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      06-17-2008, 04:28 AM   #23
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Quote:
Originally Posted by jm1234 View Post
Perhaps I should be more clear, I think it is fair and valid to conclude that the "system is going to be robust to changes in spring rate" (it is actually guaranteed because even stock spring have some variability) but you do so without attempting to put bounds on the statement. I'd bet the N54 engine is robust to changes in boost pressure too. Surely you don't believe that was a system design criteria and/or that any spring rate or type will work just fine? It's a question of degrees and nobody seems to want to put any numbers to any of the changes discussed.

The conclusion that the system is robust to changes in damping when damping is the controlling variable is nonsensical. The damping rate is clearly designed to change and to do so over a much greater range than a traditional damper so the comment doesn't make sense when placed in the same context as the spring rate which is not designed to change.

I wish you luck with your modification and if you say that the specific springs installed do not ruin the ride I'll take your word for it. I still think that the system will not take any spring rate and that some spring rates will cause a crappy ride regardless of the system. I also think that the effect on damper life is an open question as no one can describe any of the pertinent design elements of the system (like how the dampers or control systems work) let alone analyze their interaction.

By dynamic system offset I'm simply referring to the offset over a known period of time. In other words, the system can calculate with a certain degree of error and over a fixed period of time what the relative (from the start of the time period) position offset is based on accelerometer readings.

By static system offset I'm referring to a measurement of the system offset with a certain degree of accuracy without regard to time. Basically you wouldn't be able to do this with an accelerometer because the buildup of position error over time would overwhelm the position measurement.
-I even guessed at some bounds on the spring rates and stated that before. My guess was basically anything that any reputable spring manufacturer would build for the car for anything outside of dedicated track/race car.

-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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Quote:
Originally Posted by lucid View Post
If you get a chance, can you measure the chassis ride height difference between the 335 and the M3? Thanks.
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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Quote:
Originally Posted by swamp2 View Post
-

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.
swamp:

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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Quote:
Originally Posted by GregW / Oregon View Post
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.
I agree with you wholeheartedly. Until a tried and true plug and play EDC coilover system comes out, I'll stick with Dinan for springs for coverage.

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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Quote:
Originally Posted by OC kid View Post
I vehemently disagree with your statement about there being "a serious question of expertise."
OK, I disagree back.

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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Quote:
Originally Posted by swamp2 View Post
My guess was basically anything that any reputable spring manufacturer would build for the car for anything outside of dedicated track/race car.

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.


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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Quote:
Originally Posted by jm1234 View Post


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?
I see why you may feel I am being contradictory but I really am not. Let me clarify. It has nothing do with how you will be driving the car.

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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Quote:
Originally Posted by swamp2 View Post
Yes the EDC system is probably "aware of" the spring constant, but the system is going to be robust to changes in spring rate, damping and ride height, virtually guaranteed.
Quote:
Originally Posted by jm1234 View Post
Perhaps I should be more clear, I think it is fair and valid to conclude that the "system is going to be robust to changes in spring rate" (it is actually guaranteed because even stock spring have some variability) but you do so without attempting to put bounds on the statement.
A specific spring rate is most likely not hardcoded in the control algorithm. Moreover, there might not even be an algorithm, but merely a lookup table of some kind that matches measured acceleration data to predetermined damper settings. That might explain the “robustness” of the system althought an algorithm can be robust as well, but will be more precise by definition. However, I would refrain from making educated guesses on how robust the system is exactly. Any such educated guess would have to have solid technical justification to avoid constituting a mere hypothesis or lead to trial and error engineering.

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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Quote:
Originally Posted by GregW / Oregon View Post
"EDC-K is available for the first time in the E92 M3. EDC-K is an option and is based on the EDC-K in the E65.

Both dampers of one axis are always activated in parallel. The valve is installed internally in the damper in the damper oil system.

The driver can choose between three settings, the controlled programs "Comfort" and "Normal", or the uncontrolled fixed setting
"Sport". The program is selected using the EDC-K button on the center console or preset via the MDrive menu and activated using the M button on the steering wheel.

The input signals come from two vertical acceleration sensors in the front wheel arches and a third sensor in the rear right-hand wheel arch.
The steering column switch cluster sends the steering angle to the F-CAN. This is transmitted together with the wheel speeds from the DSC to the PT-CAN and evaluated in the EDC-K control unit.

The longitudinal, lateral and vertical accelerations calculated as a result are used as a basis for regulation. The EDC-K button signal enters the junction
box and is transmitted to the EDC-K on the PT-CAN." -- BMW Aftersales Training - Product Information

The only thing that is controlled is the shock damping, both compression and rebound. The following graph illustrates the damping force on the vertical axis "A" (compression below & rebound above) related to the damper piston speed on the horizontal axis "B". Controlling ride height would take some sort of air suspension or such. Also, the M3 does not have active anti-roll bars like the Range Rover and some other vehicles. Still, it is quite effective.
Thanks for the info Greg. You always come up with solid specs when needed.
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      06-18-2008, 09:41 AM   #33
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Quote:
Originally Posted by Gearhead999s View Post
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.
Thanks for confirming this. Yes, we did work out the tire height issue earlier on another thread, and if I remember correctly, it did account for about 1cm of the difference.

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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Quote:
Originally Posted by lucid View Post
A specific spring rate is most likely not hardcoded in the control algorithm. Moreover, there might not even be an algorithm, but merely a lookup table of some kind that matches measured acceleration data to predetermined damper settings. That might explain the “robustness” of the system althought an algorithm can be robust as well, but will be more precise by definition. However, I would refrain from making educated guesses on how robust the system is exactly. Any such educated guess would have to have solid technical justification to avoid constituting a mere hypothesis or lead to trial and error engineering.
I have been very clear that my statements about EDC are hunches and educated guesses, of course I have no proof. That being said I am confident enough in them to place bets! We do have some evidence that the "control system" is nothing more than a lookup table. BMW published that long ago and Greg posted it above. Of course that does not mean that the table is not supplemented with other parameters. Guesses are good and needed at this point is simply because folks want to swap springs and there is no conclusive answer nor proof of the affects both on longevity and performance. You may feel the need to be very conservative and rigorous, I generally do as well, but not in this case. Ultimately, like I said, without cold hard facts directly from BMW there will never be 100% certainty nor "rigor" in this discussion.
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      06-18-2008, 11:08 AM   #35
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Quote:
Originally Posted by swamp2 View Post
I have been very clear that my statements about EDC are hunches and educated guesses, of course I have no proof. That being said I am confident enough in them to place bets! We do have some evidence that the "control system" is nothing more than a lookup table. BMW published that long ago and Greg posted it above. Of course that does not mean that the table is not supplemented with other parameters. Guesses are good and needed at this point is simply because folks want to swap springs and there is no conclusive answer nor proof of the affects both on longevity and performance. You may feel the need to be very conservative and rigorous, I generally do as well, but not in this case. Ultimately, like I said, without cold hard facts directly from BMW there will never be 100% certainty nor "rigor" in this discussion.
Well, I guess the "rigor" in any such online discussion is simply to acknowledge that we are all shooting crap here in the absence of more information, which is fine, educational, and entertaining. Regardless, the issue is more about how one goes about sizing the risk associated with doing any kind of serious modification to the car. In that regard, I am pretty conservative indeed when dealing with an expensive car that is under warranty. After the warranty expires, I'd be much more willing to experiment.

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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Quote:
Originally Posted by lucid View Post
Well, I guess the "rigor" in any such online discussion is simply to acknowledge that we are all shooting crap here in the absence of more information, which is fine, educational, and entertaining. Regardless, the issue is more about how one goes about sizing the risk associated with doing any kind of serious modification to the car. In that regard, I am pretty conservative indeed when dealing with an expensive car that is under warranty. After the warranty expires, I'd be much more willing to experiment.

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?)
Agreed.

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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Quote:
Originally Posted by swamp2 View Post
I ended up selling my old E36 M3 before I had time to make good on my bet.
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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Quote:
Originally Posted by lucid View Post
A specific spring rate is most likely not hardcoded in the control algorithm. Moreover, there might not even be an algorithm, but merely a lookup table of some kind that matches measured acceleration data to predetermined damper settings. That might explain the “robustness” of the system althought an algorithm can be robust as well, but will be more precise by definition. However, I would refrain from making educated guesses on how robust the system is exactly. Any such educated guess would have to have solid technical justification to avoid constituting a mere hypothesis or lead to trial and error engineering.
Note: I have a masters and part of a doctorate in control system design. For several years I designed aircraft model control systems for wind tunnel usage and then control systems for the wind tunnels themselves. I have no specific knowledge of suspensions, just feedback loops.

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:
Originally Posted by lucid View Post
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...
I would doubt the behaviors are too complex to model practically. NASA, Boeing, etc... model airplanes in great detail (many flexing components, high stress, fluid dynamics to get the airflow and vortexes, multiple control surfaces, etc...). They still spend many hours testing but it's generally to validate the models as much as to fine tune the systems. Bridge builders/skyscaper architects build models with many more flexing components than a car suspension. It's easier to test ideas on a car and make mistakes than to do so in other applications so there may be less reliance on fine tuning the models. NASA will spend decades testing models in wind tunnels and then in the air, it doesn't mean there is no model. BMW could build a model, they probably have. Testing is to validate the model (which is not 100%, no model is) and fine tune the design, not replace the model. Having said this, there is no system that you can model "comprehensively" if you are anal enough about defining the word comprehensive.

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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Quote:
Originally Posted by jm1234 View Post
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).
... AND the system is designed to be robust to a variety of real world operating parameters and very likely a decent range of spring/roll bar stiffnesses.

Quote:
Originally Posted by jm1234 View Post
Any control algorithm if connected to a system with different parameters will not operate as designed.
Not true. A system can be designed to be able to work with a wide vaariety of physical contants affecting the dynamics of the system. The above is making an ENORMOUS assumption that they did not bother to test EDC with a range of spring and roll bar combinations. I think that assumption is definitely incorrect. Of course they did, seem almost trivially obvious to me. Spring swapping during real world testing occured just as much as it does during testing of a non EDC car.

Quote:
Originally Posted by jm1234 View Post
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.
Want to bet? M5 and M6 also have EDC and vendors have claimed their springs are fine on the system and users have swapped with no deleterious effects. There is no reason to believe the M3 is any different. I care about the math and subtleties myself too, but here that is not what matters. All we care about is does the system work and does it last.

Quote:
Originally Posted by jm1234 View Post
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.
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.

Quote:
Originally Posted by jm1234 View Post
I would doubt the behaviors are too complex to model practically.
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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Quote:
Originally Posted by jm1234 View Post
Note: I have a masters and part of a doctorate in control system design. For several years I designed aircraft model control systems for wind tunnel usage and then control systems for the wind tunnels themselves. I have no specific knowledge of suspensions, just feedback loops.

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.



I would doubt the behaviors are too complex to model practically. NASA, Boeing, etc... model airplanes in great detail (many flexing components, high stress, fluid dynamics to get the airflow and vortexes, multiple control surfaces, etc...). They still spend many hours testing but it's generally to validate the models as much as to fine tune the systems. Bridge builders/skyscaper architects build models with many more flexing components than a car suspension. It's easier to test ideas on a car and make mistakes than to do so in other applications so there may be less reliance on fine tuning the models. NASA will spend decades testing models in wind tunnels and then in the air, it doesn't mean there is no model. BMW could build a model, they probably have. Testing is to validate the model (which is not 100%, no model is) and fine tune the design, not replace the model. Having said this, there is no system that you can model "comprehensively" if you are anal enough about defining the word comprehensive.

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...
NASA and Boeing are in a different industry. They don't have any options other than to spend hundreds of millions of dollars to model whatever they can. But please don't tell me they can model everything. There is a guy next door to my office who does CFD models all day long to model very specific flow issues. It takes him months to come up with an "acceptable" model for a constrained case. Modeling buildings is also a different ball game. Yes, there is dynamic loading, but the loads are much more predictable. A car manufacturer will not invest the kind of resources NASA or Boeing invests into modeling the dynamics of a car. If testing was simply to verify the model, they would not need to spend thousands of hours driving the car around acquiring data. I’ve watched some clips on the development of the M3. Their test driver is an engineer. He explicitly said that the design engineers rely on him to assess handling and make suggestions to improve the behavior. Why would that be necessary if they had an accurate model which simply needed verification? Why can’t BMW engineers arrive at a comprehensive accurate model after some iteration so that they can take the test driver off-line? Cars have been around for a long time. Although I am not a dynamics expert, I think you are oversimplifying the variation in potential road and driver inputs and their interactions--the sheer number of scenarios that can entail even if you have an accurate model. And, I doubt that the ECU is doing any kind of deterministic calculation in real-time that includes a general system model of the car, but of course, neither of us knows that. (The reason is that a car is not an unstable aircraft that needs to be actively managed to stay in the air). Computational models are the place to start and they have clearly transformed the way engineers work, but reality is complex and one discovers and learns a lot about the phenomenon while testing; testing is not just to validate the model. Models can and do change during testing. There is nothing wrong with that. (I am not advocating the type of approach the F1 "guru" made to solve the suspension problem although it makes a case about how complex a car suspension system can be if you want every bit of control).

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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      06-19-2008, 12:32 PM   #41
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NASA and Boeing are in a different industry. They don't have any options other than to spend hundreds of millions of dollars to model whatever they can. But please don't tell me they can model everything. There is a guy next door to my office who does CFD models all day long to model very specific flow issues. It takes him months to come up with an "acceptable" model for a constrained case. Modeling buildings is also a different ball game. Yes, there is dynamic loading, but the loads are much more predictable. A car manufacturer will not invest the kind of resources NASA or Boeing invests into modeling the dynamics of a car. If testing was simply to verify the model, they would not need to spend thousands of hours driving the car around acquiring data. I’ve watched some clips on the development of the M3. Their test driver is an engineer. He explicitly said that the design engineers rely on him to assess handling and make suggestions to improve the behavior. Why would that be necessary if they had an accurate model which simply needed verification? Why can’t BMW engineers arrive at a comprehensive accurate model after some iteration so that they can take the test driver off-line? Cars have been around for a long time. Although I am not a dynamics expert, I think you are oversimplifying the variation in potential road and driver inputs and their interactions--the sheer number of scenarios that can entail even if you have an accurate model. And, I doubt that the ECU is doing any kind of deterministic calculation in real-time that includes a general system model of the car, but of course, neither of us knows that. (The reason is that a car is not an unstable aircraft that needs to be actively managed to stay in the air). Computational models are the place to start and they have clearly transformed the way engineers work, but reality is complex and one discovers and learns a lot about the phenomenon while testing; testing is not just to validate the model. Models can and do change during testing. There is nothing wrong with that. (I am not advocating the type of approach the F1 "guru" made to solve the suspension problem although it makes a case about how complex a car suspension system can be if you want every bit of control).

P.S. The video was cool. But, without crunching the numbers, how can you say swapping springs will not do that!?
Buildings and bridges undergo predictable loads? Google Tacoma Narrows (awesome video) for the most obvious bridge example and think about what happens to a building during a fire, earthquake, hurricane, tornado and/or impact (planes and cranes being recent impact examples). What loads do you think a car undergoes that are as complex in comparison?

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.

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If testing was simply to verify the model, they would not need to spend thousands of hours driving the car around acquiring data. I’ve watched some clips on the development of the M3. Their test driver is an engineer. He explicitly said that the design engineers rely on him to assess handling and make suggestions to improve the behavior.
I've never met a test driver/pilot that didn't think the world rested on their shoulders. Seriously though, there is a difference between subjective design changes within defined parameter ranges and objective design changes to achieve objective design goals. It's the difference between fine tuning and course tuning.
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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?

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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.
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      06-19-2008, 01:13 PM   #43
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Buildings and bridges undergo predictable loads? Google Tacoma Narrows (awesome video) for the most obvious bridge example and think about what happens to a building during a fire, earthquake, hurricane, tornado and/or impact (planes and cranes being recent impact examples). What loads do you think a car undergoes that are as complex in comparison?
Tornados and plane crashes would be difficult to model for a building as well. I was referring to the loading conditions a structure would experience on a daily basis--the way your car would driving around a track without crashing into anything--not the extreme cases.

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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.
Of course they do. Where did I say, they don't? The issue is at what level of precision? Do you really think the models they build account for every millimeter of movement of the chassis for 99.99% of the behaviors the car will exhibit under different loading conditions? Is that level of precision necessary for a passenger car (it clearly was for the F1 car in the example I gave earlier)? Even if it is, is it even possible? There is cost/benefit trade-off there. And I think you are mixing up the aerospace industry's requirements with the automotive industry's requirements.

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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.
Yes, you got me on this one.

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Originally Posted by jm1234 View Post
When an engineer speaks of model validation he is talking about an iterative process not a one time check.
Yes, I said the same thing.

Quote:
Originally Posted by jm1234 View Post
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.
This is obvious. That is why it is cheaper to use models than running a million real life tests. However, the inputs are a part of the simulation together with the model. And virtual inputs are not going to be as representative as the real world conditions. It is perfectly possible that when the model is tested with real world inputs, issues that could not have been found with the model using virtual inputs surface. I don't know, maybe we are actually saying the same thing here.

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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.
I disagree with your use of the term "subjective" criteria. Why won't you simply agree that models can be faulty for various reasons and produce invalid outcomes? This is often caught in testing and corrected. There is nothing subjective about that. Do you really believe BMW would spend thousdans of hours logging data in this car just to make sure it "feels" right? They are also improving the objective performance measures when they are testing. Again, we don't have comprehensive ontological knowledge of all things/systems, and the real world is a complex environment.

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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      06-19-2008, 10:41 PM   #44
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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.
Trying to get back to some common ground, agreement or disagreement, this is where I seem to differ significantly from both you and jm1234. I've said it before but I will summarize again.

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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