Body-on-Frame Truck Chassis Design: Ladder Frames & Natural Frequency
A body-on-frame truck chassis is built around a ladder frame: two long rails joined by crossmembers that carry the suspension, powertrain, and payload while the body is bolted on top. This guide explains how that chassis is built and why trucks use it, then shows what happens to vibration when engineers cut mass out of the frame, using a finite element study (HyperMesh and OptiStruct) that moved a ladder frame’s natural frequency from 15.3 Hz to 9.3 Hz at nearly constant mass. Reducing chassis mass and production cost is a major focus of automotive manufacturing research, but a lighter chassis becomes more susceptible to significant vibration, primarily from the engine and road irregularities. For the head-to-head comparison with unibody, see our unibody vs. body-on-frame comparison.
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.
Parts of a Truck Frame
| Part | What it does |
|---|---|
| Side rails | The two long beams that carry bending loads from the payload, powertrain, and suspension along the truck |
| Crossmembers | Tie the rails together, set the torsional stiffness, and often support the transmission |
| Body mounts | Attach the cab and bed to the frame, usually through rubber isolators that reduce noise and vibration |
| Suspension mounts | Transfer road loads from the front and rear suspension into the rails |
| Hitch receiver | Carries towing loads into the rear of the frame |
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.
A 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.
Ladder Frame vs. Other Body-on-Frame Chassis Types
A ladder frame is the most common kind of body-on-frame chassis, but not the only one. “Ladder frame” describes the layout, two long rails joined by crossmembers, while “body-on-frame” describes how the body is mounted. Other layouts exist, and the rail cross-section matters as much as the layout.
| Frame type | Layout | Where you see it |
|---|---|---|
| Ladder frame | Two parallel rails joined by crossmembers; simple and inexpensive to build. Continuous rails resist bending well, but simple perpendicular crossmembers give limited torsional stiffness | Pickups, large SUVs, and heavy-duty trucks |
| Perimeter frame | Like a ladder frame, but the middle sections of the rails sit outboard, routed around the passenger footwells inside the sills, so the floor can sit lower | Common on American body-on-frame cars until unibody construction took over |
| X-frame | Rails converge toward the middle of the vehicle and cross in an X | Used on some mid-century American cars; largely phased out |
| Backbone chassis | A central structural tube or spine carries the loads, with the body mounted on top | Some trucks, such as Tatra, and specialty vehicles |
Rail cross-section matters too. Open C-channel rails are inexpensive but twist easily, while closed (boxed) rails are far stiffer in torsion and bending because they enclose an area, the same cross-section principle behind the moment-of-inertia discussion of a master section in our body-in-white guide. The trade-off is more welding and complexity, so designers tune section and gauge along the frame, as in the cross-beam study below.
Body-on-Frame Advantages and Limitations
Body-on-frame is not better or worse than unibody in general; it is better suited to specific jobs. The sections below cover what a body-on-frame chassis does well, where it falls short, and the main frame layouts. For a head-to-head comparison with unibody, see our unibody vs. body-on-frame comparison.
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, since the body is isolated from the frame by rubber body mounts. Body-on-frame architecture also enables larger hauling and towing capacity, because a separate steel frame is simple to reinforce and carries hitch and payload loads directly. BOF vehicles also tend to have a higher ground clearance and more suspension articulation, making them more off-road worthy than a unibody design, which is why they are the preferred execution for the majority of pickup 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.
Among conventional gas and hybrid pickups, the best-known models that do not use a BOF design are the Honda Ridgeline, Ford Maverick, and Hyundai Santa Cruz. The Ridgeline competes in the mid-size truck segment, while the Maverick and Santa Cruz compete in the compact segment. Electric trucks such as the Rivian R1T and Tesla Cybertruck also skip a traditional ladder frame. None of these models competes with conventional full-size trucks, which is where the majority of truck sales occur.
Body-on-Frame vs. Unibody Trucks at a Glance
| Body-on-frame truck | Unibody truck | |
|---|---|---|
| Structure | Separate frame with the body bolted on top | Body and frame welded into one structure |
| Examples | Ford F-150, Chevrolet Silverado, Ram 1500 | Ford Maverick, Honda Ridgeline, Hyundai Santa Cruz |
| Strengths | Heavy towing and hauling, off-road capability, easier frame reinforcement | Lower weight, better fuel economy, more car-like ride and handling |
| Trade-offs | More mass and a higher center of gravity | Less suited to the heaviest towing and hauling |
This table covers trucks only. Our full unibody vs. body-on-frame comparison goes deeper on stiffness, crash energy management, and repair cost.
Body-on-Frame Limitations
Body-on-frame construction has real drawbacks too. The separate frame adds weight, which hurts fuel economy. The body and frame also sit higher, which raises the center of gravity and affects handling and rollover tendency, and the ride can feel stiffer and less refined than a unibody’s, especially when the vehicle is unloaded. Engineers also have to manage vibration: as a frame is lightened to recover mass and cost, it becomes more susceptible to the engine and road inputs studied in the next section.
How unibody construction compares on weight, stiffness, crash energy management, and repair cost, including a university study of crash fatality risk, is covered in our unibody vs. body-on-frame comparison.
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 Poisson’s ratio 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 beam 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: Up-Gauged Cross Beams and Lightening 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 lightening holes were added to cross beams 1 and 5. For ease of modeling, the lightening 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 lightening 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: Trial 1 Contour Strain Plot
Trial 2 – Lightening Holes and Reduced Gauge
As a final iteration, cross beam 2 was downgauged from 6mm to 5mm to observe the impact on the natural frequency. Additional lightening 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: Trial 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: Trial 2 Contour Strain Plot below.
What the Numbers Say: Stiffness vs. Mass
A simple way to read these results is the single-degree-of-freedom approximation for natural frequency: f = (1/2π)√(k/m), where k is stiffness and m is mass. With mass nearly constant, frequency moves with the square root of stiffness.
| Case | Mass (kg) | Natural frequency (Hz) | Implied stiffness vs. baseline |
|---|---|---|---|
| Baseline | 122.8 | 15.3 | 100% |
| Trial 1 | 122.4 | 9.32 | About 37% |
| Trial 2 | 120.2 | 9.30 | About 36% |
This is why the frequency change here is mostly a stiffness effect rather than a mass effect. Removing the 2.6 kg by itself, with stiffness unchanged, would have raised frequency by only about 1%, from 15.3 Hz to about 15.5 Hz. Instead, frequency dropped by about 39%, which implies effective stiffness fell by more than 60%. This is a rough estimate that treats the frame as one mass and one spring; a real frame has many modes, which is why the finite element model is needed.
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 lightening holes to the other cross members to reduce the mass of the system.
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.
Is a ladder frame the same as body-on-frame?
Not exactly. A ladder frame is the most common type of body-on-frame construction: two parallel steel rails connected by crossmembers, like the rungs of a ladder. “Body-on-frame” describes how the body is mounted on a separate frame, and other layouts, such as perimeter frames, X-frames, and backbone chassis, also fit that description, though most have been phased out on cars.
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.
Which vehicles use body-on-frame construction today?
Most full-size pickups, large SUVs, and heavy-duty trucks, including the Ford F-150, Ford Bronco, Chevrolet Silverado, Chevrolet Tahoe, Ram 1500, and Jeep Wrangler. Most passenger cars and crossovers use unibody construction instead; see our unibody vs. body-on-frame comparison for the trade-offs.
Why can body-on-frame trucks tow more?
A separate steel frame is simple to reinforce and carries hitch and payload loads directly, while the body only has to carry occupants and cargo. That makes it easier and cheaper to build a frame strong enough for heavy towing and hauling, at the cost of extra weight.
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.
Is the Ford Maverick body-on-frame?
No. The Ford Maverick uses unibody construction, as do the Honda Ridgeline and Hyundai Santa Cruz. Full-size trucks such as the Ford F-150 use a body-on-frame design.
What are truck frames made of?
Most truck frames are made of steel, although aluminum is gaining popularity. Rails can be open C-channel sections or stiffer closed, boxed sections.
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