Stability controls, what exactly happens

For future reference, can someone describe what happens when the stability control kicks in. All it can do, as far as I know, is kick in individual brakes. For example, say I’m cornering too fast on wet pavement and would slide with stability control turned off.


If it matters, 2015 Forester.

To make it simple, and I’m not an automotive engineer: The vehicle’s multiple sensors detect an event that indicates the vehicle is sliding. Depending on the sophistication of the system, these include wheel speed sensors (to determine if a wheel is spinning or turning slower than others, which would indicate a loss of traction, a slide, or locked up brake), yaw sensors, indicating if the vehicle is rotating around an axis (as if it is spinning out), and taking into account throttle and brake inputs, the speed you’re going, and possibly even the outside temperature. If the event meets the criteria to be detected as a skid, the system will actuate one or more of your brakes, typically while decreasing engine horsepower, to arrest the skid.

This happens very fast and is usually quite effective, though it’s not going to save you if you hit a sheet of ice or are doing something really stupid.

Here is the Wikipedia article on ESC:

Another thing the system can do is reduce the throttle.

My very rough understanding is that there are both traction control and stability control systems in general, although I don’t know anything about your car specifically.

A traction control system is simpler. It just detects that a wheel is spinning and brakes that wheel in order to send more power to the wheels with traction.

A stability control system is fancier and presumably more expensive. It uses sensors to detect things such as the steering angle and the yaw rate of the car in order to determine if the car is actually going in the direction that the driver is aiming it. If not, then it brakes wheels as needed to try to get the car back under control. I know that many people here like to complain about electronics that help the driver, but I believe I’ve read that studies have shown stability control systems to be pretty effective in preventing accidents.

You might want to read this:

If you can find a large snowy parking lot nearby, you could experiment with some of these systems.

@oblivion gave a pretty good description for a guy who is not an engineer.

I am an engineer and I’ve worked on such systems, so I’ll add a couple of extra details. The stability plus ABS systems have pumps and valves to rapidly release and apply each individual brake for ABS stops. If you turn the steering wheel (read by the steering sensor) and the car doesn’t follow (called understeer) the yaw sensor sees that the car didn’t turn and no cornering acceleration was created (sensed by accelerometers). The computer applies one or 2 brakes only on the wheels on the INside of the turn. That causes the car to turn as the driver commanded with the steering wheel. If the cornering acceleration doesn’t increase like it should, then the computer determines you don’t have ANY traction so it stops applying the brakes so you leave the corner nose first instead of door first. Hitting a tree with the nose is safer than with the door. All this happens many times a second applying individual brakes is impossible for you to do as a driver but quite possible with these systems. It takes care of tail slides (called oversteer) too.

Mustangman: thanks. But I’m still unclear on one point: If you are on the edge of losing traction in a turn, how does applying one or two brakes help? Seems to me it would push you into the skid.

(PS, I’m also an engineer)

Related: Auto race drivers and mechanics are always on the search for ways to go around curves faster. If selective brake application helps you get around a curve faster, then you would think someone would have rigged up a race car so the driver could selectively apply brakes.


It all depends on the situation. A combination of reduced throttle and braking the tire(s) not nearing a skid is used.

Stability control has been banned from racing for just the reason you mention, it gives an artificial advantage. Again, much is done with throttle control, not just braking.

@BillRussell Aha, THAT is food for discussion! The edge of losing traction is rarely a case of both tires losing traction at the same time and only the outside tires. The inside tires are not the most heavily loaded ones and tire traction is non-linear with load. I.e., if you double a tire’s load, you don’t double the traction it has on a given surface. So with that, understeer is where the outside front tire is out of traction but the rear has some left. The inside tires have quite a lot left so you drag the inside rear or front brake a bit and and like an unintentional brake drag, it pulls the car in that direction so the understeer is corrected. If the car oversteers, the outside rear tire has no traction left so you lightly add outside front brake to rotate the car back straight. OS is a bit touchy to correct since the outside front tire is involved and near the limit of traction you don’t have much left. Electronic shocks work better for that.

The electroinic systems are outlawed in most racing circles although ABS only is legal in sports car racing in the US. Circle track racers, especially on dirt, sometimes put a valve in their right front brake lines to shut off the RF brake so they can stab the brakes to rotate the car left into the turn! McLaren Formula 1 racers used to have what they called a “fiddle” brake that the drivers used to brake either the left OR right rear wheel to correct understeer or as traction control coming out of corners but as @texases said, that has been outlawed.

^I always wanted to have “tractor brakes” on my RWD truck to brake the rear wheels individually, as a traction aid…but I suppose a locker diff would be a better solution.

Would you like to elaborate on your disagreement @MikeInNH?

Would you like to elaborate on your disagreement @MikeInNH?

@Mustangman - What are you talking about. I haven’t even contributed to this thread.

My login shows that you disagreed with my last post here. The one before meanjoe’s tractor brake post. Sorry if that is in error.

Miky must have hit the disagree button inadvertently. I know that I have done it occasionally too.

thanks all


My login shows that you disagreed with my last post here. The one before meanjoe's tractor brake post. Sorry if that is in error.

O must have accidentally hit the button. I read your post and I agree completely.

@mustangman, I removed Mike’s disagree.

A few months after buying his new 2007 Honda CRV my dad decided one morning to see if he could get the system to intervene if he hit the gas while going around a slightly icy corner. This was going from the private road onto a 2 lane county road with a right-hand turn onto the county road. All that happened was that the ESC light on the dash lit up but the Crv barely slid if at all. Dad was impressed. This was the first vehicle in the family fleet with stability control so he wanted to see how it worked.

I’d suggest an icy parking lot for that, @wolyrobb. A journalist that drove a demo car I was hosting described it as “the hand of God” yanking him back in line. You can’t repeal the laws of physics with these systems but you can dance right up to the edge.

@Mustangman the corner in question was almost as good, but yes an empty lot would have much more room. If dad hadn’t been only doing around 15mph when he did the test he might have felt more of an effect,the nearest parking lot would have been about 4-5 miles away (my folks live out in the country a little bit). A co-worker did use a nearby empty shopping center parking lot one snowy morning to find out exactly how her new Jeep Liberty would respond in the snow and ice. Improved her snow driving skills quite a bit at 2am.

These systems are very effective. I actually find it annoying sometimes that you can’t completely turn off the stability control on my car without a lot of bother. It’s a RWD car and the stability control and traction control often spoil the fun.

Turning a corner on a wet or slippery road, give it a little gas to make the rear slide out a bit and Thunk! No more slide. It’s like a stern, humorless chaperone. (I swear I never yell: “Hey y’all, watch this!”)

Overall though, I think this is a brilliant safety invention. Like smoke detectors and ground fault interrupters, it has and will continue to save a lot of lives. Whether or not these inventions thwart natural selection and the pruning of the shallow end of the gene pool I’ll leave as an exercise for the reader :smile: