The Impact of Side Airbags Expert Overview

The performance of airbags in side impact vehicle collisions is often the subject of litigation. In this article, Automotive Engineer Chase Nalls provides an overview of side airbag design, anatomy, crash testing, and applicable safety standards.

Side Airbag Performance & Safety

According to the Insurance Institute for Highway Safety (IIHS), as of 2023, approximately 22% of vehicular deaths occur in side impacts.¹ Vehicles that obtain a “good” rating in their side impact crash test have a 70% lower risk of death than vehicles that obtain a “poor rating.”²

Side Impact Occupant Protection

Providing protection for vehicle occupants in side impact collisions presents a challenge to vehicle engineers. Compared with frontal impacts, there is a relatively shallow crumple zone for deformation between the occupants and the closest vehicle side-structure.

A stiff side and roof structure combine to protect the occupant from injury by maintaining survival space and dissipating the force of the collision away from the occupant. Safety cage designs, such as those used by Volvo, Mercedes, and Subaru for decades, are at the forefront of side impact occupant protection.

In addition to energy-absorbing side structures, the side impact airbag has proven to be one of the most effective methods of improving vehicle crashworthiness in side impact collisions, especially those with head protection.

Widespread implementation of side impact airbags began in the 1990s and was not limited exclusively to luxury manufacturers; for example, a combination head/torso seat mounted side airbag was introduced as standard equipment on the 1999 Hyundai Sonata. In 2012 more than 95% of passenger vehicles sold in the U.S. were equipped with side impact airbags as standard equipment.

There are three distinct designs of side impact airbags:

• Seat or door mounted thorax bag
• Seat mounted combination head and thorax bag
• Roof rail mounted side airbag inflatable curtain

As their names imply, these bags are designed to protect different regions of the occupant’s body. Many cars now offer both inflatable curtains and pelvic bags to provide occupant protection from head to hip in side impacts.

Federal Safety Standards

Since 1973, side impact test procedures and performance targets have been dictated by Federal Motor Vehicle Safety Standard (FMVSS) 214: Side Impact Protection. For over 20 years, compliance with the standard was confirmed through a static test of the door and structure by measuring the load and deformation to the system with a hydraulic ram.

Starting in 1994, dynamic testing with a Moving Deformable Barrier (MDB) was phased into fleet certification requirements. Since its inception, the ability of the MDB to relate to real-world collisions has been subject to debate due to its “bumper” profile height of 13 to 21 inches; several inches lower than a typical truck or SUV bumper.

The lower bumper height increases the likelihood that collision forces will be applied to the relatively strong rocker panel structure of the struck vehicle, potentially misrepresenting the likelihood of devastating injuries caused by cabin intrusion of higher truck/SUV bumpers.

In 2009, FMVSS 214 was changed to include:

1. An oblique angle pole impact
2. New Anthropomorphic Test Devices (ATD or crash test dummies) with upgraded bio-fidelity in side impacts
3. New injury metrics, including Head Injury Criteria (HIC) for the first time.

The modified 214 was phased into the fleet certification requirements, with full implementation in the 2015 model year. A manufacturer can also test their vehicle using the same setup, but at a higher MDB velocity (62 kph versus 54 kph) to attain a star rating. These star ratings are used to help customers compare the crashworthiness of different vehicles under the New Car Assessment Program (NCAP).

Curtain airbags have also proven to be effective in mitigating the risk of partial and complete ejections. They can remain inflated for more than 10 seconds, containing occupants through multiple rolls of the vehicle.

In 2011, the National Highway Traffic Safety Administration (NHTSA) implemented FMVSS 226: Ejection Mitigation. This federal regulation aims to protect occupants from partial or complete ejection through the side windows in side impacts or rollover events.

FMVSS 226 requires that four locations at each side window of a vehicle be impacted by an 18 kg headform, traveling laterally and horizontally, at 16 and 20 km/hr.

The countermeasures must prevent the impactor from traveling further than 100mm beyond the surface of the window. The 20 km/h impact occurs at 1.5 seconds after counter measure deployment and the 16 km/h occurs at 6 seconds after counter measure deployment.

This requirement essentially mandates that roll-overs deploy curtain airbags in most new vehicles.

IIHS Side Impact Crash Test

IIHS is a non-profit organization that operates independently of the auto manufacturers and the federal government. In response to its own research, the IIHS devised a side impact test and ratings criteria and began using it in the evaluation of vehicles in 2003.

The IIHS test regimen differed from the FMVSS 214 in the test speed, impact angle, and MDB design. The IIHS MDB design was modeled to simulate a light truck or SUV; the “bumper” profile was 4 inches higher off the ground, the “face” of the MDB was 12 inches taller, the MDB replaceable “face” was stiffer, and the MDB weighed an additional 300 pounds as compared to the FMVSS 214 MDB used in certification testing.

In 2021, the IIHS updated the side impact testing, increasing the weight of the MDB by an additional 900 lbs and increasing the impact speed by 6mph. This MBD effectively subjects the tested vehicle by 82% more energy than the IIHS’s original test.

The IIHS rates each vehicle’s performance in their side impact testing based on injury criteria to the ATD and the performance of the vehicle’s structure as measured by the intrusion profile.

Side Airbag Forensic Investigations

Forensic casework involving airbag performance often seeks to determine 1) if vehicles were properly equipped to protect occupants against reasonably foreseeable crash incidents and 2) if airbags performed in a manner that is consistent with manufacturer's specifications and industry standards from the date of manufacture. Robson Forensic approaches these investigations with tenured automotive engineers with hands-on experience designing, testing, and manufacturing modern vehicles.

For more information, submit an inquiry or call us at 800.813.6736.

Sources

1. Insurance Institute for Highway Safety, Highway Loss Data Institute, “Fatality Facts 2023 Passenger vehicle occupants,” https://www.iihs.org/research-areas/fatality-statistics/detail/passenger-vehicle-occupants
2. Teoh, Eric; Lund, Adrian, “Traffic Injury Prevention (TIP)” Insurance Institute for Highway Safety, Highway Loss Data Institute, https://www.iihs.org/research-areas/bibliography/ref/1607

Featured Expert

Chase Nalls

Automotive Engineer & Crash Expert

Chase Nalls is an automotive engineer specializing in the design and performance of airbag and restraint systems. He applies his expertise to forensic casework involving crash reconstruction and… read more.