Custom Valving Coilovers: How to gauge shim stack and shock force graphs
 

CURRENT STEP: First revalve
Currently this project is stopped. Megan shocks don't fit the Bilstein pistons and I'm currently not willing to invest the time and money in figuring out how to shim the stock pistons to be digressive. If I continue in the future I'll post here.


This first post will have a lot of explanations/links to pertinent stuff. To preface, I'm rebuilding a set of coilovers for the track because I go through with my bad ideas.


Bilstein has a 78 page pdf covering valving online with diagrams I'll link to below that labels each diagram with rebound and compression at 20.5in/s. It's very helpful to get a baseline from where to alter your shim stacks from.
https://media.discordapp.net/attachm...466&height=612
(So for this, the rebound force at 20.5 in/s 3000N and the compression force is 1000N)

Bilstein PDF (just alter the url to "11to20", "21to30", etc to get more pages. For the last pages use "71to78")
http://p143198.mittwaldserver.info/f...nual_1to10.pdf


Below is a Github repo with a calculator I made to get spring rates and critical damping. I've been using it to do all the math for this since I'm lazy. I'm probably going to update it from C++ to python and add a GUI with tkinter.
https://github.com/PlasticLover/ValvingCalculator



Now a bit of explaining of how all this damping stuff works
Springs: Very simple. Cars use linear springs. F=-kx. Force increases proportionally to distance from equilibrium. These are position sensitive. Most cheap springs are not ideal springs (they don't increase proportionally, they increase exponentially). Swift springs are as close as you can get to an ideal spring. There's a good post on it here: https://www.northamericanmotoring.co...gs-dynoed.html


Dampers: Not as simple. Your dampers create damping force by restricting oil flow through a piston. Technically it's the piston moving as the rod moves but it's still restricting oil flow. This means that these are velocity sensitive. They literally damp the spring oscillations. This is why your shocks need to be valved for the springs, they have to work together properly. The 14k springs you swap onto your coilovers will be very underdamped. Technically it's good to have coilovers underdamped, but not THAT underdamped. You generally use about 65% of the critical damping force before digressing to about 30% (NOTE: this is for TRACK APPLICATION. Rally suspension is way different, so this doesn't apply to that at all). This is a good blog on damper setups: http://farnorthracing.com/autocross_secrets19.html



A lot of the spring setups I'm using might sound weird, but they're not. The rear motion ratio on our cars is way lower than the front, so a 4k spring up front is more firm than a 4k spring in the rear. RCE talks about it here: https://www.ft86club.com/forums/showthread.php?t=9936
Basically wheel spring rate is what matters when you want to set the rear a little softer. We use this equation:

Kw = keff * (MR^2)

where Kw is wheel spring rate, keff is the spring rate, and MR is the motion ratio. You can see that motion ratio is squared, so a value that's a little smaller is actually a lot smaller. This is why some coilovers/lowering springs for ours cars are actually firmer in the rear. They're softer at the wheels, but at the coil they're firmer. This is good. People who sell suspension like that have probably done the math to make them work properly.


For the exact geometry values and more in depth explanations, there's a good thread here: https://www.ft86club.com/forums/showthread.php?t=26661

I guess I'll update this thread. Lots of progress since my last post, I know you're all happy to hear. Figured out a lot of the math and how shim stacks work. I'm updating the first post as I go with more information. I currently have a set of Megan EZ Street coilovers sitting all opened up I'm working on (good god this is a long process, everything is threaded on so tight. Also FYI shock fluid smells disgusting, I don't recommend opening your shocks up).


This is what I'm aiming for:
FRONT 4k @ 1.74hz: 40.79lbs/in/s (crit)
REAR 5k @ 1.64 hz: 54.27lbs/in/s (crit)
OR
REAR 4.5k @ 1.56 hz: 51.48lbs/in/s (crit) (if i can grab some 4.5k swifts, they seem harder to find)



I might do 5k front 6k rear, which is closer to a 2hz front 1.85hz rear setup, it just depends on how the shim stacks dyno. Shim arrangements are much less of a science than I thought. I guess I could use Restackor but I'm cheap and lazy so Bilstein PDF guessing will do for now... (actually dyno time is expensive, maybe i will get Restackor, who knows:sigh:)





I can upload some cool pictures in the coming days that show how your damping adjusters work and what's inside of BC/Megan coilovers (they seem to be very similar, not sure if BC is worth the price honestly. Just my speculation though).

Thanks, this is all just over my head, but it's interesting enough to keep trying to understand!

It's honestly not that complicated, it's just hard to find technical topics on car setup and whatnot. It's mostly a bit of math and knowing certain geometry about the car.

Here's the pictures. This is the front rod on a Megan EZ Street coilover for a GT86. Just to preface, the way your damping adjustment works is by letting oil through the center of the piston, relieving damping force. It's effectively an adjustable bleed shim.


https://cdn.discordapp.com/attachmen...578/image2.jpg
The rebound side of the piston. This is one of the orifices that oil goes through to adjust damping.


https://cdn.discordapp.com/attachmen...668/image1.jpg
And the compression side. You can see the lack of shims, as the compression has less damping force. Those holes are the other end of the damping adjustment. I don't know if there's a check valve to actually act as 1 way adjustment, or if it flows both ways and it adjusts both compression and rebound.

These are valved linearly, so you can see the shims getting smaller the farther out they go. Digressively valved shims are stacked differently. There's a diagram of them in the first post. Linear valving is, in my opinion, not as good as digressive as high speed impacts have proportionally high damping forces, unlike digressive valving, which digresses to absorb harder impacts. This is why I'm going to be valving them digressively in the future.


https://cdn.discordapp.com/attachmen...178/image0.jpg
This is the wonky spring setup on the coilovers. They have 6k fronts and 4k rears, which has like a 5.5k front wheel spring rate and 2.3k rear wheel rate, which is pretty weird. Reversing these almost seems like a better spring setup. I got these used, which is why the paint is all coming off. It's probably also why the collars were all seized.


I'm kind of busy again preparing for another college semester and moving out, so I'm not sure how regularly I can update this. I should be able to at least revalve this damper and send it to get dyno'd soon, so I guess expect that sometime.

What is your plan to validate you valving, do you have access to a shock dyno?

I don't. I'm planning to ship them to someone who does have a shock dyno. I'm considering calling Fortune Auto since they're reasonably local. If not, I'm sure someone else can.

I'd be shocked if there wasn't a shock dyno in the Atlanta area, lots of road racing around there, maybe pinging some local Facebook scca/nasa groups could point you in the right direction?

Good point, I'll make sure to check it out, thanks.

You're doing good work. One note, it's not just the shims specs that can make the valving digressive. The piston and any shim preload (if your piston allows that) will have an effect as well.

- Andrew

iirc you may be able to use a Bilstein digressive piston in the BC/Megan monotube, I think they're the same size.

I'm aware, I have Bilstein 46mm COB pistons without the preload spring thing, so they should act as double digressive pistons. And by shim preload, do you mean the part on the piston or the preload shims? Or something else entirely?


Thanks by the way

Yep, 46mm monotubed shock. I have bilstein pistons sitting downstairs and their respective 12mm ID shims

Another pseudo-update. Figured out what shim stack I'm gonna toss in. This is roughly based off of the Bilstein 275-110D shim stack.
*all of this is in millimeters, diameter x thickness


Rebound side
[22 x 3]
[16 x .5]
[36 x .5]
[742B2] *bleed shim, 2 divots
[28 x .1]

[PISTON]

[28 x .1]
[743B2] *bleed shim, 4 divots
[36 x .35]
[15 x .5]
[15 x .5]
Compression side




I know I could use the adjustable damping on the Megans, but I want to at least see what this does before putting in solid shims in place of the Bilstein bleed shims


Also MCS is in the Atlanta area, I'll be calling them after these are done. Thanks strat61caster