electrons
Posts: 48
Joined: Mon Mar 21, 2016 4:49 pm

BMW i3 vs. Chevy Bolt -- Structural Efficiency ?

Thu Mar 04, 2021 4:13 pm

Any engineers or techies out there that can explain why the i3 doesn't do better? Check my analysis & 'splain, if you're into structures at all.

As many of you know, the i3 has carbon fiber composites making up just about all the upper parts of the chassis. Aluminum & a little steel are used on the rolling chassis, low to the ground, so most of the body is advanced composites, not metal.

The 2017-2022 Chevy Bolt uses all-steel for its body-in-white, with only doors, hood, fenders, and 1 or 2 more parts, all non-load-bearing really, being aluminum.
Mostly steel like almost all cars on the road. Not even as advanced as the aluminum upper-body structure on Ford F-150's for pete's sake.

OK, so composite body vs. steel body. Which results in weight savings? I calculated what a BMW i3 would weigh if it were identically sized to match a Bolt (they're only 4% different anyway, but I take it into account).

A Bolt fits into a rectangular box of 419 cubic ft, and an i3 fits into a box of 403 cubic ft, so the Bolt is bigger by a factor of 419/403 = 1.04, or only 4% bigger.
An i3 (no ReX) weighs 2,972 lbs, so scale it up by 4% more to get 3,091 lbs if it was the same size as a Bolt.
Add extra mass to match the 960 lb massive battery in the Bolt. Actually just add 385 lbs to an i3 for the delta difference in battery pack weights between the 2 cars. (Remember Bolt has a much longer range & kWH battery, so you gotta make that adjustment of course.)

So that puts our slightly-expanded (by 4%) BMW i3 with the more massive battery pack at 3,091 lbs + 345 = 3,476 lbs for our new i3.
Now, the Bolt weighs 3,563 lbs, almost the same!!!! And the Bolt has a much stronger roof crush strength (IIHS.org 5.74 vs. 4,72) !! Can anybody explain this?

eNate
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Re: BMW i3 vs. Chevy Bolt -- Structural Efficiency ?

Thu Mar 04, 2021 10:01 pm

I'm hoping this thread come up with some good theories (not that it's bothering me -- but you've got me curious!).

And as I was mulling over ideas, I checked, and the Bolt has slightly higher horsepower and torque than the i3, so it ought to have a roughly comparable drivetrain weight.

About the only worthwhile difference I could come up with is the Bolt's unibody construction vs. the i3's body-on-frame. But after some web searches, I could only find mentions of "considerable weight savings" without any actual data. The best modern day equivalent I could think of is the Honda Ridgeline pick-up (unibody) vs. a Toyota Tacoma, similar HP, bed size, and seating, and they're roughly equal weight.

'course there's the heavy glass paneled lift gate that's got to be worth — what, 800 lbs? :)
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alohart
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Re: BMW i3 vs. Chevy Bolt -- Structural Efficiency ?

Thu Mar 04, 2021 11:54 pm

A 2014 U.S. BEV's curb weight was listed as 2,640 lb. on BMW USA's Website. That must have been for a Mega World with no options and 4 of the lightest 5" wide pizza slice wheels and 155 mm tires. The 120 Ah battery pack is only 88 lb. heavier than the 60 Ah battery pack. Features that were optional in 2014 but standard now include DC fast charging, heated seats and battery pack, universal garage door opener, and Comfort Access, all of which add a bit of weight but unlikely enough to reach 2,972 lb. The heat pump was standard in 2014 but optional now which would reduce the curb weight. I don't know any LCI changes that would have increased the curb weight significantly in 2018. I wonder whether BMW is now using Europe's curb weight standard that includes a 165 lb. driver. Anyway, I am not confident that a new BEV weighs 2,972 lb.
Aloha,
Art

2014 BMW i3 Arravani Grey, Giga World, Tech + Driving Assist, Parking Assist, DC Fast Charging, JuiceBox EVSE

electrons
Posts: 48
Joined: Mon Mar 21, 2016 4:49 pm

Re: BMW i3 vs. Chevy Bolt -- Structural Efficiency ?

Fri Mar 05, 2021 11:08 am

OK, my mass numbers might be off. I'll take another look at it.

Car and Driver reviewd a 2014 BMW i3 years ago when it was all new, smaller battery, etc., and said:

"BMW calls its skateboard the Drive Module, and fashions it almost entirely from aluminum. The framework surrounding the 450-pound lithium-ion battery pack, the front control arms, and even the bolts that attach the chassis to the body are all aluminum. Only the rear suspension links are steel. Atop the Drive module sits the Life module, which is crafted primarily from carbon fiber. The body structure and roof panel are made from that stuff, while the body panels are composite. Throw in some other lightweight materials, such as a magnesium dash brace and hemp-reinforced interior plastics (maybe that’s why the drive program was conducted in Amsterdam), and the whole thing (with range-extender) weighs about 2900 pounds. That’s roughly 450 less than a Nissan Leaf and in the same mass neighborhood as a Chevy Spark, an entirely conventional car that is a bit smaller than the i3. Remember that business about BMW core values? Naturally, those 2900 pounds are evenly distributed between the front and rear axles." -- https://www.caranddriver.com/reviews/a1 ... ve-review/

I think the 450 lb battery pack weight they mention (22 kWH battery from 2014) includes all the metal hardware surrounding the battery, not just the cell weight, BTW, it's the whole unit battery pack. ...... For comparison, the 21 kWH LG Chem pack in the Ford Focus Electric of that same era was 640 lbs, so 450 lbs sounds like they could be leaving something out. Not sure. Or it just beat the old LG Chem pack by 200 lbs(?).

When the IIHS crushed the roof of a 2019 BMW i3 (not ReX), they actually weighed it on their own scales at 2,920 lbs, & since they do that in the U.S., I assume that was without a driver, IOTW, a U.S. style "curb weight".

Similar to a 2019, a 2020 BMW i3 (not Rex) is listed by Car and Driver as "2,972 lbs", and they now say a 2014 i3 (not ReX) is 2,860 lbs.

So going from 2014 to 2020, the higher 120 Ah battery packs do add mass, as expected.
Here, from the Deep Blue BMW i3 120 ah battery Torqeedo will sell you, straight from the BMW-Dingolfing factory, is:
Image
.... which is the complete stressed-member Battery Box weighing 613 lbs. Compare to a Bolt's battery box at 960 lbs (or 947 lbs for their latest version according to GM), also a stressed box package. We could also add in a little extra BMW i3 structure to handle our imaginary battery swap for the bigger Bolt battery, probably around 30 lbs extra structure needed for bracing.

From what I can see, getting info from multiple sources, the numbers I used originally are right.

electrons
Posts: 48
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Re: BMW i3 vs. Chevy Bolt -- Structural Efficiency ?

Fri Mar 05, 2021 11:14 am

Maybe view it another way.
Take a Bolt & remove the battery. That's 3,563 - 960 lbs = 2,603 lbs.
Take an i3 & do the same: 2,920 lbs (IIHS.org's weighed value) - 613 lbs (Torqeedo) = 2,307 lbs
To be fair, as mentioned above, the Bolt is 4% bigger than an i3, 2307 × 1.04 = 2,399 lbs

They're already within about 200 lbs of each other.
Now, lets add 31 lbs to the i3 to thicken the rocker panel frame area to hold the bigger (Bolt's) battery.
Now the BMW i3, expanded by 4%, and capable of handling a massive 960 lb battery like the Bolt has, is 2,430 lbs.
Let's now give the BMW i3 the ability to score a roof crush standard as big as the Bolt's ((IIHS.org 5.74 vs. 4.72). I'll guess we need 100 lbs of thicker A,B,&C pillars on the i3 to get there. So that makes 2,530 lbs.

That puts us at:
Bolt: 2,603 lbs, no battery on board.
i3: 2,530 lbs, no battery & no ReX.
Diff = 73 lbs, not much.

I'm accustomed to seeing a real strength-to-weight advantage when using CFRP vs. HSS (& UHSS). Back in the 80's I was working on the AV-8B Harrier program, and the new carbon composite wing & forward fuselage saved only 10% mass, yet the wing could be loaded up 50% more than the old AV-8A Hawker-Siddeley all-metal version. Much stronger wing. (I was working on unrelated AV-8B engineering tasks, not on composites; not involved directly.)
Last edited by electrons on Sat Mar 06, 2021 12:14 pm, edited 1 time in total.

agzand
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Re: BMW i3 vs. Chevy Bolt -- Structural Efficiency ?

Sat Mar 06, 2021 12:50 am

I think you have it right there, BMW body is 300 lbs lighter, other adjustments are not relevant and do not contribute much to the weight difference. Also compare the sound of shutting the door or tailgate in i3 to the sound of Chevy, there is another 100 lbs in there.

It is also driven by design of the car, BMW doesn't have a fixed B pillar, that will add to the weight. If they had used a fixed B pillar probably they could exceed the Chevy roof strength and reduce weight at the same time. It is like a convertible, hey no roof so it must be lighter, but usually convertibles are 250-300 lbs heavier than the identical coupe.

eXodus
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Re: BMW i3 vs. Chevy Bolt -- Structural Efficiency ?

Sat Mar 06, 2021 4:33 am

electrons wrote:
Fri Mar 05, 2021 11:14 am
Let's now give the BMW i3 the ability to score a roof crush standard as big as the Bolt's ((IIHS.org 5.74 vs. 4.72). I'll guess we need 100 lbs of thicker A,B,&C pillars on the i3 to get there. So that makes 2,530 lbs.

I'm accustomed to seeing a real stength-to-weight advantage when using CFRP vs. HSS (& UHSS). Back in the 80's I was working on the AV-8B Harrier program, and the new carbon composite wing & forward fuselage saved only 10% mass, yet the wing could be loaded up 50% more than the old AV-8A Hawker-Siddeley all-metal version. Much stronger wing. (I was working on unrelated AV-8B engineering tasks, not on composites; not involved directly.)
Former Firefighter with Highway duty here: IIHS Crashtests do not really good translate into real crashes.

A rollover is no static scenario like the roof strength test. The CFRP of the i3 is probably very likely strong enough to protect the occupants in a roll over.
Most rollovers are more then one.

I think Volvo is the only manufacturer which tests their cars for multiple roles. Volvo has a team which goes out and studies real crash sites and optimizes their cars for the real world. While most others do the minimum to get 5 stars.
For example - the 2003-2015 XC90 did only have 4 stars - but almost nobody ever died in this car - which is Volvos mission.
While plenty of people died in other 5 star cars. Any 20 year old Volvo is still better then any todays build pickup for passenger safety. If with lots of stars and awards.

I would suggest going on Copart or any of the other car wrecking websites and look at crashed i3s. So far I have not seen one collapsed life module. (how the CFRP cell of the i3 is called) Compare this with a few Bolt pictures:

https://www.copart.com/lot/56141560/201 ... ca-vallejo

I see intrusion into passenger compartments all the time.

While crashtest are important - it is actually more important to know how you product is fairing later when people really mess up.

Literally the only thing I think is lacking in the i3 - are the seats. The head restraint and back is not that great. I'm looking to fit in some better rated seats.

electrons
Posts: 48
Joined: Mon Mar 21, 2016 4:49 pm

Re: BMW i3 vs. Chevy Bolt -- Structural Efficiency ?

Sat Mar 06, 2021 5:43 am

No B-pillar, that's it. That's what's going on here. If BMW built that in, the i3 would get better side impact and roof strength. Still would just equal a Bolt though. B-pillars do add mass. Back to square one.

Also, carbon fiber really only works well when loaded in tension, not compression.

No doubt the BMW i3 has adequate Roof Strength, certainly in single-vehicle roll-over accidents. It's good enough. The i3 has a 4.72 SWR.

What is the SWR? (Strength-to-Weight Ratio)

It merely measures the ratio of the force required to crush the roof down 5", to the curb weight (to normalize it). Define this as "Breaking the Roof".

Let's look at this simple thing another way:
How much force does it take to break the BMW i3's roof? 4.72 x 2,920 lbs = 13,782 lbs crush force on the top of the roof
Now for the Bolt: 5.74 x 3,563 lbs curb weight = 20,452 lbs

There is a lot of difference in 20k vs. 13k lbs. The Bolt, with it's old-fashioned majority steel body, can handle 7,000 lbs more force.
Last edited by electrons on Sat Mar 06, 2021 6:54 am, edited 2 times in total.

electrons
Posts: 48
Joined: Mon Mar 21, 2016 4:49 pm

Re: BMW i3 vs. Chevy Bolt -- Structural Efficiency ?

Sat Mar 06, 2021 6:22 am

agzand wrote:
Sat Mar 06, 2021 12:50 am
I think you have it right there, BMW body is 300 lbs lighter, other adjustments are not relevant and do not contribute much to the weight difference.
The Bolt is bigger, both inside & out, compared to an i3. That 4% difference counts. Then, the extra structure (bracing) needed in the i3 to carry a bigger battery like Bolt has (960 lb brick vs. 613 lbs) requires extra mass.

Build in a B-pillar on the i3, and now the BMW i3 is as heavy as a Bolt.

Adjustments add up.

1. Bolt is bigger, add 4% to the i3's weight to compensate.

2. A few more pounds added to the i3 to brace a heavier Bolt-like battery. I guessed 40 lbs needed, maybe it's 30 lbs, not much either way.

3. B-pillars (to meet roof strength parity with the Bolt) could be about 50 lbs extra total (2 needed).

4. Put the heavier, higher kWH battery from the Bolt into an i3 at this point, and the weights, Bolt vs. i3, are about identical.

agzand
Posts: 43
Joined: Tue Dec 22, 2020 3:00 pm

Re: BMW i3 vs. Chevy Bolt -- Structural Efficiency ?

Sun Mar 07, 2021 12:41 am

i3 has a massive B pillar. It is not fixed to the roof though. So it doesn't provide the same roof strength. It is probably much heavier than a fixed B-pillar of the same material. Providing a big aperture adds to the weight, it does not save weight.

As I said above a convertible doesn't have a metal roof but it is much heavier than a coupe of the same model. And obviously it has lower roof strength.

My guess is that an i3 with 66 kWh battery will weigh around 3200 lbs, still 300-400 lbs less than a Bolt.

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