Very difficult question, because there is no set answer. What type of dampers are you using and what does the manufacturer recommend? Whatever that is, it's probably a bit on the safe side.
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?
It is a set on a set of motorcycle forks, but the if there is any type of "rule" governing the largest OD using some type of formula then it would be universal I would figure.
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.
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?
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.
One thing that I am specifically working on is a good rebound stack.
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 does the low speed adjuster effect? Is it a needle and seat that changes bleed?
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!
Been wrapped up in a catastrophe, so that is why I disappeared for a time there.
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.
On the rebound I have been thinking about trying this -
(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
spdingtkts,
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.
Just looking at the stack you've laid out, I think you might have too little low speed. With no preload and a large step from the sealing to the next shim, it will crack off the piston port very easily, even with 3 sealers. Uncovering the port even 0.05mm will allow a lot of oil to flow. Think of what a small amount of oil the needle/seat adjuster flows and how much it effects the damping.
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.
I have installed some 25 mm cartridges in some Showa forks. I am not really working with anyone right now that is worth all the money and trouble to do it though.
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?
On the dual stage stack front, I was thinking about what Morewing said and I think that using a 12 mm OD in between the stacks is too small. I think a 14 mm would be a better place to start.
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.
Edrush,
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
Wow, this is an epiphany! (I hope I spelled that better than "facinating")
$@%&!, 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.
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.
