heinzboehmer's 2002 Topaz 6MT Coupe

[heinzboehmer]

For the load case, you can just calculate the bucking load for the beam. All you need is diameter, length, wall thickness and material. Since it’s notched, it will fail below this number because it has an initiation point. That way you can use a real number and get a real safety factor.

Added to TODO list!

I would leave the ribs for bonding area, and leave the extended surface to stiffen up the windshield sheet to avoid peeling adhesive as much as possible. That’s the place that makes the most sense to go overkill to me. You could also tab and slot the machined part and sheet then just weld it up. That would allow you to ignore bonding area internal to the part.

You bring up an excellent point that I did not consider yesterday. We ran the analysis assuming the top surface of the aluminum piece was fixed in place, not bonded. I'll mess around to see if I can produce something useful that takes into account the adhesive area.

Also, ran some super quick tests in CAD just to get an idea of the theoretical min weight of the assembly. Baseline is the overkill design, which weighs 579 g total.

First up, extended surface fully removed, but everything else the same. This saves 111 g (19%):



Next up, extended piece removed, ribs removed and wall thickness decreased to 2mm. This saves 235 g (41%):



This means that a realistic minimum weight I can get this assembly to is 400-450 g, which raises an interesting question: do I really care about saving max ~200 g? The weight is being added to a terrible spot in the chassis, but the potential tradeoff of saving the 200 g is ending up with a subpar piece that will fail and essentially require a chassis replacement. It might make more sense to focus my attention on saving weight in other pieces (CF cabin air filter housing and firewall plug for instance).

I still want to run the FEA on everything (because new toy to play around with), but I'm not sure I'll actually end up making any changes to the design.

[Bry5on]

I would leave the ribs for bonding area, and leave the extended surface to stiffen up the windshield sheet to avoid peeling adhesive as much as possible. That’s the place that makes the most sense to go overkill to me. You could also tab and slot the machined part and sheet then just weld it up. That would allow you to ignore bonding area internal to the part.

For the load case, you can just calculate the bucking load for the beam. All you need is diameter, length, wall thickness and material. Since it’s notched, it will fail below this number because it has an initiation point. That way you can use a real number and get a real safety factor.

[heinzboehmer]

Thats some great news! Really looking forward to this especially if and im sure it will work with the OE cabin filter housing (i have the OE CSL One). Also I love the cleaner looking engine bay this will give and the OE CSL intake will look so much nicer on Full display. Thanks for working on this.

Thanks! Looking forward to having this done and installed on the car as well.

I don't think this will work with the stock cabin air filter housing, unfortunately. Gonna need to chop it up to make clearance. Ultimately, I want to print a mold and make it out of prepreg CF, but that's a project for the future.

Also, just to be clear, I plan on running this in addition to the stock strut bar, but I guess you could run it in place of if you wanted to.

[0-60motorsports]

Sat down with a friend (who works for an OEM) and he walked me through doing FEA on my design. Learned a ton, so documenting here for my own sake.

We didn't really know what the typical loads that these braces would see were, so instead we approached the analysis by using the max load they'll ever see. Assumptions follow:

1. BMW designed the E86 braces to bend in a crash, so the max load my design will ever see is just below the load that causes the braces to fail.
2. The braces must fail below the shear strength of the fasteners that hold them to the chassis, otherwise the feature would be useless.
3. BMW fastens these braces with class 10.9 M10 bolts on the E85/E86 chassis. Shear strength for the bolts is 23.2 kN, which is the absolute theoretical load limit for the braces (real limit is below that due to the bending).
4. Braces are at a 46.4 deg angle outward from the centerline of the car towards the strut towers.
5. Materials for all my pieces are 6061 T6 aluminum. Figured we would do the analysis on the worst case (in terms of 6061 vs 7075) just to see.
6. 6061 tensile yield strength is 265 MPa (bit of extra padding cause why not).

With all this in mind and a bit of math, we get that the max x component (front to back) of the load is 16 kN and the max y component (side to side) is 16.8 kN (for each brace).

Plugged all this into the FEA tool, set up all the connections and constraints and done. Analysis came back saying what every mechanical engineer I've shown this to has said: my design is way, WAY overkill.

I forget the actual numbers, but the piece that bonds to the windshield has an insane safety margin (think 10 or so). Also, as expected, the piece that the braces bolt to sees the most load by far, but was still only seeing around half the tensile yield strength for peak load. Keep in mind that all these numbers come from loads that the brace mounting points will never see, since the braces are designed to bend before the fasteners fail.

What this all means is that I can make this design significantly lighter, so lots of iterations coming up!

The following is the initial list of TODOs:

• Remove that aluminum extension on the passenger side. I thought this would help with distributing the load to the sheet, but simulation said it was basically useless.
• Decrease wall and rib thickness. It will be interesting to find a good tradeoff between overkill and too little bonding area.
• Play around with the thickness of the flange that the studs press into.

I'll likely target a safety factor of 1 for future simulations, since the normal loads the assembly will see are way below the peak loads I'll be plugging in to the SW.

And just for fun, a couple images I found online to substantiate the claim that the braces will bend before the fasteners fail:

Thats some great news! Really looking forward to this especially if and im sure it will work with the OE cabin filter housing (i have the OE CSL One). Also I love the cleaner looking engine bay this will give and the OE CSL intake will look so much nicer on Full display. Thanks for working on this.

[heinzboehmer]

Sat down with a friend (who works for an OEM) and he walked me through doing FEA on my design. Learned a ton, so documenting here for my own sake.

We didn't really know what the typical loads that these braces would see were, so instead we approached the analysis by using the max load they'll ever see. Assumptions follow:

1. BMW designed the E86 braces to bend in a crash, so the max load my design will ever see is just below the load that causes the braces to fail.
2. The braces must fail below the shear strength of the fasteners that hold them to the chassis, otherwise the feature would be useless.
3. BMW fastens these braces with class 10.9 M10 bolts on the E85/E86 chassis. Shear strength for the bolts is 23.2 kN, which is the absolute theoretical load limit for the braces (real limit is below that due to the bending).
4. Braces are at a 46.4 deg angle outward from the centerline of the car towards the strut towers.
5. Materials for all my pieces are 6061 T6 aluminum. Figured we would do the analysis on the worst case (in terms of 6061 vs 7075) just to see.
6. 6061 tensile yield strength is 265 MPa (bit of extra padding cause why not).

With all this in mind and a bit of math, we get that the max x component (front to back) of the load is 16 kN and the max y component (side to side) is 16.8 kN (for each brace).

Plugged all this into the FEA tool, set up all the connections and constraints and done. Analysis came back saying what every mechanical engineer I've shown this to has said: my design is way, WAY overkill.

I forget the actual numbers, but the piece that bonds to the windshield has an insane safety margin (think 10 or so). Also, as expected, the piece that the braces bolt to sees the most load by far, but was still only seeing around half the tensile yield strength for peak load. Keep in mind that all these numbers come from loads that the brace mounting points will never see, since the braces are designed to bend before the fasteners fail.

What this all means is that I can make this design significantly lighter, so lots of iterations coming up!

The following is the initial list of TODOs:

• Remove that aluminum extension on the passenger side. I thought this would help with distributing the load to the sheet, but simulation said it was basically useless.
• Decrease wall and rib thickness. It will be interesting to find a good tradeoff between overkill and too little bonding area.
• Play around with the thickness of the flange that the studs press into.

I'll likely target a safety factor of 1 for future simulations, since the normal loads the assembly will see are way below the peak loads I'll be plugging in to the SW.

And just for fun, a couple images I found online to substantiate the claim that the braces will bend before the fasteners fail:

[heinzboehmer]

Mooore changes.

First up, moved the front fastener bosses to also be in line with the load paths. I hadn't done this before because I had misunderstood the rule of thumb for placing holes near edges of a piece (remember, I don't actually know what I'm doing):



Test fit for everything printed. Love that mcmaster has models for all their parts, makes designing the clearances super easy:




(Yes, those are M10 threads grafted onto an M8 bolt + washer combo. The piece that gets bonded under the windshield is designed with M10 threads for timeserts, but the bolts holding the brace mounting point will be M8. Easiest way to get everything assembled and test fit fastener head clearance at the same time is with these frankenfasteners.)

I also updated the brace cut templates and made the first modification to one E86 brace:



Unfortunately, the hole is a couple mm off center. I guess that's what I get for trying to eyeball the drill hole from a sharpie mark instead of double checking with calipers. I'll likely redo this in the future, but gonna leave for now cause I'm mostly using these parts to validate my designs. Good news is that these braces came on every E85/E86, so there are tons available.

Here's the assembly on the car. Note the washer to space the brace down. I modeled the boss as tight to brace as possible and left the studs long so that I could decide on optimum spacing in real life. Upcoming iteration has a slightly taller boss for better fastener clearance:



That angle actually exaggerates how close everything is, it's not terrible with the washer in there (still super tight in the grand scheme of things though!):



I ended up landing on approximately two washers of boss surface extension:



For the test fit above, I printed low profile cap head fasteners, but I'll be able to get normal sized fasteners in there with the extra clearance, for maximum strength.

And here's where the brace lands at the strut tower side after the trim and all the other adjustments:



The point where those two silver lines intersect represents the furthest (center) point I can put a fastener at, so that its bearing surface stays fully within the flat portion of the brace. It just barely works! Should also have enough space to slot the new hole in the brace to allow for chassis/alignment differences.

Finally, here's the latest version of the design. Boss extended, real ribs modeled and fastener clearances in check with the taller cap heads:

[heinzboehmer]

Now I’m clearer on what you are doing. Carbon could absolutely be superior to aluminum. Easy Composites did a test on forged carbon, its tensile strength not that much lower than a 2x2.

Interesting, I'll have to look into it

[bigjae46]

Interesting.

There's two very distinct load paths for this piece though, so wouldn't the random fiber orientation be a detriment? Compared to orienting the fibers so the part is stronger in those directions.

Now I’m clearer on what you are doing. Carbon could absolutely be superior to aluminum. Easy Composites did a test on forged carbon, its tensile strength not that much lower than a 2x2.

[heinzboehmer]

Designed a super quick fit test piece to check clearances and angles of everything:



I put the outer surfaces of the dark blue test piece right where I thought they would interfere with the HVAC components and gave myself juust enough clearance for tools. Here's everything on the car:



You can tell I measure/transcribed incorrectly because I had to remove the blower motor cover to test

Also, not gonna lie, swapping the blower motor is gonna suck with this in place. Totally doable (after unbolting the dark blue piece), but its gonna be really tight.

Next came the most exciting moment of this whole project. This is actually gonna work!



Also, test fit revealed a bit of extra room that I didn't know I had, which is great. It allows me to adjust the position of the fasteners slightly and put the rear fastener right in line with the load paths.



(the two circles along the dotted lines represent where the fasteners for the brace will be)

As you can see, I also got rid of that top rib, as it really wasn't doing much. The two diagonal ribs beside it should be able to handle all the longitudinal load the mount will see.

Next up was actually designing the attachment point for the braces. Here's what I came up with:



This will also need to be machined out of Al, but should be fairly easy to make.

Also, the brace mounting points are designed so that I can press in two M10 studs. Doing studs instead of threads for bolts allows me to bring the braces as close to the main mount as possible, which is highly desirable.

The funky shape of the surfaces the E86 braces bolt to are a function of fastener clearance as well as clearance for the braces themselves. Braces are not perfectly flat up top, so had to design for that:



And here's how everything will fit together:




This angle really shows just how tucked up the braces are, there's not a lot of room to go much higher:



Next round of prototype pieces are being printed now. Excited for the next test fit!

[heinzboehmer]

More feedback from people who know what they're doing incorporated:


Pocket is designed to be machined out with a 6mm diameter ball end mill. Fillets are 5mm radius.

With the new changes, there is a ton of material on the bottom surface for the CF sheet to bond to. CAD says the bonding surface is 48% of the entire bottom surface area of the part (3942 mm2 vs 8174 mm2). Feel a lot more comfortable bonding it like that.

Weight is up slightly, but still hovering around the 1-1.1 lb mark.

[heinzboehmer]

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.

Interesting.

There's two very distinct load paths for this piece though, so wouldn't the random fiber orientation be a detriment? Compared to orienting the fibers so the part is stronger in those directions.

[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.