Nerdly discussion about air springs

[lolachampcar]

I've not had to get into the nuts and bolts of air springs before. I've been exposed to a lot of street car related suspension stuff since getting my first MS; a lot of it is new to me given I've not worked on street cars for over thirty years. The idea of using rubber bushings both as isolators and bearings was interesting. Air springs are another interesting item and some recent posts have got me wondering if I am missing something.

MS uses what amounts to an air bag built into the upper strut mount as a spring on the active air cars. The dampers are 50mm Bilstein (twin tube although this is just a guess on my part) fixed valving units. What has me puzzled is the idea that the suspension module has control over anything but ride height. Recent posts about "updating the air suspension programming for the P+ option" and others suggesting improvements or changes in suspension performance via over the air software updates have me scratching my head a bit.

As I understand them, air springs are a simple air bag. Add more air and the bag gets bigger thus the car goes up. Unless there is a some creative geometry, there is a fixed amount of area that works to "push down" on the damper housing and support the car. PV=nRT suggests that you can have a rising spring rate but this is fixed at design time by the air volume and how that volume decreases as the damper is compressed.

Does the above sound like an accurate understanding of the air system? If it is, I really do not see how software updates can do anything but change ride heights and perhaps put the car in a different region of a rising rate spring function.

Thoughts?

 

[db8ek]

Lola - I recently had a Ranger visit, where I learned that with a P85+ upgrade, they need to reprogram the traction control portion of the code in order to deal with the new suspension setup. There coulld still be advantages to be had with reprogramming the air springs, and maybe there is some of that going on too, but at least reprogramming the traction control gives a plausible reason for the software update.

 

[lolachampcar]

Now TC tweaks makes a bunch more sense. Small changes in allowable slip percentage can make big differences in the way the car handles hard acceleration and moving to different sized tires front to rear would drive such a change.

 

[Gizmotoy]

I can certainly see tweaking the TC with the bushing change/wider wheels.

Your understanding of the suspension follows what I understand as well. Most automakers with air suspensions that vary the feel of the car accomplish this by adjusting ride height with the air, and compliance with adjustable dampers. With fixed dampers, all the car can modify is the height. Indeed, in a performance-only setting, you'd choose adjustable dampers over air every time if it was an either/or proposition.

Truthfully, I think Tesla ended up with the air suspension because they decided the average buyer should not be expected (trusted?) to handle a car sitting so low to the ground in town. So they compromised handling, or perhaps more accurately road feel, to get adjustable ride height. Then, once committed, they set about making the best of the decision by launching the + tweaks.

I suspect a future generation of the S will include something like the highly-regarded magnetorheological dampers in the CTS-V/Corvette/Acuras/etc. Given they're controlled by electromagnets and not hydraulic pumps, they'd be a good fit. Further, they could probably be paired with both the air and coil suspensions.

 

[Dreamin]

Now TC tweaks makes a bunch more sense. Small changes in allowable slip percentage can make big differences in the way the car handles hard acceleration and moving to different sized tires front to rear would drive such a change.

Agreed, the TC recalibration is likely for the larger diameter rear tires, not the suspension bits per se.

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I suspect a future generation of the S will include something like the highly-regarded magnetorheological dampers in the CTS-V/Corvette/Acuras/etc.

That would be a nice future S suspension upgrade option. It's amazing how we'll the Caddy handles given the size of the car.