The company’s CEO, Elon Musk, announced the design change in an article on Medium.com, saying that all Model S vehicles built after March 6 have the shield. Owners of cars built before that date may have the shield added upon request.

Hitting road debris is inevitable, NHTSA documents say. By raising the car’s ride height through a software upgrade last fall, and adding the titanium shield, Tesla reduced the likelihood and danger of such a strike, the agency concluded.

The agency’s defect investigation office “believes impacts with road debris are normal and foreseeable,” the documents say. “In this case, Tesla's revision of vehicle ride height and addition of increased underbody protection should reduce both the frequency of underbody strikes and the resultant fire risk. A defect trend has not been identified. Accordingly, the investigation is closed.”

Musk vigorously defended the safety of the Model S last year, even after regulators opened their formal investigation on November 15. In today’s article, he said the change would have value, by “minimizing owner inconvenience in the event of an impact and addressing any lingering public misperception about electric vehicle safety.”

“With a track record of zero deaths or serious, permanent injuries since our vehicles went into production six years ago, there is no safer car on the road than a Tesla,” Musk wrote. “The addition of the underbody shields simply takes it a step further.”

Extra strength

The first of the fires that prompted the NHTSA investigation occurred Oct. 1 in suburban Seattle, when a driver ran over metal road debris. NHTSA said Oct. 24 that it found no evidence of a safety defect, but opened an investigation after a second fire occurred on Nov. 6 when a Model S ran over a metal tow hitch in Smyrna, Tenn.

In each incident, the driver walked away without injury before the fire grew intense. Tesla tried to douse any criticism by publishing testimonials from the cars’ owners.

“This experience does not in any way make me think that the Tesla Model S is an unsafe car,” Juris Shibayama, the driver from the Tennessee accident, wrote in a blog post published Nov. 9 on Tesla’s Web site. “I would buy another one in a heartbeat.”

The shield that Tesla added to the Model S to prevent such fires has three layers. The first is a rounded, hollow aluminum bar capable of deflecting objects; the second is a plate of military-grade titanium, and the third layer is an aluminum extrusion that absorbs more energy from an impact.

Tesla said it conducted 152 tests using the shield and found it prevented any damage that could cause a fire. The company said the tests included placing steel structures in a position to impale the battery pack, as if a car was driving “at highway speed into a steel spear braced on the tarmac.”

Added weight can be a particular problem for electric vehicles, because it cuts into driving range between charges. Tesla said the added weight would reduce range by about 0.1 percent. That is equivalent to one-third of a mile for a Model S with the 85 kwH battery pack, which gets an estimated 300 miles between charges.

PR crisis averted

By resolving an investigation by NHTSA, which can order a car company to issue a recall if it has a safety-related defect, Tesla seems to have cleared its biggest public relations challenge since the company’s creation.

Batteries are inherently less fire-prone than gasoline, experts say, and less prone to the type of fast-burning fires and explosions that tend to kill occupants. Yet the novel nature of Tesla’s fires thrust them into the public eye and threatened to undermine confidence in electric vehicles, despite Musk’s protestations.

After the fires, Tesla broadcast an electronic software update to its cars, raising the ride height of the Model S to reduce the risk of debris damage. NHTSA investigators questioned whether that would protect against larger pieces of debris, agency documents show. Then, in a March 10 meeting, Tesla told NHTSA investigators “it would conduct a free-of-charge service campaign” to add more protection to the undercarriage of the Model S.

It remains unclear whether NHTSA would have closed its investigation without that move by Tesla. A spokesman for NHTSA would not directly respond when asked that question. The spokesman pointed to the agency’s prepared statement, which says the new shield being used by Tesla will reduce the risk of debris damage and fires.

But throughout the investigation, Musk made clear he did not intend to have regulators order a change in design.

As he declared at a New York Times-sponsored conference in November: "There definitely won't be a recall."