Hardcore truck people probably don’t think of the 2017 Honda Ridgeline as a “real truck,” but I just towed a classic Ford Mustang over the Appalachians with one. So why aren’t there more trucks in the U.S. built on a unibody platform like the Ridgeline? I talked to engineers at Honda and Toyota to find out, and now I’m wondering why every automaker—including Nissan, Toyota, and GM—isn’t doing it this way.
Body-on-frame construction—where the powertrain, drivetrain and body are all bolted to a strong ladder frame foundation—was the prominent method of building cars for decades. That began to change after 1941, when Nash introduced their 600, a mass-produced car that incorporated its body and frame into one piece called a “unibody.”
Unibody quickly became the go-to method for designing sedans and small cars, but SUVs and pickups kept their dedicated frames to better handle payload, towing and off-roading. Then 1984 came around, and AMC, with their Jeep Cherokee XJ, showed the world that unibody SUVs can be as tough or tougher than their ladder-framed counterparts. Today the vast majority of sport utilities in the U.S. are frameless.
Over the last decade, unibody crossover and SUV sales have been at all-time highs, with many automakers entering the market for the first time to get in on the fun—but mid-size unibody trucks have been virtually nonexistent.
But recently, demand for mid-size trucks has gone up, and automakers hoping to join in on the segment will need platforms. Most of them have large unibody crossover platforms ripe for the picking, ready to be made into mid-size trucks. And Honda just showed that, with modern engineering tools, it can be done and it can be done well.
Will the rest of the mid-size truck segment follow Honda? And why is literally every other mid-size pickup sold in the U.S. still built on a body-on-frame platform?
For answers, I spoke with the chief engineer of the Toyota Tacoma, Mike Sweers, as well as Jim Loftus and Steve Behm, the chief engineer and a body development engineer of the Honda Ridgeline, respectively.
Their input helped me understand the tradeoffs automakers have to face when deciding between body-on-frame and unibody. And the result of those conversations has me wondering why the heck everyone—including Nissan, Toyota, and GM—is still using body-on-frame in this segment, forgoing the improved handling, ride quality, and interior volume inherent to unibody construction.
Towing & Payload
The first thing that many truck owners think when they hear “unibody pickup” is towing: “How well can a unibody vehicle really handle high input loads from a trailer that I tow once or twice a year?,” they wonder.
The answer is that, given the relatively low trailer tow ratings in the mid-size truck segment (under 7,000 pounds for gasoline-powered trucks), a unibody can absolutely get the job done.
To be sure, a frame can make designing a truck for trailering much easier, as Toyota’s Mike Sweers told me. “By disconnecting the body structure from the rolling chassis, strength for towing is increased via increasing chassis strength in areas that display higher input forces,” he said.
But considering unibody vehicles like the Jeep Grand Cherokee EcoDiesel can out-tow pretty much any body-on-frame truck in the mid-size pickup segment, clearly there’s nothing inherent to a unibody that makes it incapable of towing mid-size truck loads.
And Sweers agrees, saying unibodies are definitely capable of towing enough to satisfy mid-size truck buyers. But he did mention that my Grand Cherokee analogy is an oversimplification, and that there’s more to it than “This unibody SUV can tow, therefore unibody trucks can tow,” because SUVs have D-pillars to take up loads, and trucks don’t.
The D-pillars and roof play major roles in tying the unibody SUV’s structure together and managing the “energy” imparted by a trailer or by payload. Cut that pillar off, and you’ve reduced the car’s moment of inertia about its bending and torsional axes, severely compromising the vehicle’s ability to handle loads. Thus, it’s harder to design a unibody truck to tow than it is to design an SUV for the same loads.
A traditional body-on-frame truck, like this F-350 below, decouples the bed and cab, and allows them to move relative to one another to take up the beefy frame’s bending and twisting motions. Have a look at the twist between the bed and cab:
With a unibody truck, the loads trying to bend and twist the truck between the cab and bed are still there, except now there’s no frame below to take up all the forces, and the bed and cab are now connected.
So to manage these loads and to prevent the bed-to-cab interface from breaking, the old Ridgeline had that strange C-pillar-to-bed transition, called a “sail pillar.”
That’s the same feature found on the Hyundai Santa Cruz concept and the dearly departed Chevrolet Avalanche. Yes, those were structural sail pillars beneath all that gaudy triangular plastic cladding, providing extra stiffness to a truck whose body (which still sat on a frame) was all one piece instead of a separate cab and bed.
So yes, designing a unibody truck to withstand high input loads is harder than designing a unibody SUV to do the same, but it can be done. And you don’t even need to use those ugly sail pillars.
One of the ways Honda did it without using those un-truck-like sails was by beefing up the new Ridgeline’s “critical unibody joint” between the cab and rear bed rail.
That critical joint, Steve Behm—one of the Ridgeline’s body development engineers— said, is key because it’s the location of a major stress concentration.
And to deal with that, he said, Honda made sure they maintained boxed sections for their bed rail, C-pillar and the back wall of the cab, culminating in the critical joint where the bed rail boxed-section ties in with spot welds, structural foam and bolts.
As Honda describes it:
Utilizing fully boxed frame members for the body sides and rear tailgate frame, the truss-style rear inner construction contributes to the new Ridgeline’s more conventional three-box design profile—allowing for the elimination of the buttress-style body structure in the forward portion of the upper bed on the previous model—while contributing to a 28-percent gain in torsional rigidity versus the previous model.
Honda’s Ridgeline tows 5,000 pounds— a respectable figure that Honda says is not limited by the unibody platform— and hauls 1,584 pounds, very close to the top of the class. So yes, designing a unibody pickup to handle trailering loads expected in the mid-size truck class is tough, but it can be done, and it doesn’t have to look hideous, either.
But beefing up a unibody to handle towing requirements has its cost.
One of the major benefits of a unibody design is its ability to offer higher stiffness with less weight. But that benefit doesn’t come to light on a pickup truck, as all of the additional structure needed to handle the stresses of towing and hauling (particularly stress at that critical c-pillar to bed joint), mean unibody trucks aren’t likely to show much of a weight-savings over their body-on-frame counterparts.
That’s why Toyota’s Mike Sweers said that yes, a unibody vehicle can tow enough for this segment, but you get to the point of “diminishing returns,” where you’ve added so much structure to bolster the body, your unibody truck now weighs the same or more than its more traditional counterparts.
Even Honda’s Jim Loftus admitted that the Ridgeline isn’t exactly a featherweight, saying that reducing bulk “wasn’t a top priority.” The Ridgeline in 4x4 guise weighs about 4,400 pounds, about the same as a similarly equipped Colorado or Tacoma.
Hard-core off-roaders will tell you that a frame is god’s gift to rock-crawling because it twists underneath the body, conforming with the terrain and allowing for increased articulation.
But while the frame’s ability to twist can indeed be beneficial for off-roading, it’s not because it helps with articulation, because the flex in the frame is minuscule when compared to the actual travel in the suspension. Not to mention, as I’ve proven, stiff unibody vehicles are plenty capable of keeping their wheels planted on uneven terrain.
The real benefit of the frame is what Toyota’s Mike Sweers refers to as “compliance.” Sweers gave the analogy of a Boeing 747 airplane wing, which is designed to deflect in order to handle high loads and to prevent fractures.
And it’s a real issue with heavily-modified unibody off-roaders. Jeep Cherokee XJs, for example, have a reputation for cracking their unibodies after severe rock crawling. But unibody cracks tends to only be a big issue in hard-core off-road conditions, as my XJ demonstrated that in moderate and even difficult terrains, a unibody can do great.
Honda says 95 percent of mid-size trucks live on the pavement. But Sweers says 45 percent of its Tacoma buyers do go off-road, and that many spend lots of money on aftermarket accessories. He says “the typical after sale spend for pick-ups, depending on region and segment, is $3000~8000,” a significant figure that Sweers says could be compromised with a unibody platform.
Will mid-size truck owners even take their vehicles on hard-core off-road courses? I doubt it, as they’ll likely be limited by their large wheelbases, overhangs, low front fascias (in the case of the Colorado), and independent suspension travel long before body strength ever becomes an issue. Sure, there are some people who rock-crawl their solid axle-swapped Tacos, but they’re a small minority.
So a unibody will do just fine off-road, just like it has in the SUV segment.
One way in which body-on-frame construction helps in the mid-size truck segment is the architecture’s ability to accommodate various bed and cab sizes.
Take the Chevrolet Colorado Crew Cab with 5'2" bed and Extended Cab with 6'2" bed: they’re two different configurations built on the exact same frame.
If you tried having two configurations on a Ridgeline, you’d have to build two entirely new unibodies—quite a large financial investment—whereas on the Chevy, you can just bolt on a different bed and cab to the same ladder.
How big of a deal is this? Well, nearly two-thirds of mid-size truck buyers are buying crew cab models, hence why Honda chose that configuration for their Ridgeline. Loftus also said that, if they wanted to, the company could totally build an extended cab version of the Ridgeline, saying the Ridgeline is essentially a modified version of the Pilot, so they’ve proven then can modify the platform successfully.
So clearly modifying a unibody platform is doable. Will it be worth the investment for less than a third of mid-size truck sales? Honda doesn’t think so.
So we know a unibody truck can tow and do moderate off-roading, but that in order to do so, it would have to weigh as much as a body-on-frame truck. Therefore, there’s not necessarily a big fuel economy benefit to a unibody truck (as showcased by the Ridgeline’s EPA numbers).
Plus, it’s harder to offer different cab and bed configurations, and it’s actually harder to fix a unibody versus a frame. So what’s the point, then?
The answer comes down to ride quality, handling, interior volume, and ease of ingress/egress.
Loftus, the Ridgeline’s chief engineer, says ride and handling was number one on their list of vehicle-level functional objectives. It’s what most people said they wanted in Honda’s customer clinics.
With a body-on-frame truck, there’s a rubber bushing wedged between the chassis and the cab. That means when a tire hits a bump, passengers feel a shock through the frame, which is damped by the bushing, sending a second “aftershake” jiggle to the cabin. This deteriorates ride quality, Loftus says.
Then there’s handling, which is a measure of how well the vehicle responds to driver input. Since unibody vehicles are stiffer, a steering input is quickly translated into vehicle response, whereas a traditional truck’s frame and body tend to flop around a bit (elastically deform) before finally doing what the driver wants them to.
Even Sweers, the Tacoma’s chief engineer, agrees, saying:
The disadvantage is that BoF construction gives up some ride comfort and handling due to overall vehicle rigidity, bending and torsional frequency.
And this benefit of unibody construction is very obvious to anyone who drives the Ridgeline, particularly when taking turns. Our Andrew Collins said after driving the Ridgeline back-to-back with its competition: “Here’s a truck with the payload capacity that would have been find for a full-sized truck ten years ago, on a car that feels like it could have come from ten years in the future.”
Besides ride quality and handling, a unibody truck offers two other important benefits over a traditional truck: interior volume and ease of ingress/egress.
Because there’s no frame, the floor of a unibody can sit much lower without compromising ground clearance, helping passengers get in and out, and also allowing for lots of flexibility in cargo capacity.
This higher interior volume was one of the first things I noticed when I took the keys to my press truck. Honda says the Ridgeline has more passenger volume than both Colorado or Tacoma, and that it has significantly more cargo volume with the rear seats folded.
But will anyone take a unibody pickup seriously?
Honda says the biggest issue with the last generation Ridgeline was that it didn’t “look” like a truck. And that’s a problem, because purchasing a vehicle is an emotional decision, and buyers need to feel like they’re buying something “beefy” and “brawny.”
The old truck, with the 135 degree angle between the bed rail and sail pillar looked a bit “off.” But now, thanks to the reinforced “critical joint,” the Ridgeline looks like a legitimate truck. Have a look at that angle of the Ridgeline above. It looks tough enough; handsome, you might even say.
The perplexing thing is that Honda decided to throw the Pilot’s fascia on the front. After all the engineering effort they made to make the Ridgeline look “truck like,” they designed a front end that looks, in my opinion, too “soft” for a truck.
Still, that front fascia was clearly designed that way out of choice, and not out of necessity. Had they put a chunkier front fascia on there, I have no doubt the Ridgeline would look like a “real truck” to 99 percent of buyers, who, frankly, don’t care if there are two frame rails sitting beneath them.
Based on my talks with chief engineers from both Honda and Toyota, I think unibody makes the most sense for this segment.
To be sure, Honda didn’t just build their Ridgeline on a unibody platform for that reason alone—it was their cheapest option. Loftus even admitted it to me, saying the company wanted to “work within the constraints of a unibody construction environment.”
The same likely holds true for Toyota: why retool their plant when they can just keep building their truck on a frame like they’ve been doing for ages?
So that’s the real question: is it worth the effort? Why change a winning formula when body-on-frame truck sales make up such a large portion of profit for the Big Three? Also, will customers really care if the truck rides better? Honda says their customers do, but perhaps a Dodge or Chevy customer couldn’t care less.
These are all hard questions to answer. But if you were an automaker looking to get into the truck game from scratch, unibody is the way to go.
Not only can car-based trucks meet the moderate trailer tow requirements in the mid-size segment, but they can totally do the job off-road, they offer better ride quality, better handling, better interior volume and better ease of ingress/egress.
Honda’s chief engineer told me that he’s “still looking for a benefit of body-on-frame,” and while I wouldn’t quite go that far (after all, I do think a frame makes it easier to offer more cab configurations, and I do see how super hard-core off-roaders might still want a frame), it does look like unibody constrution is the future. We’ll likely start seeing more of them, particularly from foreign automakers whose plants are tooled for unibody vehicles.
And it won’t be terrible.
Top photo credit: Honda