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Old 08-03-2006, 12:11 AM
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How critical is the roof skin on a unibody car?

I want to make a carbon fiber center roof skin for my 2000 Audi. What im thinking is laying up the carbon fiber on top of my existing roof then popping it off and cutting the center of the metal roof out and leaving a 3/4 inch flange around the outside and using panel bonding adhesive to glue the carbon fiber down... if i leave all of the bracing intact and just remove the skin will the car still be structurally sound? heres a pic to show the roof panel although my car is not a wagon its the same idea just not as big..


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Old 08-03-2006, 02:38 AM
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i realy dont suggest that u do this youll most likely ruin the structural integrity of the car, audis have amazing side impact protection because EVERY PANEL on the car is structural

replacing the roof with CF is just as smart as driving the car without any roof panel at all

and this is coming from someone who did 5 years of collision repair for an audi/ vw dealer. ive seen audis hit EVERY possible way, including having a 18 bay storage garage that was build out of 2x6 and 2x10's collapase under 18 inches of wet snow

the chryslers in the garage were no taller then the top of the doors, the audis had an extra foot of headroom

Last edited by lowROLLERchevy; 08-03-2006 at 02:46 AM.
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Old 08-03-2006, 05:09 AM
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The skin itself lends no integrity to the car at all.

About 15-18 years ago Duramix and the Canadian insurance companies
did extensive crash tests with GM and Ford involved and found the door skins, quarter skins and roof skins in a crash if the three were not on the car made no difference in the integrity if the car when crashed.

This was done to get approval for the shops in Canada to bond these panels instead of welding and these panels were classified then as non-structural.

Of course GM and Ford knew this but the Canada insurance companies has to have it proved to them for approval.

Edit:
I think the test was done in 1988-89 and also should add Allstate was the lead tester and testing was done at the Allstate facility's in I think Arizona.
Also floor pans and trunk pans in a full frame car are also considered non-structural.

Last edited by BarryK; 08-03-2006 at 05:27 AM.
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Old 08-03-2006, 06:40 AM
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Floor pans and trunk pans are Non structural??

I think they are smoking something in Arizona. Any shaped metal will add structural intergity to an assembly. I think Allstate was trying to save some repair bills!!

Think of how much structure is in a floor pan! The floor pans provide "shear" strength. To simplify it, if you were to hit the right front, where the impact was felt by the frame horn, the floor pan would help triangulate and disperse the impact. Granted, it would be the weakest at the body mounts, but still stiffens it non-the less.
In drag racing suspension clinics, they are VERY cognizant of the floorpans. If a "door car" removes the floor pan, they want you to install a series of tubular bracing, to keep the right frame rail from moving backwards. I was suspicious of this, but saw a 9 second car on a frame table, that was in fact skewed. "Doorslammers" by David Morgan, goes into this in depth.
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Old 08-03-2006, 08:39 AM
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Beenaway, notice that Barry said "floor pans and trunk pans in a full frame car are also considered non-structural".

I still think that is pushing it because there are hits that miss the frame (go over it) and that floor is playing a part in absorbing energy. On a unibody, you bet your butt that floor is structual!

As far as the roof, I have to tell you, Even on an Audi I really don't think the roof panel is doing much of anything. It is like the front fenders on a Honda, they weigh less than your wifes purse for goodness sakes. I call these panels "Paint holders" because that is all they are doing. The roof panel is just like that, the SIDE structure, is what holds the roof over your head when you roll it on it's top, it certainly isn't that skin.

I just replaced the roof skin on a 2005 Honda, it had side air bags and very good support from the front and rear headers. But the skin, come on now, it did NOTHING. It was a "paint holder".

Brian
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Old 08-03-2006, 08:40 AM
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Carbon fiber is almost 3 times stronger than steel in tensile strength. But it has minimal compressive strength and is very brittle. If you can orient the strands to go in as many directions as you can, and glue it to all of the structural members that are part of the roof, with a glue that will have the same adhesive and cohesive strengths as the steel, you would effectively replace the roof panel as a stressed panel.

BUT!!! I don't think it would behave at all like the original steel panel would in a crash though, as it would not absorb the impact the same as the steel.

If you are adding a roll cage to your car to protect you as well as that car currently will, then I woulds say go for it.

If you are concerned less for your safety in a crash, then I'd say go for it.

if you only want to save weight and add some "wow" to your car, then I'd say go for it. (I'm not even sure you would save all that much weight, unless you bagged your laminate)

The surface will not look near as smooth as the molded parts ,unless you flow coat it with resin and sand/polish it after. Sanding through the resin into the CF will make an ugly blemish that you wont be able to fix.

Although CF is used in making the safety capsules that are found in speed boats and f1 stuff, I don't think you can use it on an engineered metal structure and duplicate all of the characteristics of the steel you are replacing.



Why don't you just buy a piece of CF and lay it up on top of your roof. It will look the same and you can fool your friends. Or better yet, just paint it on. I saw a thread somewhere about doing the faux paint thing to look like a CF laminate.

That's what I think.

Mikey
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Last edited by powerrodsmike; 08-03-2006 at 11:05 AM. Reason: I remembered something
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Old 08-03-2006, 08:51 AM
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Thats a good idea Mikey! Don't know why I did not think of that myself.



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Old 08-03-2006, 11:23 AM
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Oh and If anyone doubts that the skin adds structural integrity to the car, go and remove all of the urethane bonded windows and cut out the roof of ANY car and watch what happens. The remaining structure will flop around like a fish out of water.

Similarly, take the plywood sheeting off of you house and see what happens.

The shear strength that the skin contributes is a great deal of a structures overall strength.

Remember the old biplanes? With the canvas covering?
Take the canvas off and watch what happens.

Pull a door skin off of an inner panel and tell me that inner panel is just as strong.

I don't mean any disrespect here, but that is what I know to be true fro my experience.

later, mikey
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Old 08-03-2006, 12:13 PM
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Well im not really going after this idea for the look really. i want to get rid of the sunroof and i want to save weight at the same time. Carbon fiber is my first choice because i know a little bit about making fiberglass and carbon fiber parts etc. and it is strong.. my second choice would be aluminum but i dont have the equipment to make a crowned roof panel out of metal. Third choice would be the dealer to order a non-sunroof skin.. but im pretty sure that would cost as much as making a CF panel and an aluminum panel..

New bmw M6 has a carbon fiber roof skin. and what else comes to mind is lowrider guys that have old honda civics etc with full sliding sunroofs..
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Old 08-03-2006, 03:20 PM
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Mikey, I agree it does "something" but it sure isn't much. On that Honda I did, we are talking 22 guage steel that is crowned. A TINY bit of side pressure the crown is increased. It doesn't do much at all. I have replaced many of these roof skins and you have to be VERY careful with the new part getting help to put it up on the car it will fold in half under it's own weight.

On a Saturn for instance the skin is glued on, just a spot weld in each corner, the rest is glue. On the idea of removing the urethaned in windows, now THAT is going to effect structual regidity. They ARE a major part of the structure. On a late model car the door shell with the skin removed is almost the same as with it on. It isn't like on an old Camaro from the 60's. The reinforcent beam is welded in to thick metal at the hinge and latch points and that door basically just as strong with or without the skin.

Now, on most of these cars the skin is supported from under with a number of braces going from side to side. These braces are again, VERY thin metal. But with them welded to each roof rail and then the skin glued down on top of them and welded on the sides it is creating some structual integrety. That carbon fiber installed in the same way gluded down to braces is going to be pretty nearly the same. By the way, this metal is so thin on the braces and skin it is VERY difficult to separate the two without bending everything all to heck. I use an "inductor" to heat up the glue so the roof can be peeled off. If you just pulled up on the skin that brace is so thin it would bend up VERY easily. The glue holding the skin (kind of a foam like product) is almost as strong as the welds.

It just isn't the same as HHS structual compontants such as the door surround apperature or roof pillars and that sort of thing.

Brian
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Old 08-03-2006, 05:58 PM
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Quote:
Originally Posted by Beenaway2long
Floor pans and trunk pans are Non structural??

I think they are smoking something in Arizona. Any shaped metal will add structural integrity to an assembly. I think Allstate was trying to save some repair bills!!
Beenaway,
No one was trying to save any money here.
Also you cannot compare a race car frame to a modern car frame.
The liability here for the insurance company that approves this system of bonding and for the automakers that give the repair its blessing as well as the manufacturer that sells the stuff is a lawyers dream.

The report when done was two 9x11 manuals that each were about 6-8 inches thick. Compiled by some of the top structural engineers in the country.
Many of cars were wreck and rolled over in tests much more demanding than the normal tests they do on new cars for your yearly safety rating of cars.

Like one Ford engineer said to me during the test if we could sell the car without skins or paint holders as Brian says it would make their job easier to design the crumple zones and the engineered spot welds.

Edit:
The one car it was a Taurus I believe and he made that statement to me that "you know if everything on that car was in tack except someone replaced the front glass and rear glass with widow ribbon instead of urethane everyone in the car would have died in that rollover".

I knew it was serious on how the glass was replaced but not that serious.

Last edited by BarryK; 08-03-2006 at 06:10 PM.
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Old 08-03-2006, 07:28 PM
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If it were me,I would make the CF roof just like you are talking about.I would "glue" it to the suport structure.If you were worried about saftey,you could allways put a cage in it later.Just my 0.02

EVIL
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Old 08-03-2006, 09:34 PM
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Quote:
Originally Posted by MARTINSR
Mikey, I agree it does "something" but it sure isn't much. On that Honda I did, we are talking 22 guage steel that is crowned. A TINY bit of side pressure the crown is increased. It doesn't do much at all. I have replaced many of these roof skins and you have to be VERY careful with the new part getting help to put it up on the car it will fold in half under it's own weight.

On a Saturn for instance the skin is glued on, just a spot weld in each corner, the rest is glue. On the idea of removing the urethaned in windows, now THAT is going to effect structual regidity. They ARE a major part of the structure. On a late model car the door shell with the skin removed is almost the same as with it on. It isn't like on an old Camaro from the 60's. The reinforcent beam is welded in to thick metal at the hinge and latch points and that door basically just as strong with or without the skin.

Now, on most of these cars the skin is supported from under with a number of braces going from side to side. These braces are again, VERY thin metal. But with them welded to each roof rail and then the skin glued down on top of them and welded on the sides it is creating some structual integrety. That carbon fiber installed in the same way gluded down to braces is going to be pretty nearly the same. By the way, this metal is so thin on the braces and skin it is VERY difficult to separate the two without bending everything all to heck. I use an "inductor" to heat up the glue so the roof can be peeled off. If you just pulled up on the skin that brace is so thin it would bend up VERY easily. The glue holding the skin (kind of a foam like product) is almost as strong as the welds.

It just isn't the same as HHS structual compontants such as the door surround apperature or roof pillars and that sort of thing.

Brian
Thanks Brian, I really think we are mostly in agreement. I said pretty much the same thing in my first post about gluing the CF skin to the supporting structure. I can't speak from a properly schooled engineers point of view,
BUT,
I would guess that if that steel roof skin is glued on with a glue that is near as strong as the steel, there has to be a reason based on some engineering requirement. .
The tensional stresses absorbed by a 5'x5' piece of 22 g steel has to be a fair bit when spread out over a few hundred square inches of bond lines.

If all the glue had to do was keep the rattles down then the manufacturer would use some cheap foam.

I can speak from some experience when I say that the CF will not behave the same as steel if there was an impact. It reaches its yield point and then it's over. Depending on the resin type and laminate thickness, it's yeild point may or may not be the same as steel.

This means that if the car was in a crash that involved the roof rails, the CF roof skin will probably not behave the same as the engineers that designed the car would need it to. I can't say for better or worse.

I'm not saying that it would result in some kind of catastrophic failure, I am not the gloom and doom kind of guy.

As was said in my first post, I think that beemdubya should go for it. Unless he has any reservations about the engineering.

AND I should add , I think it would make the overall structure of the car more rigid, (not much tho)and consequently handle better.


I will take your word about the newer cars structure as far as the inner door, I have not yet cut one of these up. (there is a early ninties cavalier that is an abandoned orphan and my landlady wants it removed. I may take the torch to it in the name of science )

Later, mikey
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Old 08-03-2006, 09:40 PM
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well if it makes a difference.. they also installed 8 airbags in this thing at the factory.. none of which remain.. its a street car but not a daily driver im trying to get a 3500lbs car down to 2850lbs or so..
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Old 08-03-2006, 10:02 PM
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Well..then....DO IT
I think I would vacuum bag it if at all possible, it will keep your resin content down.
I have found also that if you put enough PVA on just about any thing other than enamel you can pull a part off.

Later, mikey
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