Unit construction has been proven to be better, but not for pickups and large SUVs. Why?
Episode Summary:
A century ago, a pickup truck buyer would purchase a vehicle engineered with a sheet steel box and a stamped steel body bolted to a ladder frame. Today, a pickup buyer is purchasing exactly the same technology. In Europe, the auto industry explored lighter, stiffer and more modern technology, monocoque construction, as far back as the 1930s and in America a steady shift away from body-on-frame in passenger cars began in 1960 and is now complete. Unit construction makes a vehicle lighter, stiffer, with better ride quality and handling than body on frame construction. Yet most pickup trucks and large SUVs still use the older technology. Why?
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Transcript of this week’s show:
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This is a Supermarine Spitfire, an iconic high-performance World War II airplane. And this is a Hawker Hurricane, a not so iconic contemporary of the Spitfire, built to perform the same tasks and using the same engine. The Spitfire was a significantly higher performance airplane, and a major reason why was its construction technique. It used a monocoque, meaning that the outer skin of the aircraft was a stressed structural member. On the Hurricane, the skin was simply a covering over a tubular frame, durable and easy to repair, but notably heavier. Light weight with strength was the secret to the Spitfire’s high-performance.
In the auto industry in Europe this advantage was recognized as early as the 1930s, with Citroen’s revolutionary Traction Avant. In America, progress moved more slowly, but a key turning point was 1960, when Chrysler Corporation’s large cars and a new generation of compact cars from Ford and General Motors switched from the Hurricane concept of a body attached to a frame, to unitized construction, essentially a monocoque. In auto production, the concept was to use stamped, lightweight sheet steel and spot weld it with cleverly designed channels and box sections to make the body stiff and strong enough to mount the drivetrain steering and suspension to it directly.
The older technique, body on frame, went back to the horse and buggy days and today it’s still seen on most pickup trucks and full-size SUVs. It’s heavier, more prone to squeaks and rattles, and produces a vehicle with inferior handling to unibody vehicles. Why hasn’t anyone tried unit construction for light trucks? Well, they have, with a notable example being the Honda Ridgeline. Compared to body on frame light trucks, it is not a sales success. Why not?
The historical reason for the persistence of body on frame in pickup trucks is the need to adapt different cab and chassis configurations to a common set of frames differing only by overall length. Regular cab, extended cab, crew cab and multiple box lengths were once common, and is certainly cheaper to build them all with common body tooling. But in an age when fuel efficiency demands light weight, and Ford has gone so far as to develop special aluminum alloys to lighten the body, why bolt it onto a heavy frame?
In the time honored tradition of engineering personnel everywhere, I blame marketing. For decades, old-fashioned technology and the perception of high strength in body on frame construction has been used by industry marketers to sell their product. In the light truck market, manufacturers wear obsolescence like a badge of honour. I once owned a Ford pickup which was advertised as featuring “massive twin I-beam construction”. It drove like a Conestoga wagon and used the same 19th century kingpin technology in the front spindles, and when I traded it for a late ‘90s F-150, the difference was night and day.
The later truck was tighter, quieter, rode better and had a higher payload and towing capacity, while burning less fuel than my old ’85. But it still used a heavy steel frame. Honda has tried and frankly failed to break the mould. How about it Ford, GM and RAM? You’re going to have to electrify these vehicles soon. How about bringing them into the 21st century in the chassis too?
