As part of the Porsche SUV safety driving program, Porsche claimed that SUVs with a unibody structure are safer than those with a trucklike body-on-frame design. A unibody vehicle uses the body panels and floorpan to form its structure, while a body-on-frame design uses a ladder-type frame as the main supporting structure. The body attaches to the frame and, while it does provide structural support, the body is not as significant to the structure as it is in a unibody vehicle.
The handling advantages of a unibody design tend to support Porsche's claim. A unibody design allows for a lower center of gravity, which means a lesser chance of rollover and a better, more stable, car-like ride. This would afford the driver greater control and could make the difference between being involved in an accident and avoiding one. And it would certainly reduce the risk of rollovers.
Though a unibody vehicle may have frame rails, those rails are tied to the vertical surfaces of the body to increase strength. In some cases, vehicles are a combination of both construction techniques
However, would unibody construction be safer in the event of a crash? When asked, Robert Shelton, former executive director of NHTSA, said no real-world studies had been done to prove that unibody SUVs are safer in a crash. But a deeper look at NHTSA's crash-test data for 2005 midsize and large SUVs tends to support Porsche's claim. As you can see in the chart below, in general, unibody SUVs are indeed safer than their body-on-frame counterparts. They score considerably better in front crash and rollover tests, and score only slightly lower in side impact tests. It should be noted, however, that many of the body-on-frame SUVs tested were older in design than most of the unibody SUVs. Perhaps with the application of newer technology, body-on-frame SUVs will be safer in the future.
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