Where is the CF roof?

[bmstyle71]

Where is the CF roof? on top of my car. HAA I'm sorry, had to break the tension

[rzm3]

Originally Posted by seriousm3

No it doesnt... its a thin sheet of CF glued to the roof of the car... it provides no structural rigidity vs. a steel piece. The rigidity comes from the steel bows running across the roof.

The cf roof is simply there for lightness and looks.

You may be wrong. If the CFRP roof didn't take any loads, BMW wouldn't have built a completely seperate facility to attch the roof to the spaceframe.
Also, here is the source:

http://www.bmwusa.com/Standard/Conte...ory/Facilities

"It's a big challenge for us to really get the [CFRP roof] adhesive right, and that means we have to control the bonding process very very tight. We have to make sure that the temperature is always in the right tolerances, that humitity is in the right tolerances, and that all the facilities are always working to perfect conditions. Because that is very very important for the stiffness of the car".

[Serious]

Originally Posted by rldzhao

You may be wrong. If the CFRP roof didn't take any loads, BMW wouldn't have built a completely seperate facility to attch the roof to the spaceframe.
Also, here is the source:

http://www.bmwusa.com/Standard/Conte...ory/Facilities

"It's a big challenge for us to really get the [CFRP roof] adhesive right, and that means we have to control the bonding process very very tight. We have to make sure that the temperature is always in the right tolerances, that humitity is in the right tolerances, and that all the facilities are always working to perfect conditions. Because that is very very important for the stiffness of the car".

that doesnt say the CF adds rigidity vs. a steel roof... it just means its difficult to bond the CF roof so it performs as well as the steel roof.

[rzm3]

Originally Posted by seriousm3

that doesnt say the CF adds rigidity vs. a steel roof... it just means its difficult to bond the CF roof so it performs as well as the steel roof.

all i am saying is that the roof, CF or steel, provides structural rigitity, which goes back to my original post: "[the roof] increases torsional rigitity [vs. not having a roof at all]".

[footie]

The CF roof was probably chosen exclusively for the E92 just to make it different, like in the way it has only a 4 seat option, the same with the X6. There is no engineering, financial or other reason for it not to be on the saloon other than to separate the two models and that is it.

[sayemthree]

e90 is for girls - they doent care bout CF looks or wight of the car!

[synclastica_86]

Okay... so let's make it clear once and for all. Apart form the weight savings that can be offset by one less happy meal from Mcdonnels, does the CF roof give any performance advantage over a steel roof?

Is it possible to make a CF roof for the E90 or E93? See, since the E93 does not rely the roof for any structual strength isn't it better to make it as light as possible?

[mixja]

Originally Posted by synclastica_86

Okay... so let's make it clear once and for all. Apart form the weight savings that can be offset by one less happy meal from Mcdonnels, does the CF roof give any performance advantage over a steel roof?

Is it possible to make a CF roof for the E90 or E93? See, since the E93 does not rely the roof for any structual strength isn't it better to make it as light as possible?

The most beneficial gain is a lower centre of gravity, which will improve handling.

The M3 sedan is quite a bit taller than the coupe, so a CF roof probably wouldn't quite have the effect as it does on the coupe...

[KenH]

"The M3 sedan is quite a bit taller than the coupe, so a CF roof probably wouldn't quite have the effect as it does on the coupe..."

The sedan is 1.4 inches taller than the coupe so therefore a lighter roof higher up would have more of an effect on the sedan's CoG than on the coupe.
The glued-on CF roof adds torsional rigidity to the bodyshell in comparison to a bodyshell with just the cross strut (and no roof or one not glued on).
This is why manufacturers (starting with FIAT on the 128 in about 1970) glue in the windshields these days. Previously they used to sit in a piece of rubber and "float" which added nothing to the torsional rigidity.

[rzm3]

Originally Posted by KenH

"The M3 sedan is quite a bit taller than the coupe, so a CF roof probably wouldn't quite have the effect as it does on the coupe..."

The sedan is 1.4 inches taller than the coupe so therefore a lighter roof higher up would have more of an effect on the sedan's CoG than on the coupe.
The glued-on CF roof adds torsional rigidity to the bodyshell in comparison to a bodyshell with just the cross strut (and no roof or one not glued on).
This is why manufacturers (starting with FIAT on the 128 in about 1970) glue in the windshields these days. Previously they used to sit in a piece of rubber and "float" which added nothing to the torsional rigidity.

Spot on. But the reason why only coupes get CF roof is something we could ask an M engineer... Maybe I should apply for a job there... oh wait I don't speak German

[Malky]

I work in engineering, not automotive but aerospace. In my particular area we deal a lot with graphite composites.

The main advantages of composite over steel is that you can fine tune composite for the specfic load charataristics. You can drastically increase torsonal stiffness by altering weave angles, combining weave angles or adding more layers. You can do this with steel or aluminum but at the cost of weight.

The other advantage of composite is that it can handle many more cycles than metal. Over time metal will fail, when constantly flexed, before composite.

[southlight]

Originally Posted by rldzhao

Spot on. But the reason why only coupes get CF roof is something we could ask an M engineer... Maybe I should apply for a job there... oh wait I don't speak German

I think that's a question only the M marketing could answer, not an engineer.


Best regards, south

[synclastica_86]

Originally Posted by KenH

"The M3 sedan is quite a bit taller than the coupe, so a CF roof probably wouldn't quite have the effect as it does on the coupe..."

The sedan is 1.4 inches taller than the coupe so therefore a lighter roof higher up would have more of an effect on the sedan's CoG than on the coupe.
The glued-on CF roof adds torsional rigidity to the bodyshell in comparison to a bodyshell with just the cross strut (and no roof or one not glued on).
This is why manufacturers (starting with FIAT on the 128 in about 1970) glue in the windshields these days. Previously they used to sit in a piece of rubber and "float" which added nothing to the torsional rigidity.

Wait... I don't really get it. Why does gluing on a CF roof help add torsional rigidity? A steel roof doesn't float too, would that give the same strength as gluing a CF roof on?

[AJBoston]

On this topic, I'm also curious about which has the greatest overall impact on performance:

1) The lower CG of the coupe with CF roof? (I think it saves something like 11lbs, so I can't imagine it lowers the CG that far?)

2) The coupe's 22lbs less curb weight? (3726 vs. 3704)

3) The difference in F/R weight distribution between the coupe (51/49) and sedan (52/48). So the sedan has 11 more lbs on the front wheels, and 26 less on the rear.

My guess is the the weight distribution is probably the only impactful difference, and even then probably only to a skilled race driver.

[Sedan_Clan]

Originally Posted by AJBoston

On this topic, I'm also curious about which has the greatest overall impact on performance:

1) The lower CG of the coupe with CF roof? (I think it saves something like 11lbs, so I can't imagine it lowers the CG that far?)

2) The coupe's 22lbs less curb weight? (3726 vs. 3704)

3) The difference in F/R weight distribution between the coupe (51/49) and sedan (52/48). So the sedan has 11 more lbs on the front wheels, and 26 less on the rear.

My guess is the the weight distribution is probably the only impactful difference, and even then probably only to a skilled race driver.

I don't believe that any of the above differences result in any significant performance advantage. The differences are very minor........

...like having a car with 295 HP versus a vehicle with 300 HP.

[addy85]

Originally Posted by sayemthree

e90 is for girls - they doent care bout CF looks or wight of the car!

You damn rights the E90 is for the girls I can fit one in the front and three in the back comfortabley without having to move my seat so they can get in Easier for the quickies also, >>>>>OPEN BACK DOOR, GIRL CRAWLS IN, I CRAWL IN, 90 SECONDS LATER, REAR DOOR OPENS UP, GIRL GETS KICKED OUT, I GET IN THE FRONT SEAT AND DRIVE OFF<<<<<

[KenH]

"Wait... I don't really get it. Why does gluing on a CF roof help add torsional rigidity? A steel roof doesn't float too, would that give the same strength as gluing a CF roof on?"

I didn't mean to imply that a glued-on CF roof would have more torsional rigidity than a welded-on steel roof, only that if it were not glued-on (or rigidly attached in some other way) the body would have less torsional rigidity.
At the start of this thread some were speculating that the CF roof was just a sheet of CF in a metal frame and that it imparted no structural rigidity at all.
I don't know whether the torsional rigidity with the CF roof is more or less than that with the steel roof but certainly the CF is lighter. And as above, glueing it in place puts back some, all or more of the torsional rigidity from not having the steel roof.

[sayemthree]

Originally Posted by KenH

"Wait... I don't really get it. Why does gluing on a CF roof help add torsional rigidity? A steel roof doesn't float too, would that give the same strength as gluing a CF roof on?"

I didn't mean to imply that a glued-on CF roof would have more torsional rigidity than a welded-on steel roof, only that if it were not glued-on (or rigidly attached in some other way) the body would have less torsional rigidity.
At the start of this thread some were speculating that the CF roof was just a sheet of CF in a metal frame and that it imparted no structural rigidity at all.
I don't know whether the torsional rigidity with the CF roof is more or less than that with the steel roof but certainly the CF is lighter. And as above, glueing it in place puts back some, all or more of the torsional rigidity from not having the steel roof.

the sheetmetal roof of a unibody car is part of the structure. if the CF is more rigid and deflects less than the steel then it provides a stiffer structure.

[rzm3]

again guys, without more information about the car, materials, etc., it's very hard to determine exactly how much performance advantage a CF roof has over a steel roof.

Performance gain? Definitely. But NOTICEABLE performance gain? Hardly. Marketing? More than you can imagine.

[addy85]

this thread has to much technical stuff for me to understand :/

[rzm3]

Originally Posted by southlight

I think that's a question only the M marketing could answer, not an engineer.


Best regards, south

, you are right south. Probably also platform/brand manager too. Those are usually the people who determines the specs of a new car. Of course marketing is a part of the equation.

[rzm3]

Originally Posted by addy85

this thread has to much technical stuff for me to understand :/

anyone care to do an finite element analysis with ANSYS?