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06-17-2008, 04:28 AM | #23 | |
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-Just FYI, I do not plan to change the springs on my own car. All is just fine and BMW have done a darn good job of compromise IMO. -It is not really nonsensical. You could easily design a simple suspension system that actively controlled damping that was not robust to the allowed range of damping. Would it make production, no, could it be designed, yes. -I more or less agree with you clarification of what types of offsets an accel can and can not measure. OC: There is an inherent problem asking folks who design springs (coils and torsion bars) for their opinions on the suitability of their own products for certain applications. There is not only possible conflict of interest but there is a serious question of expertise. |
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06-17-2008, 06:59 AM | #24 |
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I finaly got the chance to measure the height difference between the cars.The M3 sits about 1" higher at the front and 1.5" at the back.This was confirmed by taking measurments at 3 different points F&R.Some of the height difference is related to the M3 having an taller tire height.
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06-17-2008, 09:53 AM | #25 | |
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Yes, I'm very aware of the potential 'conflict of interest' from manufacturers like Bilstein, H&R, Koni, etc., when asking for an official answer about adaptability with EDC from the companies themselves, it's akin to asking for an official answer from BMW re: installation of aftermarket components.....but we're not asking about suitability, "if the springs will work with the system" we are asking specific functionality questions of individual components within the system. I vehemently disagree with your statement about there being "a serious question of expertise." I would venture to say that Bilstein or Koni engineers are competent and could provide valuable information in regards to the specific questions put forth by jm.
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06-17-2008, 10:18 AM | #26 |
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Aftermarket EDC springs
I'm not a huge Dinan fan, but this is a case where I would be tempted to get EDC springs from them, if offered. At least you would be assured they or BMW would warrant shock failures. I would guess these are pretty spendy dampers.
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06-17-2008, 01:15 PM | #27 | |
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Similar topic - anyone know when Dinan will release anything aside from their wheels for the our cars? |
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06-17-2008, 04:35 PM | #28 | |
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It all depends on who (what level) you actually get to talk to. Some of the shock guys will have designed their own active systems. That would help. But still their system could be entirely robust to such changes where others may not be. You will almost never get to talk to those guys anyway (they are hidden away at the design/engineering centers). You will get some application or support "engineer" to talk to at best and a sales guy at worst. Unless they have had specific training on this topic they will probably be guessing. BMW will almost for sure never tell us all the facts, without those anything else we will get, from folks selling springs or shock or whatever, all of the advice will be educated guesses. |
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06-17-2008, 06:40 PM | #29 | |
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Are you saying in the first paragraph that any spring from a reputable spring manufacturer that is built for the car for non track/race application should be OK? In the second paragraph, are you saying there is an inherent problem asking the "folks who design springs" (spring manufacturer?) for their opinion on the suitability of their product for certain applications (like non track/race or something else)? Help me out here, it sounds to my ears as if you are contradicting yourself unless you are saying that it's OK to ask the spring manufacturer if their spring will work for road driving but not OK for racing? |
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06-17-2008, 09:57 PM | #30 | |
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I believe, in my own educated opinion, that any spring made for the car by any reputable manufacturer for a non EDC car will work just fine with an EDC car. Again limiting the set of springs to non ultra low and non race springs. Of course with the caveat that the spring mounting diameters and other dimensions are compatible. My educated opinion is based on a variety of factors, my academic work in math and physics, my work as a mechanical engineer and suspension designer (bicycles BTW, not cars). All that being said I do not believe that most folks you or I can talk to at an OEM or aftermarket suspension company will be able to definitively tell you that any particular spring is guaranteed to work flawlessly with the EDC system either in terms of its function nor longevity. The availability of this information and the certainty of it may certainly evolve over time, depending on how much information BMW will part with and how much testing is done independently of BMW. |
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06-18-2008, 09:29 AM | #31 | ||
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Having said that, the funny thing is, even BMW must be using a good deal of trial and error engineering when sorting out the suspension systems (hence the thousands of hours spent testing the car on the ring, etc). The range of inputs and behaviors are too complex to model comprehensively. I saw a documentary on one of the F1 teams trying to work through suspension issues they were having. They crunched numbers endlessly and could not explain the telemetry data. Then they hired a suspension “guru”, flew him to site, and the guy poured over the data for a couple of days and finally figured out what was wrong, but he was working from the data and not the model. Well, I guess one can argue that they had a crappy model...
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06-18-2008, 09:39 AM | #32 | |
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06-18-2008, 09:41 AM | #33 | |
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Someone posted pics of his lowered M3 here: http://www.m3post.com/forums/showthread.php?t=149525 As I said on that thread, I think that is overdone. Of course, the owner claims significant handling improvements, which I seriously doubt. Very few people will spend big bucks on a mod and then say it made things worse. For aesthetic reasons, I personally would like to see the car ride about 1.5cm lower at the most after seeing those pics. But that is such a small difference that it wouldn't be worth it to mess with the suspension.
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06-18-2008, 10:49 AM | #34 | |
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06-18-2008, 11:08 AM | #35 | |
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P.S. You wanna bet $100 on this? I don't know. That guy still owes me $200 on the pricing bet, but I know you would honor your word if you were to lose. (taking you up on the bet is a joke in this case as I don't even know what we would be betting for or against. What happened to your bet with Jason?)
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06-18-2008, 04:35 PM | #36 | |
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I am conservative as well. I would be more concerned about the aero/cooling effects of a CSL style trunk lid or a twin canister aftermarket muffler than swapping out springs. I suppose I would bet but the only way we could tell would be to compare the longevity of a group of non resprung EDC cars with a group that was resprung. You can imagine how tough it would be to get much data or reliable data. Basically the bet is non verifiable IMO. Nice of you to think I am good on my bets. I ended up selling my old E36 M3 before I had time to make good on my bet. Lucky for me Jason was very chill. Perhaps letting him take my car out for a bit of M-DCT testing helped satisfy the payment (although he never made such an association). Swamp the m3post.com welcher |
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06-18-2008, 04:55 PM | #37 |
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Damn! I was literally about to send you a message asking if your E36 was up for sale because I want to gut one for the track. I've been looking the past two weeks, but cars around here are rusted out, so I would prefer a California car...
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06-18-2008, 10:32 PM | #38 | ||
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I doubt if you had access to the source code that there is a variable that's called spring rate. The point is that the control algorithm is designed to operate in conjunction with the as designed system parameters (including the stock springs). Any control algorithm if connected to a system with different parameters will not operate as designed. The source code could have a variable in it called spring rate and you could adjust it for the new system. I doubt it does because I doubt BMW built this system to be used with any spring. It's a mathematical point, functionally the system may operate exactly the same with different springs. Lookup tables are not inherently robust or stable. Step changes in any control surface are inherently destabilizing because they introduce a frequency spread of noise into the system. Positive feedback may exist at any frequency. The larger the step, the greater the noise and the greater the likelihood of exciting a positive feedback loop. If a lookup table artificially limits the quantity of control values then this will increase the size of the step change. They could indeed use lookup tables but it would be because the system is inherently stable, not because the lookup table is. Quote:
FYI - I've seen a million dollar model rip itself to pieces in seconds and fly into a wind tunnel turbine because the programmer got a number wrong in a large control matrix. Small changes in feedback loops can (not will) have a dramatic impact. I saw this video of an out of control feedback loop on a car suspension. I don't think changing the springs would do this... |
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06-19-2008, 02:21 AM | #39 | ||||
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Completely agree on this one. Sure you model will never be perfect nor capture all effects but it would be pretty easy to get a darn good model that is plenty good for realistic virtual testing. |
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06-19-2008, 07:24 AM | #40 | |
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P.S. The video was cool. But, without crunching the numbers, how can you say swapping springs will not do that!?
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Last edited by lucid; 06-19-2008 at 08:30 AM.. |
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06-19-2008, 12:32 PM | #41 | ||
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Car companies spend years and tens of millions developing a new car design. There is both time and money for modeling. Not as much as in other industries but the models are simpler. A system controller would not include a model of the controlled system in it's design. A controller would be built based on a model of or even the real controlled system. I never said the ECU would be running a model of the car. When an engineer speaks of model validation he is talking about an iterative process not a one time check. The sheer number of scenarios is exactly the reason that a model is more helpful than real world testing. You can run a model through the same set of thousands of hours of "real world scenarios" much cheaper and faster than real world testing. There comes a point later in the design process where real world testing is more valuable but at that point you have limited the parameter ranges and are designing for subjective criteria. Quote:
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06-19-2008, 01:11 PM | #42 |
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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? 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. |
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06-19-2008, 01:13 PM | #43 | ||||||
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That said, I actually agree with you on not messing with the stiffness of the springs. That has been my position from the beginning. Whoever designed this system has spent a lot time in optimizing--by using models and real world data--and I think one would take a chance by assuming everything would work fine by swapping springs. Moreover, a system working fine might be a system far from its potential peak performance.
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Last edited by lucid; 06-19-2008 at 03:13 PM.. |
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06-19-2008, 10:41 PM | #44 | |
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I won't be messing with my springs personally, but I strongly suspect and would wager (could a reasonably verifiable and concrete proposition be formed) that any non race spec spring from any reputable manufacturer will work fine with EDC and will not cause any decrease in reliability. I also think that if all you want is better lap times lower and stiffer springs would likely provide that, EDC or no EDC. The entire system is a compromise and I personally like that - comfort and performance. This is where it would be incredibly tough to best the all OEM components set up - improving comfort and improving performance. Can we at lesat separate the ride height issue from the spring stiffness issue? I think I "proved" with a simple thought experiment that a spring that only changed the ride height could not be "detected" by the EDC system nor enough to alter its performance. |
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