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      06-19-2008, 02:11 PM   #42
jm1234's Avatar

Drives: E90 M3 Jet Black/Tan/SA 6MT
Join Date: Mar 2008
Location: Alpharetta, GA

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Yes, I'm making assumptions about what a controls engineer would do based on my experience as a controls engineer. I thought that might be helpful to the discussion of what happens if you change the components inside a closed loop control system. If it isn't a closed loop system then I'd appreciate someone posting the design details instead of the guessing I see here. I still don't even know how the dampers work although Greg's post seems to indicate there is a magnetically actuated variable orifice valve in the damper which might be the answer or might not.

Please do not pull sentences out of paragraphs, that takes the meaning out of context and destroys the point. I don't really want to get into a tit for tat type exchange. I'm not trying to be right, just state what I thought was a perspective with some unique merit.

When I say I don't think that BMW designed the system to be used with ANY springs I mean just that. I don't think you believe you can take any spring at random from any spring shop anywhere and put it in an M3 and it will work fine. My point was as trivial as that. Sometimes, it's helpful to start with a statement both parties can agree on before moving on to establish the point of disagreement.

Again, I thought we had already established the fact that the controller is likely designed to operate on a system with a range of parameters. When I say the controller will not operate as designed on a system with different parameters, I'm talking about parameters that are different than the range of parameters it is designed to work with. Do you really want me to be more prolific?

Drawing reliability conclusions from that fact that vendors have sold products that people have installed is only pseudo legitimate. How many vendors have sold how many springs to how many owners and how long have they been installed to be with what sort of driving habits and how many EDC failures? Without followup it's the same as concluding that chipping a 335 is fine "because vendors have sold products...". I've already admitted that swapping springs is "probably" fine but you still seem to be hammering away as if nothing could possibly go wrong. Do you believe that?

Originally Posted by swamp2 View Post
In essentially a 1 DOF mass/spring/damper system with two controlled variables (compression and rebound damping - both in a fairly narrow range)? Don't think so. A look up table or look up function would likley be just fine.
Just because "it would likely be just fine" doesn't mean it's more robust. A lookup table is less robust. That is all I said and I explained why. There is feedback through the entire frame and chassis of the car so there are at least 2 DOF. The system is more complex than you indicate and you also ignore any subsystems to control the damper. In that video I just posted you can see that energy is transferred from the front brakes, through the front suspension (which compresses causing the car to rotate around the cg) and into the rear suspension. Half the time you guys appear to be arguing that the system is so complex that it can't be modeled or explained by simpletons like me and then the system is so trivial that you can control it with a lookup table. Perhaps a lookup table is fine but I think it's because the system is simpler and more robust/stable than indicated and not because it's so complex. Regardless, a lookup table is less robust because of the reasons stated earlier.