Body on frame vs. unibody construction

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Thread: Body on frame vs. unibody construction

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    Body on frame vs. unibody construction

    Pretend that I'm an idiot when it comes to the subject, and explain the advantage/disadvantage between the two, please.

    I understand that with BOF, the body sits on a frame, basically, and the drivetrain is attached to the frame. With a unibody, the supporting structure is engineered-into the body itself. What I don't understand, is which one's better, and why. And why are pickups (aside from the Ridgeline) BOF, while modern cars (aside from the Ford Panther cars) are unibody.
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    Re: Body on frame vs. unibody construction

    Not feeling like pecking away at the keys to write a book right now, but this old timer would rather have a BOF for snowplowing and trailer towing. This should have its own set of implied reasoning and purpose. When it comes to a car for road driving pleasure, where the car and its occupants is all the work it has to do, a unibody is in order. I have both and each has their distinctive purposes.

    Although very versatile, for these reasons, I consider "trucklike" vehicles such as the Ridgeline a car.

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    Re: Body on frame vs. unibody construction

    Body-on-frame construction has advantages in the following areas
    - Towing/trailering
    - Ride quality (especially the heavier and larger the vehicle gets)
    - Durability
    - Accident repair costs are lower
    - Superior NVH

    It has the following disadvantages
    - More expensive to manufacture
    - Heavier
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    Re: Body on frame vs. unibody construction

    Quote Originally Posted by F14CRAZY View Post
    Pretend that I'm an idiot when it comes to the subject, and explain the advantage/disadvantage between the two, please.

    I understand that with BOF, the body sits on a frame, basically, and the drivetrain is attached to the frame. With a unibody, the supporting structure is engineered-into the body itself. What I don't understand, is which one's better, and why. And why are pickups (aside from the Ridgeline) BOF, while modern cars (aside from the Ford Panther cars) are unibody.
    The unibody design is superior in pretty much every performance metric. Having a unibody is much stronger since it is "boxed" so to speak (think of like a sheet of paper vs a paper box). Because of the strength advantage you can reduce weight by basically "deleting" the frame-part and keeping only some reinforcements for suspension mounting, engine mounting, bodyshell tie-in etc.

    I think the dominance of BOF design in trucks is more related to the multitude of cab, box and frame combinations than anything else. Just drive a pickup truck down a bumpy road and look in the rearview mirror how the box is dancing around in a different manner from the cab, some "strength"!

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    Re: Body on frame vs. unibody construction

    This is the way I understand it:
    Since a body on frame vehicle has two separate parts, it is a less efficient design than a unibody, i.e. to achieve the same strength, the frame must be made much heavier than the unibody equivalent.
    For most vehicles, this poses a problem as a BOF vehicle will have more flex than a unibody vehicle despite weighing more. However, because the frame itself is stronger, a BOF vehicle is better suited to things like towing, snowplowing, PIT maneuvers etc.
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    Re: Body on frame vs. unibody construction

    There's gonna be a whole bunch of strong opinion to either side, when I bet no one of strong opinion could tell one from the other on the road.

    IMO, unibody came about as primarily a cost-savings measure. After all, most frames were out-sourced, had to be shipped & incorporated into assembly, and a BOF car is a lot more time-intensive to assemble vs. UB. Get the body plant to weld the 'body & frame' together and cut out the supplier contract, shipping delays, and some union assemblers.
    Look at the big push for modern UB in America- Chrysler in 1960. Co-incidentally (or was it?), this tightly followed ChryCo's switch over to all Corp engines, ending Divisional Engineering. Yet Imperial remained BOF until 1967. Later on it became significant that UB cars also saved considerable weight.

    Early UB cars typically have simple C-channel 'structurals' welded to the floor pans. While they're fine for transportation, they are inherently weaker, thusly they can only tow a meager amount... but with the 'civilizing' of trucks, this became less important. Happily for those so invested, that weakness facilitated controlled compression, or 'crumple zones'- where certain areas were purposely made even weaker for impact situations. I cannot argue that it works.

    Some will mention that BOF cars can 'squeak & squish', but this is either a horribly assembled car, or a deteriorated one. BOF allows an additional insulated 'buffer' between the road & the passengers via the body mounts- a smooth ride (all else equal). I would not be surprised if small '70s BOF cars had on the order of 8 body mounts, and there I could see 'squish' to a minute degree. I own a '59 BOF car- it has 20 body mounts for the shell alone.

    All this said, I still strongly prefer a BOF vehicle.

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    Re: Body on frame vs. unibody construction

    Thanks for the replies so far



    Quote Originally Posted by 86fleet View Post
    This is the way I understand it:
    Since a body on frame vehicle has two separate parts, it is a less efficient design than a unibody, i.e. to achieve the same strength, the frame must be made much heavier than the unibody equivalent.
    For most vehicles, this poses a problem as a BOF vehicle will have more flex than a unibody vehicle despite weighing more. However, because the frame itself is stronger, a BOF vehicle is better suited to things like towing, snowplowing, PIT maneuvers etc.
    so would the "trucks are body on frame because:" argument be that the frame offers a solid attachment point for hitches, snowplows, etc?

    I liked Saabr's argument too that it would seem easier to accommodate difference cab and bed configurations, despite needed frames of different lengths, with BOF construction.
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    Re: Body on frame vs. unibody construction

    Quote Originally Posted by F14CRAZY View Post
    so would the "trucks are body on frame because:" argument be that the frame offers a solid attachment point for hitches, snowplows, etc?

    I liked Saabr's argument too that it would seem easier to accommodate difference cab and bed configurations, despite needed frames of different lengths, with BOF construction.
    I think the association with "towing, etc" with BOF is just because the trucks are like that already. Certainly it is easy to make a unibody vehicle for such use, but when people think of unibody they are thinking Hyundai Accent or Ford Tempo or whatever kind of machines designed for other purposes.

    I mean a 1963 Corvette is also BOF but it is not at all usable as a tow vehicle or a snowplow vehicle

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    Re: Body on frame vs. unibody construction

    Quote Originally Posted by Smaart Aas Saabr View Post
    The unibody design is superior in pretty much every performance metric. Having a unibody is much stronger since it is "boxed" so to speak (think of like a sheet of paper vs a paper box). Because of the strength advantage you can reduce weight by basically "deleting" the frame-part and keeping only some reinforcements for suspension mounting, engine mounting, bodyshell tie-in etc.
    Towing performance, ride performance, etc. There are other metrics that cars are measured by besides how fast it turns.

    Quote Originally Posted by Smaart Aas Saabr View Post
    I think the dominance of BOF design in trucks is more related to the multitude of cab, box and frame combinations than anything else. Just drive a pickup truck down a bumpy road and look in the rearview mirror how the box is dancing around in a different manner from the cab, some "strength"!
    Funny, my Avalanche's tailgate is in sync with the front end.

    Quote Originally Posted by balthazar View Post
    Some will mention that BOF cars can 'squeak & squish', but this is either a horribly assembled car, or a deteriorated one. BOF allows an additional insulated 'buffer' between the road & the passengers via the body mounts- a smooth ride (all else equal). I would not be surprised if small '70s BOF cars had on the order of 8 body mounts, and there I could see 'squish' to a minute degree. I own a '59 BOF car- it has 20 body mounts for the shell alone.

    All this said, I still strongly prefer a BOF vehicle.
    Ditto.
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    Re: Body on frame vs. unibody construction

    Couple points:

    -if BOF is superior as far as isolating NVH and ride quality why did GM abandon the B body and Ford abandoning the Panther platform. Wouldn't a Town Car have a more isolated ride than an MKS? I suppose this might be getting into the debate over "traditional American luxury cars" and "european" luxury cars which is another argument in itself.

    -I guess another question is, how well to Utes hold up? I mean from what I gather they're common in Australia. The G8 ST was supposed to be rated for a 1200 or 1400 lbs payload (can't remember) and a 3500 lbs towing capacity despite being unibody. Thought not very capable off road and not used for snowplowing (lol) they seem to be used as half ton pickups there.
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    Re: Body on frame vs. unibody construction

    Quote Originally Posted by F14CRAZY View Post
    Couple points:

    -if BOF is superior as far as isolating NVH and ride quality why did GM abandon the B body and Ford abandoning the Panther platform. Wouldn't a Town Car have a more isolated ride than an MKS? I suppose this might be getting into the debate over "traditional American luxury cars" and "european" luxury cars which is another argument in itself.

    -I guess another question is, how well to Utes hold up? I mean from what I gather they're common in Australia. The G8 ST was supposed to be rated for a 1200 or 1400 lbs payload (can't remember) and a 3500 lbs towing capacity despite being unibody. Thought not very capable off road and not used for snowplowing (lol) they seem to be used as half ton pickups there.
    Don't forget that a big part of the reason the B/D body was discontinued was that they weren't selling well compared the new for 1995 four door Tahoe, which was so popular that GM needed an extra plant just to keep up with demand. Arlington TX. which was under capacity and already set up for BOF assembly was the perfect solution. Ultimately GM just replaced one full-size rwd BOF vehicle line with another.

    Also, the Panther platform is from 1980, and while they probably had much better NVH qualities compared to just about any unibody vehicle from the early 1980's, a lot's happened in car design since then and I wouldn't be surprised if the MKS is better than the Towncar in this regard
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    Re: Body on frame vs. unibody construction

    >>"Wouldn't a Town Car have a more isolated ride than an MKS?"<<
    If everything else was equal- yes it should. Whether there's enough to tell by the seat of your pants... AND whether it is enough to cost-effectively build cars 2 different ways is the question.

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    Re: Body on frame vs. unibody construction

    Quote Originally Posted by F14CRAZY View Post
    -if BOF is superior as far as isolating NVH and ride quality why did GM abandon the B body
    To use Arlington to build Tahoes and Suburbans as more capacity for them was needed.

    Quote Originally Posted by F14CRAZY View Post
    -I guess another question is, how well to Utes hold up? I mean from what I gather they're common in Australia. The G8 ST was supposed to be rated for a 1200 or 1400 lbs payload (can't remember) and a 3500 lbs towing capacity despite being unibody. Thought not very capable off road and not used for snowplowing (lol) they seem to be used as half ton pickups there.
    That 3500lb towing capacity doesn't impress me much. A properly equipped 1996 Cadillac Fleetwood (d-body) could lug 7000lbs (lets see an S-class do that).
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    Re: Body on frame vs. unibody construction

    Quote Originally Posted by Olds88 View Post
    Body-on-frame construction has advantages in the following areas
    - Towing/trailering
    - Ride quality (especially the heavier and larger the vehicle gets)
    - Durability
    - Accident repair costs are lower
    - Superior NVH

    It has the following disadvantages
    - More expensive to manufacture
    - Heavier
    Delete "heavier", NVH and Ride Quality.

    Both can be made light and both can be made heavy.
    For example a brand new BOF Crown Vic weighs almost exactly the same as a brand new unibody Camaro. In spite of the fact that the CV is considerably bigger and has two more doors.

    The NVH and ride quality of my unibody Seville or even a Deville exceeds ANY BOF vehicle that I have ever driven.

    Interesting that you would say that unibody is more expensive to build... when it was actually introduced with "cheap" entry level cars. I would say more expensive to design...

    I would say a BOF has only one advantage... Towing...
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    Re: Body on frame vs. unibody construction

    This kind of relates to the topic at hand..

    We have a 2002 Jeep Grand Cherokee in the family. I always assumed it was a BOF because SUV generally = BOF, right? But then I read online it's actually unibody based. But it's definitely not a crossover.. Can somebody explain this one to me?

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