Vetting a better NASCAR window net mounting system

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The window safety net (right) is intended to keeping flailing hands inside the interior of the car while also serving as a shield to protect the driver from intruding debris. NASCAR Nationwide Series driver Travis Pastrana is shown during practice at Kansas Speedway in October 2013.

First required by NASCAR in 1970, the driver’s window safety net has continually evolved and improved, playing an important role in protecting racecar drivers from injury. Originally used to help keep the driver’s upper torso, head, and arms inside the interior of the race vehicle during a crash event, the role of the driver’s window net in stock car racing has transitioned to keeping flailing hands inside the interior of the car while also serving as a shield to protect the driver from intruding debris.

Engineers at NASCAR's Research & Development Center in Concord, NC, performed three separate window net and window net mounting tests and used them to design an improved window net mounting system for the 2013 season.

In 2005, the NASCAR rulebook recommended the use of window nets that complied with SFI 27.1. SFI 27.1 consists of two drop tests onto a horizontal fixed window net. The window net is mounted in a horizontal plane by two solid steel dowels of 0.625 in (15.9 mm) diameter. The first drop test consists of a 3-in (76-mm) diameter steel ball weighing 4 lb (1.8 kg) dropped from 2 in (51 mm). The center of the ball is aligned with the geometric center of a webbing opening near the center of the net. This test is repeated three times on three different webbing openings.

The second drop test consists of an impact bag made of leather or other heavyweight material. The bag is 10 in (254 mm) in diameter at the bottom and is long enough to contain 175 lb (79 kg) of “OO” or smaller lead shot. The bag is dropped from a height of 26.5 in (673 mm) directly over the center of the net. Three out of four nets are required to pass each drop test. Passing is defined as preventing the steel ball from passing through the webbing as well as preventing the bag from passing through the net. The impact bag drop test must also not separate more than 50% of the inner cross-section webbing joints, the webbing ends to border joint, and the border material stitching at the dowel loops.

While the SFI 27.1 window nets themselves were found to be very robust, the existing 2012 window net mounting assemblies needed improvement to achieve the goal of lowering the risk of debris penetrating the driver’s window net.

A window net mounting assembly (Assembly #1) consisting of 0.375 in (9.5 mm) diameter round solid steel top bar and latch assembly was tested with provided upper mounting tabs. The lower mounting bar consisted of a 1-in (25-mm) wide steel flat strap mounted to the chassis on each end and the center. This system represented an exemplar window net mounting assembly from a NASCAR series vehicle in 2012 and prior. A peak load of 425 lb (193 kg) was achieved prior to the rear top mount sliding through and separating from the rear keyhole mounting tab. The front latch remained secured to the tab.

A second window net mounting assembly (Assembly #2) consisting of a 1.0 in (25 mm) wide by 0.125 in (3.2 mm) thick solid flat steel top bar and latch assembly was tested with the provided upper mounting tabs. The lower mounting bar consisted of a 1-in-wide steel flat strap mounted to the chassis on each end and the center. This system also represented an exemplar mounting assembly from a NASCAR series vehicle in 2012 and prior. The top bar separated from the rear top mount due to sliding through the keyhole mounting tab. The front latch remained secured to the tab. The recorded peak load was 74.4 lb (33.7 kg).

The final version of the NASCAR designed window net mounting assembly (Assembly #3) featured top and bottom bars consisting of 0.75 in (19 mm) diameter steel tubing. The top bar separated from the forward top mount at a recorded peak load of 2184 lb (991 kg).

Assembly #3 displayed a stiffness of 1035.4 lb/in (181.33 N/mm), which was approximately 7.5 times greater than Assembly #1 and approximately 26 times greater than Assembly #2. The failure mechanism in both Assembly #1 and #2 were due to top bar beam bending, which permitted the top bar to slide through the top rear mount, eventually causing separation. The failure mechanism in Assembly #3 was due to overloading of the top forward latch mechanism.

Assembly #3 sustained greater than four times the energy of Assembly #1 and greater than 66 times the energy of Assembly #2. Assembly #3 sustained slightly less energy than would be imparted to a system during the SFI 27.1 testing.

The majority of load-carrying improvement shown with Assembly #3 resulted from the large increase in the moment of inertia of the top bar.

A secondary source of improvement in load-carrying capability for Assembly #3 was from the addition of the top bar rear insert. The top bar rear insert decreases the ability of the top bar to slide through the top bar rear mounting sleeve. The addition of this mechanism did not change the normal operation of existing window nets, which drivers are accustomed to.

In total nine quasi-static tests were conducted during the development and refinement of Assembly #3, baseline testing, and test fixture development. The force deflection plots of the three previously discussed assemblies as well as three additional tests used during the development and design process of Assembly #3 is shown in the accompanying chart. It is shown that two of the Assembly #3 Development tests (A & C) displayed higher peak loads than the final reported Assembly #3. However, the stiffness of these previous assemblies was not as large as that of the final Assembly #3 as seen from the slopes between displacements of 2 and 3 in (51 and 76 mm). The stiffness is an important factor for the system as displacements in excess of 5 in (127 mm) will begin to contact most driver’s seats, depending on their seating position preference.

The implementation of the window net mounting system shown in Assembly #3 provides an enhanced barrier against debris for drivers while also preventing hand and arm flailing injuries. It was incorporated into the 2013 NASCAR Sprint Cup, Nationwide Series, and Camping World Truck Series vehicles.

This article is based on SAE International technical paper 2014-01-0508, “Design, Development and Testing of an Improved Stock Car Driver’s Window Net Mounting System,” by John Patalak, Senior Manager, Safety & Structure Engineering, NASCAR; Thomas Gideon, Director, Safety, R&D, NASCAR; and Don Krueger, Lead Fabricator, NASCAR Research & Development Center. It will be presented at the SAE 2014 World Congress on April 8 as part of the “Occupant Protection: Occupant Restraints” technical session.

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