Mercedes-Benz ESF Safety Car: More, Better, Smarter Airbags!

March 5, 2010

Last summer we wrote briefly about the Mercedes-Benz ESF-2009 experimental safety concept vehicle, including its remarkable under-car airbag, which deploys only during a crash to help slow it down.

It's a 2009 Mercedes-Benz S400 Hybrid full-size luxury sedan that's been fitted with several new and experimental devices to increase the car's passive safety during an accident.

Almost all of the technology shown will make its way into production cars at some point.


First since 1974

Yesterday, we spent two hours interviewing project manager Michael Fehring and climbing all over the car, which was built to revive the company's long tradition of high-tech safety prototypes, lapsed since 1974.

Some of the technologies aren't particularly visible, like automatic headlights that adjust the brightness of a "partial high beam" to compensate for oncoming vehicles.

But the most notable thing about the ESF may be its use of inflation technology in new and different ways. Not only does it have a full suite of airbags, plus some new ones, it uses gas cylinders to inflate both steel door beams and a large bag under the car.

Some highlights:

Mercedes-Benz ESF 2009 safety concept car, inflatable metal door side-intrusion beams

Mercedes-Benz ESF 2009 safety concept car, inflatable metal door side-intrusion beams


Looking at ways to reduce the weight of the car, the ESF team created a new type of side-intrusion beam inside the door skin. Current beams must fit within the door skin, meaning they are heavily engineered to be strong within a very narrow space.

The startling innovation here was to create an inflatable steel beam, using the same gas cylinder that inflates current thorax airbags. By increasing the beam's diameter as much as 2 inches, the beam becomes structurally stronger using much lighter weights of steel.

The inflation is triggered by side-looking radar sensors, part of Pre-Safe 360, an all-around accident detection system. Only 20 to 30 milliseconds is needed to inflate the beam, which bulges out the door skin significantly--unimportant if a side impact is pending.

According to Fehring, each of the four new inflatable beams was 0.5 kilogram (1.1 pound) lighter than the current design, cutting almost 5 pounds of total vehicle weight.

mercedes benz esf 2009 experimental safety vehicle 008

mercedes benz esf 2009 experimental safety vehicle 008


While front-passenger airbags are now standard in new cars, they're less than perfect. Unlike the driver's seat, where the need to steer keeps the driver a predictable distance from the steering wheel, passengers can adjust their seats far forward or aft--and they do.

The ESF team sought to make the passenger airbag smarter by using the seat position sensor to control how much the bag inflates. Four straps inside the bag control its thickness, keeping it smaller when the seat is far forward, but letting it fully inflate if the seat is back.

And this appears to be one ESF innovation that's close to showing up in Mercedes-Benz vehicles. Although he wouldn't say when, Fehring said the size-adaptive airbag "will come very quickly" into production cars.

An even nearer-term innovation is adjustment for the passenger's weight, based on a seat-compression sensor, by opening or closing a supplemental vent in the bag. Heavier passengers need "harder" bags, so the vent stays shut; lighter riders are cushioned by a "softer" bag whose gas escapes more quickly through the open secondary vent.

Mercedes-Benz ESF 2009 safety concept car, driver seat vertical central head curtain

Mercedes-Benz ESF 2009 safety concept car, driver seat vertical central head curtain


One problem in both side impacts and rollovers, Fehring said, is that even occupants belted tightly in their seats can have their extremities flung around.

To keep the driver's head in place, the ESF has a very stiff vertical side curtain that inflates from shoulder level upwards. It keeps the head in position, helping to "cocoon" the driver and prevent driver and passenger heads from knocking together.

mercedes benz esf 2009 experimental safety vehicle 007

mercedes benz esf 2009 experimental safety vehicle 007


Rear passengers have no frontal airbags; they're simply too hard to engineer into the front seatbacks. Seeking ways to reduce injuries to the collarbones, sternum, and thorax of rear-seat riders, the ESF engineers looked at ways to distribute the forces of their seat belts over a wider chest area.

The result was an inflatable seatbelt, or "beltbag," to spread decelerative loads across twice the surface area of a standard belt. Fehring said the team spent a great deal of time on "the haptics and optics" of the design, to ensure it wasn't unpleasant to put on or wear.

The belt bag is contained inside a strap that is just twice the thickness of regular seat-belt webbing, though Fehring noted that measuring comparative injuries from a seatbelt and a belt bag was a challenging problem for which instrumentation doesn't yet exist.

Mercedes-Benz ESF 2009 safety concept car, Inter-Seat Protection system activated in rear seat

Mercedes-Benz ESF 2009 safety concept car, Inter-Seat Protection system activated in rear seat


Similar to the front-seat vertical air cushion, the ESF team created an Inter-Seat Protection system to keep rear-seat passengers properly located. It's one of the few new technologies, in fact, that doesn't actually inflate.

Instead, a pair of padded bolsters at the top of the rear seat center pops up, out, and forward to prevent each outboard passenger's head from moving toward the center.


The most futuristic idea in the ESF is the "braking bag," a large standard airbag covered in rubber over a steel mesh. When a collision is imminent, the bag (located just ahead of the front axle) deploys, inflating downward and actually bouncing the car upwards.

This has several advantages: It slows the vehicle further by adding contact area beyond simply the four tire patches; it stabilizes the vehicle; and it raises the front end to compensate for "dive" to provide a better height match between two colliding vehicles.

The video below shows the braking bag in action, though Fehring stressed that "it needs a lot of work" before the company will even consider deploying it in production cars.

The Car Connection
See the winners »
The Car Connection
Commenting is closed for this article
Ratings and Reviews
Rate and review your car for The Car Connection
Review your car
The Car Connection Daily Headlines
I agree to receive emails from The Car Connection. I understand that I can unsubscribe at any time. Privacy Policy.
Thank you! Please check your email for confirmation.