heinzboehmer's 2002 Topaz 6MT Coupe

[heinzboehmer]

Got the full SimSolid analysis going to check the stresses on the adhesive. Largest load was with both braces in either tension or compression:



Since everything is either gonna be steel or aluminum, I think I'm gonna use 3M 07333. Datasheet says that ultimate tensile strength of the adhesive is 35 MPa, so we're good! Will run this latest design through all my mechanical engineer friends just as a final check, but it's looking pretty final!

[heinzboehmer]

Did what George Hill suggested and ran the analysis on the earliest version possible. Note that this version is actually fairly recent, but unfortunately, I hadn't modeled the brace attachment point in any previous versions. Previous ones did not have the fasteners in line with the load paths, so they would have definitely performed worse.

I'm not gonna show all the cases like I did before cause it's gonna cause a bunch more clutter. Instead, I'll show the worst one, which turned out to be the "one tension, one compression" case. Material was set to 6061-T6 for all of the following.

Stresses:




Piece that attaches to the windshield is still alright, which isn't super surprising, as this was one of the overkill versions. The other piece is definitely not alright, though! Max stress is higher than the yield strength of 6061.

Safety factor:




Same story here, windshield piece is fine, attachment point is not. 0.87 is not good.

Definitely cool to see the progress!

[heinzboehmer]

Clearances looking good, ready for a test fit:

And finally, ran some quotes and the one that made the most sense was right around $950 all in for 6061-T6 (materials, labor, taxes, shipping, etc.) and $1000 for 7075-T6. $50 extra for 7075? Uh, yes please!

Couple things I should also mention:

Some fab shops (the type that seem to sponsor every single electronics-related youtube video) are way, WAY cheaper than others. Like $300 for everything, it's insane. However, none of these cheap vendors specify T6 for their materials, so who knows what they're using. I think I'm not gonna risk it and instead I'll go with a vendor that tells me the exact material composition of what I'll be getting.

Also, I ran some quick quotes to compare cost for the laser cut + machined version of the windshield piece I briefly discussed above. Bottom line is that I don't think it's worth it, unless you're good at welding aluminum.

For 7075-T6:

• Machined in one piece: $641.42
• Multiple pieces: $409.49 (machined main body) + $40.25 (laser cut bottom sheet) + ~$100 (welding + machining bottom flat labor) = $549.74

So roughly $100 less for the windshield piece if you go the multiple piece route. Even less if you use a cheaper vendor for the machining. I'm personally gonna stick to the fully machined version. Yes, it seems a little crazy to carve away that much material for the flat part, but the extra overhead involved with the other approach is not worth the $100 saved for me.

All this is to say that you could technically get this all made for <$300, but I'm not going to.

[heinzboehmer]

IF you still have time on your simulation trial it might be fun to see how your first design stacks up against the final design. Would show what you learned and how it affected everything starting with a design that "seemed" good to now.

That's an excellent idea. I'll run that analysis soon.

[George Hill]

IF you still have time on your simulation trial it might be fun to see how your first design stacks up against the final design. Would show what you learned and how it affected everything starting with a design that "seemed" good to now.

[heinzboehmer]

I've had good luck with these ties so far with my set of Bry5on ducts. Only street and one track day so far though - fingers crossed.

https://www.mcmaster.com/6614K57/

Thanks! That's basically exactly what I bought

Should have known better than to throw some cheapo ties at them...

[heinzboehmer]

Back to chassis mods. Be warned, there are LOTS of pictures in this one, but as previously mentioned, my FEA SW is on a free trial, so documenting all the results here

Discussed the design of the piece that bonds under the windshield with Bryson and we (mostly him, if I'm being honest) came up with some alternative designs/construction methods.

First did the buckling calcs in an effort to get some more realistic loads for the braces. Approximated wall thickness by measuring the thickness of the flat part, dividing by 2 and then rounding up to the nearest value that made sense (flat is wider than OD of bar, so I'm assuming it's squished out a bit). Flat measured in at 2.75 mm, so I used 1.5 mm for the calcs:

E = 2 x 10^11 Pa (these things are made out of some sort of steel)
I = (pi / 4)(r2^2 - r1^2) = (pi / 4)(0.01275^4 - 0.01125^4) m^4 = 8.175 x 10^-9 m^4
L = 0.45 m
K = 1 (you can see in the above Z4 crash images that the brace buckles right after the flat, so counting that as rotation free and translation fixed)

Plugged that all into Euler's formula:

F = (pi^2 * 2 x 10^11 * 8.175 x 10^-9) / (1 * 0.45)^2 = 79.686 kN

Which is, unfortunately, ~3x higher than the shear strength of the bolts used on the Z4. i.e. Useless

Guess that notch weakens the braces much more than I expected, so time for some more involved analysis. Talked to Bryson and he walked me through the statics of this problem. I don't presently feel comfortable enough with the theory to try and explain it, so I'll just use the number he arrived at for the FEA

Statics says it will take ~21 kN to buckle the notched brace, which tells us the max x component (front to back) of the load is ~15.8 kN and the max y component (side to side) is ~13.9 kN (for each brace).

Alright time to test the new designs. First up, a boxed + welded version of the previous design:



In this design, the CF sheet is replaced with an aluminum one, the ribs are entirely removed and wall thickness is decreased to 2 mm. The two pieces are meant to be welded together, which is what gives it its strength. Despite this being constructed entirely out of aluminum, it's lighter than the previous design! Weighs in at 450 g (0.993 lb) for all three pieces. FEA results follow:

Both braces in compression:




One brace in compression, one in tension:




Both braces in tension:




(Ignoring brace attachment point piece for now, as it needs work at this point. More on that later.)

As you can see, the piece that gets bonded to the chassis is a beast. Biggest stress concentrations are at the bosses, but those can likely be mitigated with some design tweaks. However, this is where I stopped iterating on this design and moved on to the other one.

Next up is a fully machinable, one piece design that gets pocketed from the front/sides instead of from the bottom. The result is a V shape that is super strong along the load paths and also cuts down significantly on weight.

This design can also be extended to be a two piece, weldable assembly (machined top part + laser cut sheet metal bottom part). This should help cut the cost due to the sheet metal part, but for now, I'm just gonna go ahead with the fully machined version.


A little heavier, but nothing that makes me want to trade strength for weight. Both pieces weigh in at a total of 499 g (1.099 lb). FEA results follow:

Both braces in compression:




One brace in compression, one in tension:




Both braces in tension:




The part that the braces mount to is the component under the most stress, but I am less concerned about this piece, as it is easily replaceable. I iterated on it a bunch based on the feedback from the previous FEA runs and this is about as good as it's going to get without having to modify the braces. As mentioned before, the braces are not flat up top and this piece has to accommodate for that. I could take a big hammer (really, a press) to them and flatten them, but this seems like it would make it significantly easier to bend the brace at the interface between the flat and cylindrical sections, so I'd rather avoid it. If I run into issues with the current design, then I'll likely have to go this route.

Quick look at safety factor analysis as well, starting with 6061-T6:

Both braces in compression:




One brace in compression, one in tension:




Both braces in tension:




Not bad. Min safety factor is 1.3 for the brace attachment point and 2.5 for the windshield piece.

Also ran this analysis with 7075-T6:

Both braces in compression:




One brace in compression, one in tension:




Both braces in tension:




As expected, it's much stronger. Min safety factor jumps up to 2.4 for the brace attachment point and 4.6 for the windshield piece.

Happy with the V design. Highly likely that this is going to be what's going into my car. I do want to run one final analysis with the full fledged SimSolid just to double check that the panel bond adhesive isn't going to be subject to loads higher than what it's designed for, but I think it'll be alright.

And finally, ran some quotes and the one that made the most sense was right around $950 all in for 6061-T6 (materials, labor, taxes, shipping, etc.) and $1000 for 7075-T6. $50 extra for 7075? Uh, yes please!

[ethan]

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:

I've had good luck with these ties so far with my set of Bry5on ducts. Only street and one track day so far though - fingers crossed.

https://www.mcmaster.com/6614K57/

[heinzboehmer]

Not sure what exact part is the issue but GSR Motorsports makes a sturdy aluminum part that holds the lower fender liner in place.

https://gsrmotorsports.net/products/spec-e46-coupe-splash-shield-brackets

Ah this brake scoop attaches to the control arm, not the stock brake duct bracket. But thanks for the link, good to know that exists,.

[bigjae46]

Not sure what exact part is the issue but GSR Motorsports makes a sturdy aluminum part that holds the lower fender liner in place.

https://gsrmotorsports.net/products/spec-e46-coupe-splash-shield-brackets

[heinzboehmer]

Wait what am I doing. I just need to print the entire tab laying down...

[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.

[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]

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.