Body-on-frame construction has become passé in the auto industry today. But in the eras celebrated by vast numbers of Hemmings
So why the switch? Was the transition to unit-body construction all about reducing manufacturing costs or is it superior? Perhaps even more importantly: Is unit-body construction safer than or as safe as body-on-frame?
Full-size, rear-wheel-drive American cars, light trucks and SUVs have stayed with body-on-frame construction into the 21st century--though there are some differences among the full-frame vehicles of today and those of the past. For instance, many early full-frame cars used a substantial X-brace between the rails to tie them together. While strong, this required high floors to clear the bracing, which subsequently gave these vehicles high ride heights. So, by the mid-1960s, most full-frame vehicles were built on a ladder-type frame in which perpendicular crossmembers tie the two rails together. This allowed floor pans to drop between the crossmembers, lowering the center of gravity.
On the plus side, full-frame cars are less likely to suffer structural damage or fatigue from corrosion over time, because it takes longer for rust to eat through a heavy steel rail than through sheetmetal. Moreover, in the event of a serious accident, there's often less work involved to repair a full-frame car. The downside to a full-frame chassis, of course, is weight, which is fuel economy's greatest enemy.
Unit-body cars rely on the combined strength of sheetmetal stampings for their structural rigidity. The vehicle's roof, floor, A-, B-, and C-pillars, the firewall, and the sills are all key components to the unit-body vehicle's skeleton. Most unit-body vehicles incorporate front and rear subframes: These are steel or sometimes formed sheetmetal frame stubs that provide a place to hang the vehicle's suspension parts. Unit-body cars are typically stiffer and lighter than their full-framed counterparts, which gives them an edge when fuel economy is a concern. But when rust attacks the body, it is gnawing away at the vehicle's entire structure. Similarly, repairing a unit-body car after an accident can involve major sheetmetal surgery that must be performed exactingly in order to get the car to track straight again.
So how safe can an older car, built almost entirely out of sheetmetal, be? In some cases... extremely.
The Mercedes-Benz W123 chassis automobiles, built from 1976-1986, were designed with safety as a priority and serve as a good example of how front and rear crumple zones with a center "safety cell" are incorporated in unit-body vehicles. The front and rear ends were designed to progressively crumple in a collision in order to absorb the energy from the impact. The central portion of the body, the "safety cell" (this concept was pioneered by Mercedes-Benz engineer Bela Barenyi and patented by the company in 1951 as "a shape-retaining passenger compartment surrounded by crumple zones at the front and rear"), relies on a reinforced roof with six pillars and heavy-duty crash beams inside the doors.
Engine and transmission mounts designed to shear on impact also help absorb some of the blow in a collision, but, more importantly, keep the drivetrain parts from entering the cockpit. Of course, seat belts and air bags, which were available in 123-series Mercedes-Benz automobiles starting in 1982, are also key to the effectiveness of the safety cell in protecting its occupants.
In the final analysis, it's unlikely that unit-body construction will ever replace body on frame in heavy-duty applications requiring high towing and hauling capacities. But due to advancements in safety and manufacturing, passenger cars with ladder frames are probably destined for the history books.