Electronic Stability Control

On October 21, 1997, the compact Mercedes A-Class became the subject of ridicule when it skid out of control and flipped over in the infamous elk test. Performed by Swedish automotive magazine Teknikens Värld, the test revealed how easily an avoidance maneuver could turn into a deadly disaster. This was a PR blunder for Mercedes-Benz especially considering this was their first foray into affordable front-wheel drive cars. One German newspaper mockingly published how the much older 1950s Communist Bloc-based Trabant passed the "A-Class killer test".

The Stuttgart headquartered auto-maker rushed to mend their reputation. Within three months, they were ready to demonstrate their refitted A-Class. In January 1998, the compact car was made to swerve hard left and right to avoid an 'elk' while remaining composed and firmly planted to the ground. The car was stable throughout without so much of a skid. Robert Colin, the test driver who originally made the car to flop over, was impressed with the results. Mercedes went on to exceed their projected sales numbers and even sold a plush moose at their dealerships.

How did they manage to salvage the car's safety reputation? They modified the suspension but more importantly, fitted the vehicle with ESC. Electronic Stability Control is a computer program that prevents loss of traction with an array of sensors and hydraulic brake actuators. During the test, ESC intervened within fractions of a second and applied corrective braking action to keep control at the hands of the driver. The system was so effective that to the onlooker, it appeared to be pure sorcery.

I wanted to blog about ESC for a long time, but I kept getting more and more ambitious. I realized that the solution was to blog only about certain aspects of it over time. Today, we're talking about the operation and effectiveness of ECS. Other subjects include the controversial history of ESC, how to properly correct slides and finally the physics behind wheel grip. All these will be detailed eventually in an article of their own. However, this post is the one with fun videos and pictures! No math equations or boring history guaranteed!

This is not intended to be a scholarly article on ESC, but rather an introductory one.

How to Crash

Cars lose control in two basic ways. Rather than explaining in minute details with advanced mechanics, I have prepared two videos which demonstrate how accidents happen due to loss of control.


Understeer is when the car takes a wider turn than what the driver commanded. This is very common on front-wheel drive cars and usually happens when someone attempts to turn to quickly and at high speed. The front tires can only provide so much grip to steer the car, if it is exceeded, the car will slide forwards.

To regain control from understeer, one must slow down carefully until the tires have enough grip available to control the car again. 


Oversteer is a much more terrifying condition because it is an unstable one. The rear-wheels lose grip and the rear-end of the car appears to go all over the place. Oversteer is much more common with rear-wheel drive cars where the grip on the rear tires can overwhelm the front. In icy conditions, it can happen to any type of car as any of the wheels can lose grip at any point depending on the slipperiness of the road and the patches of ice/mud.

Correcting oversteer requires skills and concentration and understanding of the car's drive-train.

In front wheel drive cars, one wants to move the weight of the car back to the rear. Usually, that means, counter-intuitively accelerate rather than brake (or at least brake very carefully). Braking the car will cause the weight of the car to be shifted forward and the rear wheels will lose grip and induce even more oversteer. Therefore, stay calm, steer against the skid and accelerate. Once control is regained, you may carefully start braking again.

In rear wheel drive cars, it is much more difficult and would require a blog post on it own (hint hint!). However, the basics involve reducing throttle carefully and making calculated adjustments to the steering to ensure that the car is in line with the direction of travel again.

How Much of a Difference ESC Really Makes

What ESC does is correct oversteer and understeer, we'll see how in a moment.

The best way to demonstrate ESC is to do it in the field. In the tests below we'll be simulating a very common occurrence where a bus is stopped in the middle of nowhere and we're going too fast to slow down. As a result, we need to swerve left and then right to avoid the bus and get back onto to the driving lane.

This emulates the infamous elk test that is done by certain car magazines to test the limits of a car. The car is first made to avoid the obstacle by turning hard to avoid it, this induces understeer due to the forces involved in turning as the car wants to remain straight due to its initial interia. Once the obstacle is avoided, the car will tend to oversteer turning back as the avoidance maneuver caused the car to lose it's rear balance from the change in interia from the first turn.



ESC on Ice

What's even more impressive is how capable the system is on ice. I have prepared a scenario where again, there's a bus stopped in our path and we have to avoid it. The difference is, we're doing it on a frozen lake!



How ESC works

As the previous videos have shown, ESC can be extremely effective. However, it's mode of function is deceivingly simple.

Cars are equipped with a variety of sensors and accelerometers which allow the ABS Module to determine what direction the car is actually going. It compares it with the intended direction that the steering wheel is pointed too. If they don't match, it means the car sliding and the system intervenes.

ESC Operation

ESC Operation

The idea behind ESC is to cancel the spinning momentum caused by the loss of control. It does this by applying brake pressure on an individual wheel to compensate for the momentum.

In an understeer situation, one needs more momentum in the intended direction of travel. To do this, the rear wheel that is on the side of the direction of the slide will have it's brakes applied achieving the desired course.

In an oversteer situation, one needs to cancel the moment caused the slide. In that case, the wheel opposing the slide in the front has the brakes applied and the course is corrected.

It's really that simple.

ESC Counteracting Oversteer (from Fifth Gear)

ESC Counteracting Oversteer (from Fifth Gear)

In addition, as we previously mentioned that the key to preventing (not correcting) most slides involves a reduction of throttle. ESC will automatically determine how much throttle to reduce in a slide intervention happens as to not worsen the situation.


However, ESC cannot break the laws of physics. If you're going too fast for a corner, you will be headed straight to the wall. ESC is a system to design to prevent loss of control, not enhance it.

ESC performs these actions repeatedly in very shorts periods of time acting much faster than a human possibly could. Usually 50 times per second. In fact, the kind of corrections made by ESC cannot made be made a human, unless they drive carefully enough not to break traction. But in panic situations one might easily make mistakes. 

ESC Today

Electronic Stability Control is now considered mandatory in new vehicles in many first-world countries. It has shown to reduce single-car accident rates by quite a huge margin as multiple studies have shown. Accident avoidance figures range from 30-50%, a massive improvement and it's easy to see why it has become one of the pioneering achievements of the automotive world like the seatbelt and airbag.

Most cars today are fitted with this technology as standard. A flashing warning light indicates that the system is actively correcting a slide. If the light is solid, it is time to visit a mechanic as the system is probably malfunctioning. Other vehicles may have a separate "ESC Off" light that either flashes or stays on if the system was deliberately turned off or malfunctioning. 

ESC Light.jpg

ESC Light

A typical dashboard ESC indicator

In many cars today, ESC cannot be fully disabled. Pressing the disable switch will only switch it off partially, usually the traction control portion only under certain speeds. This is usually because engaging the system means that the wheels will not spin in stuck snow or mud situation because the system is detecting a loss of traction. In order to allow the wheels to spin slightly to hopefully get out of the deep mud pit, the traction control will be disabled at low speeds.

For the really brave ones, some cars have essentially 'cheat codes' to fully disable the system if the car doesn't allow it already. A quick google query will find you some results.

I hope this gave a small glimpse on a life saving technology that we often don't even notice. Next time you press and hold the ESC Off button, think about what protection you will be losing.

Learning More

The Wikipedia article on the subject is actually quite short on the subject of ESC (and sorely misses points on the history aspect) and I haven't found something in-depth enough yet.

I found a few more interesting videos describing vehicle dynamics and ESC and I recommend a watch:

All the above simulations were provided by the BeamNG.drive vehicle simulator.