Dubai Build (& CF Roof Discussion)

[///M0F01SD]

I think it only makes sense in a caged race car.

I'm not sure how much structural integrity comes from the roof skin vs. the internal roof
support structure.

If this goes through, no prepreg... either vacuum bagged or resin infused with a foam core.

Prepreg in an autoclave would yeild a lighter part (Improved Carbon to resin ratio), I just can't
seem to fit an autoclave in my garage

I'm glad you're at least considering to vacuum bag it. I did forget to mention that there is a weaved prepreg material that has a low temp cure (>200*F). You can get away with a made shift tent & hairdryers for that cure.

TBH, my main concern is the mounting points to the frame. If these areas are not compacted down & built up enough then there will be no strength to it whatsoever. There's plenty of adhesive out there that will do the job of attaching the metal bits to CF. But where your CF to metal intersection is my concern of weakness.

P.S., I'm not trying to knock down your intentions, I'm just giving you my opinion and advise. I do wish you were closer to SD as I would like to lend a hand on the project and having an autoclave & oven would not be impossible.

Rey

[chutrain]

This should be an application for only caged cars. I would never do this to a street
car and jeopardize structural integrity. These cars can still roll over, and last thing I
need is the roof to completely cave cause I liked the carbon look. Bonding and securing
the panel is not an issue. It should be bonded with adhesive and riveted into place.
The main concern with this is how to blend everything into the front half of the car
where the A pillars and the windshield are, and not making it look like a hack job.

To be honest, the amount of money and time spent into making it work, I think it will
rate at the bottom of the weight savings per dollar ratio. There are so many other
things that can be done. I think yanking out the sunroof and mechanisms and
welding the panel shut will make the weight savings between the two negligible,
unless you are competing 10/10ths for money.

I rather looking into doing the CF driveshaft for our low torque cars.

[Jake]

True. If you're after a CF roof for the looks there are more appropriate options (CF laminate to steel roof).

Jake

[Mike K.]

The main concern with this is how to blend everything into the front half of the car where the A pillars and the windshield are, and not making it look like a hack job.

There are adhesives & fillers designed to do this provided you cover with paint or modify the A pillars for a clean joint.

There is some structure advantage that will be lost in a cage car. Most cages are attached (welded) to the roof in several areas for strength. Bonding the CF roof to the cage will never be as strong as a weld to a steel roof.

Mike K.

[lucid]

Most cages are attached (welded) to the roof in several areas for strength. Bonding the CF roof to the cage will never be as strong as a weld to a steel roof.

When you say roof, what do you mean exactly? I just checked my cage and roof skin in my E30 M3. There is a structural cross member (not a thin sheet of metal) spanning the roof connecting to vicinity of the B pillar on each side. Are you referring to cages being welded to such cross members? If so, I can see how that would help because those cross members (obviously there are cross members between the two A and C pillar as well) clearly carry the majority of the load. The thin flat sheet of metal, the "skin," tacked onto those structural members is flimsy to the extent that you can wobble it by pushing down with 3 fingers.

[Brendan]

When you say roof, what do you mean exactly? I just checked my cage and roof skin in my E30 M3. There is a structural cross member (not a thin sheet of metal) spanning the roof connecting to vicinity of the B pillar on each side. Are you referring to cages being welded to such cross members? If so, I can see how that would help because those cross members (obviously there are cross members between the two A and C pillar as well) clearly carry the majority of the load. The thin flat sheet of metal, the "skin," tacked onto those structural members is flimsy to the extent that you can wobble it by pushing down with 3 fingers.

Yeah, I agree with this. I think a Cored-CF roof might be stiffer than what's there now (Assuming you keep the cross-member, or reproduce as part of the roof). My cage is welded to the B-Pillars, A-Pillars, and the frame above the windshield, not the roof or roof cross-member.

My Car:

[Mike K.]

When you say roof, what do you mean exactly? I just checked my cage and roof skin in my E30 M3. There is a structural cross member (not a thin sheet of metal) spanning the roof connecting to vicinity of the B pillar on each side. Are you referring to cages being welded to such cross members? If so, I can see how that would help because those cross members (obviously there are cross members between the two A and C pillar as well) clearly carry the majority of the load. The thin flat sheet of metal, the "skin," tacked onto those structural members is flimsy to the extent that you can wobble it by pushing down with 3 fingers.

It's not as flimsy as you may think. Look at any aircraft wing; wood, aluminum or composite. If you press on any section of any wing that does not have a skeleton under it, it's just like that flimsy steel roof - it's useless. The skin on wings started out as cloth or paper in the old days, & models are still made that way. Did you ever notice all the "no step" decals on a wing? But when you bond it to the skeleton, it's becomes very strong that is what all the spot welds & glue are doing to your steel roof. If you look close you notice all three pillars are ultimately one piece & the thin roof makes it extremely strong torsionally, just like the wing. In order to install a composite roof & have the rigidity, that skeleton has to be removed or reduced because of the thickness of the skin. You cannot weld to, all of your bonding will be achieved via adhesives.

To make the roof correctly you need a core of foam, aluminum, or balsa wood and build layers (Hexacore or solid variants). This creates a light, rigid part, the thicker the part the stronger it will be. If you utilize that structure as it is, your roof will be higher than a steel roof. Or you have to reduce the skeleton & this will also reduce the rigidity. In addition, you cannot weld to it so you can only bond or rivit between the composite roof & skeleton.

One other option is to remove the core materials & build the roof with a blend of CF weaves. This will mimic the core method, although thin, the final product will be heavier. I'm also not smart enough to figure out the proper blend / weave option

Mike K.

[lucid]

Yeah, I agree with this. I think a Cored-CF roof might be stiffer than what's there now (Assuming you keep the cross-member, or reproduce as part of the roof).

Right. Looking more carefully at the pics from the Dubai build, it seems like that cross member between the B pillars might have been replicated in the CF panel geometry. It is not a flat sheet there, and the change in cross section will increase its stiffness if not its strength. Since we don't have a cutout of the roof panel, we can't see any potential changes in material thickness there; it is possible that thickness might be increased as well.

So, the CF panel might actually have more favorable stiffness and strength characteristics than the OEM steel structures it replaces, but then the issue becomes the method of attachment to the chassis (and the cage) as discussed already in this thread.

[lucid]

It's not as flimsy as you may think. Look at any aircraft wing; wood, aluminum or composite. If you press on any section of any wing that does not have a skeleton under it, it's just like that flimsy steel roof - it's useless. The skin on wings started out as cloth or paper in the old days, & models are still made that way. Did you ever notice all the "no step" decals on a wing? But when you bond it to the skeleton, it's becomes very strong that is what all the spot welds & glue are doing to your steel roof. If you look close you notice all three pillars are ultimately one piece & the thin roof makes it extremely strong torsionally, just like the wing. In order to install a composite roof & have the rigidity, that skeleton has to be removed or reduced because of the thickness of the skin. You cannot weld to, all of your bonding will be achieved via adhesives.

You point is valid, but it actually illustrates why the wing skin is functional and does not tear off, etc; because of the supporting skeleton. On the roof of the car, the thin sheet of metal spans a relatively wide cavity (2'x3.5' behind the center cross section and about that ahead of it) and loading conditions are different. I doubt that I can deform the skin of a wing by pressing on it gently with 3 fingers although I have never tried it. That is different than standing on it.

Actually, there is a simple proof for the relatively low structural functionality of the roof skin: the sunroof. If it the skin is such a critical functional structure, BMW would not have cut a gigantic hole through it and introduced a major discontinuity in the material without reinforcing it.

I don't understand what you mean by the pillars being one peice. They might be one (or several) stamped part(s), but the stamped part(s) vary significantly in cross section.

[Mike K.]

Actually, there is a simple proof for the relatively low structural functionality of the roof skin: the sunroof. If it the skin is such a critical functional structure, BMW would not have cut a gigantic hole through it and introduced a major discontinuity in the material without reinforcing it.

I don't understand what you mean by the pillars being one peice.

BMW did support the sunroof. They punched the hole & rolled material inward, this helps ad the strength back. Plus BMS MS never raced or supplies a sunroof car for racing. The sunroofs where only added to sell cars at the dealership.

The structure going across at the B pillar, is also located at the A & C pillar. It is than welded to each side of the car above the side windows. So what you end up with is a very large rectangle above all the windows with a reinforcement going across the B pillar.

[lucid]

BMW did support the sunroof. They punched the hole & rolled material inward, this ads the strength.

The rolled lip does not add significant strength to the structure. It adds some stiffness. Basically, strength is about the yield load and stiffness is about deflection. But there is still a gaping hole in the roof: ~90% of the skin in front of the center support beam.

The structure going across at the B pillar, is also located at the A & C pillar. It is than welded to each side of the car above the side windows. So what you end up with is a very large rectangle above all the windows with a reinforcement going across the B pillar.

Completely agree. This has been my point all along.

[Jake]

The rolled lip does not add significant strength to the structure.

I disagree that the rolled lip doesn't add significant strength. Using your example, imagine pushing on the edge the hole without a lip. It would move quite a bit. But with the rolled edge you could barely flex the same area. Regardless, the important factor here is torsional strength not whether you can flex it out of plane with your hand.

A rolled edge is a common structural edge reinforcement. All quality lightening holes in high performance sheet metal structures include a small lip to help negate the missing material and improve its stiffness. In fact its usually stronger with a properly designed lightening hole.

The framing you speak of needs the skin to function. That's the basic concept of a semi-monocoque design. Either skin or framing on their own is flimsy, but together they form a strong stiff structure. The aircraft wing analogy is a good example.

Note that a lot of aircraft structure design uses bonding and riveting. In fact welding is normally avoided in favor of riveting. The Lotus Elise is a great example of this concept in the automotive world. It's almost completely bonded and riveted together, and its very stiff. That's what I'd do with the roof. Bond an aluminum rib to the roof across the B pillar section and rivet it to the main hoop with a matching vertical rib.

Jake

[lucid]

I did say that the lip would add some stiffness (meaning in bending). You didn't quote that. It will have no effect on strength in tension, so the loading condition matters. The issue is that the lip cross section does not span the entire roof and the vertical dimension is small (~.5") relative to the other dimension of the roof. It is probably there to keep the sunroof from deflecting the skin.

With regards to the wing, the thin sheets need to be supported with closely packed ribs. Of course, the spacing is a function of material properties, thickness, cross section, and loading. The cavity the roof skin spans appears to be relatively large, and the material is thin and flat (which was the point of my example of pushing it down). As we all agree, it does provide some structure (and the debate is how much), but I wouldn't lose sleep over it in this case. There is no basis for saying the frame that has been referred to by several people here will not function without the skin. If that was the case, many bridges and buildings would collapse.

Also, monocoque design doesn't mean using thin flat sheets of metal. I keep on saying this: cross section matters. You can press and join flat sheets into relatively complex shapes to increase their stiffness. That is not the case with the roof skin. In a properly caged car, I can't see how the skin itself is a major contributor. Consider the wall thickness and cross sectional arrangement of the cage tubes, and then the thin roof skin sheet spanning 3.5'.

I never said anything against the efficacy of bonding or riveting.

[RAD2LTR]

I'm not sure but wouldn't it be easier to just do a top out of Aluminum? Yes carbon would be lighter if it were paper thin, but you need some material there to give it some strength. Either way you are trying to bond dissimilar materials to steel. You can't weld it. You can however bond it on. I think Aluminum might be a better and cheaper way to go. You can rivet it on (obviously it will require some cleaning up of the tops of the riviets) with a bonding agent added as well. Depending on the type of Aluminum used, it can be very rigid as well as strong. Just ask all the Alfa Romeo GTA racers (I'm talking the 1600 Stepnose GTA of the early '60s) Carbon is funny stuff and if not done correctly, you might as well not do it at all.

If all you are doing is filling in the sunroof hole, I'd still go with Aluminum, its just easier to work with. You still can't weld it to steel, but you can't with CF either. You also have to worry about galvanic corrosion between CF/Aluminum and steel (easily fixed with a layer of fiberglass between the carbon and what ever its bonded to. If this step is skipped, you end up with the carbon delaminating from what ever its attached to) The aluminum might be ok if you use a bonding agent, but it will try to bond with the steel anyway. (Ask anyone who has left a steel bike frame out in the rain all winter with a bare aluminum seat post with no grease on it. Odds are, the seat post becomes one with the steel frame.)

Will

[Jake]

I did say that the lip would add some stiffness (meaning in bending). You didn't quote that. It will have no effect on strength in tension, so the loading condition matters.

It does actually. Twist a flat sheet of paper. Then then try again but this time cut a hole in it and roll the edges. There will be an obvious resistance to deflection in torsion compared to the flat version, especially in the area of the hole.

In that light, a sunroof top is probably stiffer in torsion with its additional framing, but it's much heavier.

With regards to the wing, the thin sheets need to be supported with closely packed ribs. Of course, the spacing is a function of material properties, thickness, cross section, and loading.

I'm sure you do understand the point. The skin and frame form and work as a semi-monocoque. It's just an "example". That is in fact how it works, not exactly the same, but that wasn't the point. A wing is typically of semi-monocoque construction, and so is our roof.

The cavity the roof skin spans appears to be relatively large, and the material is thin and flat (which was the point of my example of pushing it down).

It's convex. Compared to a flat piece of sheet metal it is much more resistant to bending forces. I think you're discounting this.

As we all agree, it does provide some structure (and the debate is how much), but I wouldn't lose sleep over it in this case. There is no basis for saying the frame that has been referred to by several people here will not function without the skin.

The basis is the general rules of a semi-monocoque structure. Is there any strength in the frame without the skin? Of course there is. The convex roof skin also has some strength by itself. The point I'm trying to make is torsional rigidity and overall strength is much higher as an assembly than the sum of each on their own.

If that was the case, many bridges and buildings would collapse.

Bridges and buildings are not monocoque, they are truss. A truss design gets its strength from its frame.

Monocoque:





Truss:

Also, monocoque design doesn't mean using thin flat sheets of metal.

"Monocoque is a construction technique that supports structural load by using an object's external skin, as opposed to using an internal frame or truss that is then covered with a non-load-bearing skin or coachwork"

The roof skin is load bearing. So is the internal frame. Thus...

"Semi-monocoque - A structure in which the loads are carried partly by the frame and stringers and partly by the skin."

I keep on saying this: cross section matters.

Agreed. But the skin itself does not need a wide cross section to function as explained above. And not that you shouldn't build it with a core, the point is that you don't have to.

You can press and join flat sheets into relatively complex shapes to increase their stiffness. That is not the case with the roof skin. In a properly caged car, I can't see how the skin itself is a major contributor.

See definition of semi-monocoque. You don't have to agree, however this is a fact.

Consider the wall thickness and cross sectional arrangement of the cage tubes, and then the thin roof skin sheet spanning 3.5'.

With respect to a caged shell, the roof skin will obviously be less responsible for overall strength percentage wise than the cage itself. I don't think anybody is disputing this.

Jake