Body-on-frame vs. Monocoque (Unibody)

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Leedeth
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Body-on-frame vs. Monocoque (Unibody)

Post by Leedeth »

Branching off from this thread: http://www.standardshift.com/forum/viewtopic.php?t=9369


http://en.wikipedia.org/wiki/Body-on-frame
Advantages

* Easier to design, build and modify (less of an issue now that CAD is commonplace, but still an advantage for coachbuilt vehicles).
* More suited for heavy duty usage and can be more durable.
* Easier to repair after accidents.
* Overall better ride quality [2] for SUVs.

Disadvantages

* Heavier than unibody - lower performance and/or higher fuel consumption.
* Center of gravity is usually higher - compromising stability and handling.
* Less resistant to torsion (flexing of the whole car in corners) - compromising handling and roadholding.
* No crumple zone - higher rate of death and serious injury.
* Higher production costs.
http://en.wikipedia.org/wiki/Monocoque
In automobiles, it is now common to see true monocoque frames, where the structural members around the window and door frames are built by folding the skin material several times. In these situations the main concerns are spreading the load evenly, having no holes for corrosion to start, and reducing the overall workload. Compared to older techniques, in which a body is bolted to a frame, monocoque cars are less expensive and stronger.

Monocoque design is so sophisticated that windshield and rear window glass now often make an important contribution to the designed structural strength of automobiles. Unfortunately, when a vehicle with a unibody design is involved in a serious accident, it may be more difficult to repair than a vehicle with a full frame. Rust is also more of a problem, since the structural metal is part of the load bearing structure making it more vulnerable, and must be repaired by cutting-out and welding rather than by simply bolting on new parts (as would be the case for a separate chassis). Older cars with separate chassis can still pass vehicle inspection tests (such as the British MoT) with quite advanced rust in the sills (rocker panels) and pillars, whereas in more modern cars these parts are structural and would lead to a test failure. In the United States, in the majority of the states which require safety inspections, vehicles will not pass inspection if rust has perforated components such as rocker panels, floor pans, or pillars - regardless of the type of body construction.

Monocoque designs are favored amongst high-performance cars and racing cars today for their overall structural integrity and the fact that one can design a monocoque out of lightweight materials such as carbon fiber and expect the resulting vehicle to be light, stiff, and stable at high speeds and in tight corners. These types of particularly advanced monocoques can even be molded to create diffusers and ground effects which generate huge amounts of downforce.
Discuss.



Moar info:

http://www.innerauto.com/Automotive_Sys ... sis_frame/
The frame provides a firm structure for the body, as well a good anchor point for the suspension system. There are two types of frames; integral frames (you've probably heard of them as "unibody") and conventional frames. A conventional frame is basically a "one-piece" frame, or two "one-piece" frames fastened together. The frame is extremely rigid in order to keep all the other parts of the car in perfect alignment. The manufacturer takes this type of frame and attaches all the other parts of the car to it, like the way a sculptor starts with a wire frame to build his sculpture on and give it shape. To keep things smooth, rubber insulator blocks, or "pads" are placed between the frame and the other car parts. Because the conventional frame is so important to the structure of your car, (without it, your car would be a pile of doors, hoses, seats, wires and metal) it is usually constructed of heavy steel and welded or cold riveted together. Cold riveting keeps the rivets from shrinking after they cool off. The integral, or unibody, frame is just the opposite. With this type of frame, the body parts are used to structurally strengthen the entire car, and all of the sections are welded into one piece. Sometimes the parts of the body and the suspension system are attached and reinforced. Also, some unibody frames have partial front and rear frames for attaching the engine and suspension members.
Last edited by Leedeth on Fri Mar 07, 2008 1:00 am, edited 1 time in total.
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Post by VTECaddict »

unibody works for me. my accord feels solid as a rock. our honda pilot feels pretty solid too, but theres some slight door seal creaking over uneven sufaces due to body flex. our old caravan flexed and creaked like no other. even my mom noticed how much more solid the pilot feels, and thats saying a lot. i think unibody works well for most cars. its a matter of design and engineering to determine how rigid or flexible the final product will be.
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Post by hockeystyx16 »

i guess ill chime in here as well.

i dont like unibody, the whole idea of not having a solid frame just doesnt sound very smart.

in my car, i can feel the chassis twisting slightly when lane jumping at high speeds, and i dont like it. thats why im gonna make myself a set of subframe ties to tie the front and back subframes together. because i dont have torque to come out the back wheels, something small, like 1" square tubing will do just fine and will make my car handle like its on rails. the extra weight will be well worth it, and it will be offset by the weight reduction i have already done.

theres a reason why all the RWD sports car owners eiher have SFCs or want SFCs on their cars. unibody is a weak and inferior design, done that way only because its cheaper. if it was cheaper to make a full frame, every car out there would have a frame.
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Post by microchip13 »

To me, I think from a design perspective, and unibody would seem a lot harder to engineer than a simple frame.
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Post by Leedeth »

Damnit, I'm pretty much going in circles here, it's confusing.

In a unibody, how do they use the doors, hood, and trunk to reinforce the structure. They're insulated by rubber seals, so they're not actually reinforcing anything, are they?

So then I assume that they must be using the other body parts of the car for rigidity...like the fender, quarter panels, rocker panels...what else?

Also, what exact part is the frame on a body-on-frame.
* Center of gravity is usually higher - compromising stability and handling.
How is the center of gravity higher on a frame?
* Less resistant to torsion (flexing of the whole car in corners) - compromising handling and roadholding.
How is a frame less resistant to this?
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Post by hockeystyx16 »

doors are not used for support, its the fenders, quarterpanels (big structural item, this is why they are welded on in a lot of unibodys), windows (front and back) roof, A,B,C pillars, etc.
this whole thread started when i said to fix the windshield before the crack gets bigger, because windshield is a support piece and it flexes with the rest of the body which will make the crack spread

in a frame, the actual frame is 2 long beams taht run the lenth of the car, with some crossmembers between them, engine mounts, suspension mounts, cabin mounts, bumpers, etc. its the backbone of the car. everything on the car bolts to the frame. the frame is the support structure, the cabin are not, they dont bear any load.
because bumpers are mounted directly on the frame, if you hit something, all the force goes straight into the frame, and you will feel it. but the trade off is a scratch on the bumper vs a smashed up front end. well worth it IMO.

heres a picture of a typical truck frame.
Image

all this talk is making me want to go down to the garage, jack my car up and start laying out the subframe ties.

i think they are saying the frame is resistant to flexing in corners, theres no way a solid metal box is gonna be less ridgid than a couple layers of sheet metal spot welded together
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Post by Leedeth »

Ok. So then in a body-on-frame, the body just sits on top of the frame and basically does nothing. Right?

In a unibody, is there even a frame at all, or is it just a bunch of pillars going everywhere like a rollcage?

It sounds like a unibody is just a body without the frame.

So then you can get a super-rigid frame by getting a frame and bolting the body on as support?
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Post by hockeystyx16 »

yeah, in a body on frame, the body just sits there, bolted onto the frame, it doesnt have to do any structural support because the frame itself is doing that.

under your car, if you look, theres 2 fake frame rails, they are about 3-5 layers of sheet metal rolled into a frame rail that runs between the 2 subframes. thats about as close to a real frame rail as unibodys get.

up front and in the back are subframes. front subframe is usually a metal box made out of real metal beams (not multi layer sheet metal) that holds the whole drivetrain, and is bolted onto the rest of the car with some bigass bolts.
in the back, is the same thing, but it houses the rear axle.
both of these subframes bolt to those 2 fake frame rails i talked about. this gives it some rigidity, but it still needs the support of the roof and the glass to stay together.

think of it this way. take a empty paper tower roll, you know how it has that carboard pipe in the middle. thats a real frame rail
now take a sheet of paper, and roll it up into a pipe. thats a multi sheet metal rail.
which one is stronger? you see.

you cant really lift the body off a car and make a frame, because the body is the frame. but what you can do and what makes a huge difference is buy or make some subframe connectors. they are pieces of square tubing, real steel tubing, that connect the front and back subframes together.
because they are the 'cardboard roll' they are much stronger than the fake frame rails, and are about as close as you can come to having a full frame on a unibody.

damn it, now im about to go downstaits and start planning the SFCs for my car. i have a general idea of what im gonna do, i just need to do a lot of measuring and planning, get some 1 inch square tubing and have at it with a welder and a grinder.
they dont make SFCs for probes, but some people had theirs custom made a few years ago, and they said it makes it drive and feel like a totally different car. and thats a usual result of putting on subframe ties.
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Post by Leedeth »

I kind of get it now, but now it doesn't make sense how a unibody can be driven hard without bending down the middle.

Here's the pic that's confusing me, it's apparently a unibody, but it looks like a framed body or something.

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Post by VTECaddict »

if a solid frame is so much stiffer, why do pickup beds flex so much in relation to the cab on uneven surfaces?
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Post by VTECaddict »

Leedeth wrote:
* Less resistant to torsion (flexing of the whole car in corners) - compromising handling and roadholding.
How is a frame less resistant to this?
a fame would be like building a square or rectangle out of 2x4's.

a unibody structure would be like building a cube or box out of 2x4's.

now picture you and a friend both lifting one of them up and twisting it. which one will deform more?

but also unibodys dont just rely on the structural beams. so now add on plywood to the sides of that cube/box. that acts as all the other stress bearing things on a unibody car like the windshield, floorpan, structure beneath the fenders, etc. now try to twist this closed box.

for a unibody car: picture taking that closed box, and directly attaching four wheels to four corners of that closed box.

for a body on fame car: picture building a flat rectangle of 2x4's and attaching 4 wheels to the corners of that rectangle. now take a closed box like the one above and nail it on top of that rectangle with the wheels.

i hope that made sense :lol:

that was just a general analogy. a lot of different factors play into a car's rigidity (unibody or BOF) such as the gauge of steel used, proper engineering and placement of structural beams and/or frame rails, the quality and quantity of welds, use of high tensile strength steel or other materials, etc.
Last edited by VTECaddict on Fri Mar 07, 2008 3:13 am, edited 1 time in total.
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Post by hockeystyx16 »

Chris, your right, it cant. it will flex down the middle. thats the core of the problem. that thing has it worded confusing.

pickup beds flex because youll be hard pressed to find a fully boxed frame the whole lenth of the truck. most trucks alternate from full box to C frame in different parts of the truck. C frame is much weaker than full box frame obvously. thats why they flex.

Ashish, thats live axle for you. all mustangs (except cobras) have live axles, thats why they handle like a dumptruck. live axle is very strong, but it has no place on a sports car
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Post by Leedeth »

OK WTF. Now I'm more confused than ever. Unibody is now supposedly stronger, but it's not a thick frame.

Do unibodys have some sort of framework underneath all the bodywork which isn't as strong as a BOF? Because I'm like WTF at how body panels and windows are able to hold the car together without cracking, shattering, or bending, etc.
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Post by comingbackdown »

And may I add that body roll and "higher center of gravity" (negligible if it's built right) aren't that much of a problem if you put a proper suspension on it? Think about this... Unibody came about in like, the '60s. Shit was still rolling on leaf springs back then... They needed unibody. With a pro-kit low suspension and shit, you don't need a unibody. If it's low enough to the ground, it won't be affected by a higher center of gravity.

Also, the fuel mileage claim is crap. The way they make a unibody, there's just as much metal in one as in a body-on-frame.

I mean, hell... They can use lighter, stronger materials then they did before body-on-frame went out of use for cars, so it could be improved upon massively... I think somebody needs to try it. From what I've heard from my dad about guys who had one and modded the suspension on it back in the day, they cornered like bats outta hell. Then again, a '70s GTO or Firebird would've been beastly anyway.

I refuse to believe that a unibody is better, especially not for something ridiculously powerful.

Also, you could make it crumple if you wanted to. If they can fire a space shuttle into space and build the ISS, they can make a working body-on-frame that will crumple if needed.
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Post by AHTOXA »

Leedeth wrote:OK WTF. Now I'm more confused than ever. Unibody is now supposedly stronger, but it's not a thick frame.

Do unibodys have some sort of framework underneath all the bodywork which isn't as strong as a BOF? Because I'm like WTF at how body panels and windows are able to hold the car together without cracking, shattering, or bending, etc.
Think of an egg. When it's whole, it's strong and can support a good weight as long as it's evently distributed.


Going back to my offroad days:

1. About seeing puck-up bed wobble on rough pavement. Some flex in the frame is allowed and needed to keep it from cracking, thus you see the beds wobble.

Overall BOF is MUCH tougher than a unibody. In the offroading world you may see some Jeep cherokees that wheel hard but often they have body stiffners because they are unibody and prone to twisting. The only exception to the BOF rule in offroading are rock buggies, but that's a whole other ball game.

As far as unibody goes... Advances have been made in unibody construction that have made it more solid and ridgid, yet still kept the crumple zones and ride comfort. I know VW/Audi have started a different welding thechnique on their car bodies with the latest gen vehicles, where the bodies are much stiffer and not as prone to flex during hard cornering.

Anyone here with a Dub will confirm this.


As far as my opinion goes, frame is great if you're gonna offroad the truck/suv or carry heavy cargo. Otherwise it's not needed due to weight and diminished handling as opposed to unibody.

That said, I loved my full frame Xterra.
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