Alex Trebek looks at you and then carefully reads the card:
“While one of them rests on a ladder frame, the other has unibody construction” and then he looks up.
If you were a Jeopardy contestant and shouted “What is the difference between an SUV and a Crossover?” Alex would announce, “Correct!” You’d win the $300, and then shoot a knowing wink at the home audience. Saying that SUVs ride on ladder frames and crossovers have unibodies is the simplest, correct-est answer.
Yet there’s another, squishier answer, sort of the vehicular version of Supreme Court Justice Potter Stewart’s “I know it when I see it” for how he identified obscenity in 1964. And I’ll get to that at the end.
First off, what’s a ladder frame?
It’s pretty close to what it sounds like—two parallel rails with interconnecting stiffeners, laid horizontally. Stood up on end, it resembles a ladder. Spanning nearly the length of a vehicle, it’s a car’s steel skeleton, holding together all its major organs—engine, transmission, suspension, wheels, and bodywork skin—in place. Ladder frames are rampant among trucks, underpinning almost all of them (except for a few oddballs like the Honda Ridgeline and Tesla’s Cybertruck). And, judging by our Jeopardy success, underpinning SUVs as well.
The automotive ladder frame is ancient genetic material. Horse-drawn carriages were built upon them, and early car-builders (notably, Studebaker) just carried on with the practice as they transitioned from animal horse-power wagons to combustion horsepower cars. This wasn’t just a stubborn, archaic tradition, though. The rutted roads of the early-Twentieth Century required huge, heavy wheels, and together with a primitive understanding of springing and damping, it was a good thing that the ladder frame flexed a bit too. It was literally part of the suspension, the whole car becoming a giant spring, with weights and mini-dampers often attached to either end to tamp-down any nasty resonances. The emphasis was on strength, not stiffness, plus, a live axle-on-leaf-spring suspension easily mated with frame-rails.
As roads became paved and smoother, wheels and suspensions could be smaller and lighter, so having a flexible frame mattered less – and finally, an outright hindrance as brands (notably, Cadillac) better understood suspension theory. For instance, ride quality is helped by minimizing “unsprung weight” (considered the wheels, tires, and axles, plus half the pivoting parts). A heavy ladder frame is the last thing you want as part of the suspension mix.
OK, so what’s a unibody?
Like a ladder frame, a unibody is also what it sounds like. Instead of underfloor frame rails doing the work while the bodywork’s simply worn like a big hat on top, here, the whole, thin-gauge sheet metal structure—usually excluding most of the bodywork skin itself—carries loads. It’s distributed. As an aside, the French word monocoque, meaning “single shell,” is the unibody’s kissing cousin, except that much of its bodywork is substantially stressed as well, like a jetliner’s. It’s a rarity, usually seen in racing. (Though in that realm, its meaning is obviously malleable; many one-piece carbon-fiber racing chassis—with non-stressed bodywork, so just like a unibody—are commonly called monocoques, a term that’s simply more popular in Europe.)
What’s good about unibody construction?
There’s a list of things. The structure’s comparatively huge cross-section (10 or more times a frame rail’s height) makes it tremendously stiffer for its weight. Or inversely, lighter for the same stiffness, which greatly improves mileage. Crash safety benefits, too, as its folded-origami construction can be designed to collapse progressively, reducing your body’s deceleration upon impact. And handling improves because the suspension can be more complex as it can attach to a more three-dimensional structure, while its connection points don’t flex around as much as the chassis distorts, letting the tires accurately do their work.
With all these advantages, why would anybody still build a ladder-frame SUV? Good question. Which is why the vast majority of the offerings out there are unibody crossovers. But there are few justifiable instances. Serious off-roading returns us to those rough roads of yesteryear, once again demanding heavy wheels and suspensions, and a strong (if not stiff) structure to attach it to. Towing loads can make a beeline through the frame rails to suspensions where the wheels are doing the pulling.
By the way—there are many other types of chassis concoctions besides ladder frames and unibodies. The 1934 Chrysler Airflow anticipated unibody construction with a hybrid blend of a tubular skeletal frame webbed by load-bearing panels. Today’s C8 Corvette is hard to categorize, being a structure composed of cast, extruded, and sheet aluminum, all bonded together (the body is a non-load-bearing envelope).
What is it—an SUV or a crossover?
And what about Supreme Court Justice Stewart’s definition? I once attended a lunch with Subaru product planners who wanted our opinion on a new product they were considering “What if we raise the Legacy wagon (it had FWD) and give it all-wheel drive?” I thought they were crazy and said so. The SUV craze had just started and Subaru wanted in on it without actually building an SUV. So was born the Outback—what we now call a crossover. And why I’m not a product planner.
If a manufacturer thinks it’ll sell more unibody crossovers by giving them tough, SUV looks that we “know when we see it,” nobody can stop them. But it isn’t going to impress Alex Trebek.
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