Body-on-frame vs. Monocoque (Unibody)

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Postby VTECaddict » Fri Mar 07, 2008 3:20 pm

hockeystyx16 wrote: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.

new F150 has a fully boxed frame. still bed flex/bounce. and if its stronger, why does ford use a C-frame on the "heavy duty" superduty pickups?
http://www.youtube.com/watch?v=EWjTbiYo3x0

i will agree that BOF is better for something like towing, since all the stress is put on the frame instead of distributed all over the body, but i fail to see how a BOF car can be stronger than a unibody in terms of torsional rigidity etc.
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Postby Johnf514 » Fri Mar 07, 2008 4:22 pm

Funny - in the copy, they said that the frame bending like that saved his life. :?:
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Postby Leedeth » Fri Mar 07, 2008 4:31 pm

Wow, good timing as well.

If it was a unibody the engine probably would've made the whole thing fall of the edge. :lol:

I think I get it now. Unibodys have better structural integrity, until some of it becomes compromised by fatigue or rust.

I still think they need to combine the two and get the best of both worlds.

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Postby JHamilton » Fri Mar 07, 2008 4:32 pm

hockeystyx16 wrote: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.


I read your other post and have some comments to add...
- Windshields provide no rigidity to the structure of the car. Annealed glass is very strong, but does not flex. It is designed to break quickly and into many small pieces. The A-pillar provides all the support in that area.
- Unibody cars have a solid frame. Much more solid than a body-on-frame car. All of today's race cars (every type I can think of) are monocoque. Most cars since the early 60's have been monocoque. Which is easier to twist, a flat piece of cardboard or a box?
- The only unibody I have been in that I could feel chassis twist from road forces is a Fox Body. I'm sure there are others out there, but I believe it is the exception more than the rule. The experience you are having with your car is probably one of those exceptions.
- STBs, on most modern cars, perform no function other than aesthetics. My STB is huge and provides no performance advantage. The only time I seen subframe connectors used (other than Mustangs) is extremely high horsepower cars. It is true that the early, modified/high horse unibody cars in the '60's would ripple their body panels. This is because the frame won't twist though, while a ladder frame will twist and rack all day long without affecting the body.
- Your cost analysis is backwards. It is much more expensive and time consuming to design and produce a unibody car than a ladder frame car. For example, a frame-on-body chassis can be produced in pieces, while modern unibodies are hydroformed. I'm not even qualified to go into the design challenges.
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Postby VTECaddict » Fri Mar 07, 2008 4:44 pm

Leedeth wrote:I still think they need to combine the two and get the best of both worlds.

Leedeth, meet Honda Ridgeline. unibody with integrated closed box ladder frame.

you people complaining about unibody just need a stronger/newer/better designed unibody car. i can go 3 wheeling without feeling any body flex or hearing any creaking.

what's 3 wheeling? http://img.photobucket.com/albums/v610/ ... re1399.jpg
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Postby Tinton » Fri Mar 07, 2008 4:50 pm

comingbackdown wrote: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. quit was still rolling on leaf springs back then... They needed unibody. With a pro-kit low suspension and quit, 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.




Something that I always found funny was that my friend's '94 Honda Accord was dropped like 2 inches, but the roofline was still like 2" higher than my car (and my car has stock springs). He dropped the hell out of that Accord, he could barely make it over speedbumps, and yet my car still had much lower center of gravity, and you could feel it while driving.

And in terms of frame design you missed 1. The Spaceframe.

This is a Fiero with the body panels completely removed:
Image

The car is perfectly rigid and drivable without the body panels on it. My cars were designed with this, it provides a strong frame with lightweight plastic body panels. Every exterior surface on my car is plastic.

Most real race cars are built on a spaceframe too, because of it being able to resist flex and still be lightweight. I may sometimes hear the frame creaking when I go over bumps, but it never actually flexes. That's another reason why Fieros are popular for engine swaps, its almost impossible to twist the frame. Some older hotrods like to twist the frame if they have too much power, due to the restrictions of a unibody setup.

If you can also see that picture, you see that the frame extends over the passenger compartment too. Its similar to having a built-in rollcage. I've heard of Fieros flipping over before, and I've seen pictures, I've never seen the roof crushed. Fieros are small cars and I've heard people call them deathtraps, but they did get 5-star crash tests due to the spaceframe , you'd be surprised how much steel surrounds you.

Body-on-frame vs. Unibody though, I don't like either.
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Postby Leedeth » Fri Mar 07, 2008 4:52 pm

VTECaddict wrote:
Leedeth wrote:I still think they need to combine the two and get the best of both worlds.

Leedeth, meet Honda Ridgeline. unibody with integrated closed box ladder frame.


http://en.wikipedia.org/wiki/Honda_Ridgeline

"The Honda Ridgeline uses unibody architecture ladder frame / unibody hybrid chassis. This design gives it 2.5 times more bending rigidity and 20 times the torsional rigidity than the standard ladder frame only type of chassis construction, while retaining the load carrying capacity of the traditional ladder frame."

OMFG that's so 1337. Sucks that there's no manual tranny though.

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Postby AHTOXA » Fri Mar 07, 2008 5:08 pm

Leedeth wrote:I think I get it now. Unibodys have better structural integrity, until some of it becomes compromised by fatigue or rust.



No, a frame is more structuraly ridgid and much more solid by nature because it does not allow for much twisting/bending.

That's why in my previous post I mentioned off-roaders using framed vehicles almost exclusively.
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Postby AHTOXA » Fri Mar 07, 2008 5:09 pm

Leedeth wrote:
VTECaddict wrote:
Leedeth wrote:I still think they need to combine the two and get the best of both worlds.

Leedeth, meet Honda Ridgeline. unibody with integrated closed box ladder frame.


http://en.wikipedia.org/wiki/Honda_Ridgeline

"The Honda Ridgeline uses unibody architecture ladder frame / unibody hybrid chassis. This design gives it 2.5 times more bending rigidity and 20 times the torsional rigidity than the standard ladder frame only type of chassis construction, while retaining the load carrying capacity of the traditional ladder frame."

OMFG that's so 1337. Sucks that there's no manual tranny though.




Yeah, try taking the ridgeline offroad. It'll twist up like a pretzel.

What the heck is 1337? English, please.
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Postby Leedeth » Fri Mar 07, 2008 5:26 pm

1337 is leet, short for elite.

Ok hold on, let's get this straight first.

A unibody essentially has a frame underneath it, with the body panels reinforcing it?

Again, this pic. What type of construction would you describe it as?

Image

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Postby AHTOXA » Fri Mar 07, 2008 5:40 pm

The pic above is unibody. The body is one piece with the main structural component. The body is the reinforcement.


If you can look under a vehicle and spot two rails with beams running accross, it's a framed vehicle. Like those 20 footer moving trucks.

A unibody vehicle does not have those and relies on other structural elements for ridgidity such as A, B, C pillars, door beams and others. The whole "skeleton" of the car acts as the load-bearing frame that resists torsional forces.

In a framed vehicle the frame takes the brunt of the abuse without the body really playing much of a role.

Here's an example: this dude I know though a friend of a friend took his dode minivan to go fishing. He decided that he wants to pull the van closer to the fishing spot se he could have music playing or whatever. He crassed a few ruts getting there and then his back door would not close properly because the unibody got twisted up while corssing the ruts.

Framed vehicle fare much better in a crossed up cituation like this:

Image
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Postby VTECaddict » Fri Mar 07, 2008 5:47 pm

i dont think BOF is stiffer. its just that in a unibody car there is little to no "leeway" for twisting. so if there is any significant chassis flex at all it will end up twisting the whole unibody and start rippling body panels, etc. which is very very bad.

a BOF car will allow more flex than a unibody, but that isnt a problem because the body itself is nearly unaffected, just the frame itself is twisting.

so its not that BOF is more resistant to twisting. its that its more durable and gives you a bigger buffer for flex before you actually start damaging anything.

and there was a picture or video of a ridgeline on two wheels (opposite corners), and all the doors and tailgate could still be opened and closed without any issues. hows that for twisting up like a pretzel? [im trying to find the picture]
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Postby AHTOXA » Fri Mar 07, 2008 5:54 pm

Sure, BOF is more resistant to twisting.

You say that the body is not affected much if frame twists, but the body is attached to the frame...

From my years of offroading and driving/seeing many different trucks I can definitely say framed vehicles are much stiffer. There is some body flex in framed vehicles, but not much at all, even in extreme cituations when you have a cross-camber case going on.

My Xterra used to flex, but by far not as much as some unibody Jeeps (Cherokee).
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Postby VTECaddict » Fri Mar 07, 2008 6:12 pm

yes, the body is attached to the frame, but usually with several giant bolts further cushioned by big rubber bushings.

therefore, the frame is the structure that takes the bulk of the stress from the suspension. the body just sits on top of it, and is not used to provide major structural reinforcement. the body is more isolated from the frame and chassis (and this is why BOF cars also exhibit lower NVH), so that if there is any flex/vibrations/whatever, the body isnt the primary structure to take it on.

a unibody channels all that stress throughout the body, so any large amounts flex will exhibit itself directly on the body by means of deforming the unibody structure.

if a BOF is more resistant to twisting, why does something like the ridgeline which has a unibody offer 20 times the torsional rigidity of a ladder frame?
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Postby AHTOXA » Fri Mar 07, 2008 6:58 pm

VTECaddict wrote:yes, the body is attached to the frame, but usually with several giant bolts further cushioned by big rubber bushings.

therefore, the frame is the structure that takes the bulk of the stress from the suspension. the body just sits on top of it, and is not used to provide major structural reinforcement. the body is more isolated from the frame and chassis (and this is why BOF cars also exhibit lower NVH), so that if there is any flex/vibrations/whatever, the body isnt the primary structure to take it on.

a unibody channels all that stress throughout the body, so any large amounts flex will exhibit itself directly on the body by means of deforming the unibody structure.

if a BOF is more resistant to twisting, why does something like the ridgeline which has a unibody offer 20 times the torsional rigidity of a ladder frame?


You said everything I said when you were talking about distributing forces between a frame and a unibody car. That's correct.

As far as Ridgeline goes, it's different. They did frame-like boxed sort of unibody. This is different. I was talking about the regular uni-body construction used elsewhere.

Regular unibody car or some SUVs do not have a regid unibody like the Ridgeline, this will twis easily under severe stress.

Really, if the BOF was more prone to twisting, would the offroad community use it as much as they do? Nope, because off-road ridgidity is everything. You can't have an off-roader felx all the time. This creates metal fatigue which will cause major stress cracks in metal, rendering the vehicle useless and unsafe.

The BOF does not flex as much offroad, thus resulting in a smaller risk for metal fatigue/serious structural compromise.
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