BMW i3 vs. Chevy Bolt -- Structural Efficiency ?

[electrons]

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]

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?

 

[alohart]

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.

 

[electrons]

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/a15113673/2014-bmw-i3-first-drive-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:

.... 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]

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.)

 

[agzand]

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]

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/56141560/2019-chevrolet-bolt-ev-lt-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]

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.

 

[electrons]

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]

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.

 

[electrons]

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

You're not adjusting for the 4% larger Bolt, among other things.

To compare vehicles that are somewhat close to each other like this to start with, you still have to add 4% mass to a BMW i3. So, 2,920 lbs x 1.04 = 3,037 lbs for the new larger BMW i3 to match the Bolt's external dimensions & interior volume.

Now add 334 lbs to get the bigger battery on our i3: 3,037 + 334 lbs = 3,371 so we now weigh 192 lbs less than a Bolt.
We'll need more bracing to handle the new massive battery pack. Guessing here that's 30 lbs needed.
We're within 162 lbs of a Bolt now.

However, if we want the stellar Roof Strength that the Bolt has, we need a genuine B-pillar due to concerns the door-B-pillar could pop out under stress.
So we've got 162 lbs to work with to add additional bracing for roof strength, trying to match the Bolt's. We can do it with 50 lbs.
We're within 112 lbs of a Bolt.
All that expensive CFRP buys us 3% weight savings. GM wins this.

 

[alohart]

All that expensive CFRP buys us 3% weight savings. GM wins this.

For those of us living in humid, salty environments, BMW wins this. There's not much to corrode on an i3 unlike on a Bolt.

 

[eXodus]

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

Correct, Carbon fiber behaves completely different then steel.
You can drive a Formular 1 car with 200mph directly into a wall and the driver survives.

Comparing the roof strength test is just wrong. That's like comparing a basketball with ball made from concrete. Sure the concrete ball will hold out a lot more crushing force.
Yet the you can roll that basketball down the side of hill and it will always stay somewhat in shape while the concrete ball shatters to pieces.

You need to actually roll those cars - and let them flip a few times. I think the i3 is better in protecting the passengers - because it's flexible - not because it's strong.

Further the carbon fiber life model of the i3 is really light - the aluminum frame it sits on - packs most of the weight.

 

[electrons]

CFRP is brittle, not ductile like steel.
CFRP is amazingly strong for it's mass, yet only in tension loading, not in compression.

I'd say the poor strength-to-weight performance of the i3 is due to:

1. Lack of an attached genuine B pillar. The thick rear suicide door pillar helps, although it pops out from the weatherstriping, not loaded much from a roof hit. It does appear to stay in place in a side collision (IIHS.org), loading up the far side of the thick section in tension.

2. A lot of compression loading occurs in a collison. BMW had to make the CFRP thick enough to take some compression, not it's forte, hence driving up the mass.

3. The Bolt uses modern high strength steels and GM is very clever with finite element analysis these days with their structures, placing in stamping folds & curves to use steel's inherent ductility for energy absorption while being stiff enough to resist deformation. Remarkable what they've done. Think about it: The Bolt carries a nearly 1,000 lb brick in it's belly, yet still only weighs 550 lbs more than a 2017 Chevy Cruze (both same passenger volume).

A lot of the aerospace applications that CFRP has performed well in involve tension loads. CFRP wing boxs & skins, with a thick enough airfoil section, can be made to handle more bending & twisting loads than aluminum, with less weight. Race cars with CFRP have bathtub or tube shapes to create tension, achieving high levels of stiffness for handling. The i3 has way too many large door openings in comparison, placing much of the loads low into the rocker panels.

 

[eXodus]

The Chevy Bolt is a great vehicle. GM knows how to built efficient cars. They mastered the bean counter industry.

You are also comparing two cars from different time frames. The Bolt came out in 2017 and the i3 in 2013 (actually more like 2012, yet took a while to ramp up production ( https://www.press.bmwgroup.com/usa/article/detail/T0124925EN_US/bmw-at-the-82nd-geneva-motor-show-2012?language=en_US) so there are 5 years of development in between.

The i3 was an industry first - the first mass produced carbon fiber car and never got much love from BMW after that.
I'm sure if they would have optimized for a successor the carbon fiber tech would have been lighter an stronger.

https://www.youtube.com/watch?v=UNMng1BbcPk

Carbon fiber was all the rage 5-6 years ago. But nobody adopted it, so no R&D money and thus no improvements.

Is the i3 a perfect car? No way. (the doors have 4 layers of various plastics/metal - not optimal so save weight, the floor tub could have been carbon instead of alum) It's a great concept car, which somehow made it into mass production. I still don't get that

 

[electrons]

Carbon fiber was all the rage 5-6 years ago. But nobody adopted it, so no R&D money and thus no improvements.

True for mass produced vehicles. Carbon fiber hit Formula 1 & aerospace & a few other areas in about 1981, so it's a 40 year old technology. I remember seeing it explode in popularity in the early 80's at McDonnell-Douglas (F-18, AV-8B, etc.) and in research at my university.

The BMW i3 saves around 15% mass compared to a modified steel Ford Fiesta chassis converted to an EV:

2017 Ford Fiesta, nearly identical interior volume as a BMW i3:
Fiesta: Curb Weight 2,720 lbs
Remove the engine + transmission + fuel system + exhaust system: subtract 400 lbs to get 2,320 lbs
Put in the BMW i3's motor & power electronic modules & copper: add 200 lbs to get 2,520 lbs
Put in the BMW i3's 613 lb Deep Blue 40kWH battery to get 3,133 lbs
Add a rocker panel droop & bigger wheels/tires & some battery mounting braces: Add 200 lbs to get 3,333 lbs
Beef up the new Fiesta EV's suspension & steering rack for the extra mass: Need 60 lbs, so now it's 3,393 lbs.
A Ford Fiesta Hatchback EV would need to weigh about 15% more than a BMW i3, or 3,393-2,920 = 473 lbs heavier as a steel Fiesta

That's a result I'd expect. I still don't know how GM's Bolt got that good.

 

[agzand]

Those who have experience with better road bikes or high end mountain bikes would know that steel is no match for carbon fiber in weight and stiffness. Aluminum is half way between steel and carbon fiber but it is not as durable as CF or steel. Even higher end metals like titanium are obsolete as a frame material when absolute performance is needed. A carbon frame weighs about 50% of a comparable steel frame. Most CF frames have a lifetime warranty as well.

 

[electrons]

When talking CFRP applications like bicycles or cars, as mentioned already in this thread, tension along the fibers is what CFRP is best at. .... Bike frames do not have collision requirements as semi-monocoque car bodies do, which need high shear & compressive energy absorption, attributes CFRP doesn't have. BMW had to thicken the side & corners of the vehicle quite a bit, adding weight. .... I blame those requirements for the failure of the i3 body to compete with a Chevy Bolt all-steel one.

I"m not sure why the Ford Fiesta did much worse than the Bolt. Could be the Al non-stressed parts on the Bolt, if used on the Fiesta, might save 200 more pounds, making the comparison closer

In a car, BMW can achieve basic chassis stiffness requirements & still fail side & corner collision requirements unless BMW adds more mass to areas taking compression and seeing shear forces at the edges of the battering rams.

Bike frame triangular shapes carry a lot of tension loads, hence they work well in that application. CFRP tubes, arranged in a triangle, performs very well.

 

[eXodus]

When talking CFRP applications like bicycles or cars, as mentioned already in this thread, tension along the fibers is what CFRP is best at. .... Bike frames do not have collision requirements as semi-monocoque car bodies do, which need high shear & compressive energy absorption, attributes CFRP doesn't have. BMW had to thicken the side & corners of the vehicle quite a bit, adding weight. .... I blame those requirements for the failure of the i3 body to compete with a Chevy Bolt all-steel one.

Two things I mentioned before:

I'm assuming that the i3 does much better in real world crashes then the bolt - from pictures from junked cars. Saw multiple collapses passenger spaces in the bolt and none on the i3. You can roll the i3 down a hill side with 10 flips and the passenger compartment stays stable. The bolt does good at the first flip - but fails after 2-3 roll overs. Guess what - the average roll over is hardly ever ONE.

Most of the weight of the i3 comes from the aluminum frame (drive module) the carbon fiber sits on. Which is completely overkill for the application.

That thing is so overbuilt - really interesting material to study.
Guess why BMW did that - it's the first "skateboard" the rumor is that initially there where like 2-3 other cars planned on that platform.
If you imagine a slightly longer 1 or 4 series type car on that frame. - It must be stronger - to still use the same parts.

 

[electrons]

Since the Bolt takes 7,000 lbs more force (to displace 5"), it's safer in rollover accidents. Junkyard surveys aren't accurate. Accidents vary.

Guess why BMW did that - it's the first "skateboard"

The first "skateboard" was in 2001 by GM. Tesla copied the concept with prototypes in 2010, production in 2012. BMW's after that. http://www.adrianchernoff.com/project/electric-skateboard-platform/ I think they couldn't fully patent it since it's too obvious though.

What was BMW thinking when they decided to build a CFRP body & sell it for way above other competitors at the time? It did have a Cool Factor, marketing. Still, most people wouldn't care or notice. BMW had to know the 40 year old CFRP tech would result in something only 2%-15% lighter than a metal vehicle. At the time (2010), Jaguar & (2012) Tesla were into Al, followed by Ford F150 in 2015. If BMW had gone all-Al, they'd get the same results at less cost.