Common sense dictates that older cars with high crash safety rankings will maintain an advantage in an accident in comparison to a similarly sized newer car with a slightly more modest lineup of safety equipment. But does the factor of age and rust alter the equation even when we're talking about relatively recent used cars?

Researchers at the nonprofit Swedish organization Villaagarnas Riksforbund and insurance company Folksam tested used cars, just over a decade old, to find out just how much age and rust affects used cars and whether the cars' crashworthiness degrades over a moderate period of time.

The organization tested multiple cars from two groups: 2004-2008 generation VW Golf and a 2003-2008 generation Mazda 6, all diagnosed with rust issues. The cars were subjected to front and side impacts according to the EuroNCAP standards, and the tests were conducted at Thatcham's British Crash Laboratory. The group chose the Golf and the Mazda 6 because they are fairly representative cars in two different segments, both popular in Europe, and because both are relatively sensitive to rust.

The aim of the tests was to examine if rust problems in crash beams and other parts of the structure effectively reduce a car's crashworthiness, and the group discovered deterioration in both cars' crash safety levels.

The VW and the Mazda were tested to the same standard that EuroNCAP used back when both cars were new -- the cars were not held to newer and stricter crash safety tests that have since emerged -- as the researchers sought to replicate the same crash safety tests of the time.

As the tests show, both cars received a lower rating in comparison to when new.

"When the Mazda 6 was tested in the frontal test, the car was deformed so that the driver's seat ended up leaning against the interior of the car and the crash test dummy hit its head in the B-pillar (the pillar between the doors)," the organization said. "At the frontal collision, the crash test dummy was thrown forward towards the airbag, which is released from the steering wheel. Then the crash test dummy rebounded in the B-pillar before going back to the car seat again. If a crash test dummy rebounds into the B-pillar, this is not considered by the EuroNCAP-model."

The Volkswagen Golf was also found to have lost some of its crashworthiness, but was only down a single point in comparison to its score when new.

"For the Volkswagen Golf, the grade was reduced according to the EuroNCAP's five-point rating scale from a weak five (33 points) when the car was new to a strong four (32 points)," the researchers said. "But the difference in crash safety was marginal with only one point difference."

The main finding, and perhaps the most significant surprise of the tests, was just how much the Mazda's crash performance deteriorated due to rust issues.

"In the case of Mazda 6, crash safety was significantly impaired," the researchers concluded. "The rating was lowered from a weak four (26 points) when the car was new to a weak three (18 points). According to Folksam, for Mazda 6 there is a 20 percent higher risk of being killed in a real accident, because of the rust."

The research group points out that no two cars' rust issues are alike, and hence it cannot be said with certainty that another used Mazda 6 with rust problems would have behaved the same way in these tests. The location of the rust issues matters, but in the case of the Mazda 6 it no longer deformed as designed and thus its crashworthiness has degraded.

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