Magic Airbag Restraint Systems

In 1996 reports were getting stronger that airbags were not the magic protective restraint system some had hoped. In several moderate speed (i.e. 12 to 20 mph) collisions the airbag deployment caused the death of a number of short stature drivers and a number of children that were seated in the front passenger seat.

Research started, throughout the industry, to find a solution to this problem. The general opinion in the industry is that the power needed to restrain an unbelted 50th percentile male in a 30 mph crash, as mandated by federal law, is more than can be survived by someone who gets into direct contact with a deploying airbag. This led to a modification of the federal standard to allow less powerful airbags to be installed in vehicles, while the industry searches for a solution that will provide the necessary protection while avoiding the adverse effects.

In search of the ultimate airbag module, Henk took to the laboratory to investigate what other factors play a role in surviving an airbag deployment if one's stature dictates that one sits close to the steering wheel. A series of tests was run with a 5th percentile female dummy (height 5'0") sitting a mere 6" (150 mm) away from the airbag module.

It was found that airbag modules that have a single piece cover increase the chance that the airbag gets caught under the chin of a closely seated occupant. Once caught there, the force of deployment will snap the head back with great force, increasing the risk of neck injury. Single piece covers are often desired by vehicle stylists. Optimizing the airbag fold such that the airbag deploys free from interaction with the occupant is an engineering challenge. It is more difficult than with module designs that have a cover that splits through the center which therefore have the engineer's preference.

Single flap driver airbag covers make airbag fold optimization more challenging. If the bag is folded incorrectly, it may lock-up under the chin of a closely seated occupant, potentially causing severe neck injuries.

An other challenge engineers face is the ongoing cost cutting to remain competitive. At approximately $3 a set, airbag tethers are frequently targeted by the bean counters. Testing showed, however, that they are a worthwhile expense to improve airbag safety. Tethers are strips of fabric, hidden inside the airbag, that span from the back to the front of the cushion. They reduce the throw of the airbag during deployment and determine the final thickness of the cushion. Without such thickness control the airbag will expand into a spherical shape. The extra thickness increases the load on the head of a closely seated, short stature occupant. The head now has to pull the body backwards which increases the neck loads.

Without tethers an airbag will balloon into a spherical shape. The increased thickness causes extra loading on the head of a closely seated occupant, potentially causing severe neck injuries.

The Magic Returns

The experience with the first generation airbag systems provide a strong motivation to find improvements. The depowering of airbags, sanctioned by the National Highway Traffic Safety Administration is just an intermediate step. While reducing the chance of airbag induced injury to the short stature drivers and young front seat passengers, it also reduces the amount of protection to larger occupants and occupants in more severe crashes. This affects the area airbags have been proven to be very successful. Henk has been on the forefront of the development of the next generation airbag systems. This culminated in his co-authorship of the BREED Technologies' TOPSTM strategy for airbag deployment. (fellow authors are Dr. Russel Brantman and Dr. Said Nakhla)

TOPSTM is an acronym for Tailorable Occupant Protection System. It combines several new and improved sensors with gas generators that have two output levels to tailor the airbag for the (crash) situation at hand. TOPSTM is unique in the industry in that it allows a phased introduction of third generation airbag systems with increasing levels of sophistication. This is important to allow cost effective development and a rapid return to full protection airbag systems.

If you go out to buy a new car after the year 2000 this is what your airbag system might comprise of:

  • A crash sensor that can discriminate multiple levels of crash severity.
  • A gas generator with a low and high output level, known as a dual stage inflator.
  • A belt usage sensor.
  • A driver seat position sensor.
  • A passenger seat weight sensor.

Furthermore there is an increasing chance that your car of choice will be equipped with seat belt pretensioners. These will take the slack out of the belt in the early stages of a crash, making the belts more effective (only if you wear them, of course). In addition to this the seat belts might incorporate a load limiter. This device will cap the seat belt load at a certain maximum, which will reduce the force of the belt on the chest and the shoulder. This reduces the chance of injuries caused by the seat belt, such as cuts and broken ribs (yes, belts can cause injuries too...)

With all this in place the restraint system can be tailored to the needs of the occupant. The airbag deployment strategy on the driver side might look like this:

Belt UseCrash Severity1 Seat Position2->Airbag Deployment
yeshighbackward->low level deployment
nomediumforward->low level deployment
nomediumbackward->low level deployment
nohighforward->low level deployment
nohighbackward->high level deployment
1) A medium crash severity is equivalent to a vehicle driving into a rigid wall at 12 mph or more. A high crash severity is equivalent to a vehicle driving into a rigid wall at 20 mph or more. 2) Seat forward position typically means the seat is in the forward most 1/4 of the seat track. Consequently, seat backward position means the seat is further back than 1/4 from the front of the seat track.

The airbag deployment strategy for the front passenger side might look like this:

Belt UseCrash Severity Occupant Weight3->Airbag Deployment
yeshighhigh->low level deployment
nomediumhigh->low level deployment
nohighlow->low level deployment
nohighhigh->high level deployment
3) The occupant weight is typically considered low if it is less than 35 kg. Above that it is considered high. With multi-threshold weight sensors the occupant weight might not be considered high until it exceeds 50 kg.

These systems will deal with the two most pressing problems:

  • Avoid deployment of the passenger airbag if a child seat is placed there.
  • Reduce the airbag deployment force for closely seated drivers.

Sensor accuracy (gray area's) and the realization that everything man makes can ultimately fail, make that the real deployment strategy is somewhat more complicated then the one described above.
Multilevel weight threshold sensors and sensor systems that determine the proximity of the occupant to the airbag module, again, will increase the complexity of the deployment strategy. Such systems are scheduled to become available after the year 2001. They will add the capability to deal with more dynamic situations, such as occupants being thrown out of position prior to the impact.

Confused ?

Has all the (media) talk about the dangers of airbags confused you to a point where you wonder whether you want an airbag at all? Are you ready to trade in your brand new car for an older model without an airbag?
Get better informed !

Start with the NHTSA web site. From there you can surf to related sites such as the Insurance Institute for Highway Safety. A list of all airbag related fatal accidents, listing make, year and model, can be found there. If you are a short stature driver (i.e. under 1.60 m, or 5'3") and you want absolute piece of mind, simply avoid buying the cars on the list. If you happen to own one (double check the year) then you can do the following:
  • Realize that these are freak accidents (there have been over 2 million airbag deployments that have caused no serious injury).
  • Increase your proximity to the steering wheel, by wearing the seat belt and possibly installing pedal extenders. If your car is equipped with a telescopic steering column, push it all the way down. Make sure there is at least a 10" (25 cm) distance between the airbag module and your chest.
  • Trade the car for a new one (which will have a depowered airbag) or for a used car that is not in the list.
If you go out to buy a new car, pay extra attention to your seating position. Certain makes and models are more suited for short people than others. The IIHS has performed a seating study on a large range of vehicles. This study showed that short stature people have more difficulty finding a good seating position in a large car than in a small car (but there are exceptions to that). Many luxury cars have telescopically adjustable steering columns that allow the short person to push the steering wheel further away. Look for that! Always transport small children in the back seat, if you have one. Use appropriate child restraint systems, such as child seats, booster seats, and regular seat belts for older children. No technology can save an unbelted child that rides in the front seat when you slam on your brakes and crash into an object at high speed.

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© 1999 HORK Enterprises