heinzboehmer's 2002 Topaz 6MT Coupe

[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.

[heinzboehmer]

I like your bracket idea, why not commission it to a machine shop and have a billet set that will last forever, unless you have to use their safety benefit (:

Mostly cost. Laser cut parts will be ~$50 after bending and powder coating. Billet part will be multiple orders of magnitude more.

I might do a billet version in the future, but I have a suspicion that I'll want to iterate on the design down the line (like always) and would rather not end up with multiple useless billet pieces.

[maupineda]

I like your bracket idea, why not commission it to a machine shop and have a billet set that will last forever, unless you have to use their safety benefit (:

[heinzboehmer]

Hell yeah, thanks. Have two sets of those, so should be easy. We'll find a time.

(Have two sets because the early mounts have press in studs vs screw in and figured the screw in ones would be easier to work with)

[Bry5on]

Easy enough. Get me a pair of strut mount castings in the next couple weeks and I’ll machine them down for you one evening.

[heinzboehmer]

I'd just be designing the sheet metal parts now if I were you

Thick flat sheet with a mild bend and some tab/slots for gussets, some quick welds and you'll be good to go.

I've been giving some thought to this and for some reason, I really want to try and keep the stock strut bar brackets. That voice in my brain that tells me to make everything look factory can be annoying sometimes...

Anyway, I think I might try going for an intermediate approach that lands somewhere in between of "small reinforcement bracket" and "full replacement". Basically do the thick flat sheet part you were talking about, but make it so that the stock piece bolts up to it and essentially acts as the gusset. Would just need to drill out the threads in the stock piece and add studs to the sheet metal so that it bolts through.

Would also likely need to machine down the "feet" on the stock parts so that the stock bar doesn't move up significantly:



Getting too far ahead of myself though, first need to finalize the design for the windshield bracket and see where the E86 braces land with those final dimensions. Then I can see move on to the (first) final design of the front brackets.