[QUOTE]Originally posted by outoforder
[B]Does anyone know for sure how the front and rear subframes are connected to the main chassis?

 

Teh subframes front and rear bolt to the chasssis. During production, the entire driveline and suspension is assembled together and then "mated" to the chassis and bolted down with about a dozen very large bolts.

 

Fullthrottle:Teh subframes front and rear bolt to the chasssis.

Yay! And about cost, check out http://www.composites.com (if your Java interpreter is up to it).

I spec'd a solid laminate sheet, and got $775 for the manufactured part, but they make 4 at a time. (group buy? ) Here's the spec I used:

197 gsm carbon plain 0 degrees
300 gsm carbon tape +45 degrees
300 gsm carbon tape -45 degrees
300 gsm carbon tape +45 degrees
300 gsm carbon tape -45 degrees
197 gsm carbon plain 0 degrees

(It's approximate, since I'm still trying to figure out how to convert from gsm units to GPa to pounds of force across the panel. I've also not determined the resistance of the resins used to heat, specifically the heat from the catalytic converter after you've finished driving the car and park it.)

0.065 inches thick, 0.511 pounds/sq ft. 36" x 100" -> 12.8 pounds for the CF. If the velcro weighs another pound or two, the brace will be a lot lighter than I expected, maybe under 15 pounds.

This might not be the right way to build the brace, since all the attach points are not at the same level under the car. Ideally we'd like the CF panel to rise to meet the higher attach points.

 

I would worry about the oilpan effect that you would have with panels that size. I think a decent
composite sandwich panel with a dual carbon/foam layer. You could more then likely get away with a simple very large vacuum setup but I doubt your mounting method would be doable. You could do a fan out on the leading edge plus add small metal mounting points and where the shear bolt links would be there for giving the bolts something to bite into while still having not fighting the weight issues. I doubt that this would be cost effective, you could tube everything significantly cheaper and more then likely not fight the cracking issues. I doubt that the speeds we reach could have a effect on aerodynamic effect on flat roads but I am reminded of the GTR and the fact that any upward presure at any speed would have disastours effecgts on the handling of the car,, also factoring in wind noise and tunnel testing you would have a prodect that woul require a lot of sacrifices.

 

You will have to vent it. CF is an insulator.

 

Stormbringer,

I can't understand your post. Oilpan effect? And what would the foam in the sandwich be for? Are you worried about damping resonances in the space between the underbody and the panel, like maybe the panel is going to howl at freeway speeds or something? And what's this "simple very large vacuum setup"? Are you referring to the manufacturing method used by composites.com?

I think you're suggesting metal plates laminated into the CF panel, then drilled for bolts that would attach the panel to the existing subframe brace bolt holes. I've been thinking about that as well, but I'm concerned about the different thermal coefficient of expansion of the metal and the CF. I would feel more confident in a solution with a tiny bit of compliance between the CF and the metal bits. Obviously, that compliance is going to allow the chassis to twist a little bit before the fasteners take up the strain, and so the compliance is something that has to be designed and budgeted, which I have not done yet.

StormBringer: ...you could tube everything significantly cheaper and more then likely not fight the cracking issues.

What does this mean? You want to run CF tubes front-to-back, and somehow attach them to the chassis? Torsional stiffness is proportional to the moment of inertia of the cross section. Reinforcing the existing chassis tube is going to get you a lot more torsional stiffness per pound than trying to build your own additive framework, which must be thin to fit under the car.

Oh, and if it's not clear yet, I've pretty much given up on any notion of getting ground effects out of this sheet. My guess is that a dead flat sheet, relative to the ground, will at most generate a few pounds of downforce by presenting a low-drag surface for a bernoulli effect. I don't think I can build anything that isn't dead flat for reasonable sums of money, because I haven't found a existing fabrication service that I can leverage, and I'm sure as heck not going to buy a vacuum oven thingy myself. Nor am I going to build my own teflon molds (unless I can see how to do that cheaply).

Instead of having the CF sheet rise to meet the subframe brace attach points, I'm thinking that I might get special front and rear aluminum subframe braces fabbed, and epoxy the CF sheet onto those. These subframe braces would be pretty much exactly the same as what DiGrappa is already producing, except the bottom of the brace would have wide flat rough pads out the outer edges, vertically aligned to the chassis frame rails. More precisely, they'd be aligned a fraction lower so that the CF sheet velcroed to the chassis rails would be snug against the aluminum.

The attachment procedure would be:

• remove any existing front and rear chassis brace.
  
• position the sheet so that the bolt holes in the aluminum visible through the sheet line up on the bolt holes in the front and rear subframes.
  
• secure and torque the bolts.
  
• bang the velcro closed.

Unfortunately, adding the aluminum bits front and back is going to bring the manufactured cost up to about $1000. I'm still thinking about that.

 

[QUOTE]Originally posted by outoforder
[B]Stormbringer,

I can't understand your post.

 

bbsilver,

I searched MatWeb for the CTE of CF and steel (car chassis).

AISI 1005 Steel (total guess, to match chassis). CTE: 12.6 um/m-degree C.
Average epoxy/CF composite CTE: 12 um/m-degree C. Values range from 9 to 14 um/m-degree C.

My guess is that the distance from the sheet to the center of elongation of the car chassis is about 15 cm, and so if I want to keep vertical hunching to less than 2mm over 100 degrees centigrade, I would need to match the CTE to within 2 um/m-degree C. Which seems pretty reasonable.

Hmmm... so why the warning about epoxying aluminum to steel? Look up aluminum's CTE... 24 um/m-degreeC. YOW!

So here's my prediction about those Cusco front-to-rear aluminum reinforcements: across 100 degrees C variation, they'll make the car center buck up and down over one centimeter. That's can't be good.