LESSONS FROM THE TRACK: CRASH SAFETY PRINCIPLES AND PRACTICES FROM THE WORLD OF MOTORSPORTS
By John P. Patalak
Whether it’s a pilot flying a fighter jet through the air at Mach 2 or a driver speeding a race car around an oval speedway at 200 mph, when it comes to crash safety, many commonalities exist in the associated physics, research, and successful protection of operators (and occupants) during sudden acceleration events. Regardless of the type of vehicle involved, there are generally two main attributes that must be addressed in crash safety—(1) the maintenance of a “survival space” around the operator throughout the crash sequence, and (2) a properly designed and sufficiently strong restraint system for the operator. Beyond these attributes, of course, there are many other principles, maxims, and best practices in applying these safety measures to specific occupant environments, acceleration magnitudes, directions, and other factors and requirements. But even here, much can be leveraged and learned by crash safety researchers in a myriad of fields.
Just like many combat aircraft pilots, drivers for the National Association for Stock Car Auto Racing, Inc. (NASCAR) often operate in a small, confined, hot, loud, and stressful environment. They are also routinely exposed to low-level (<3 G’s), sustained (>1 s) lateral and vertical accelerations while navigating high-banked racetracks at high speeds and experiencing severe accelerations during on-track crashes. Accordingly, many common characteristics and approaches exist in the driver safety research and development (R&D) efforts that are occurring in the fields of professional motorsports, aircraft/ aerospace, and even the general automotive industry.
MOTORSPORTS AND MILITARY AVIATION: A SHARED HISTORY IN CRASH SAFETY
The modern NASCAR driver restraint system is highly customized for its motorsport environment; however, its operating principles, origins, and developments are actually rooted in military aviation safety research. In the 1940s and ‘50s, Air Force colonel, flight surgeon, and early aerospace medicine and safety research pioneer John Paul Stapp (shown in Figure 1) conducted a series of test studies assessing the effects of rapidly applied accelerations on the human body [1]. Dr. Stapp—who would come to be called “the fastest man on earth”—and other test volunteers would be strapped onto a rocket sled and shot down tracks at high speed (in excess of 600 mph) to place high deceleration forces on their bodies. The significant contributions of Dr. Stapp to our understanding of these effects benefitted not only aircraft pilots but also race car drivers, passenger car operators/occupants, and others.
Figure 1. COL Stapp and Sonic Wind, the Rocket Sled That Made Him “The Fastest Man on Earth” (Photos Courtesy of the U.S. Air Force).
In 1959, Martin Eiband summarized the work of Dr. Stapp in a National Aeronautics and Safety Administration (NASA) literature review of human tolerance to rapidly applied accelerations [2]. Over the following decades, many other researchers likewise quantified human acceleration tolerances, often coinciding with the development of increasingly advanced restraint systems for successful protection. In 2002, Rolf Eppinger also published an excellent collection of restraint maxims (summarized in Table 1) [3].
After World War II, the approach to improving motorsport driver safety was often based on driver’s instincts rather than the application of science and engineering. By the 1990s, however, as increased emphasis and regulation were placed on motorsport safety by sanctioning bodies, drivers, car manufacturers, etc., the science, engineering, and test methods of the general automotive industry were increasingly applied to motorsport safety research efforts [4].
Today, more than 7 decades after COL Stapp’s historic high-G rides, many new tools to study injury mechanisms have become available for safety researchers in a wide range of applications. These tools include the rapid advancement of computer processing technologies that allow for complex human body modeling and simulation. For example, the Global Human Body Models Consortium (GHBMC) and the Toyota Total Human Model for Safety (THUMS) are two commonly used human body models for today’s crash safety studies [5, 6]. These types of models have allowed for new levels of research and exploration, delving deep into nuisances far beyond what the traditional anthropomorphic test devices (ATDs) have been capable of capturing.
With the sudden deaths of four NASCAR drivers in less than 1 year, tremendous efforts surrounding driver safety in professional motorsports occurred during the first several years of the 21st century. The primary areas identified for improvement focused on the driver restraint system, the vehicle, and the racetrack. The most well-known of these driver deaths was that of NASCAR champion Dale Earnhardt (pictured, along with his father and son, and some of the driver safety features of their respective eras, in Figure 2). The 20th anniversary of this incident, which occurred on the final lap of the Daytona 500 in February 2001, provides a poignant opportunity to look back over the last 2 decades of NASCAR driver safety development and highlight some notable principles and practices that are relevant not only for the motorsports industry but also for safety researchers in military aviation and other fields.
Figure 2. NASCAR Driver Safety Advancements. Left to Right: Ralph Earnhardt (1953), Dale Earnhardt (1981), and Dale Earnhardt, Jr. (2009) (Photos Courtesy of ISC Archives).
While there are thousands of projects and areas of research that could be highlighted herein, the following four items (which are listed in no particular order) are especially notable in that each of them represents a step-function improvement to overall NASCAR driver safety:
- The Steel and Foam Energy Reduction (SAFER) Barrier
- Head and Neck Restraints
- Seats
- Seatbelt Restraint Systems.
SAFER BARRIER
Most oval racetracks traditionally used rigid, reinforced concrete walls to contain race vehicles on the track. In 1999, however, the Midwest Roadside Safety Facility (MwRSF) at the University of Nebraska-Lincoln began developing the SAFER Barrier. The barrier works by increasing crash displacement, which extends the duration of crash pulses, thereby reducing the peak accelerations experienced by the vehicles and drivers. As shown in Figure 3, energy-absorbing foam modules placed behind high-strength steel skin deform upon impact, allowing for this additional displacement and enabling the SAFER Barrier to lower peak vehicle accelerations by 30 to 80% [7].
Figure 3. Traditional Rigid Concrete Wall (Left) vs. SAFER Barrier Stroking (Right) Crash Tests.
HEAD AND NECK RESTRAINTS
The use of head and neck restraints virtually exploded after Dale Earnhardt’s widely publicized death at the Daytona 500 in 2001. What many people do not realize, however, is that a handful of the 43 drivers in that race were already wearing the now-well-known Head and Neck Support (HANS) device [8]. Developed in the late 1980s, the HANS device (shown in Figure 4) is still the predominant head and neck restraint used by NASCAR drivers [9]. During a frontal impact, the primary purpose of any head and neck restraint is to limit head excursion and neck kinetics. The driver’s seatbelt restraint system applies restraining loads to the pelvis and thorax. These restraining forces, along with the mass of the driver’s head and helmet, can induce dangerous neck loads during frontal impacts. A head and neck restraint thus limits this cervical loading by providing an alternate load path between the skull and the thorax.
Figure 4. HANS Device (Photo Courtesy of Simpson
Performance Products).
In the case of the HANS device, this alternate load path is accomplished through bilateral anchors on the helmet that are connected to HANS tethers. The HANS tethers connect to the HANS collar behind the drivers’ helmet. The collar is integral to bilateral yokes, which are fit underneath the driver’s shoulder belts. This system has proven highly effective in coupling the skull, through the helmet, to the torso via the shoulder belts, thereby limiting cervical neck loads and head displacement. Furthermore, ongoing design improvements have continued to increase the device’s effectiveness while also reducing the driver’s awareness of—and associated distraction/irritation by—the presence of the device when behind the wheel [10].
Since 2005, all NASCAR drivers have been required to use a NASCAR-approved head and neck restraint. The first step of the NASCAR approval process is for the device to pass the SFI Foundation (SFI) 38.1 test specification [11]. SFI 38.1 requires sled testing with a Hybrid III 50th-percentile male ATD, which is used to evaluate the performance capabilities of head and neck restraints. ATD criteria such as peak neck axial force and Neck injury criteria (Nij) are both limited in SFI 38.1.
SEATS
In the early decades of NASCAR racing, a driver’s seat was typically the factory stock seat. Modifications and additions to provide lateral supports and stiffen seat structures were then made to the stock seats as speeds increased. Eventually, purpose-built race seats, primarily fabricated from aluminum, replaced the stock seats. While these initial aluminum seats improved driver comfort and support during normal driving, they were still structurally insufficient during crashes. Thus, early in the 21st century, biomechanically based work was undertaken to quantify seat strength requirements [12]. This seat research was continued using sled and quasi-static testing, resulting in the creation of SFI 39.1 [13].
During side and rear impacts, the seat itself provides the primary means of driver restraint. Because NASCAR seats are rigidly mounted at the base and shoulder level, rear impact performance is already well established. So SFI 39.1 focuses its test requirements on lateral stiffness, specifically setting minimum performance criteria for lateral head, shoulder, and pelvic stiffness [14].
In 2015, all drivers seats were required to be “all-belts-to-seat” (ABTS), meaning that all seatbelt restraint system mounting anchorages must terminate within the seat structure itself rather than the vehicle chassis. ABTS seats use shorter seatbelt lengths, permit improved seatbelt geometry, and eliminate seatbelt pass-through holes in seat structures. In addition, minimum performance criteria were implemented for the ABTS seats [15].
The interiors of all seats and head surrounds are also required to be lined with SFI 45.2 energy-absorbing foam [16]. This foam effectively couples the driver’s body to the ABTS seat structure by filling any voids between the driver’s body and the seat and custom-fitting the seat to each individual driver. This fitting is initially done by positioning the driver inside of a seat shell with foam shims. The driver is positioned on top of a plastic liner, which is then filled with a liquid foam. The liquid rapidly expands and hardens, producing a form-fit foam liner. This liner is then trimmed and sanded to the driver’s preferences. While this liner can be used as-is, in most cases the liner is then scanned and reproduced with CNC machining from SFI 45.2 foam billets.
Likewise, leg restraint is accomplished with bilateral leg supports that attach to the bottom of the seats and extend to the driver’s feet. A padded knee knocker also separates the driver’s knees and prevents knee-to-knee contact and potential injury during lateral impacts.
The combination of appropriately strong lateral seat supports and the energy-absorbing foam effectively couples the driver’s body to the seat across large bony surface areas. Through effective coupling, early in time, peak forces on the body are reduced. By providing complete lateral restraint from the head to the feet, local body articulations, deformations, and surface pressures are also decreased. Figure 5 shows computer-aided design (CAD) images of a typical carbon fiber composite ABTS seat and SFI 45.2 foam insert.
Figure 5. CAD Images of a Carbon Fiber Composite ABTS Seat (Left) and SFI 45.2 Foam Seat Insert (Right).
SEATBELT RESTRAINT SYSTEMS
Even though seatbelts were typically not available in passenger vehicles when NASCAR ran its first race at Daytona Beach, FL, in February of 1948, rule no. 34 of NASCAR’s first rulebook wisely required that all drivers use seatbelts. And as with passenger vehicle and aviation seatbelt restraint systems, the NASCAR seatbelt restraint system has become increasingly sophisticated and effective over time.
In the 2000 NASCAR race season, five-point seatbelts were required, consisting of left and right shoulder and lap belts with a single crotch (negative-G) belt. As seatbelt restraint systems were improved, however, additional belts or mounting points were added. These additional points were typically used to reduce the time over which an individual belt could produce a restraining load in particular directions. The new mounting points were placed for specific crash directions, thereby promoting the maxim of earlier coupling of the occupant. Figure 6 illustrates the evolution of seatbelt restraint systems that occurred from 2000 through 2015.
Figure 6. The Evolution of Seatbelt Restraint Systems From 2000 to 2015 (From Left to Right: Five-, Six-, Seven-, Eight-, and Nine-Point Systems).
Currently, only seven- and nine-point seatbelt restraint systems are permitted in NASCAR. These systems use three crotch belts instead of the five-point system’s single crotch belt. The center crotch belt, which is referred to as the negative-G belt, is centered laterally, is anchored forward of the groin, and follows the shoulder belt line down the driver’s chest. This negative-G anchor location provides highly efficient vertical restraint during rollovers since the negative-G belt does not need to move to immediately react a tensile load. The two outboard crotch belts are referred to as antisubmarine belts. These belts have anchors beneath the driver’s buttocks, which wraps the belt webbing around the inner thighs.
Combined with the lap belts, the antisubmarine belts are effective in reducing forward motion of the pelvis during front impacts and helping to promote proper positioning of the lap belt across the strong anterior superior iliac spine of the pelvis. This forward pelvic restraint reduces groin contact injuries from the negative-G belt during frontal impacts. These three crotch belts, with their specific mounting locations, also effectively reduce chest deflection during frontal impacts and head excursion during vertical impacts [17].
Another way to follow the maxims of increasing the time over which restraint forces can be applied and applying the largest possible restraint forces as quickly as possible in a crash is through the use of webbing pretensioner systems. These systems, which are found in many passenger vehicle seatbelt restraint systems, remove belt slack and lock the belt retractor moments before a driver’s body begins to move relative to the vehicle during a crash. Similarly, aviation ejection seat systems sometimes use haulback devices to help position limbs as well as pretensioners to tighten webbing moments before ejection.
Because NASCAR crashes often include multiple impacts during a single crash event, and because NASCAR drivers commonly receive pit road service and continue racing after experiencing minor impacts (many of which would trigger pretensioner firing algorithms), NASCAR’s seatbelt restraint system does not currently include pretensioners. Instead, drivers manually pretension their restraint systems. Typical motorsport seatbelt pretensions have been measured at 91.2 N (20.5 lbs) for shoulder belts and 110.3 N (24.8 lbs) for lap belts [18]. Similar to head and neck restraints and seats, the seatbelt restraint systems must also comply with SFI performance requirements.
SUMMARY
As the speeds and capabilities of race cars, passenger cars, airplanes, and other vehicles continue to grow, so will the need for improved and different occupant crash protection research and equipment. While the protection maxims for such work are currently well-established, there remain nearly endless opportunities to iterate, revise, modify, and improve existing safety systems for better adherence to those maxims. In addition, the leveraging of newer and increasingly advanced tools, such as numerical simulation, along with time-tested approaches in the field, promises to help researchers and developers continue to improve the protection of vehicle operators and occupants, regardless of the type of vehicle in which they travel.
ABOUT THE AUTHOR
Dr. John Patalak is the Senior Director of Safety Engineering for the National Association for Stock Car Auto Racing (NASCAR). He specializes in occupant crash protection and biomechanical and mechanical engineering and has extensive experience in accident investigation, occupant safety research and development, crash testing, sled testing, quasi-static testing, vehicle crash databases, experimental design, and computer modeling. Dr. Patalak currently oversees all aspects of NASCAR driver safety, including accident and injury investigations, safety equipment research, testing and approvals, computer modeling efforts, and the NASCAR crash database. His work has resulted in many peer-reviewed journal publications, several patents, and multiple awards and recognition, including the Society of Automotive Engineers Ralph H. Isbrandt Automotive Safety Engineering Award. Dr. Patalak holds a B.S. in mechanical engineering from The Pennsylvania State University and an M.S. and Ph.D. in biomedical engineering with concentrations in biomechanics from the Virginia Tech – Wake Forest University School of Biomedical Engineering and Sciences. He is also a licensed Professional Engineer in North Carolina.
References
[1] Ryan, C. Sonic Wind. New York: Liveright Publishing Corporation, 2015.
[2] Eiband, A. M. “Human Tolerance to Rapidly Applied Accelerations: A Summary of the Literature.” NASA Memorandum 5-19-59E, Lewis Research Center, Cleveland, OH, June 1959.
[3] Nahum, A. M., and J. W. Melvin (editors). Accidental Injury: Biomechanics and Prevention. 2nd ed., Springer-Verlag, New York, 2002.
[4] Melvin, J., and J. K. Russell. Developments in Modern Racecar Driver Crash Protection and Safety-Engineering Beyond Performance. SAE International, 2013.
[5] Decker, W. B., D. A. Jones, K. Devane, M. L. Davis, J. P. Patalak, and F. S. Gayzik. “Simulation-Based Assessment of Injury Risk for an Average Male Motorsport Driver.” Traffic Injury Prevention,vol. 0, no. 0, pp. 1–6, doi: 10.1080/15389588.2020.1802021, 2020.
[6] Smith, R. D., S. Hayashi, Y. Kitagawa, and T. Yasuki. “A Study of Driver Injury Mechanism in High Speed Lateral Impacts of Stock Car Auto Racing Using a Human Body FE Model.” SAE
International, doi: 10.4271/2011-01-1104, 2011.
[7] Bielenberg, R., R. Faller, D. Sicking, J. Rohde, J. Reid, K. Polivka, and J. Holloway. “Initial In-Service Performance Evaluation of the SAFER Racetrack Barrier.” Proceedings of the 2004 SAE Motorsports Engineering Conference and Exhibition, no. 724, pp. 1–6, doi: 10.4271/2004-01-3526, 2004.
[8] Ingram, J. CRASH! From Senna to Earnhardt: How the HANS Helped Save Racing. RJP Books, 2019.
[9] Hubbard, R., and P. Begeman. “Biomechanical Performance of a New Head and Neck Support.” SAE Technical Paper 902312, 1990.
[10] Gramling, H., P. Hodgman, and R. Hubbard. “Development of the HANS Head and Neck Support for Formula One.” SAE Technical Paper 983060, 1998.
[11] SFI Foundation Inc. “Head and Neck Restraint Systems.” SFI Specification 38.1, 2015.
[12] Gideon, T., J. W. Melvin, and P. Begeman. “Race Car Nets for the Control of Neck Forces in Side Impacts.” SAE Technical Paper 2004-01-3513, 2004.
[13] Patalak, J., and J. W. Melvin. “Stock Car Racing Driver Restraint – Development and Implementation of Seat Performance Specification (2008-01-2974).” SAE International Journal of
Passenger Cars – Mechanical Systems, doi: 10.4271/2008-01-2974, 2008.
[14] SFI Foundation Inc. “Stock Car Type Racing Seat (Custom).” SFI Specification 39.1, 2015.
[15] Patalak, J., and T. Gideon. “Development and Implementation of a Quasi-Static Test for Seat Integrated Seat Belt Restraint System Anchorages.” SAE Technical Paper 2015-01-0739, 2015.
[16] FI Foundation Inc. “Impact Padding.” SFI Specification 45.2, 2013.
[17] Patalak, J., T. Gideon, J. W. Melvin, and M. Rains. “Improved Seat Belt Restraint Geometry for Frontal, Frontal Oblique and Rollover Incidents.” SAE International Journal of Transportation Safety, vol. 3, no. 2, pp. 2015-01–0740, doi: 10.4271/2015-01-0740, 2015.
[18] Patalak, J., M. Davis, J. Gaewsky, J. Stitzel, and M. Harper. “Influence of Driver Position and Seat Design on Thoracolumbar Loading During Frontal Impacts.” SAE Technical Paper 2018-01-
0544, doi: 10.4271/2018-01-0544, 2018.
