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The aluminium alloys used to produce the castings behaved differently in sand and metal molds and often failed to meet Tesla's criteria for crashworthiness and other attributes.

The breakthrough Tesla has made centres on the how the giant molds for such a large part are designed and tested for mass production, and how casts can incorporate hollow subframes with internal ribs to cut weight and boost crashworthiness.

The subframes in a car underbody are typically hollow to save weight and improve crashworthiness.

That has been deemed prohibitive by automakers - especially as a design might need half a dozen tweaks or more to achieve a perfect die from the perspective of noise and vibration, fit and finish, ergonomics and crashworthiness, the sources said.

In 1983 the agency launched the New Car Assessment Program, which publicly rates car models’ “crashworthiness” during a collision.