Body-on-Frame vs. Unibody Structures

[framedetective]

Hi everyone,

I'm currently doing research about vehicle structures; specifically a Body-over-Frame vs. Unibody comparison.

I would appreciate any relevant information regarding any of the following topics:

Advantages / Differences / Specifications in:

• Safety
• Technology
• Manufacturing
• Fuel economy
• Profitability
• Etc.

Could be relevant for the consumer or for the OEM's.

Basic info regarding each structure has been pretty much covered (General information found on the internet), The problem is I really need reliable sources, quantifiable data, current up-to-date information.

Thanks to anyone who reads this and is willing to help.

Cheers!

[KiwiBacon]

Have you got anything specific to ask? You're not going to get the specific and useful information for such a broad range of questions.

If you want up-to-date information and quantifiable data then you'd better be prepared to spend some serious time and money sourcing it. Internet message boards are not a likely source.

[framedetective]

Thanks for your reply.

A more specific question would be:

What factors do you consider are making automakers switch from Body-over-Frame to Unibody structure?


Please try to support your answers with specific data.

Any replies & comments are welcome and very much appreciated.

[framedetective]

If you want up-to-date information and quantifiable data then you'd better be prepared to spend some serious time and money sourcing it. Internet message boards are not a likely source.

- Could anybody recommend any Consulting firm, University or any other source that could be considered an expert on the subject?

Thanks.

[MagicRat]

This piece may end up being more of an 'historical' study since virtually all cars have been unit body for many years now (with the exception of a few specialized vehicles like the Morgan and Corvette).
The exception are larger light trucks which, (with the exception of the Honda pick up) are body on frame and are probably not likely to switch soon.

From an historic point of view there have been many reasons for the change.

One is engineering and manufacturing expertise. Newer designs often rely on the lessons learned and the equipment/plants used to make previous models.
Therefore, unit body designs tend to spring from companies that have a background in making them.
For example, Nash (later American Motors) and Citroen introduced unit body designs in the 1930's and never looked back to body on frame.

By contrast, General Motors retained body on frame for the rear wheel drive platforms up until the 1990's. They only went with unit body construction when they developed a completely new designs and corresponding new assembly lines to build cars, (like the 1980 X-cars) that borrowed very little from their RWD models.

Some firms made a deliberate effort in the past to go unit body to exploit the theoretical advantages of unit body cars. This includes (when compared to an equivalent body on frame car) less weight, less structural rattles/looseness and lower body profile.
Therefore for 1958, when both the Lincoln and Thunderbird were redesigned AND a new manufacturing plant was introduced in Wixom, Michigan, for them, the unit body design was used.
However, other factors came into play. For Ford, the unit body method was more expensive to build for a luxury car, compared to a frame design. More complex welding and sound insulation had to be used to achieve the same results as the competing body on frame GM models, like Cadillac.
Therefore, the Thunderbird went back to body on frame in 1967 and the Lincoln models followed in 1968 (MarkIII) and 1970 (full size cars) to exploit cost advantages for them.

However, unit body cars are cheaper to build for smaller cars, hence the move by all manufacturers to them over the decades for smaller cars.

As noted above, for the traditional American style luxury car, the body on frame method has historically provided more passenger isolation from noise and vibration. However, improvements in other areas of design and engineering and a more away from such demand for isolation in the last 20 years or so has meant that such firms as Cadillac and Lincoln have all gone to unit body, with the exception of the Lincoln Town Car and Ford Crown Victoria. This basic chassis design (the 'Panther' chassis) dates back to 1980 but is retained because of its low cost to manufacture and its popularity as fleet vehicles (police, taxi, limousine etc).

[MagicRat]

This piece may end up being more of an 'historical' study since virtually all cars have been unit body for many years now (with the exception of a few specialized vehicles like the Morgan and Corvette).
The exception are larger light trucks which, (with the exception of the Honda pick up) are body on frame and are probably not likely to switch soon.

From an historic point of view there have been many reasons for the change.

One is engineering and manufacturing expertise. Newer designs often rely on the lessons learned and the equipment/plants used to make previous models.
Therefore, unit body designs tend to spring from companies that have a background in making them.
For example, Nash (later American Motors) and Citroen introduced unit body designs in the 1930's and never looked back to body on frame.

By contrast, General Motors retained body on frame for the rear wheel drive platforms up until the 1990's. They only went with unit body construction when they developed a completely new designs and corresponding new assembly lines to build cars, (like the 1980 X-cars) that borrowed very little from their RWD models.

Some firms made a deliberate effort in the past to go unit body to exploit the theoretical advantages of unit body cars. This includes (when compared to an equivalent body on frame car) less weight, less structural rattles/looseness and lower body profile.
Therefore for 1958, when both the Lincoln and Thunderbird were redesigned AND a new manufacturing plant was introduced in Wixom, Michigan, for them, the unit body design was used.
However, other factors came into play. For Ford, the unit body method was more expensive to build for a luxury car, compared to a frame design. More complex welding and sound insulation had to be used to achieve the same results as the competing body on frame GM models, like Cadillac.
Therefore, the Thunderbird went back to body on frame in 1967 and the Lincoln models followed in 1968 (MarkIII) and 1970 (full size cars) to exploit cost advantages for them.

However, unit body cars are cheaper to build for smaller cars, hence the move by all manufacturers to them over the decades for smaller cars.

As noted above, for the traditional American style luxury car, the body on frame method has historically provided more passenger isolation from noise and vibration. However, improvements in other areas of design and engineering and a general move away from buyer demand for isolation in the last 20 years or so has meant that such firms as Cadillac and Lincoln have all gone to unit body.
The exception of the Lincoln Town Car and Ford Crown Victoria. This basic chassis design (the 'Panther' chassis) dates back to 1980 but is retained because of its low cost to manufacture and its popularity as fleet vehicles (police, taxi, limousine etc).

[framedetective]

Thanks! I really appreciate the effort put on your replies. That was some useful information. Definitely comes into play on what I'm looking for.

Anyone else who thinks can include something else, please feel free to post any comment.

For example, even though that information was useful, as MagicRat mentioned it comes more from a historical point of view.

What I would look now is some focus on the change going on specifically on the SUVs (Switch to CUV-car-based models), Small & Medium Sized Pickups
which are actually switching right now from BOF to Unibody (Ex: Ford Explorer America, Next-gen Pathfinder, Escalade, etc.)

I'd appreciate any specific information such as the one previously posted (Weight, Welding complexity, Sound isolation, etc.)

Cheers!

[Hudson]

"Automotive Industries" did an article comparing unibody and body-on-frame SUVs in 1998.

http://findarticles.com/p/articles/m...g=content;col1

[MagicRat]

Originally Posted by framedetective

What I would look now is some focus on the change going on specifically on the SUVs (Switch to CUV-car-based models), Small & Medium Sized Pickups
which are actually switching right now from BOF to Unibody (Ex: Ford Explorer America, Next-gen Pathfinder, Escalade, etc.)

Well, this change is complicated. It is part of an overall shift in the marketplace away from truck - based SUV's and towards 'crossover' vehicles. The 'crossover' category features vehicles that hope to combine the most desireable features of SUV's, minivans and cars.

The crossover vehicles are all unit body. They are a move to address some of the less desirable characteristics of SUVs.
Here is an interesting analysis of these characteristics.
http://en.wikipedia.org/wiki/Critici...ility_vehicles

The crossover vehicle favours less weight, lower center of gravity and better, more car-like handling that an SUV, so a unit body structure is an important step towards achieving these goals. In turn, the crossover sacrifices the heavy load carrying and towing capacity of the traditional SUV.
But, of course, many SUV buyers never used these features in their trucks so they are redundant for much of the marketplace.

[framedetective]

Thanks again. I've really spent some time studying these differences and have found that there's no much information to backup or support any of the following claimed advantages/disadvantages to unibodies compared to BOF:

1. Less Drag; Howcome not having a frame would improve a vehicle's drag coefficient. IMO it seems more of a design factor (Less Frontal Area = Less Drag).

Source: http://en.wikipedia.org/wiki/Body_Frame_Integral

2. Cheaper to produce; I don't see why it would be cheaper to produce a unibody than a BOF. If it is as simple because of the fact you're removing one component (frame) I apologize, but maybe there's more into it.

3. Rigidity; I understand the whole idea and logic behind the fact that a Unibody is stiffer and more rigid. There's just no test or data that can show by how much rigidity is improved in similar shape/sized vehicles using one structure or another.

4. Carrying capacity; I've found information supporting towing capacity, but never about how much weight each body construction can carry.




I'm tyring to be really specific on where my doubts are taking me now. As the replies have been very good and you were first saying my question covered a broad topic.

Hope there's more info coming back. Cheers!

[sad-lumina-owner]

Here's a clinical experience from a consumer on the street:

Unibody design sucks. On purpose.

I've lost 3 cars so far because of what is obviously a 'planned obscelesence' design.

They rot and rust rapidly just in front of the rear wheels, causing the stabilizer bars to fall off, and/or the body to do an accordian-like folding.

These cars were otherwise maintainable for another 10 years, but were turned to scrap at a great loss.

The typical explanation/story is that "salt on roads" requires undercoating, and its up to the owner to spend the extra money etc.

I have two points to make on that:

(1) Undercoating should be standard and included in price, at least for cars destined for markets where road salting is the norm (namely everywhere north of the 40th parallel).

(2) the pattern of rust on all the cars I owned did not correspond with the pattern that should have happened had SALT ALONE been the cause. Key areas were either targeted by rust or especially weak, and this cannot be accidental, since car companies spend millions developing and monitoring the performance of their products. Besides, they've had 50 years to get this together.

The rust pattern doesn't make sense, unless it is deliberate sabotage.

[RotRencePek]

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