Maybe, maybe not. I’m not a design engineer, and so my insight as a manufacturing engineer limits me appreciably. I debated adding notes about the possibilities of building a unibody pickup truck with the same performance as a body on frame, and decided not to. I’ll give it a shot anyway, and start off by saying it’s probably not practical (compromises will have to be made).
Remember that a unibody body structure has serious structure built into it, especially in the underbody and front end (engine box) subsystems. These have to bear the motor and road load, and any additional load (such a trailering), as well as provide a lot of the crash protection. While the upper body plays its part in rollover protection and side impact (of course), the underbody is still responsible for most of the strength.
Modern underbodies are made from sheet metal. There are lots of ways to stiffen sheet metal, but in general they’re all variations of (1) adding another layer of sheetmetal, or (2) creasing the sheetmetal. For example seat belt anchors on a 0.75 mm mild steel floor may be welded to a 1.8 mm high strength steel reinforcement. Or a rail section will have three main creases – creating an open box – that runs along the length of the underbody. Adding the floor closes of the box, but it’s still flexy compared to a chassis frame that’s an actual, closed box. Because the body is part of this underbody, the entire car can twist in multiple axes.
Boxes in particular are how a chassis frame become so rigid and resistant to flex; the rails and cross rails are nearly completely closed box sections, and they’re GMAW welded instead of spot welded. There may still be some flex, but not nearly as much as a sheet metal underbody. And if there is some flex, the body and bed are decoupled from this anyway and so not subject to the flex.
As a result of the strength the chassis frame brings to the car, the bed and cab have a lot less structure. Certainly doors still have boron intrusion beams, and modern a pillars and roof rails are usually boron steel or hydroformed, but the floor is just a floor. Because there’s no underbody structure, the whole closed cab weights a lot, lot less than a unibody car of similar spatial volume.
Finally, can we build a unibody pickup that is as strong as a body on frame? Sure, but this is where we start to compromise. Do we want to allow the cab and bed to flex? Do we limit the maximum service (towing, plowing, bed payload)? Do we want to add weight? Do we want to add 12 hours per vehicle to build a unibody underbody system? Will we lose sales because GM makes fun of the unibody pickup? Really, how much does it cost?
Assuming we want all of the same load performance, the only real way to substitute the chassis frame with a unibody equivalent is to massively, massively overbuild the underbody. Maybe we use (expensive!) hydroformed rails and cross rails. Instead of a perfectly serviceable 0.7mm floor we add strength by using 1.0 mm (weight! cost!). There’s no chassis providing protection, so now we add weight and cost to the engine box area.
I’m sure we could do it if it made economic sense, but to answer that conclusively and address all of the cost questions definitively would take a cross functional team (product, manufacturing, purchasing, marketing) about a year of research, and involve thousands of person hours. In fact, this is pretty much the process that any new vehicle program has to go through before it’s approved by the Board of Directors.
(In the case of my company, we decided to remove the weight by using a lot more aluminium.)
