Tesla: “Model S is the safest car ever tested”

August 21st, 2013 § 0 comments

A pretty bold claim by what I consider to be the preeminent car manufacturer but they have some impressive numbers to back it up. Only 1% of all cars tested by the National Highway Traffic Safety Administration (NHTSA) achieve 5 stars and the Model S actually broke one of the machines used to test the car.

Of note, during validation of Model S roof crush protection at an independent commercial facility, the testing machine failed at just above 4 g’s. While the exact number is uncertain due to Model S breaking the testing machine, what this means is that at least four additional fully loaded Model S vehicles could be placed on top of an owner’s car without the roof caving in. This is achieved primarily through a center (B) pillar reinforcement attached via aerospace grade bolts.

Model S - White
Since the Model S uses a rather innovative battery pack they are able to mount it below the floor pan giving it a very low center of gravity. As a result the independent testing facility had to use non-standard means to even get it to roll over.

The Model S was also substantially better in rollover risk, with the other top vehicles being approximately 50 percent worse.

Tesla is quick to point out that it’s quite possible to game the testing score by strengthening the car in the locations used by the machines.

After verifying through internal testing that the Model S would achieve a NHTSA 5-star rating, Tesla then analyzed the Model S to determine the weakest points in the car and retested at those locations until the car achieved 5 stars no matter how the test equipment was configured.

The press release also contains some pretty interesting details about crumple zones and the side pole intrusion test which, according to Tesla, is considered one of the most difficult to pass. Here is footage of a 2012 Jeep Liberty failing it.

[T]he Model S was the only car in the “good” category among the other top one percent of vehicles tested. Compared to the Volvo S60, which is also 5-star rated in all categories, the Model S preserved 63.5 percent of driver residual space vs. 7.8 percent for the Volvo. Tesla achieved this outcome by nesting multiple deep aluminum extrusions in the side rail of the car that absorb the impact energy (a similar approach was used by the Apollo Lunar Lander) and transfer load to the rest of the vehicle. This causes the pole to be either sheared off or to stop the car before the pole hits an occupant.

Much like the recent Hyperloop Alpha proposal it’s clear that Mr. Musk, who found SpaceX and is currently head of product design at Tesla, is willing to ignore accepted standards to create what he believes is a better product. Writing for MIT Technology Review Kevin Bullis shines a light on what sets the Model S battery apart from the rest of it’s competition.

Tesla’s choice of these small lithium-ion batteries is, arguably, one of its most important strategic gambles. Established automakers have chosen larger battery cells—they make engineering a battery pack simpler, since you need fewer of them. But the larger cells, because they contain more energy, are also more dangerous. So automakers use less energy-dense battery materials that are more resistant to catching fire. Trying to offset the lower energy density, automakers chose flat cells because they pack together more densely, but such cells cost more to manufacture.

a Model S with everything removed but the frames, wheels, battery and engine.

Tesla Model S with everything removed but the frames, wheels, battery and engine.

Not only are the smaller cells cheaper, thanks in no small part to the mobile computing industry, but they have better energy density. It’s not as good as gasoline yet but they are ahead of the pack. Lastly these cells have a safety benefit as well.

Choosing the smaller, cylindrical cells also gave Tesla more flexibility in packaging the cells. Large, flat cells will deform in a collision and possibly catch fire, so other automakers have had to find places within the car where the battery would be out of the way in a crash. That meant using up some passenger or cargo space.

Which takes us back to the Tesla Press Release.

The Model S lithium-ion battery did not catch fire at any time before, during or after the NHTSA testing. It is worth mentioning that no production Tesla lithium-ion battery has ever caught fire in the Model S or Roadster, despite several high speed impacts. While this is statistically unlikely to remain the case long term, Tesla is unaware of any Model S or Roadster occupant fatalities in any car ever.

I’m sure Tesla has cherry picked some of numbers to make the car look as good as possible, it’s a press release after all, but it’s hard to argue with the overall results.

Of all vehicles tested, including every major make and model approved for sale in the United States, the Model S set a new record for the lowest likelihood of injury to occupants. While the Model S is a sedan, it also exceeded the safety score of all SUVs and minivans.


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