heinzboehmer's 2002 Topaz 6MT Coupe

[bigjae46]

That timing should work. I can print you a piece and send you that.

To be completely honest, I'm not sure that forged carbon is gonna be the best material for this application. This piece will see a decent amount of load and the random nature of the forged carbon makes me a bit uneasy. Would gladly hang the piece on the garage wall if you're still interested in the practice!

As long as there are no voids in the part and there is a high carbon:resin ratio - 60% - forged carbon will be stronger than the sheet metal it will be mounted to. Forged carbon would probably have better performance given the randomness of the fiber orientation.

The part won’t elongate or stretch which I’m not sure is an issue here. Carbon has poor elongation and strength after the carbon yields. Also is not as tough as steel.

[heinzboehmer]

^that sheet metal version doesn't look like it's going to fit in a brake
(not manufacturable)

Oh it for sure won't. Would likely have to leave one of the bends halfway and finish it off on the bench.

[Obioban]

^that sheet metal version doesn't look like it's going to fit in a brake
(not manufacturable)

[heinzboehmer]

Also, been learning as much as I can about designing this piece for strength, while also having it be light and relatively easy to manufacture. Lots of learning to go, but I'll get there. I'm very much the wrong kind of engineer to be designing these kinds of things, but luckily, I have a decent chunk of mechanical engineer friends (a lot of them automotive as well)!

There are a few things that are of concern to me right now (there's likely more points of concern, but I have yet to identify them...):

1. The ribs are placed randomly. I tried my best to put them in the load paths, but the corners of the piece don't line up with them, so there's a bit of a compromise there. I don't know what the implications of this are, need to figure that out.
2. The surface area for the CF to bond to is decreased because of said ribs. Again, not sure what the full implications of this are, maybe it's fine? But I will sleep better when I know what analysis to do to make sure it's good enough.
3. My fillets and bosses are picked kinda randomly. I just guessed at wall thicknesses and stuff for them in the version posted above. I've gotten some feedback from friends and they look much better now, but I need to keep these constraints in mind going forward.

Anyway, in an attempt to address the above concerns, I quickly drew up a couple (wacky) alternative ideas for the mount. Figured I'd document them even though I might not implement either.

First up, a split machined version of the mount:





The idea behind this is that I can keep the full surface for the adhesive to bond to on both the top and the bottom. The two halves would have to be machined with no threads, bonded together along the ribs, post processed (tap holes, clean up any excess adhesive) and then get bonded to the CF sheet.

The extra material on the bottom only adds ~50 g, which sound like a worthy tradeoff. The problem is that I don't actually know how splitting this thing in half and then bonding it back together will affect its strength. Again, more learning required.

Second alternative is to make the entire thing out of sheet metal. Would look something like this:



This design would also require two end caps to box in the sides, along with a bunch of locating tabs everywhere so that it's easy to fixture for welding. Some sheet metal ribs inside along the load paths would probably not be a bad idea either.

Big issue with this is that even without the end caps and ribs, the design already weighs roughly the same as the machined + CF version. Making it this way would really only help with cost and I'm not exactly trying to optimize for that factor with this project (especially after seeing that the CNC'd quote was <$500). Assembly would also be harder as it requires knowing how to weld well instead of just smearing a bunch of adhesive on.

Anyway, alternative #1 is appealing, but more thinking is required before any decisions are made.

[heinzboehmer]

What's your timing? I have 2 more roofs, hood and a mold, and a dashboard mold, and front thrust plate for George Hill to do - 3-4 more weeks for me?

I have about 20lbs of casting resin and it has a limited shelf life. So far, I plan to a compression mold for race seat floor mounts, jack pad, and maybe seat mounts. I should have plenty of material left to do your mount. If you want to give it a try then all I ask is to get rid of the ribs on the backside...I'd have to fill all of that stuff in which would take forever. Estimate about 2-4 weeks to get done. Again, it's going to cost less than $50 in materials...if that.

It would be an honor to contribute something to this build!

That timing should work. I can print you a piece and send you that.

To be completely honest, I'm not sure that forged carbon is gonna be the best material for this application. This piece will see a decent amount of load and the random nature of the forged carbon makes me a bit uneasy. Would gladly hang the piece on the garage wall if you're still interested in the practice!

[bigjae46]

What's your timing? I have 2 more roofs, hood and a mold, and a dashboard mold, and front thrust plate for George Hill to do - 3-4 more weeks for me?

I have about 20lbs of casting resin and it has a limited shelf life. So far, I plan to a compression mold for race seat floor mounts, jack pad, and maybe seat mounts. I should have plenty of material left to do your mount. If you want to give it a try then all I ask is to get rid of the ribs on the backside...I'd have to fill all of that stuff in which would take forever. Estimate about 2-4 weeks to get done. Again, it's going to cost less than $50 in materials...if that.

It would be an honor to contribute something to this build!

[D-O]

"Just went with what the eyecrometer said looked good."

I am using that one. Far superior to the old "calibrated eyeball."

[heinzboehmer]

Make sure you put the carbon in compression if you want it to take load. You’ll need a shoulder area under the carbon on both sides as it will need to be ‘pinched’ - this means your machined bosses should certainly be thinner than the carbon sheet. Frankly, that sheet should probably just be aluminum so you don’t need to worry about bearing area on the carbon. But still make the boss shorter than the thickness of the sheet for sure. I can sketch this for you some time in person if you want the joint to perform as well as it can.

Yep! The bosses are slightly thinner. Once again picked some random number, it's one of the things on the TODO list that I still need to figure out.

Open to making it out of Al as well, just picked carbon cause of the weight.

And yes please! In person design review would be awesome.

[Bry5on]

Make sure you put the carbon in compression if you want it to take load. You’ll need a shoulder area under the carbon on both sides as it will need to be ‘pinched’ - this means your machined bosses should certainly be thinner than the carbon sheet. Frankly, that sheet should probably just be aluminum so you don’t need to worry about bearing area on the carbon. But still make the boss shorter than the thickness of the sheet for sure. I can sketch this for you some time in person if you want the joint to perform as well as it can.

[heinzboehmer]

Some more progress. Design is getting close to final, just need to validate the hell out of it before I send it out to get manufactured and bond it permanently to my chassis.

First up, quick test fit of the braces with the planned cuts marked on them (the 3D printed cut line guides worked great!). Tight, but doable:



Here's approximately where they will land. Silver dots are the locations of the fasteners and the point where they interfere now will be the virtual point where they intersect once cut down:



Attachment points land right at the bottom of the windshield, just as planned!

After the test fit, I got lost in CAD land for a while. Crawled out of there with this in hand:







There is absolutely no science to the ribs. Just went with what the eyecrometer said looked good. Similarly, the wall thickness and radii at the corner of the ribs were chosen essentially at random. I need to validate fitment and general design first, then I'll worry about the more nuanced manufacturing constraints.

Terrible engineering practices aside, the piece should weigh 477 g (1.052 lbs) with the pocketed design! Right in line with my target weight (well, sorta, more on that in a bit).

Prototype is printing out now, with the final (hopefully) tweaks added after a fit check against Bryson's scan. Thanks again for that! Will be super useful from here on out.

Looking pretty good so far:




Also, ran a quick quote out of curiosity and wow, getting this thing CNC'd (in 7075!) is gonna be way more affordable than I was expecting. Was budgeting ~$1k just for the Al piece. I need to double check that the material offered is actually what I want (website is a bit vague), but happy with this:



I was originally going to design a sheet metal version of this piece as well, so that I could compare costs, but definitely not gonna bother with that now.

I should also mention that the piece is designed to only need to be refixtured once. It can be machined by having the toolhead come in from the direction of the red arrow first, refixturing, then having the toolhead come in from the direction of the blue arrow. Could also swap the order of red and blue if the bottom threaded holes are useful for fixturing.


Speaking of threaded holes, you'll notice that the bottom of the assembly has a bunch of random ones in there. Their purposes are the following:

1. The holes that don't poke through the carbon will have bolts sandwiching the CF sheet to the Al piece, helping to keep the adhesive at its optimal 0.2 mm thickness while curing, as well as acting as mechanical fastening when cured. The shallow cylinders along the ribs and above the piece measure 0.2 mm tall, so as long as the sheet is bottomed out against them, the adhesive should cure optimally.
2. The holes that do poke through the carbon are attachment points for a third piece, which is what will actually have the attachment points for the braces. This is yet to be designed, as I have to figure out the angles of everything, but I figured it was worth separating out from the main piece, since it's very likely that I'll iterate on this design sometime in the future. This three piece design will also allow much better access to the components underneath the whole assembly, for serviceability reasons. Note that the three threaded holes are meant to be put in use with time sert inserts installed. Will make them stronger and less likely to strip. Design accounts for the dimensions of those.

Unfortunately, having that third piece does mean that the weight of the entire thing will be >1 lb

Think it's worth it for the reasons mentioned above though, so I'll just live with it. On the bright side, I'm pretty sure I can design this third piece to be laser cut + bent + welded, so that should make it fairly light.

Anyway, all up to speed now. See you when I redesign the entire thing once again!

[Bry5on]

For Heinz:


And for fun :

[bigjae46]

Hell yeah! Let me know how that process works out.

Also, doing it at no charge is crazy. I'd at least pay for the materials

I'd be using waste carbon that I chop up then $5 in epoxy. I think it would be fun!

[heinzboehmer]

I am about to try mold casting. If it's successful, just send me a part. I can cast it and make a forged carbon part from it. I do it no charge because you're doing cool shit.

Hell yeah! Let me know how that process works out.

Also, doing it at no charge is crazy. I'd at least pay for the materials

[bigjae46]

Yeeeaah

I guess it's more a matter of when than if. Fortunately they don't look too hard to remake out of sheet metal. And then I can put the mounting points for everything where I actually want them.

But at this point I kinda just want to see how long these will last

I am about to try mold casting. If it's successful, just send me a part. I can cast it and make a forged carbon part from it. I do it no charge because you're doing cool shit.

[heinzboehmer]

Been thinking about adhesive choice for the windshield piece. I think I've pretty much decided on it being made of CF and 6061 Al, so that helps narrow it down a bit.

I kinda want to go with 3M 07333 because I can get it easily and it's sold in smallish package, so won't end up with too much waste. However, 3M says it's only applicable for Steel and Aluminum, with no mention of CF (see attachments for more). So I think I'm just gonna go with the Sika 7888 L10 again. It's kind of a pain to get and comes in a humungous package, but it meets all my criteria.

Anyone planning on doing a carbon roof soon that wants to split the adhesive? Would be nice to do my own roof at the same time, but I don't think the timelines are gonna line up, unfortunately.