Body-on-Frame: Truck Design
Body-on-frame and unibody are the two main ways a vehicle’s structure can be built, and the choice affects everything from towing capacity to ride quality to how the frame vibrates on the road. This guide breaks down what body-on-frame construction means, how it stacks up against unibody, and—since one of the biggest engineering challenges in truck chassis design is controlling a body-on-frame chassis’s natural frequency once you start cutting mass out of it—how automakers work to reduce that vibration. One of the key areas of focus in automotive manufacturing research is the reduction of chassis mass and production costs. However, when you begin to reduce the mass of the chassis, the chassis becomes more susceptible to significant vibrations, primarily stemming from the engine and road irregularities.
Truck chassis refers to the automotive body structure commonly used on trucks: ladder frame construction/ Body-on-frame (BOF) plus the driveline. This Truck chassis, sometimes referred to as a rolling chassis, exhibits natural frequencies that manifest in various modes of vibration. Among these modes, the first, second, and sixth are considered global vibrations, while the rest are categorized as local vibrations. This article will study how to influence the natural frequencies of a traditional body-on-frame/ ladder-style assembly of the truck chassis.
Chassis vs. Frame vs. Body: What’s the Difference?
Before comparing construction types, it helps to separate three terms that get used interchangeably: chassis, frame, and body. The frame—also called a ladder frame when it uses two parallel rails joined by crossmembers—is the structural skeleton that everything else bolts to. The chassis is the frame plus the mechanical systems mounted to it: suspension, steering, powertrain, and axles. The body is the sheet-metal shell that surrounds the passenger compartment. When the body sits on top of and bolts to a separate frame, that’s called body on frame construction, or body-on-frame (BOF) for short. In unibody (unitized body) construction, the body and frame are welded into a single structure instead, so there’s no separate frame at all.
What is the meaning of body-on-frame?
Body-on-frame (BOF) construction goes back to the early years of the automotive industry, when wealthy individuals would buy rolling chassis from automakers such as Bugatti or Rolls Royce. They would then turn around and go to a coachbuilder/body-maker who would custom-build the passenger-carrying compartment of their automobile. As a result, no two vehicles were the same, since each vehicle had both DNA from the auto manufacturer and styling from the coachbuilder.
Before the 1940s, virtually all automakers designed vehicles with body-on-frame architectures. Today, body-on-frame designs are still used on pickup trucks and large SUVs. A body-on-frame construction is one in which the structural load-carrying metal frame is separate from the body-in-white. All the chassis and powertrain components mount directly to the frame, creating what is called a rolling chassis/ ladder chassis.
The rest of the vehicle would then get assembled to the upper structures of the Body-in-White to create the “coach” which would be bolted to the rolling chassis during what is known as the body drop or marriage in the assembly process.
Ladder frame truck chassis
The truck chassis stands as a vital component, imparting strength and stability to the vehicle in diverse conditions. Serving as the backbone of any truck, these frames deliver both strength and flexibility, with steel being the predominant material, although aluminum is gaining popularity for its construction. Given that the chassis forms the foundational support for heavy-duty vehicles, its primary role is to safely carry the maximum load across all operational conditions. Additionally, it must absorb engine and driveline torque, withstand shock loading, and adapt to twisting on uneven road surfaces.
To achieve this performance, automakers must meticulously design and rigorously test the construction of vehicle frames. The ‘ladder’ type frame construction is specifically designed to provide robust support for the body and payload, while simultaneously offering torsional flexibility, particularly in the region between the gearbox cross member and the cross member ahead of the rear suspension.
The 2021 Ford Bronco’s rolling chassis, an example of modern body-on-frame truck chassis design engineered for the torsional flexibility that helps the frame absorb off-road loads.
This chassis flexibility is essential, as a rigid frame is more susceptible to failure compared to a flexible one capable of ‘weaving’ when subjected to challenging conditions. Furthermore, a torsionally flexible frame has the added advantage of reducing suspension loading when the vehicle encounters uneven surfaces.
Is body-on-frame better than unibody?
When comparing different types of automotive structure designs, such as space frame vs unibody vs body-on-frame (BOF) designs, it’s important to remember each type of architecture has its advantages and disadvantages. Of these three, the most popular type of vehicle construction for the vast majority of passenger cars /trucks is either unibody or BOF.
| Factor | Body-on-Frame (Ladder Frame) | Unibody |
|---|---|---|
| Construction | Separate frame with the body bolted on top | Body and frame welded into one structure |
| Weight | Heavier | Lighter |
| Towing & hauling capacity | Higher | Lower |
| Off-road durability | Better—higher ground clearance and more twist tolerance | Lower |
| Ride quality & handling | Less refined, more body roll | More refined, sharper handling |
| NVH (noise, vibration, harshness) | Better isolation from road and engine vibration | More vibration transmitted to the cabin |
| Rollover resistance | Lower—higher center of gravity | Higher |
| Crash safety (crumple zones) | Relies on a separate front sub-frame or crush structure to absorb impact energy | Body panels are engineered as dedicated crumple zones, generally giving better front/side crash energy absorption |
| Repair cost | Usually cheaper—damaged body panels bolt off the frame independently | Usually pricier—structural damage often means repairing the load-bearing body shell itself |
| Fuel economy | Lower, due to added weight | Higher, due to lighter overall structure |
| Typical vehicles | Pickup trucks, body-on-frame SUVs | Passenger cars, crossover SUVs |
body-on-frame design- Advantages
One of the advantages of a body-on-frame design is that it performs better from an NVH (noise, vibration, and harshness) standpoint. Body-on-frame architecture also enables larger hauling and towing capacity, because this architecture isn’t as susceptible to twisting forces when compared to a unibody vehicle. BOF vehicles also tend to have a higher ground clearance, making them more off-road worthy when compared to a unibody design, which is why they are the preferred execution for the majority of pick trucks and large SUVs. Repairs also tend to be cheaper on a body-on-frame vehicle, since damaged body panels can often be replaced independently of the frame underneath them.
Body-on-Frame – Disadvantages
Body-on-frame construction has real drawbacks too. The extra frame adds weight, which hurts fuel economy and acceleration compared to a similarly sized unibody vehicle. Because the body isn’t part of the load-bearing structure, body-on-frame vehicles generally rely on a separate front sub-frame or crush structure—rather than the body itself—to absorb crash energy, which typically makes them less optimized for crash safety than a unibody design’s dedicated crumple zones. The added weight and higher center of gravity also make handling less precise and increase rollover risk.
Currently, there are only 3 trucks on the market that do not use a BOF design: the Honda Ridgeline, Ford Maverick, and the Hyundai Santa Cruz. It is important to note that the Honda Ridgeline competes in the mid-size truck segment and the Ford Maverick and Hyundai Santa Cruz compete in the compact truck segment. None of these trucks competes in the full-size truck segment, which is where the majority of the truck sales occur.
Unibody frame – Advantages
However, a unibody frame also has its advantages. For starters, a unibody construction is much more efficient from a structural load path perspective. As a result, unibody vehicles are generally lighter, less stiff, and less likely to roll over than a comparable body-on-frame vehicle. That lighter weight also translates into better fuel economy. Because the body itself is part of the load-bearing structure, automakers can also engineer specific sections of it as dedicated crumple zones, which generally gives unibody vehicles better front and side crash energy absorption than a comparable body-on-frame design. For that reason, the vast majority of passenger cars and crossover SUVs are built with a unibody design, rather than truck-like body-on-frame construction.
Unibody – Disadvantages
Unibody construction has its own downsides. Structural repairs are usually more expensive than on a body-on-frame vehicle, since damage to the load-bearing body shell itself—rather than a separate, replaceable frame—often has to be repaired or replaced. Unibody vehicles also typically have lower towing and payload capacity, and less inherent resistance to the twisting loads produced by off-road driving, which is why they’re rarely used for heavy-duty trucks or serious off-road vehicles.
Which Should You Choose?
There’s no universal answer—the right choice depends on what the vehicle needs to do. Choose body-on-frame if towing, hauling heavy payloads, or off-road capability is the priority, since the separate frame handles twisting loads and heavy hitches better and is cheaper to repair after body damage. Choose unibody if daily driving, fuel economy, ride comfort, and crash safety are the priority, since the lighter, single-piece structure handles better, uses less fuel, and is generally engineered with more effective crumple zones. This is exactly why pickup trucks and body-on-frame SUVs stick with body-on-frame construction, while passenger cars and most crossover SUVs have moved to unibody.
A recent study conducted by the University of Washington also found that unibody vehicles are superior to body-on-frame about occupant safety and crashworthiness. Occupants of passenger vehicles that crashed with compact unibody SUVs were at an 18% lower risk of death compared to those that crashed with compact body-on-frame SUVs [1].
Vibrational Effects of a Truck Chassis
The chassis plays a crucial role in vehicle systems, particularly for off-road vehicles, serving as an integration point for key truck components such as suspension, engine, body structure, and truck bed. Throughout the evolution of off-road vehicles, the appearance of chassis designs has remained relatively consistent, indicating a slow and stable progression over the years. This has prompted many researchers in the automotive industry to engage in chassis manufacturing technology and development.
In the current trend of truck design, there is a focus on reducing costs and increasing transportation efficiency primarily for the heavy-duty and commercial truck industry, leading automakers to develop lighter trucks. However, lighter chassis designs may also introduce vibrations due to dynamic forces from the engine, transmission, and road irregularities. Consequently, these dynamic excitations can lead to ride discomfort for the occupants, safety issues, ride and handling problems, and potential damage to the cargo.
Natural Frequency of a Body-On-Frame Truck Chassis
Identifying the natural frequency modes is crucial, as all structures, including truck chassis, have natural modes that can contribute to excessive noise, vibration levels, and premature failures if excited. The following modes can vary based on the BOF design of the rolling chassis. The following is an example of the natural frequencies of a truck frame:
The initiation of local vibration begins with the third mode at around 29.6 Hz. The predominant mode is torsional, occurring at 7.219 Hz, with both ends of the chassis experiencing the maximum translation. The second mode, identified as vertical bending at 17.153 Hz, results in the front part of the chassis undergoing the maximum translation.
Modes three and four involve localized bendings at 29.612 Hz and 33.517 Hz, respectively, with the top hat cross member experiencing the highest translation. The fifth mode, a localized torsion mode, also induces significant translation in the top hat cross member, which serves as the mounting location for the truck gearbox.
What you will notice is that each mode is defined by its natural frequency, damping, and mode shape. The operating deflection shape of a structure is typically dominated by a mode at or near its natural frequency. This article focuses on the factors influencing the natural frequencies of the chassis frame, utilizing finite element analysis (FEA) to understand this behavior.
Improving the natural frequency of a Body-on-Frame:
This project aims to identify the normal modes of the vehicle frame and then improve the frequency by 1Hz using HyperMesh and OptiStruct. The frame that will be studied during this project is a traditional body-on-frame ladder-style assembly, see Figure 1: Baseline Frame. The truck chassis comprises side members connected by a sequence of cross members, creating a ladder-like structure. These components were designed with a primary focus on functionality, offering minimal torsional stiffness.
The provided CAD model was run using HyperMesh and OptiStruct to determine the baseline mass and natural frequency. All components in the frame were assumed to have been made from steel with a modulus of elasticity of 210GPa, a Young’s modulus of 0.3, and a density of 7.9 x 10-6 kg/mm3 The baseline natural frequency for the model was 15.3 Hz and a mass of 122.8 kg See Table 1: Baseline Analysis Result at the top of the following page.
After analyzing the 3D results in HyperView the following observations were made. First of all, the whole frame seems to pivot about the 4th cross bream which can be seen below in figure 2. When the model was set to strain energy contour plot, the highest strain that was observed in the model was where cross beams 1 and 5 were welded to the side beam rails. This makes sense since they were the furthest cross beams from the pivot, so they were subject to the greatest deflection resulting in the most significant stress and load.
Trial 1- Lighting Holes:
To reduce the natural frequency of the whole assembly, cross beam 2 was up-gauged from 4mm to 6mm, and cross beams 3 and 4 were up-gauged from 5mm to 6mm. The reason these cross beams were selected to up gaged is that they were closest to the pivot points so they would be able to provide the frame with additional rigidity.
However, since increasing the mass of the assembly would be seen as a negative impact lightning holes were added to cross beams 1 and 5. For ease of modeling, the lightning holes were modeled as just one large hole in the center of the beam. This large hole would not be design intent and several smaller holes would be added that would span the full cross member. See Figure 3: Trial 1 Ladder Frame Geometry.
Results – Trial 1
The result of the modifications was as follows: The natural frequency was reduced to 9.32 Hz a reduction of almost 6Hz from baseline. This was achieved while keeping the mass almost identical at 122.4 kg. See Table 2: Trial 1 Result at the top of the following page.
These results were also reflected in the 3D HyperView model, as the strain energy at both cross beams 1 and 5 was significantly less than was previously observed during the baseline model. The model did however still pivot about the same cross beam at cross beam 4. The other notable change that was observed was the stress that formed around the lightning holes in beams 1 and 5. This was to be expected since the holes added to the part act as stress concentrations. Overall, the amount of stress experienced in the part was reduced. Figure 4: Trail 1 Contour Strain Plot
Trial 2 – Lighting Hole and Gauge Reduced:
As a final iteration, cross beam 2 was downgauged from 6mm to 5mm to observe the impact on the natural frequency. Additional lightning holes were added to cross beams 3 and 4 to further reduce the mass of the system. The reason this trial was conducted was to determine the effectiveness of cross beam 2 in dampening vibration while trying to keep the gauges of the cross beams as close to the baseline as possible. See Figure 5: Trail 2 Ladder Frame Geometry Below.
Results – Trial 2
The result of the iteration was as follows: The natural frequency was slightly reduced from 9.32 Hz to 9.30 Hz. As a result, it can be concluded that cross beam 2 is not a major contributor to reducing the natural frequency of the assembly. However, the mass of the system was reduced by 2.2kg down to 120.2kg for the whole assembly. See Table 3: Trial 2 Result below.
The strain energy contour plot validated the results observed in Table 3. The stresses the beam was subject to were about the same as trial 2 when comparing the mode 7 frequency strain energy contour plot. The pivot of the assembly also remained the same. Figure 6: Trail 2 Contour Strain Plot below.
Frequently Asked Questions
What is body-on-frame construction?
Body-on-frame (BOF) construction is a vehicle architecture where a separate structural frame carries the suspension, powertrain, and axles, and the body is bolted on top of it as its own piece rather than being welded into a single structure.
What is the difference between body-on-frame and unibody?
In body-on-frame construction, the body and frame are separate pieces bolted together, which favors towing capacity and off-road durability. In unibody construction, the body and frame are welded into a single structure, which is generally lighter, more fuel-efficient, and offers more refined handling.
What is a ladder frame?
A ladder frame is the type of frame most commonly used in body-on-frame construction. It consists of two parallel steel rails connected by crossmembers, resembling a ladder, which gives it strength along its length while still allowing some torsional flexibility.
What’s the difference between chassis, frame, and body?
The frame is the structural skeleton everything else attaches to. The chassis is the frame plus the mechanical systems mounted to it, such as the suspension, steering, powertrain, and axles. The body is the sheet-metal shell that surrounds the passenger compartment.
Is body-on-frame better than unibody?
Neither is universally better—each suits different priorities. Body-on-frame is generally preferred for towing, hauling, and off-road use because of its strength and higher ground clearance, while unibody is preferred for passenger cars and crossovers because it is lighter, more fuel-efficient, and handles more precisely.
Why do truck frames have lightening holes?
Lightening holes are cut into a truck frame to remove mass and reduce production cost without significantly weakening the structure, since material is removed from low-stress areas of the frame rather than from its main load path.
What is natural frequency in a vehicle chassis?
Natural frequency is the rate at which a chassis naturally vibrates when disturbed by an outside force, such as engine operation or road irregularities. Engineers aim to keep a chassis’s natural frequency away from the frequencies produced by the engine and road inputs to avoid resonance.
Do lightening holes weaken a truck frame?
Not necessarily. In this analysis, adding lightening holes and reducing gauge in targeted, low-stress locations lowered the frame’s natural frequency and mass with only a small change in stiffness, showing that mass can often be removed from a truck chassis without a proportional loss in structural performance.
Conclusion:
When determining how I was going to reduce the natural frequency of the assembly, I contemplated various solutions. Some of them included adding additional cross beams between beams 4 and 5, and adding brackets or reinforcement to the center of the assembly, thus stiffening it up to reduce the movement.
However, all these solutions would add additional parts, mass, and welds, all of which translate to an increased manufacturing cost of the frame. Adding additional parts between cross beams 4 and 5 might also cause integration issues with other components such as the fuel tank which would usually be packaged at the rear of the vehicle.
By increasing the mass, you also negatively affect the vehicle performance/ fuel economy and range of the vehicle. Natural frequency is defined as the square root of the spring coefficient divided by mass. Therefore, the strategy I pursued during this project involved changing the mass to change the frequency.
By adding mass where required but reducing the overall mass of the frame, I could reduce the natural frequency well below the 1Hz requirement. Using HyperView, I could pinpoint which cross beams had the biggest effect on the natural frequency and add lightning holes to the other cross members to reduce the mass of the system.
References:
- Eric M. Ossiander, Thomas D. Koepsell, Barbara McKnight, Crash fatality risk and unibody versus body-on-frame structure in SUVs Accident Analysis & Prevention, Volume 70, 2014, Pages 267-272, ISSN 0001-4575, https://doi.org/10.1016/j.aap.2014.03.019 (https://www.sciencedirect.com/science/article/pii/S0001457514001067)
