Ballistic testing of NASCAR windshields spurs change for 2013 season

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The damage to a monolithic NASCAR vehicle windshield is shown after a broken piece of brake rotor struck it while at speed on the racetrack. The debris originated from a vehicle contacting the retaining wall ahead of the vehicle shown.

For the National Touring Series—Sprint Cup, Nationwide, and Camping World Truck—NASCAR has always incorporated a complete windshield mounted in a steel frame to protect the driver. As the personal safety equipment in NASCAR has progressed, so too has the windshield.

While normal on-track debris can include trash such as paper, dirt, rubber, and insects, it can also include larger, denser pieces of debris. These pieces of debris are occasionally the result of on-track incidents involving contact between vehicles and/or the track retaining walls. Examples of possible types of debris from these incidents include wheel assemblies, body panels such as hoods or decklids, rear coil springs, or pieces of broken brake rotors.

Much effort has been focused on retaining these types of components. NASCAR requires that each National Touring Series vehicle be equipped with tether systems to reduce the risk of debris detaching from the vehicles. Specifically, each front wheel assembly is tethered to the vehicle chassis with redundant front spindle tethers. Redundant tether assemblies are also required on the hoods and decklids. When used, items such as onboard television broadcast cameras located in the rear vehicle fascia are tethered to the vehicle chassis. Rear coil springs are tethered between the chassis and rear axle.

However, some types of debris cannot be secured to the chassis in this manner, such as broken pieces of brake rotor. Further, debris such as full aluminum beverage cans, originating from spectator areas, and birds may pose potential strike hazards to the windshields.

While no driver injuries have been suffered from debris striking the windshield, an opportunity for improvement was identified, pursued, and implemented for the 2013 NASCAR racing season. (A paper on this topic will be presented April 17 as part of the "Motorsports Engineering [Part 1 of 2]" technical session at the SAE 2013 World Congress in Detroit.)

From 2000 through 2012, NASCAR vehicles were required to use a hard-coated polycarbonate windshield of 0.25 in (6.35 mm) thickness.

While standardized impact testing protocols are available, an adapted version, specifically designed to analyze the challenges presented in motorsports, was developed. Ballistic testing of the baseline and new laminate prototype windshields was conducted.

The test windshields were stationary, and test projectiles were propelled at the windshields using compressed air. Windshields were tested when installed in both actual vehicle windshield beds as well as in a rigid windshield bed test fixture.

Full aluminum beverage cans were propelled at the baseline monolithic windshields at 200 mph (293.3 fps). These windshields were installed in a 2011 NASCAR Sprint Cup car and a 2011 NASCAR Camping World Truck.

During testing it was observed that the can tumbled significantly prior to impact with the windshields. It was also noted that the cans ruptured within the first millisecond of contact with the windshields, prompting immediate venting of their fluid mass. None of the aluminum beverage can tests resulted in passage of projectile shrapnel or liquid into the interior of the vehicles. However, the baseline monolithic polycarbonate windshields suffered fractures in all tests.

For steel projectile ballistic testing, a rigid steel windshield frame was constructed to serve as a testbed. This fixture allowed for quick angle adjustments and was robust enough for repeated tests.

The projectile used for this testing was a solid steel slug made from nominal 1.25-in (31.8-mm) outer diameter mild carbon steel bar stock. Slugs were saw-cut to 2.5 in (63.5 mm) length. The striking end of each slug was then faced in a lathe, prior to weighing. Slugs used for the testing weighed 0.86 lb (0.39 kg). At 200 mph, the test slug had 1148.8 ft·lb (1557.6 J) of kinetic energy. This projectile was intended to represent broken pieces of brake rotor and other metallic debris.

The windshield frame was oriented to position the test windshields at 37° from horizontal. Both the monolithic and laminate windshields were tested. All test windshields had a single layer of Mylar tear-off applied.

The steel projectile passed through the monolithic windshield but did not pass through the laminate windshield. Analysis of the high-speed video reveals that the steel projectile was traveling at 110 fps after passing through the monolithic windshield. This equates to 161.6 ft·lb (219.1 J) of kinetic energy. While the monolithic windshield allowed the projectile to pass through it, the projectile kinetic energy was reduced by more than seven times. As a result of testing, the laminate windshield is being implemented for 2013 NASCAR vehicles.

Simply increasing the thickness of a single-layer monolithic polycarbonate windshield is feasible only to a point. Acceptable optical clarity becomes increasingly more difficult to achieve as material thickness increases. Secondly, as the windshield weight increases, more emphasis must be placed on windshield retention.

The laminate test windshield was also evaluated by the NASCAR Track Services team to ensure no new extrication equipment or procedures would be necessary. Test cuts of the laminate windshield were successfully conducted using existing extrication saws.

This article is based on SAE International technical paper 2013-01-0801 by John Patalak and Thomas Gideon, both of NASCAR.

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