So I mentioned in @Kitchen 's build log that I was tinkering with suspension damper tuning so here’s some follow up on that. I will preface this with this disclaimer:
I don’t know what I’m doing, feel free to correct me!
With that out of the way I think from what I have seen it is pretty straight forward on how to increase/decrease damping, how to make your damping regressive/linear or progressive in shim stack controlled pistons/valves.
There are exceptions depending on specific damper designs, see this thread: charger 2 damper modifications | Mountain Bike Reviews Forum
psa: many arguments there and keyboard warriors, not my cup of tea. I don’t know much about that kind of thing so I will refrain from commenting on any specific designs.
Here’s a video I found very useful for getting my head around how shim stacks work:
I think the real struggle is to figure out what behavior you actually want from damper and then without being able to measure the damper on a dyno how do you quantify what the changes you are making are doing in real world terms?
For this I have been messing around with some spreadsheets, here’s my current version:
It calculates the chord length of each shim based off the clamping shim diameter and uses a modified hooke’s law equation to estimate how much stiffness it adds to the stack. Add them all together and you get a number I call “meaningless value”. It is only good for comparing 2 stacks, and it ignores any staging. I’m not sure it is correctly weighted for very small diameter shims either but as stated at the beginning, I don’t know what I am doing, I’m not an engineer! Feel free to use at your own risk, and if you can improve upon it and open source it, then go hard.
If you want to spend some money and get a real product look up shim restackor. I didn’t want to spend the money on it as that would cut into my materials budget! Maybe if I end up diving further down the rabbit hole one day.
https://www.shimrestackor.com/
I wanted to test out the changes I made in my damper before posting, and last weekend I managed to get out and do about 12 laps at the bike park with the shim stack I had modified.
My bike is running a 216x63 shock and 194mm of travel with a whopping 3% progression, this is quite highly leveraged meaning that almost any shock I put on it out of the box will be under damped for this frame. Live and learn, select the right size shock on your bike when you are designing it folks!
Anyway on my calculator the original shim stack that came in the shock scored 9524 in my spreadsheet. I added a 15*0.14 to the stack which brought the score up to 12783, about a 34% increase.
On testing it is a lot better, I was having a lot of what I would describe as “hard” bottom outs previously, now I am pretty sure I am bottoming out at appropriate times but its not a hard bottom out as the damper had slowed the bike enough that the bumper softening the last bit of the stroke.
I think I probably need to reshim the high speed rebound now as I am running a fairly stiff 600lb spring and in order to restrict total oil flow enough at high speed you end up with a very slow low speed rebound at the top of the stroke, but its not really much of a issue so it will probably be put off until its time to do some servicing.
Anyway that’s about all I can think of for now, if anyone else had experiences shuffling shims or insights on how dampers should work please share your knowledge!