Cars: explain what electronic stability control does.

I’m about to buy my first new car in ten years. It’s equipped with electronic stability control, which I understand to be mandatory on all late-model cars. My current car does not have it. My wife’s 2YO car has it, but I haven’t had an opportunity to do deliberately stupid things behind the wheel to see how it reacts.

Some background reading gives a general idea: it’s watching steering angle, yaw rate, forward speed, and lateral acceleration, and depending on what’s happening to all of these variables it will activate the brakes at one or more wheels in an attempt to help the driver “maintain control.” But I’m struggling to determine how it will react in various scenarios.

Suppose I enter a cloverleaf interchange (rated for 25 MPH) at 75 MPH. My car begins grossly understeering, and I start moving toward the outer edge of the road. What is ESC’s goal for me here, and how will it accomplish it?

Likewise, suppose my car begins grossly oversteering. Presumably ESC’s goal si the same as the previous scenario, but how will it accomplish that goal here?

Obviously in both of these scenarios there’s no hope of staying on the road; the laws of physics just won’t permit that to happen. Is ESC then just going to keep the nose of the car pointed in the general direction of travel, so that when I leave the pavement I will hit objects with the grill instead of hitting them with my door (or rolling the car over)?

I enjoy playing in cars (dad used to find empy snow-covered parking lots and do donuts with us when we were kids), and I’ll certainly be experimenting with it next winter (snowy parking lots enable less energetic skids than clean/dry pavement), but I’m curious to understand as much as I can before then.

You’ve got most of it and there are many explanations of varying technical level out there to read. In a nutshell, most stability control systems use the selective application of the brakes to try and keep the car pointed forward, where it can power out of a skid or spin. Some more sophisticated ones do more subtle things with electronically adjustable suspension, and many will intervene in the throttle control, sometimes even goosing it to try and pull the car straight.

It’s trying to turn what may be an uncontrolled skid into a controllable one, or into no skid at all. The assumption is that a car going straight is likely to stay on the road, or can be more easily corrected by a driver to stay on the road, or, barring all other fixes, will hit something head-on, which affords the most protection to the driver and passengers.

Which is pretty much what you’ve already worked out. If you want to know the specific algorithms… I’m sure they’re out there somewhere and about as comprehensible as the instructions for go.

And to piggyback a question -

How does is ESC affected by the driver attempting to respond to the situation? Easiest example - an oversteer (fishtail) situation and the driver steers into the skid, will the stability control work with the driver?

And for that matter, what if you are trying to cause an ‘unstable’ situation. Again a fishtail is the easiest example - say you are trying to hold the car in a fishtail around a corner? If it’s rear wheel drive I assume the stability controls first move would be to cut the throttle.

I know not all ESC will work the same but I am curious about how they respond in general or specific examples.

I can’t speak for all systems but the ones I am familiar with work like this:
Understeer: reduce the throttle, brake the inside front wheel to transfer load to the outside front wheel to increase its grip.
Oversteer: brake one front wheel and possibly the inner rear wheel to recover traction ( I have to go back to my teaching notes for more detail)
The system recalculates very 7ms and can operate the hydraulics about 15X per second. It is faster than you are.

As far as trying to hang the tail out in a corner good luck. The system will detect it before you do and take steps to prevent it.
Again maybe not all systems but the ones I am familiar with.

On more “sporty” cars, most ESC systems will let you get the back end out to a certain degree. I used to have a Mazda RX-8 and that would allow a bit of exuberance going round roundabouts etc, but if you overdid it you could feel it gathering the back end in.

I did a track day in that car that included a skid-pan session. It was actually very hard to make the car spin even on a skid pan, and even trying my hardest to spin it, with the ESC on. Then the instructor said “Let’s try that with the electronics switched off” and I was into a 720-degree spin as soon as I hit the throttle

But to the OP - ESC will, of course, not let you break the laws of physics. If you go into a cloverleaf at 75mph you will not come out of it well. Remember, also, that the ESC doesn’t know where the edge of the road is. If you are understeering, it doesn’t know when you’re about to go off the road. As I understand it, it will do its best to get the car heading straight, in the direction that the front wheels are pointing.

Edit:

Better turn traction control off then!

There are two ways to steer a car. The most common is by turning the front wheels.

The other way is how tracked vehicles, like bulldozers, Bobcats or tanks turn. If the wheels on one side turn at a different speed relative to the other side, the vehicle will turn.

There are a ton of tiny details that have to be taken into account for the system to work properly, but ESC steers the car into the desired direction by accelerating the wheels on one side (if possible, not all transmission setups can do that) and braking them on the other side.

I had a fast car with ESC and it saved my ass several times

There was one show on the Velocity channel (US) last night (2/25/2013) called 5th Gear from the UK that demonstrated ESC systems on a variety of vehicles from a subcompact to a good sized delivery van. The program hosts tried hard to spin or roll the cars/truck but were unable. Very impressive. I understand these systems are mandatory on all vehicles in the EU (I’m sure regulated by size) in 2014.

I read somewhere that ESC is the greatest advance in automobile safety since the invention of the seat belt, that’s how much it improves survivability of skids.

As long as people understand what it can and can’t do, and don’t try to compensate for its presence by taking greater risks than they otherwise would. See risk homeostasis. For a specific discussion of risk homeostasis observations surrounding the implementation of ABS, click here; I’ll wager something similar happens that will be found to mitigate the anticipated benefits of ESC.

The computer could could grab one of 4 wheels independently (which you can’t) to prevent yaw as well as prevent you from locking up the wheels which gets you into trouble in the first place when you try and steer with them locked up.