I'm a chassis engineer and I develop adaptive suspensions systems. This will be long.
(This post was originally written in response to a question asking about the general limitations of adaptive suspension. When this ended up on the front page, all the discussion before my post was removed. Therefore, most of this post talks about the downsides, or things it CAN'T do. However, there are lots of benefits to these systems as well, or Automakers wouldn't be spending hundreds of dollars to put them on their cars.)
- Yes, your aftermarket wheels/tires/springs/bars/spacers will make a difference
- These systems are very complex, once in a while they may do strange things under some outlier road/driving conditions, but in general are quite robust if well tuned
- They can't react perfectly to very small/very sharp bumps
- Requires a really good tuning engineer to fully optimize. Bad tuner = bad ride/handling.
- There are lots of bad tuning engineers out there.
THE LONG VERSION:
How they work:
These systems (excluding Magneride) usually work by having passive valves just like any shock absorber, and an active valve in parallel that makes the damping force change.
The ECU has a simulated model of the vehicle including all the suspension parameters (tire stiffness, spring/bushing rates, suspension geometry, wheel/body mass, etc.). Information from various sensors is run through this model to predict what the car is going to do, then the ECU sends a command to the shock, in the form of an electrical current.
When driving down a smooth road, the active valve is fully open, and the damper is in full soft**, or some "steady state" setting close to it. When the sensors detect a large swell, pothole, choppy pavement, etc., or if you turn the steering wheel or brake or whatever, the ECU does the math and sends some current to the active valve solenoid for a split second. The valve gets stiffer, the damping force increases to deal with whatever is going on, then it drops back down to the soft setting. This all happens in fractions of a second.
**(Note that "full soft" is /= comfort mode. Full soft means no current being sent, no active control, absolute minimum damping force. Comfort mode typically has some level of active control, just less of it than normal and sport modes)
1. Road conditions
Very Small Bumps
Some events, like pavement cracks, roads with a coarse texture, or very small, light undulations, are too small or too fast to detect and/or react to. In these cases, no current command is sent, and the performance will depend on how the full soft mode was tuned. Also, instant response to every road input isn't really ideal either. If the damper reacts to every single tiny bump on the road, it can cause the ride to become busy and jerky. For example . . . Magneride has much faster response time potential, but they have to put an artificial threshold there anyway to prevent this issue.
Very Sharp Bumps
Larger events that are very sharp, like your pavement transition or a manhole cover, occur too quickly for the damper to react in the ideal manner. Its possible that the full soft mode isn't soft enough for these bumps, but it's more likely that the harshness you feel is actually a result of not enough damping
at the instant that you first hit the impact. With lower damping, the tire kicks off the impact rather than enveloping it, and you get more/faster wheel motion than you would if you had more damping. This limitation will affect the body's reaction to the initial kick as well as how the wheel and body settle after the bump. If you bring in more damping faster at the initial impact, you can improve the impact quality on sharp bumps, but again, the ride can get jerky and busy on other types of roads. It's still all a compromise.
2. Adaptability and Hardware
The model in the ECU isn't perfect, and how not-perfect it is depends on many things. One trend now is to reduce the number of sensors on the car to save money. When we're tuning, we have all kinds of high precision accelerometers, height sensors, yaw sensors, GPS, etc. on the car to measure what it's doing. But the production car may only have 4 height sensors to go by, and they may be less precise than we would like. So we have to do some fancy math to know what the car is doing, which means it takes longer and the result is less accurate.
As far as your wheels and tires go:
The difference between your old and new wheels may not seem like much, but in engineering terms, an 18mm offset difference in the rear is HUGE, and will definitely affect the accuracy of the model. Say for example that 18mm equates to a 10% increase in suspension lever arm length. That means the damper will have to make 10% more force in order to get the same level of control as before, but the ECU doesn't know that!
. That's before we talk about width, diameter, tire stiffness, scrub radius, etc, etc.
3. Lots of things going on at once
If you hit a pothole in a corner while braking over a large swell, the ECU has to prioritize all those different events to decide what command to send. 1st priority would be to make sure the car is controlled and safe, then to prevent wheel/tire/body damage in the pothole, then other comfort and handling concerns.
4. Complexity of tuning
Semi-active shocks are some of the most complex, non-linear, un-model-able mechanical components on a modern car, and the car is EXTREMELY sensitive to very small changes in their behavior. These systems can cost the OEM up to $1000 or more, which is a big chunk of change for just 4 shocks and some sensors. When we're tuning these things, we make changes on the order of a couple pounds of force
, and that results in big changes to the car's behavior.
My point is that these things are very difficult and time consuming to tune, and if the tune isn't robust and the algorithm doesn't properly account for all roads and conditions, the car might do strange things under some circumstances. IMO, only in the past couple years has the hardware, software, and engineering capability come together for semi-active suspensions to be really good. Even then, it depends HEAVILY on the tuning engineers skills and knowledge.
I believe the F30 Adaptive Dampers are made by Tenneco. I haven't driven one of their systems, but from what I know, this system is a bit behind the current state of the art. (Edit: this technology is advancing very quickly these days, so their system on the 4 series or 2 series will have a different tune and may use different valves or software, and may be better than the F30.)
Sachs has some valve-based systems that are really good, and GM's latest Magneride cars are really good. Things are advancing pretty rapidly in my field right now, so within the next couple years these systems should be even better.
My Personal Thoughts . . .
All said and done, a GOOD adaptive system can be much better than any passive system if properly tuned. I've heard mixed reviews of the F30 system, and I'm having a hard time finding one locally to test drive. Regardless, I've decided that my next car will have an adaptive system . . . I've just got to find one that meets my high standards. Unfortunately the stuff I'm working on now won't be on the market for a few more years, so I'm kind of stuck hoping the other guys have tuned something that I like.