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05-31-2008, 03:44 PM | #1 |
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Why do some say changing springs destroys EDC shocks??
OK guys :
Let's hear the technical, intelligent answers to this! ie: Not my friend's car did this or I heard over on the M5/M6 forum that this happened. Let's talk physically and how it relates to the components involved about how lowering springs may or may not hurt or reduce the life of an EDC damper (ie: shock absorber). My thought : The people saying damage may occur have some ignorance on the topic or if indeed dampers have had issues it's just a coincidence that it was on an EDC equipped car. My reasoning : EDC is reading sensors in the car to automatically adjust the compression/rebound settings of the damper. The system has no effect/control upon the spring itself. In fact with lowering springs the travel of the damper is now less. Because the EDC is constantly making adjustments to the suspension of the car I assume it is reading the travel of the damper as one of its parameters in deciding what to do (the other sensors/parameters it's using are perhaps irrelevant in this discussion), and it should be able to account for whatever spring is in there within reason. I don't see how this could shorten the life of a damper within the realm of EDC - it is constantly changing and responding anyway. If a damper was designed for a specific spring rate, then the discussion seems to have merit (for example, typical aftermarket coilover setups). Also I don't know the rates of the H&R or RD lowering springs vs. the OEM springs, can anyone comment specifically what those are? When H&R says that "the spring kit harmonises perfectly with the adjustable dampers" I assume it to mean the rates are similar. What thoughts are out there?! |
06-01-2008, 12:11 PM | #2 | |
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06-09-2008, 03:37 AM | #3 | |
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Meaning, Fspring + Fdamper = Ftotal. or, Fs + Fd = mass of car x acceleration where, Fd = - damping coeff x relative velocity of the two connection points of damper Fs = - spring rate x displacement of spring If you were to weld the damper rigidly, the spring would not experience any force as it would not experience any displacement. Conversely, in practice, the spring will experience most of the force and compress and support the weight of the car except when the mass starts to oscillate due to suspension inputs and dynamic loading due to acceleration/deceleration. Then the damper ports will move relative to each other and the damper will experience force as well, dissipating energy and damping the oscillations in the process. That is a mass, spring, damper system in short. For a given value of mass, and predicted inputs to the system, engineers solve the differential equation relating those 3 variables and inputs, and pick spring rate and damping coeff values to control for the displacement of the mass. Although it is true that EDC can vary the damping coefficient, it can do so within a certain range, which is dictated by the physical characteristics of the damper. If one of the system variables changes, say the spring rate, the damping coefficient will need to change to achieve the same desired mass displacement, or for EDC, vary within a different range than stock. A couple of things can go wrong: 1. The new damping coeff range might not be physically attainable depending on what the damper is capable of producing. 2. The new damping coeff range is physically attainable, but within a range that will produce higher forces on the damper than anticipated on average, resulting in premature wear.
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06-09-2008, 11:11 AM | #4 | |
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06-09-2008, 04:27 PM | #5 |
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I was thinking about this over lunch ... let me know what you think lucid.
I'm questioning whether the damping coefficient really matters or not when we swap out springs. When we swap in springs they're typically shorter in length and have a higher spring rate. By higher I'm guessing a higher spring rate means it's requires more force for the spring to travel x distance than it would for the stock spring to travel x distance. Anyway ... say we have 2 identical spring lengths with different spring rates (stock springs A and higher spring rate B). When a spring has a higher spring rate why would that cause more wear than the lower spring rate? Say A & B drive over the same bump at the same speed resulting in the same amount of total force exerted on the system and X amount of travel for A and Y amount of travel for B. Is it safe to say that X will be greater than Y? I would have thought yes. So if X > Y the dampers travel less for the higher spring rate. How does this cause premature wear on them? I can understand how it could if the springs are shorter and the average travel location on the dampers is different. Maybe I'm missing something.
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06-10-2008, 03:09 AM | #7 | |
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damping ratio = c / (2 sqrt(k x m)) another parameter is the natural frequency, w0, which is: w0 = sqrt(k x m) If the damping ratio is 0, then the system is critically damped, and will return to a steady state the soonest without oscillating. If it is > 0, the system is over-damped, and will take longer to return to steady state (how much longer depends on how overdamped it is), but will still not oscillate. If it is < 0, the system is under-damped, and will overshoot (the spring would compress more than a critically damped system) and oscillate until it returns to steady state at its damped frequency (a function of wo and the damping ratio), but not as fast as a critically damped system. So, to answer to your question about X and Y, one would need to know not only kA and kB, but also cA and cB. In other words, if car B is under-damped, and car A is critically damped, it is "possible" for Y < X despite kb > kA (car B having stiffer springs). Now, coming back to why replacing stock springs with aftermarket springs that have a higher k might end up resulting in higher damper forces. If you look at the damping ratio equation above, you will see that if the EDC wants to maintain a specific damping ratio in a given situation--say, it wants critical damping and therefore a damping ratio of 1--and if k is now higher because of the aftermarket springs, then c needs to be higher as well. Now, we know that Fd = - c x v, and we also know the c of the aftermarket system will be higher. Then, the question is if the velocity of the aftermarket system is higher, equal or lower than the velocity of the stock system. To find that out you can differentiate the displacement response of the critically damped solution, but there might be an easier way. I think you can compare the natural frequencies of the aftermarket and stock systems since that solution is a function of the w0 and time only. So, if the aftermarket system has a higher w0, and if both systems are critically damped, the aftermarket system should experience higher velocities, and therefore, higher damper forces. And since higher k will result in higher w0, that seems to be the case. I don't do this for a living (not my area of expertise), and am thinking through what I learned in a basics dynamics course 15 years ago. So, you or anyone else is welcome to chime in and critique my thinking on this. Also, this is a simplified "model" of what really happens. In reality, there are 4 wheels experiencing different inputs but are connected to the same mass, the wheels and suspension parts have mass, stiffness and damping, and inputs vary in size and duration. So, to figure out a real situation, you need to turn to people like Swamp who codes computer simulations, and use those tools. Even then, simulations go so far and sometimes do not explain what needs to be explained.
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06-10-2008, 03:37 AM | #8 | |
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1. Want better performance than stock on the street. Can't do it. Can't beat M engineers by swapping springs and shocks. 2. Want better performance at the track. Possible since the car, despite being performance oriented is not optimized for the track. However, I personally do not want to commute in a car optimized for the track. 3. Lowered look. I would like to see the car about an inch lower, but I don't yet feel comfortable doing this. I would like to understand why the car is riding as high as it does stock. There might be flow issues. Clearence does not seem to be issue as the 335 seems to ride lower (although I have not confirmed this for sure).
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06-10-2008, 10:40 AM | #9 | |
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06-10-2008, 04:51 PM | #10 | |
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06-10-2008, 07:27 PM | #11 |
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Thanks for the post lucid. I'll need to jot some of that down on paper and ponder some more. I do agree that the M sits a tad high for my taste. I originally thought it was perfect, but I was walking to my car the other day and said "could be a tad bit lower".
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06-10-2008, 11:54 PM | #12 |
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The M3 rides higher because of two reasons.
1) The front diffuser is lower to the ground and BMW/NA requires a certian height for curbs, since the front diffuser is lower and the curbs are the same height the car needed to be raised a bit. 2) The cars are also a certian height not for ulitmate shock travel or for some other suspension issue (control arms being in a neutral position) but for the simple purpose that the DOT requires headlights to be a certian height level as well. So if you want to lower it 1" or so, it will be fine, anything more than that, you simply change too much in the engineering of the ssupension.
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06-11-2008, 07:59 AM | #13 |
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06-11-2008, 08:57 AM | #14 | |
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06-15-2008, 11:20 AM | #15 |
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I'd guess that the EDC in "normal" mode is a closed loop control system and thus the damping ratio is the controlling variable and not a constant. In comfort mode I would assume a slightly underdamped system and in race a critically damped system (or perhaps even slightly overdamped). In normal mode though it's a question of what is being controlled? Is it ride height at each wheel (to control compression/extension); is it roll/pitch frequency/rate; is it lateral/longitudinal acceleration; is it a combination; or something else entirely? Any portion of the control system based on ride height is going to be negatively impacted by changing the ride height. The control system will see a constant downward offset which could (just guessing here) result in a constantly overdamped system. Portions of the control system based on roll/pitch/acceleration should incorporate spring rates and by changing the spring rates you will impact the system response but how would be more difficult to predict.
I don't see how you can take a controlled system and swap parts to change the response values and not negatively impact the performance. It's comparable to open/restricting the intake and exhaust on the engine but not changing the computer and fuel maps. The car might "look" cooler with lowered springs but unless you change the dampers; control system; and/or keep the car in a static suspension setting (comfort or race) then I think you'll suffer a variably compromised ride with difficult to predict consequences. I'm not predicting dire things just crap suspension feel (too stiff, too soft, bouncing, bottoming out, excessive roll, etc...) some portion of the time while in "normal" mode. Does anyone know the details of how BMW's adjustable dampers work? I'm still curious and some of you seem to have a lot of engineering knowledge about the car. |
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06-15-2008, 01:08 PM | #16 | |
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I think the questions posed in your first paragraph are important ones. I do know that piezoelectric technology incorporated into shock absorbers is not new, there must be something published on this. Hopefully someone can enlighten us. Just to confirm though, from what I understand, comfort is adaptive as well...Can someone validate this?
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06-15-2008, 01:29 PM | #17 | |
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"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.
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06-15-2008, 02:57 PM | #18 | |
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A small thought experiment can help convince you that this is correct. What about ride height variation caused by max/min load cases (i.e. fuel and 1 vs. 3 or 4 passengers and possibly cargo). There will absolutely be a large noticeable change is ride height based on min/max loads and guess what EDC system will work just fine with a damper "offset". It has to. BMW may or may not tell us the truth on this topic. As well the spring makers like to do BS like call springs "EDC compliant". I have no solid proof but strongly suspect a spring that would work and feel reasonable on the car (i.e. slightly shorter and somewhat stiffer) will very likely not adversely affect the EDC systems function nor longevity. On the ride height issue: I am pretty pleased with with the cars ride height and looks. The rear wheels (19's) fill the wells beautifully and perfectly IMO. The front look slightly less "right" to me but definitely not nearly enough of an issue for me to muck with it, decrease ride quality, increase scraping and generally make the car more track like and less daily driver like. |
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06-15-2008, 10:52 PM | #19 | ||
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06-16-2008, 01:13 PM | #20 | |
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Your second paragraph indicated some serious misconceptions about how a suspension system works in term of the goals and effects of both the spring and damper (be it a static or dynamic damper). 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. |
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06-16-2008, 07:28 PM | #21 | |
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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. |
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06-17-2008, 12:11 AM | #22 |
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After almost 5k miles on RD Sport - No noticeable degradation of shock functionality.
subscribed.
Perhaps we can get an inquiry by email to an engineer at Racing Dynamics, H&R, or even Bilstein and have the responses to jm's questions copied to this thread?
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