Originally Posted by lucid
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.
Originally Posted by lucid
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...