Body-on-Frame vs Unibody Design

Body-on-Frame vs Unibody Design

Last Updated July 29, 2020 | C.J. Tragakis
Contents

Unibody and body-on-frame describe two different platform configurations of car and truck design. Evolving from horse-drawn buggies, the earliest cars were body-on-frame. Yet in the mid-20th century, this began to change.

Today, almost all passenger vehicles have switched to unibody designs. Despite the overwhelming majority of passenger vehicles being unibody, there are instances where body-on-frame vehicles are preferred.

What Does Body-on-Frame Mean?

A body-on-frame vehicle consists of a separate body and frame that are connected by mounts. Body-on-frame vehicles are also called ladder-frame due to the shape of the underlying frame. Modern body-on-frame passenger vehicles tend to be either SUVs or trucks.

Ladder Frame of a Car

The Jeep Wrangler, Toyota 4Runner, and Ford F-150 are prominent examples of current body-on-frame designs. Some prefer to call any body-on-frame vehicle a “truck,” even if it doesn’t have a bed.

If all SUVs are body-on-frame, you might be wondering why the vast majority of cars are unibody. After all, SUVs are all over the roads in the U.S. While many people refer to cars like the Ford Escape and Honda CR-V as SUVs, they’re technically not. By traditional definition, a vehicle is only an SUV if it is body-on-frame. The Escape and the CR-V are both unibody crossovers, or crossover SUVs.

BMW i3 Body and Frame
A cutaway showing a body-on-frame design. This BMW i3 is an example of an ultra-rare non-SUV body-on-frame vehicle.

As an example of the contrast, the new 2021 Ford Bronco is a body-on-frame off-roader. It’s a true SUV. The upcoming Bronco Sport, on the other hand, is a unibody crossover designed for on-road use.

Body-on-Frame Pros and Cons

Most of the pros related to body-on-frame are in terms of strength and off-roading. This capability comes at the cost of on-road performance, where worse handling and heavier weight are negatives.

Better Off-Road Capability

The biggest advantage of body-on-frame vehicles is their off-road ability. Higher torsional flexing allows the platform to better articulate over uneven terrain. It can twist and move independently of the body.

Additionally, the frame is better at soaking up jarring bumps that would otherwise be transmitted through the entire body. Body-on-frame vehicles are usually mounted higher too, which means better ground clearance.

Higher Towing Capacity

The ladder frame design typically used in body-on-frame platforms excels at towing. This frame style will usually have a heavier, more robust construction. For this reason, pickups and large trucks are almost always body-on-frame.

Modularity and Lower Accident Cost

By nature, body-on-frame platforms are heavily customizable. This means the manufacturer can put a new body on an existing frame with relative ease. Additionally, the modular nature can make it easier to replace parts after an accident.

Less Noise, Vibration, and Harshness (NVH)

Body-on-frame vehicles also provide more isolation from the road, which often means less NVH. This can improve ride comfort. However, a modern luxury vehicle with lots of effort put into NVH reduction will still be more comfortable than a basic pickup truck. Still, a body-on-frame can give drivers that “old-school SUV” feel of being separated from the road.

More Body Roll and Worse On-Road Handling

Being more isolated from the road comes at a cost. The same design that gives body-on-frame vehicles better off-road capability also means they won’t handle as well. Body roll will be more noticeable, especially because these vehicles often have a higher center of gravity.

Less Safe in a Crash

Having the body mounted on the frame means that trucks and SUVs cannot dissipate the force of a crash through the entire structure. They don’t spread the impact as well, meaning that more force is transferred to occupants.

Unibody vehicles have the advantage of dissipation through their larger frame. However, the heavier mass of a body-on-frame vehicle can make it somewhat safer in car-to-car accidents.

Pricier to Build

Body-on-frame vehicles are generally more expensive to make than unibody ones. This additional cost is passed on to consumers. Moreover, manufacturers know that enthusiasts and workers are willing to pay more for capability. Off-roaders and trucks are more expensive than unibody vehicles in most cases.

What Does Unibody Mean?

A unibody construction means that the vehicle’s chassis and body are one connected piece. Unibody designs are by far the most common today, due to their advantages in production.

Unibody Car Design

Almost all cars and crossovers are unibody, while most trucks and “true” SUVs remain body-on-frame. A notable exception in the U.S. is the Honda Ridgeline, the only unibody pickup truck for the time being. It performs basic truck duties well, but compared to something like the Jeep Gladiator, the Ridgeline isn’t as good for off-roading or towing. Still, Honda’s truck has fared much better than Ford’s ill-suited unibody pickup trucks from the early 1960s.

The simplest unibody design is what’s called a monocoque. This French word means “singular hull,” and describes vehicles with a seamless body, chassis, and skin. It’s a design rarely seen in road vehicles apart from very high-end supercars.

Another prominent example of monocoque design would be Formula-1 cars. The majority of unibody cars on roads today aren’t quite as unified. They still have separate pieces like hoods, doors, and fenders.

Unibody Pros and Cons

Modern unibody design works best for most driving applications. The superior fuel economy and driving attributes make them comfortable. They also absorb crash impacts better than body-on-frame, but are often more expensive to repair. The only real downside in terms of performance is having less off-road and towing capability.

Greater Rigidity and Better Handling

In modern unibody design, the pillars of the car connect the roof and the chassis. This adds rigidity, which offers several benefits.

One is improved handling. Unibody vehicles will handle better than body-on-frame vehicles due to the reduced flex. As we’ll see below, the structure is also lighter and better at absorbing impacts.

Lower Weight and Cost

The unibody construction means a lighter overall weight, which improves fuel economy. Unibody design also has simpler production, with less material needed. This is cheaper for manufacturers and can mean lower prices for the consumer.

Unibody Structure Absorbs Crash Impact Better

Another benefit of the rigid frame is safety. If the vehicle is involved in a crash, the unibody frame will better absorb and dissipate the impact than a body-on-frame setup.

More Expensive Repairs

There are a couple of downsides to unibody designs. Though they handle crashes better, they’re more difficult to repair. The connection of multiple areas of the body makes it difficult to replace just one section. This means repairs can be significantly more expensive.

Less Viable Off-Road

The other disadvantage isn’t as important for most drivers, but unibody cars are worse at off-roading and towing. Even with AWD or 4WD systems, their design isn’t well-suited for the torquing and twisting forces of off-road driving. Towing capacities are almost always less than body-on-frame designs, as unibodies don’t have the strength of a ladder frame.

Is Body-on-Frame Safer than Unibody?

Body-on-frame vehicles are typically seen as less safe than unibody designs. There are several reasons for this. The first is that they are heavier. Though this means they have more mass to absorb impacts, it also means that braking and handling are worse. This can make it tougher to avoid accidents in the first place.

Additionally, a body-on-frame vehicle handles worse at higher speeds due to its higher center of gravity. This results in a greater chance of a rollover crash. Finally, because the body and frame are separate, crash impacts are not dissipated as well as a unibody.

Which Is Better: Body-on-Frame or Unibody?

For the most part, drivers don’t need to “choose” the type of body and frame their vehicle has. If they want a car, they’ll get a unibody. If they want a truck or off-road vehicle, they’ll get a body-on-frame.

There are exceptions once you get into the SUV and crossover segment. But for the vast majority of drivers out there, a unibody vehicle makes sense. If you want smooth handling and fuel efficiency but still want a large vehicle, crossovers get the job done nicely.

For those that tow or enjoy rugged overlanding, rock-crawling, or mudding, body-on-frame remains king of the hill.

Sources: Engineering Explained | Auto Review Hub

Body-on-Frame vs Unibody Design

Although most drivers don’t think twice about it, there are many differences between unibody and body-on-frame designs. Each has pros and cons, with body-on-frame vehicles standing out for their robust off-road capability.