madmax199

iTrader: (20)

Jesus, I dropped by to say hi and you guys have me involved in all kind of stuff

You don't even need to get technical to understand why 65% is needed.
Critical damping is the least amount of damping needed to completely eliminate oscillation up and down as quickly as possible without too much bouncing . The up motion of the spring is controlled by rebound and the down motion by compression, 65% critical is about perfect in both directions(rebound and compression) to achieve that balance I talked about.

Overdamping is exerting a force that's not allowing the spring to move freely through it's range of motion. I said that upward of 70% critical was overdamped, you could still technically valve for 70%+ critical, if your heart desire, but the shock would be acting as extra springs and that's not ideal. Underdamping is also the exact opposite where you don't exert enough force and the spring oscillate up and down more than they need(remember old school luxury american cars infinitely floating up and down while in motion). The 65% critical, regardless of the phase(rebound/compression) is the nice sporty balance between the two.

catman78

iTrader: (15)

Anyways, I shot an email to Robispec and they came back with to following:

Increase rebound control by 30%

I sent them the same requirements DD with some HPDE

Iowa999

iTrader: (1)

Let me be clearer. I already knew everything that you said (and then some). I have modeled the suspension of the 2G DSM and have a full set of spreadsheets for the same. You do not have to give me any more background information, although, of course, you are quite welcome to.

I have asked you a very specific question twice that you haven't come close to answering; I will try a third (and last) time.

Shocks usually have different damping in rebound and compression. Therefore, the damping can be a different proportion of critical in each of the two directions. I assume that we agree on this point.

When you suggested that the damping be 65% of critical, did you mean 65% of critical in both directions or an average of 65% across the two directions or what?

Iowa999

iTrader: (1)

This is surprising to me. Everything that I have seen and read suggests that Bilsteins need more low-speed compression. Don't get me wrong ... I'm a huge fan of tons of rebound - take at as far as you can without packing down - but I would have expected to see something about the low-speed compression, as well. I wonder if they are incapable of upping the low-speed compression without ruining the wonderfully digressive compression curve (which is what gives Bilsteins their comfy street ride).

FERNO

iTrader: (4)

Honestly this just comes off as rude. I wouldn't hold my breath for a response.

Good luck with your valving.

Iowa999

iTrader: (1)

Oh, I'm not. But not because I may have been rude when asking the question for the third time.

FERNO

iTrader: (4)

Cool then.

Not sure where the aggressive undertone is coming from. This is a free forum so while I understand the frustration of not getting a question answered to your satisfaction, you're responsible for managing your own expectations.

You admitted you could have been clearer the first two times you asked your question. In my mind, any miscommunication is as much your fault as anyone's. I'd say madmax199 was attempting to answer, and while he made a comment about being sucked into this, it was in jest as evidenced by the grin. He actually took time to recommend something that he found "nice" on his Evo, and to get that kind of attitude in return is uncalled for in my opinion.

If you are the kind of person who has blueprinted a 2g suspension and filled a spreadsheet with info, then why even ask a 3rd time. Call Ramone at Bilstein and get it done man. Then post your knowledge to someone else who may or may not appreciate your efforts. Pick your battles man and don't sweat the small stuff.

FWIW, Swift, Robispec, et al engineer their MR parts around the factory valving, so you'll probably be just fine with the factory spec rebuild.

Iowa999

iTrader: (1)

I apologize for the tone. I'm frustrated and acting out. I'm just not the type of person who ever takes unexplained advice; heck, I'm known for being incapable of taking any advice. But this definitely rules out sending my shocks off to someone with the request for "65%-of-critical damping" when I don't even know what that means.

If I'm being obtuse and you're telling me that Ramone has heard this 65% "rule" and could tell me what it means, then, of course, I will call him, instead of pestering people here.

FERNO

iTrader: (4)

Well, I agree there. I'm not sending mine off asking for anything other than stock without knowing why.

Iowa999

iTrader: (1)

Actually, I lied. (I'm a very bad person.) I would take advice without explanation in two cases (related to cars): if John Mueller made a suggestion about set-up or if Sean Caron made a suggestion about driving.

madmax199

iTrader: (20)

That came out a little abrasive

Let's go past your wonderful personality and make an attempt to answer the question in a more detailed fashion because that answer may help someone else that deserves it(It's safe to say that nothing will please you)!

Our discussion is becoming about % critical damping. We are referring to the low piston velocity range(0-3 in/sec), which is the range of chassis leaning, and NOT tire tracking over bumps. This is the range where people try to set the magical critical damping value for both rebound and compression (not average, not anything else, both curves).

The reason for that arbitrary number is because it works. The people that went through the process of actually computing critical damping will tell you, that you need to know the equivalent mass of each item in the suspension and the wheel/tire combo to get get a good aproximate of what's needed. Why the equivalent mass? Because you are looking at the suspension and wheel/tire combo as a point mass-spring-damper system centered through the center of the wheel's transverse plane. It is a time-comsuming exercise to compute each member's equivalent mass and it's not for everyone. However, after you have the equivalent mass, choosen your desired damping coefficient and spring rate, then you need to look at finding a solution to the free vibration with viscous damping (dashpot) differential equation.

Critical Damping Constant = C = 2*square-root(equivalent spring rate X equivalent mass)

Thus, for a 65% critically damped suspension, you need to make the low-speed equivalent damping coefficient Ceq = .65*C .

Then by using the suspension's motion ratio, find the damping coefficient you need and use the damper dyno to fine tune the low speed control(both direction, rebound and compression).

You do all that to only realize that every time, every car, you always end up within that 65%-70% critical damping range. The lower end of that range (65%) works well on bumpier surfaces(most tracks and auto-x courses I have seen) while the upper end of the range is more suited for smooth tracks and cars with full wind tunnel tested aero packages that can create big downforce. So, from experience, those who have done the leg work started throwing around that magical 65% critical damping as a nice compromized number that get's anyone very close the ideal shock valving without too much work. I'm sure you still will not be satisfied since it's the same answer I gave you twice already but with a few details.

BTW, don't bring asymetrical damping curves and shocks with independant rebound/compression adjustments because that's another topic by itself and it makes things almost impossible to control accurately for anyone that does not have exotic testing capacity. That's all I can give you without going off topic Douche!

Iowa999

iTrader: (1)

Thanks for the clear answer to my question.

While I cannot argue with what people have said works for them (other than to ask what else they have tried), from a theoretical perspective, I would question the sweeping suggestion of 65% in both directions for a couple of reasons. First and foremost, how quickly you want the system to return to default depends on how quickly you are loading/unloading the suspension. This includes both steering changes and bumps in the surface. The more rapid the inputs, the less rebound damping you can run and/or the more compression damping you might want, in order to avoid packing down the springs. Conversely, when inputs are less frequent and the surface is relatively smooth, you can shift more towards rebound and away from compression, now trying to keep the loadings on the outside tires more consistent (since no surface is perfectly flat). It's this kind of more complicated logic that made me wonder whether the rule was meant to apply in both directions. It seems a tad too simplistic to me. While I can wrap my head around the idea that the average of the two should always be around 65%, it would seem better to have the two directions change as a function of course design (from autocross to sweeping roads) and surface conditions (from bumpy roads to smooth tarmac).

I don't consider this off-topic. The original question was about revalving some Bilsteins. The OP can ask for changes in either or both of low-speed compression and rebound. The difference between the two seems highly germane to me.

[Douche?]

edit: equal compression and rebound damping makes no sense to me at all....

catman78

iTrader: (15)

OK guys, lets get back to the subject for a moment.
Any idea as to why Robi came back with:

Increase rebound control by 30%

What's the pros/cons am I gaining anything over re-valve stock?

Iowa999

iTrader: (1)

As I said in Post #5, I'm surprised. Most people ask for more low-speed compression, as well, to make the shocks more like HDs than how they come on an MR. The only reason that I can think of for them not doing this is that they are worried about also increasing the high-speed compression and ruining the ride. I would ask him flat out: "why not more low-speed compression, as well?"

But, again, I'm a huge low-speed rebound fan. You will get faster weight transfer and, as long as you don't drive on washboard roads, I'm pretty sure 30% more isn't going to pack the springs down.

madmax199

iTrader: (20)

Then, you can enter the world of assymetrical valving every time you go out since it's seems that you're up for it(baller). Just revalve to match every track/surface or start playing with true rebound/compression independant adjustable dampers with no crosstalk between the two curves(be ready to spend 5k+ and have the capacity to really tune them for the conditions, everytime you autocross).