In a body-on-frame vehicle, the vertical surfaces are simply bolted to the frame.

Passive Safety Features

Not every accident can be avoided. Should you get into an accident, it's best to have the following safety features on your side.

Vehicle Body Structure

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.

NHTSA Crash Test Scores

Airbags

Since 1994, the government has required all cars sold in the United States to have a front driver-side airbag, and dual front airbags have been required since 1997. By the 2007 model year, all U.S. cars will be required to have advanced front airbags that inflate with greater or lesser power according to the needs of the occupant. Sensors that determine the occupant's size and position, whether a seatbelt is in use, and the severity of the crash all determine the force with which the airbag is deployed. These airbags, already in use in many vehicles, are safer for children and smaller occupants than the current single-stage airbags.

Studies show that front airbags aren't always enough, though. Side impact airbags are especially helpful in the event of a side collision. According to a 2003 study by the IIHS, head-protecting side airbags accounted for a 45 percent reduction in risk of death in side crashes. Torso-protecting side airbags reduced risk of death by 11 percent in the same study. Based on this data, safety-conscious buyers will obviously want to include head-protecting and/or torso side airbags on their SUV shopping lists.

Seatbelt Pre-tensioners and Force Limiters

Seatbelt pre-tensioners, which take the slack out of seatbelts quickly and automatically under heavy braking or in a frontal crash, are standard in most vehicles. Pretensioners help ensure that occupants get the best possible protection from their seat belts. Check to make sure the vehicle you are buying has them.

When pretensioners work, they can make the belt quite taut, possibly leading to chest injuries in the event of a crash. That's where seatbelt force limiters come into play. Force limiters let a little bit of slack back out of the belt. They are designed to work with the airbag to help spread frontal crash forces across the occupant's body, thus reducing the risk of injury.

Next, we'll check out some tips for driving an SUV.