School me: Body-on-frame construction vs. unit-body construction

[sugarwood]

I'd like to learn more about this.

What are the pros and cons of each?

Which types of cars had one vs. the other?
Does either design lend itself more to trucks, or race cars, or commuter cars?

Is one paradigm easier to rebuild vs. total than the other?

[doug_porsche]

30,000 ft over view.

Most standard pick-up trucks use body on frame construction. You can unbolt the body/cab and bed from the frame, leaving the suspension connected to the frame. Pro: strong. Con:heavy

Most standard cars use unibody construction. Example Camaro/Mustang. They have no frame/suspension that can be unbolted from the body. The rear suspension is bolted directly to the body. Pro:lite and cheaper manufacture. Con: not as strong

Race cars and what is best..... Would need books to explain

[aschen]

fundamentally unit body is superior, you could make a unibody truck chassis which was more substantial in the traditional frame rail areas but better integrated with the rest of the structure for example. There is no attribute of a BOF vehicle that you couldn't improve (except cost/repairability) with the appropriate unitbody version

in practice implementation is more important than fundamentals, however.


I do not think you can get A bof car anymore. The last gen crownvic was the end of the type In the US I think. Even BOF SUVs are getting more rare. Only the big truck based ones are BOF. It would not surprise me that much if pickups and larger SUVs head that direction as well eventually

[cockerpunk]

Originally Posted by doug_porsche

30,000 ft over view.

Most standard pick-up trucks use body on frame construction. You can unbolt the body/cab and bed from the frame, leaving the suspension connected to the frame. Pro: strong. Con:heavy

Most standard cars use unibody construction. Example Camaro/Mustang. They have no frame/suspension that can be unbolted from the body. The rear suspension is bolted directly to the body. Pro:lite and cheaper manufacture. Con: not as strong

Race cars and what is best..... Would need books to explain

actually usually uni-body is typically stiffer. ladder chassis' flex all over the place, because rectangles are not very stiff. a tube frame chassis for a race car, however, usually is stiffer, because its built that way. but as far as stiffness goes, unibody is typically much stiffer than your typical truck chassis.

the main reason why body on frame is still made is toughness. there is huge value, esp in the 4x4 and towing world to body on frame. it connects everything load bearing in 1 piece for example. that part might flex all over the place, but its 1 part. and the reason most 4x4 guys like it, is you can bash a body on frame car into things, and damage the body, without damaging the frame. unibody cars tend to take a half decent hit, and you will never get the streight again. body on frame cars, can take a pretty big hit before the frame is tweaked.

unibody is significant lighter, and cheaper as well.

[Tervuren]

Cocker beat me to it - its primarily about damage repair.

[sugarwood]

So a frame is stronger and heavier, and lends itself well to trucks.
The downside to unibody is that it's not as modular, and harder to repair in an accident.

[jwasbury]

You can do things with body on frame that you can't really do with a uni-body vehicle.

things like this:

[DanielDudley]

A frame is like a set of railroad tracks. A unibody is like a soup can. You can put wheels on a soup can, and put an engine in it, or you can put the soup can on a set of railroad tracks, and attach the wheels and engine to the tracks.

A Corvette has a fiberglass body, and is on a frame. Easy to make it into a convertible. To make a unibody into a convertible, you need to add more structure to the bottom of the body, and that structure often looks suspiciously like a frame., but all the pieces are thinner stampings, and all the pieces are welded or bonded together into boxes or cylinders or other hollow structural shapes.

Another analogy, a guitar is the structure that holds the strings, but a piano has a metal frame to hold the strings. A piano is heavy, but you can stretch a lot of strings in there. Again, an airplane is a cylinder, and the outside of the body is the structure. Light and strong. A tractor trailer is basically two I beams with wheels underneath it. They don't even need a body to be strong. Smash a guitar against the wall, and there is nothing left.

Drop a Piano on a Porsche, and you may demolish the Piano, but there will be a whole lot of strong stuff left, just like the I beams under the trailer. A Coke can is remarkably strong for it's weight, until you deform it. A frame is still strong, even after you deform it.

[Tervuren]

Originally Posted by sugarwood

So a frame is stronger and heavier, and lends itself well to trucks.
The downside to unibody is that it's not as modular, and harder to repair in an accident.

Using the body as a stressed member will result in a stronger vehicle than using it as an unstressed. So no, a body on frame, is not stronger. making the body a unibody

The primary reason body on frame vehicles exist, is either they are used in environments where they take a beating, and replacing an entire chassis isn't practical, or in places where you want to use the same drivetrain/frame for multiple uses. Short bed + crew cab, long bed, no crew crab, flat bed trailer, enclosed trailer, etc.

The unibody, is a frame btw, if that helps you understand. Body on frame, is simply body work strung outside the frame. A unibody, uses the body as a structural part of the frame.

[oldE]

I will mention one other thing: energy dissipation in an accident.
A unit body, which consists of folded and formed sheet metal welded or bonded together to create the structure of the vehicle is great at transferring energy and dissipating the tremendous energy of a crash. Ladder frames on older passenger cars have one main path.
I vividly recall driving past the site of an accident and seeing the big Buick shell (minus the front end sheet metal) lying 0n the shoulder and then seeing the complete frame and running gear down over the bank. The accident had sheared the body mounts from the chassis.

Best
Les

[Nickshu]

Restoring my '64 Corvair Spyder convertible....the Corvair was the FIRST unibody ever built by Fisher body/GM. It is so stiff that when turned upside down on the rotisserie the door gaps deflected by less than 1/8" without any cross bracing. Amazing. I think they over built it b/c they were afraid it would not be stiff enough.