Quote:
Originally Posted by jm1234
Agreed, but not for the reason stated. Getting position offset from an accelerometer is trivial. Speed and distance sensors for running are based on foot mounted accelerometers and give fairly accurate results in a very complex environment (running feet as opposed to a rolling chassis). Dynamic position offset is not the same as static position offset and accelerometers wouldn't be up to detecting static offset and thus wouldn't respond to lowered springs.
Bear with me here, I might have something wrong. The point of the suspension is to keep the car level (with respect to the road for ideal tire contact) yet still allow the tires to track the pavement (not too stiff, so the tires do not hop and/or the passengers don't spill their coffee). Controlled suspensions do this by adjusting damping as little as possible to achieve a level ride. If I have this right so far then to suggest this task can be done dynamically without knowing the spring rate isn't plausible. Spring rate, distance of spring from the c.o.g. and lateral rotational inertia are the three key system variables and changing any will cause the control system algorithms to be wrong. Those algorithms are undoubtedly based on a system model containing little more than those three system variables. The system would likely be wrong by a little bit for small changes in the spring rate but wrong is wrong and will have a negative impact on ride as considered "ideal" by BMW. There are no objective measures so we can't say how wrong or if it's detectably wrong. An owner might very well like the ride better and even drive faster with the suspension but that doesn't mean the EDC system is operating as designed. If the EDC works by changing duty cycles then new springs could conceivably wear out the system early. Conceivably isn't the same as definitely but so far no one here has posted any knowledge of how the dampers work. Somehow the normal setting results in damping that isn't the same as either sport (can be stiffer) or comfort. I would say that there is more than binary variability and thus the possibility that the system cycles (to effectively emulate multiple settings) or contains multiple settings. If it is switching damping rates more often because the spring rates changed the dynamics then that impacts long term reliability. Again no comment on the degree of impact, just that it exists. I'm not trying to scare anyone into not lowering their car. Just realize that it's an expensive (just guessing here) closed loop system and you are changing the variables and the warranty isn't going to cover any damage. Personally, I think that for small changes in the spring rate you'll be fine and not have a noticeable change in the life of the system or ride. But, I acknowledge that my opinion is not based on any real knowledge of the system and I've done enough controls system design to know it's difficult to predict what might happen when you don't know the details of the controlled parameters (three accelerometers positioned how, measuring what), the control surfaces (how do the dampers work) or the control algorithm (PID, sliding mode, fuzzy logic, yada, yada, yada).
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Running accels work based on the periodicity of an acceleration signal not based on position. I stick to my claim that is it tough and inaccurate to get absolute position from an accel. Also think you are confusing yourself (and me) with terms like "dynamic position offset". The damper has an initial static set point and the accelerometer can not really measure that. Once it oscillates you can integrate acceleration to get velocity and position but it is not very accurate. I've done it, have you?
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.