shim stack (dia. and thickness) that is on the fork, and what do you want to change about it?
RE: Clamping Shim OD
I am only interested in playing around with clamping shims at this time as I have played around with fork oil viscosities and am familiar with their effects.
I have not really tuned by adjusting the clamping shim and as with everything in racing I am looking for something that will reduce lap times and hopefully yield better tire wear.
RE: Clamping Shim OD
CS = Centering Shim
RS = Ring Shim
I started out with -
(1) .15 x 17
(1) .15 x 14 CS
(1) .20 x 17 RS
(4) .15 x 17
(1) .20 x 10
.05 mm preload on shim stack
The fork has an external low speed adjuster that has a range of 2.5 turns. This worked good, but as the rider went faster and faster we had to turn the low speed adjuster in more and more.
So I made a change to:
(1) .15 x 17
(1) .15 x 14 CS
(1) .20 x 17 RS
(5) .15 x 17
(1) .20 x 10
.05 mm preload on shim stack
While it was better - lap times continued to drop and tire wear was good, but it was not enough.
So I went with this:
(1) .15 x 17
(1) .15 x 14 CS
(1) .20 x 17 RS
(7) .15 x 17
(1) .20 x 10
.05 mm preload on shim stack
At the same time I installed this stack I also changed the springs to a higher rate (from .95 to 1.0). Again lap times dropped and tire wear improved.
What I want to develop is a stack that will give the same initial blow off as the first stack (possibly slightly less even) but has the same damping at faster shaft movements as the last stack.
The reason for this is that the track we are racing at has some transitions that really upset the suspension and I want to try a "softer" initial blow off to see if that will help.
I think the only way I a going to accomplish that is with a dual stage stack.
What do you think?
RE: Clamping Shim OD
The stack that you are running has no progression. It's all blow off. You have the sealing shim, the preload spacing, the preloaded portion of the stack, and then then clamp. There's nothing to give you any 'slope' once the preload is 'broken'.
It looks like everything you've done has increased low speed damping. This can be good for track undulations, but is usually a no-no for 'quick' bumps like pavement transitions, seams, pot-holes, etc.
I would try to get some shims to bridge the gap from the 17mm part of the stack to the 10mm portion. That will give you more slope in the higher speed sections of the curve. If you go back to your initial preload, stack, then that will give you the blow-off you want.
Just as a guess, you can try:
(1) 0.15 x 17
(1) 0.15 x 14 CS
(1) 0.20 x 17 RS
(4) 0.15 x 17
(1) 0.15 x 15
(1) 0.15 x 13
(3) 0.20 x 11
Using a 2mm step in shim sizes will make the stack feel a little more progressive after the preload breaks instead of just dumping open. You can increase the clamp shim 1mm since you are taking quite a bit of shim (3-17's) out and it makes a nice 2mm step down from shim to shim. I usually run several clamp shims so that at very high shaft speeds, the stack can get completely out of the way of the ports in the piston. There should be some sort of big washer limiting overall shim flex that sits on top of the clamp.
I've ran 2 stage stacks with limited success. Before you try that, maybe go for a cantilever stack. Use a 15mm diameter ring shim with the stack preloaded over the top of that. This will let the sealing shim crack off the piston which lets it get over the little stuff, but has a preloaded portion of the stack for larger track undulations. I've had a lot better luck with this type of arrangement.
You only have about 1,000,000 combinations of stuff to try. Should take too long!
RE: Clamping Shim OD
The low speed adjusters are just needles that effect a bleed orifice.
I would agree with the no progression assessment on the stack.
You want the low speed with the bikes as low speed translates to feel for the front tire. I do not know if you have ever ridden modern sport bike at a fast pace but it is all about rider feel. Not really like a car.
You need enough low speed to give you the feel, but not too much or then it get harsh.
You are right about the high speed. To much high speed and it is harsh over the bumps.
Another reason some go with a stiffer low speed stack is to help prevent dive under braking.
RE: Clamping Shim OD
(3) .15 x 17
(1) .10 x 12
(1) .15 x 17
(1) .15 x 14 CS
(1) .20 x 17 RS
(3) .15 x 17
(1) .20 x 10
RE: Clamping Shim OD
What piston type/size are you using. From shim stack it sounds like a a 20mm piston. You may try a two stage stack & vary the separator shim dia & thickness. As you said though alot comes down to rider feel.
Morewing, the cantilever stack is something I have not seen before but makes sense.
RE: Clamping Shim OD
With a 2 stage stack, I've always seemed to end up with too little low speed for a certain amount of high speed. By the time I get the low speed where I want it, the high speed is way too much. I think off-road guys get it to work, but on a track, I never really have.
My advice is to stick with a relatively simple stack. It's way easier to go slower with shock changes than it is faster. Regardless of what you end up running, keep good notes, and let us know how it worked.
RE: Clamping Shim OD
Ohlins uses a really stiff spring on the check valve on the rebound piston. They use a longer conventional coil spring set up on those.
The last time I was in the forks (02 GSX-R750) I did double up on the wave washers on the check valve.
Who are you working with in the AMA if you do not mind my asking?
RE: Clamping Shim OD
RE: Clamping Shim OD
htt
Look at the 2006CSS & 2006RSS valves, also make a finer tapered needle for rebound.
RE: Clamping Shim OD
You really cannot see too much in the pics though unfortunately.
RE: Clamping Shim OD
This is a facinating conversation. But I must confess I am having some trouble following it.
I also confess that I am not in the suspension industry. I have a now old CR500 which came stock with some wicked high speed compression. Over many years I have tried to improve it. When the aftermarket failed me I tried to design a better system, but never came up with anything that I had complete confidence in structurally. To date I have not overcome the design, IP, and financial challeges (I do live on an engineer's salary) and I haven't spend any time pursuing it in the last couple of years.
But I am still facinated and have been thinking about continuing my design work. Or at the very least continue to improve the suspension on my old beater CR500 (which I love dearly).
What is a centering shim and a ring shim? Kayaba MX forks (at least through '03) have regular old shims and after reading this thread I am wondering if road bikes don't have a significantly different damping system. Is the compression valve design the same on road and MX bikes? I also don't see any way to preload the shims on the compression stack. Is this possible on a road bike, or was I just not following the converstation correctly.
Thanks in advance.
Ed
RE: Clamping Shim OD
A centering shim & ring shim are the means to preloading the shim stack when the piston is not constructed to preload the stack. Please note that preloading the shim stack is not always a necessity & often comes down to rider preferences. This is done by using a ring shim (e.g. 17mm od, 15.1mm id & 0.20mm thick) that is slightly thicker than a centering shim (15mm od, 6mm id & 0.15mm thick)thus the remaining base shims are pre-loaded by 0.05mm. This has the effect of pressurising the cartridge to a higher level before the shim stack begins to blow off compared to a non preloaded stack. As morewing mentioned above there is over 1,000,000 various combinations & sometimes the problem is not always the shim stack. Also sometimes a compromise is the best outcome.
Regards,
MB
RE: Clamping Shim OD
$@%&!, this solution is so obvious and simple!
One more question - can you recommend a shim supplier? I didn't realize .15 mm shims were an option and I haven't seen ring shims at my local Race Tech dealer. But since I didn't know what to look for maybe I just missed them.
Many thanks!
RE: Clamping Shim OD
You can get the ring shims from www.traxxion.com. You should be able to order miscellaneous shims from them also.
They have them for 20 mm and 25 mm as those are the sizes used in street bike forks.
If your pistons are one of those sizes, you will also need to know if you need a 6 mm ID or and 8 mm ID. Most of the Showa stuff uses a 6 mm ID. Kayaba uses a 6 mm id on the 20 mm stuff and an 8 mm on its 25 mm stuff.
RE: Clamping Shim OD
Working in a vacuum is a tough deal. I think you guys may have re-ignited my interest.
Just kidding about the kiss. How about an appreciative handshake?
RE: Clamping Shim OD
RE: Clamping Shim OD
I may have to buy someone in our machine shop a bottle of Jack Daniels. If I make the ring shims thick I can always fill up the interior with centering shims to adjust the preload.
RE: Clamping Shim OD
Don't forget that the centering shim will form part of the shim stack & you have to take it into account. Also from my limited experience (some of the others here may be able to provide more information) pre-loaded shim stacks are not universally used. You also may want to consider a two stage stack. What is the exact problem you are experiencing & what is your current shim arrangement.
RE: Clamping Shim OD
The OD on the Traxxion preload ring for the 25 mm pistons is 22 mm.
RE: Clamping Shim OD
My problem was too much high speed compression. My suspension worked great in round whoops and for jumping, but any square edge bumps hammered me. The front beat me to death and the rear wanted to pile drive my head into the ground.
I tried a dual stage shim stack in the front and it resulted in very poor performance. It is possible that I just missed the mark, but it was so bad I wrote it off as a feasible approach.
I also tried Delta Valves in the front, then finally drilled out both stock "pistons". Current stack is a straight progressive (in size) of one each 24-22-20-18-16-14-? mm OD. I can't remember the smallest diameter. All .1 mm thick. As I recall the stock setup had two 24mm and two 22 mm shims. As you can see I am never going to make it as a tuner, even for my own bike, unless I start taking good notes.
The front end is pretty good now with the bleed valve shut off. From a philsophical standpoint I want to try some preload in the front and I need to stiffen it up a touch anyway, since I believe the orginal problem was the pistons more than the stack.
By the way, this is a 95 CR500 (I think the only year with Kayaba). I recently rode a new CRF450 and decided the motor was just as brutal as my 500, so I lost my desire to upgrade. I didn't push the CRF suspension because I had just wadded my bike I didn't want to wad up the nice man's new CRF. I was also experiencing some pain.
RE: Clamping Shim OD
Also the clamping shim (smallest shim) od will have a great influence on your shim stack. What is your piston / cartridge dia & shim id.
RE: Clamping Shim OD
Crazy thing. I wrote damn near the exact same thing and posted it. For some reason it didn't show up. You covered it anyhow.
RE: Clamping Shim OD
The compression stack that you have seems to be very light. Does your fork run a mid-valve. This can make the fork feel harsh for all but the most aggressive riding.
RE: Clamping Shim OD
My front end is pretty light, but if I can soften the rear to get some load off the front and also get rid of that nasty high speed kick I think I will be close.
Yes, my bike has a mid valve. The damping rod has a piston on the end with a rebound stack. On the top side of this piston is a check washer/shim (or two) with a scrawny coil spring behind it. I concluded the midvalve did not provide any compression damping except that created by the orifices in the piston, which I drilled out. I wanted the damping controled by the shim stack, not orifices. Now that I have removed the anodize from the piston orifices I suspect the holes may start growing in size.
Why do you say the midvalve can make the the suspension harsh? I do believe I fall into the "old and slow" rider catagory.
The problem with suspension design/tuning is that there are way to many variables. If it was simple the tuning wouldn't be nearly as much fun, but the riding would be a lot more fun.
RE: Clamping Shim OD
btw I think stock compression stack for a 1995 CR500 is,
22 x 0.1
22 x 0.1
21 x 0.1
21 x 0.1
20 x 0.1
19 x 0.1
19 x 0.1
18 x 0.1
17 x 0.1
16 x 0.15
14 x 0.15
12 x 0.15
11 x 0.25
Which part of the valve did you drill, also standard KYB 25mm valves are steel, did you buy the bike second hand.
The standard transfer ports on KYB rebound valve are very large & should not affect the compression circuit to a noticable effect. From your description it sounds like you drilled the rebound circuit. Also removing anodising will not cause the holes to grow in size unless it is where the shims are seated on the valve.
Port sizing does not need to be as large a some people think. Often it is used to control the way the shims deflect. You can design pistons to be digressive though to being velocity dependent to give characteristics that you are after, but this sort of stuff is usually used only at factory team level.
Also if the rear is kicking hard, how? I suggest you sort out the rear before you start on the front.
Regards,
MB
RE: Clamping Shim OD
You may very well be right on the diameters and arrangement, but I do not remember two 19's. I remember two of the two largest diameters.
I am pretty sure I drilled both the compression valve ports and the midvalve piston.
I do have a midvalve and I shortened the travel on the midvalve check shims and I think I added another shim to stiffen it up. The check shim arrangement confused me. I didn't think it would have any significant effect on damping. It's nice to have some confirmation that those shims do have a purpose beside just being a check valve.
I will work on the rear before I do any more work on the front. The rear end kicks on sharp edged bumps. Every once in a while it kicks me in the arse and scares the pants off me. Plus I now have a balance problem. I was very suprised the Delta Valve didn't solve that problem because it is a blow off valve.
At this point I am a bit skeptical about digressive piston designs. But I would probably need to spend many hours studying my fluid dynamics book (or build a test bench) to form an intelligent opinion.
I bought the bike new.
RE: Clamping Shim OD
My shock only had 70 psi, which I suspect is due to some Delta Valve leakage. Recently I have not been able to find a decent rebound setting, and for now I am blaming it on the low pressure.
The stock internal stack is (95 CR500):
format = (# of shims) OD x thickness
--Rebound side--
(3) 18 x 1.0 washer
(1) 21 x 3.0 washer
Shims:
(1) 25 x .31
(2) 26 x .3
(2) 28 x .3
(2) 30 x .3
(2) 32 x .3
(2) 34 x .3
(2) 36 x 3
(1) 26 x .1
(10) 36 x .15
--Piston/Valve body--
(12) 40 x .15
(1) 34 x .1
(1) 28 x .1
(3) 40 x .3
(2) 38 x .3
(1) 36 x .3
(1) 34 x .3
(1) 32 x .3
(1) 30 x .3
(1) 28 x .3
(1) 26 x .3
(1) 24 x .3
(1) 41 x 4.5 washer (thick sucker)
--Compression side--
Rebound plan: I will move the 26 mm shim to the middle of the 36mm shim pile. This should make the rebound more progressive. I am very tempted to make the rebound side an orifice type by getting rid of the shim stack and spacing that very thick 41 mm washer X distance from the valve. But I have no idea what X should be and if I take a guess I am almost certian to be re-valving after my first ride.
Compression strategy: The dual stage stack seems to explain the high speed kick. So I will remove the 34 and 28 OD shims to eliminate the dual stage. I will need to remove some 40 mm shims also, but I am not sure how many. Hmm, this is a difficult decision. I think I'll remove two of the .3 thick 40 mm shims.
Yes, those valve bodies are steel. I will probably drill out the compression ports in the valve a bit just to make sure they aren't contributing to my problem.
By the time any of you read this everything should be buttoned up, so I guess this post is mute. Sure hope my guess is correct on how many 40 mm shims to remove. I took out a lot of stiffness, but then then again the rest of the stack will be there for support (which it wasn't until the first stage bottomed out). Something tells me I will be able to use any info you guys might be willing to share when I re-valve again!
This certianly is a brain teaser. Is this a parallel or a series system? Some of both I guess.
Wish me luck.