Unless you are driving a pick up truck or a full sized SUV it is likely your vehicle’s structure is defined as a unit body. If you do have a pick up truck or full size SUV based on a pick up truck chassis it is probably a body-on-frame construction. What is the difference and why should I care?
In the beginning body-on- frame construction was de rigueur. The supporting structure of the body of a vehicle was a separate frame. The body simply attached to this frame. The frame was the strength of a vehicle. The components like fenders, doors, roofs, floors etc. were only there to define the shape and function of the vehicle. They were not designed to support any load other than their own weight. The frame was the back bone of the vehicle. The most common design is called a ladder frame. As you can imagine the frame is essentially two long rectangular rails held together by several cross members like steps on a ladder.
Body-on-frame vehicles have now been replaced by unit body vehicles other than the aforementioned pick up trucks. Unit body construction saves weight and adds strength.
Unit body construction is much more complex.
The strength and thus support of unit body construction comes from the body of the vehicle as a unit. Almost all the components of the vehicle contribute to its strength. The floor, the roof, the pillars, firewall (separates the vehicle interior from the engine compartment) and rocker panels are designed to essentially be the frame of the vehicle. These parts are usually all spot welded together.
The unit body vehicle is usually much stiffer and lighter than its body-on-frame predecessor. Unit body design does have weaknesses. A unit body is much more difficult to fix after an accident. Any loss of shape after an accident can be very difficult to straighten. Damage to one section of the vehicle flows more easily through the whole vehicle and getting everything straight again can be a challenge.
The unit body design lends itself better to safety without as much weight increase as a body on frame design. Since the body components like the roof and pillars are made to support the vehicle they also form a stiffer and stronger cage around the occupants by design. Engineers then design the sections around the centre cage to crush under impact and thus absorb energy without transmitting it to the occupants.
When your unit body vehicle is at your chosen service provider for maintenance a good inspection of structural integrity is important. Face facts; from day one your vehicle is subject to corrosion especially if it is used during the winter months. Salt, water and metal are a recipe for rust.
There are many more areas on a unit body vehicle that as they corrode can compromise the structural integrity of the vehicle. Believing that the rust you are seeing at the corner of the door sills is only cosmetic could be a costly mistake. The back of that door sill could be the attachment point for the rear suspension. Not far from it could be the bolt for the seat belt mounting point. Any onset of ‘rust through’ could effect the structural integrity of your vehicle. Caught early enough, it may be cost effective to repair but the repair of structural components must retain the same strength as was originally designed into the vehicle.