A unibody frame offers the advantages of a lowered center of gravity, increased economy and a safer crash test safety rating. The lowered center of gravity offers improved handling, response and control while decreasing the possibility of a rollover.
Unlike the previously used body-on-frame design, where the body of the vehicle is a separate entity that is bolted onto a metal frame, the unibody combines both frame and body in a single piece, adding reinforcements in strategic locations. This reduces the weight of the frame and allows the unibody to be designed to crumple in specific ways, channeling the kinetic energy released from a crash around the passenger cell.
By evaluating crash-test data from the National Highway Traffic Safety Administration compiled in 2005, in general mid to full-size SUVs that use unibody construction score much better in frontal and rollover accidents while scoring slightly lower in side-impact crashes. However, since the unibody is one piece, accidents are more costly to repair.
Some vehicles, such as trucks, still use the older body-on-frame method of design because its modular design allows for easy replacement of broken body parts. Also, the frame is stiffer in twisting, which is a necessity for vehicles that often go off-road or are towing heavy loads.