heinzboehmer's 2002 Topaz 6MT Coupe

[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:

[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]

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.

[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]

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.

[heinzboehmer]

Alright, signed up for the 14 day SimSolid trial. Integrates very nicely with Onshape, but it's $675 per month once the trial ends, so I've got a deadline for finishing this project now!

[heinzboehmer]

Drove the car a good amount today and broke a couple things.

First up, the zip tie that holds the right side brake duct upright broke and allowed it to pivot down. Was scraping nicely on the wheel when turning:




Will need to print another one and reattach it with higher quality zip ties, that Bryson recommended.

I also snapped the top piece of my phone holder:



Of course it landed inside the dash, so retrieving it was fun:




This is a tricky piece to print since it needs to be strong along multiple orthogonal axes. I considered experimenting with printing it at an angle, but instead decided to try this approach:



Blue piece will be printed upright (as seen in the screenshot) and yellow will be printed laying down. Yellow is designed to press fit into blue and should help to keep it from breaking again in the same way it did today.