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A Tesla Model S burns after damage  (Source: AJ Gill/YouTube)
Tesla wins high ratings in collision tests, but reportedly committed errors in its armor design

When it comes to Bayerische Motoren Werke (BMW) AG's (ETR:BMW) electric i3 Coupe, interest in the vehicle, coupled with BMW's decision to boldly put the Coupe out in the hands of consumers for testing has led to a great deal of press -- some of it mixed. 
 
When it comes to safety, the vehicle's credentials have been called into question.  Some have questioned whether EVs in general, much less the i3 Coupe, are safe.
 
I. Battery Fires and the Need for Armor
 
After my CES test drive of the BMW all-electric entry-level luxury vehicle, I had a deep discussion with BMW engineers and sales staff.  This discussion cast some light into the vehicles design, and why BMW is convinced that it has designed a safer EV.

i3 Coupe low wide

Our conversation was set against the backdrop of growing reports scrutinizing the safety of EVs.  From a simplistic standpoint, such reports are understandable.  As the computers industry and aircraft-builder Boeing Comp. (BA) can attest, lithium-ion batteries can be highly flammable if they overheat or short-circuit thanks to damage, design flaws, etc.
 
Tesla Motors Inc. (TSLA) learned that lesson the hard way.  While its Models S earned industry leading crash-test scores, it was plagued by at least three recent battery fire incidents.  Some of these incidents boiled down to user error after drivers hit objects, penetrating the battery housing and damaging the cell.  The vehicle informed the users that they shouldn't drive, but they did anyways -- and it let them.


Tesla Model S

But some culpability does lie with Tesla as well.  Specifically it equipped a vehicle with very low clearance with only a quarter-inch (6 mm) thick aluminum panel over the batteries, which lie along the floor in the middle of the vehicle.  One our regular readers, Reclaimer77 pointed this flaw out, commenting:

Depends on what the metal in question is. In this case Tesla opted for aluminum.

http://blog.caranddriver.com/tesla-model-s-fires-m...

So when the ~5,000 pound Model S runs over something, and a thin soft piece of aluminum is all that stands between all that force and the batteries - it's no wonder.

Aluminum is just a very poor choice for such an application. I understand the need to save weight, but come on.... Musk needs to do a recall and replace every aluminum plate with a steel one.

A Model S is seen here after its weak aluminum plate is penetrated by debris.
[Image Source: Petrus Breedt]


The "old rule of thumb" is that steel is twice as strong, but three times as heavy as aluminum. 

II. The Cost of Using Weaker Aluminum Alloys

Alloys can improve the elastic modulus of aluminum, fighting penetration.  A clue to what alloy Tesla might have used comes in publicly released comments that 25 tons of force were required to punch a 3 inch hole. 

Tesla Model S hole punch
Objects have only been able to punch through because Tesla used a cheap, light aluminum plate. [Image Source: NC State/Lattice Energy LLC]

Assuming a clean punch, the theoretical relationship between tensile strength is:

F= pi * d * t * s --> s = F/(pi * d * t) ;
where d = diameter of hole, t= thickness, s = tensile strength, F= force

25 tons ≈ 222 kN ; 3 in = 0.0762 m ; 1/4 in = 0.00635 m

s = 222,000 / (3.14 * 0.0762 * 0.00635) = ~146 MPa

So we know that the tensile strength of Tesla's armor plate was at most 146 MPa -- a strength that suggests that it is not steel.  Tesla later acknowledged that it used aluminum in the plate, but the question remained what alloy was used.  Some alloys are almost as strong as steel -- others not so much.  Tesla's armor plate seemed to fall under the  "not so much" category.

Tesla Model S
Only a thin sheet of aluminum stands between the battery pack and the road in the Model S.
[Image Source: GM-Volt]

A publication [PDF] in AluReport casts further light on the metal used in the armor plate:

Now resistance spot welding has joined the rank of large-scale aluminium joining technologies already mentioned: DeltaSpot® by Fronius...

Tesla Motors also uses the state-of-the-art DeltaSpot® resistance spot welding process in its Limousine Model S to produce a large number of two-and three-layered joints between aluminium sheets, sections and castings [4].

All of the large-scale application mentioned above, however, relate to established AlMg and AlMgSi wrought alloys and AlSiMg die-casting alloys [4].

So this tells us Tesla used a wrought aluminum plate of an aluminum alloy with magnesium or magnesium+silicon.  Looking at various alloy fact sheets [PDF], and knowing that the Tesla plate has a tensile strength of under 146 MPa, it's clear that the automaker didn't even use the strongest available alloys (which likely were more expensive).

An analysis by Lattice Energy LLC suggests that had Tesla used ferritic stainless steel, instead of a low-cost aluminum alloy, it would have been able to withstand up to 71 tons of impact force on the plate without breaking.  And that's not to mention that even stronger aluminum alloys have lower fatigue strength, meaning over time it will be weakened to a greater degree from repeated non-penetrating blows.

III. Tesla Had Options, Chose the Cheap Route; BMW Picks Safety Over Cost-Savings

There are alternatives -- such as using a thinner plate, but backed by an aluminum space frame (a technique that could produce a stronger sheet), or using more expensive alloy blends.

It'd be misleading to say cost was the only reason Tesla made this choice.  As mentioned, aluminum may be only a third as strong (roughly) as steel, but it's also half the weight.  Switch to steel would likely add around 800 pounds to the vehicle weight, reducing the range and performance.  But even if you make that argument, it seems foolish to trade safety for performance in a mass market vehicle. 

Elon Musk
Tesla chose cost, speed, over safety. [Image Source: AP]

Factor in that more expensive alloys could have deliver steel-like strength with the weight advantages of aluminum, and you come to see that Tesla indeed threw caution to the wind and chose a cheap option.  Now it's paying the consequences.

Tesla's choice is all the more baffling in that it otherwise designed a vehicle that has great frame integrity, putting up incredible numbers in various crash tests.  It'd be like painting the Mona Lisa, only to scribble a pine tree next to her in quill pen.  Tesla deserves praise for the safety of the ovearall design, but rebuke for its choice of armor plate.

While the i3 Coupe doesn't really need to be as damage resistant due to its substantiall higher clearance than the Model S, it goes the extra mile, where Tesla chose the cheap route.  Namely, BMW chose to put a quarter-inch thick high-strength steel armor plate over its batteries.

Steel vs. Alumnium
For those who skipped Eng. 101, here's a demonstration of why steel is a better choice than aluminum for armor applications. [Image Source: Lorenz Industries]

You won't see this on any crash test under current standards (which regulators may look to revise after the rash of recent battery fires from multiple manufacturers), but by better safeguarding the battery pack from damage, the i3 Coupe is indeed in some ways safer than Tesla's Model S.

IV. Debunking the Mini Myth

BMW also wanted us to debunk some of the misinformation that some confused readers were spreading on the original blog.  Specifically, one common misconception among many people is that the i3 Coupe is "built on the Mini platform."

One reader wrote:

It's a Mini with an electric motor.

Another wrote:

Yeah. You're off to a bad start when you base it around a Mini platform.

Mini Cooper
To say the i3 Coupe is a Mini Cooper (pictured) is pretty misleading.
[Image Source: HD Wallpapers]

The confusion is understandable.  BMW does own the Mini nameplate.  And unsurprisingly some recent editions of the Mini Cooper (e.g. the Paceman) shares some style cues with the i3 Coupe.  And it's true the drivetrain is a descendant of the Mini E -- the electric Mini variant that was sold in 2008.

But the Mini E and BMW i3 Coupe do not use the same drivetrain.  BMW in fact has a had three major drivetrain iterations, since 2008.

V. The Evolution of the EV at BMW

BMW's first drivetrain was found on the Mini E -- essentially a Mini Cooper equipped with a mod kit.  While the Mini E was an "official" BMW, fans had been making their own electrified Mini Coopers by several years at that point.

So the Mini E was a start, but it wasn't terribly revolutionary.

The Mini E Drivetrain


[Images Source: Mini/BMW]


Mini EThe 2011 Mini E in Germany [Image Source: Matt Blume/Wikimedia Commons]

The Mini E featured:
  • Vehicle Size
    • A 3,300 lb (1,225 kg) vehicle load
    • 97.1-inch (247 cm) wheeel base
  • Electric Motor
    • 200 hp
    • 160 lb-ft of torque
    • Front-wheel drive
    • 0-60 mph 9 s
  • Battery
    • Replaces the trunk and back seat
    • 35 kWh
    • 5,088 cells /48 modules (106 cells per module)
    • Air-cooled
    • Charge Time
      • @110 V : 18-20 h
      • @220 V : 3-5 h
    • U.S. Environmental Protection Agency (EPA) range of 99 mi (160 km)
 
The BMW Active E Drivetrain
 
At the 2010 North American Internation Auto Show (NAIAS) in Detroit, Mich. BMW showed the next step in its EV development path.  Built on the 1-Series platform, the BMW Active E sedan was a major step forward.

Notably it introduced the under-the-seat style flat battery pack which is also used by Tesla.  Looking at the battery pack, motor, and charge times, it's much closer to the i3 Coupe than the crude Mini E.

BMW Active E

BMW ActiveE
1 -- Control electronics        2 -- High voltage cable
3 -- Battery: tunnel module  4 -- Battery: rear module

The BMW Acitve E featured:
  • Vehicle Size
    • A 3,968 lb (1,800 kg) vehicle load
    • 172.2-inch (437 cm) wheeel base
  • Electric Motor
    • 168 hp
    • 184 lb-ft of torque
    • Rear-wheel drive
    • 0-60 mph 8.5 s
  • Battery
    • Under the seat
    • 32.0 kWh
    • Larger format cells
    • 192 cells/3 modules (64 cells per module)
    • Water-cooled
    • Charge Time
      • 4-5 h at 230 V, 32 A
      • 8-10 h at 230 V, 16 A
    • U.S. Environmental Protection Agency (EPA) range of 99 mi (160 km)

The i3 Coupe Drivetrain
 
In July 2011 BMW unveiled the i3 and i8 electric vehicles in concept form.  Remarkably little has changed about these vehicles as they reached production, other than leverage advances in battery cells, and other minor improvements.

The i3 refines the Active E's two-part battery pack, merging it into a single flat sheet in the tunnel region, protected by the aforementioned sheet of solid steel.


 
BMW i3 Coupe

BMW i3 Coupe

BMW i3 Coupe drivetrain
 
BMW i3 Coupe
 
BMW i3 exterior side
The 2014 BMW i3 Coupe EV [Image Source: Jason Mick/DailyTech LLC]

While the i3 Coupe has:
  • Vehicle Size
    • A 2,635 lb (1,195 kg) vehicle load
    • 101-inch (257 cm) wheel base
  • Motor
    • 170 hp
    • 184 lb-ft of torque
    • 0-60 mph 7.2 seconds
    • Rear-wheel drive
  • Battery
    • 22 kWh battery
    • Refrigerant-gas cooled
    • Large-format cells
    • 96 cells/8 modules (12 cells per module)
    • Charge Time
      • 6 h at 240 V, 32 A
      • 8-10 h at 240 V, 16 A
    • (tentative) range of 80-100 miles in "comfort" mode, 90-110 miles in "Eco Drive" mode
BMW's Head of North America Operation and Strategy further confirmed that this is actually BMW's third generation electric drivetrain, drawing from both the 1 Series and Mini lines.

The i3 is a true third generation electric drivetrain.  If you must compare the most accurate comparison would be to say that i3 Coupe is somewhat like a riff on Mini in terms of the hat (body stylings), but in terms of the base is an improved version of the 1-Series electric drivetrain (from the ActiveE).

Major design improvements over the Active E include the confinement of the battery to a tighter under-the-seat package, and a downsizing of the control electronics.

i3 Coupe charging on DC

The refrigerant cooling is another new addition, and one that BMW says makes the vehicle safer.  If the steel armor is punctured, the refrigerant gas will quickly escape leaving the vehicle undriveable.  While Tesla does include flame-resistant layers between its cells, its liquid coolant leaks slower when punctured allowing drivers to foolhardily drive off in the injured vehicle.

Both BMW and Tesla use flame-retardants in their battery packaging and feature modular designs, which prevent fires from spreading.

VI. Third-Generation Electric Powertrain Aims for the Top

Mr. Harb bragged:

In terms of [Tesla's Model S] it's using technology similar to our Mini E.  That's two generations behind... In terms of maximizing the benefits of an electric powertrain, I don't think anyone is close to us.

That's a pretty bold statement, so let's examine the Tesla's Model S stats:
  • Vehicle Size
    • A 4,647 lb (2,108 kg) vehicle load
    • 116.5-inch (296 cm) wheel base
  • Motor
    • 416 hp
    • 443 lb-ft of torque
    • 0-60 mph 7.2 seconds
    • Rear-wheel drive
  • Battery
    • 60 kWh battery (in basis configuration)
    • Water cooled
    • Small cells
    • ~7,000 cells /16 modules (440 cells per module)
    • Charge Time
      • 6 h at 240 V, 32 A
      • 8-10 h at 240 V, 16 A
    • (tentative) range of 80-100 miles in "comfort" mode, 90-110 miles in "Eco Drive" mode
The Model S is clearly a performance beast with a 310 KW motor.  By contrast, the i3 Coupe relies on a lighter 125 KW motor. The Tesla vehicle's battery is also 2.7 times the capacity of the BMW.  But based on the specs you could argue that the BMW is equally efficient (or even more efficient) than the Model S, albeit with only two-fifths of the horsepower/torque and with about half the range.

More objectively, though, the Tesla, BMW, and Nissan Motor Comp., Ltd.'s (TYO:7201) Leaf EV are all arguably "third generation" powertrains as the LEAF saw a major bump in a 2013 model year and the original leaf was preceded by an internal test powertrain.  Likewise Tesla's Model S is the third generation model, descended from the Roadster powertrain, which in turn was descended from early prototypes.


 Tesla Roadster and Model S [Source: Tesla Motors]

Unsurprisingly, all three matured electric drivetrains share some common optimizations -- namely, putting the battery pack beneath the drivers and passenger's seats and module battery pack designs.  Like the Leaf, the i3 Coupe uses a smaller battery to drive a lighter vehicle.  But like the Model S it's rear-wheel driven (like the Model S), versus the front-wheel driven Leaf.  Rear wheel drive is one reason why at lower speeds the i3 Coupe is able to feel somewhat sporty.

Nissan Leaf
Nissan Leaf EV

The one area where the BMW i3 Coupe scores a clear win over Tesla and Nissan is in the safety of the battery pack. 

But what about traditional crash test safety and recent criticism regarding the i3 Coupe?  Let's look at what kind of ratings the BMW i3 Coupe has received thus far and what they mean.

VII. Crash Tests, and the Recent European Score


One gold standard of how safe a vehicle is can be found in the National Highway Traffic Safety Administration (NHTSA) crash safety ratings.  One concerned reader on the previous piece writes:

Poor reporting?

The NHTSA gave Tesla Model S 5 stars in all categories.

The BMW i3 got 4 stars ratings in the Euro NCAP. Even a Nissan Leaf got 5 stars in the Euro NCAP (4 stars in NHTSA).

Let's dig into this criticism.  First, the 2014 BMW i3 Coupe has not yet received a safety rating from the NHTSA.  Don't believe me?  Hop on over the to the NHTSA crash safety page and see for yourself. 

Second the European crash test examines two things that the American crash tests don't -- pedestrian safety and safety assist.  The primary reason that the BMW i3 was bumped down to 4 stars in the European crash test ratings is they scored low in each of the categories.

The Safety Assist category particularly hurt the i3 Coupe, which scored a 55 percent.  Had it not been hit so hard in this category it almost surely would have achieved a five star overall rating.  I'm basing this on General Motors Comp. (GM) who scored lower in every other category than the i3 Coupe, but managed to earn the extra star via a gleaming 86 percent rating in the Safety Assist rating.

Makes sense right?  Well, not entirely.  You see, the safety assist category is based on whether or not a vehicle checks off a series of features many of which fall under the "safety nanny" category, such as speed limiters and seatbelt warnings. 

The i3 was knocked points for lacking a seatbelt warning, which I suppose is slightly bad, but really bears no impact on how a belted passenger is affected by a crash.  More ridiculously, the i3 was knocked points for not having a speed limiter.  Having driven the i3, I really don't think that was a valid critique.  After all, the car is pretty much inherently speed limited to a top speed of 80 miles per hour or so (in comfort mode, 70 mph or so in "eco" mode).

VIII. Europe EV Crash Tests -- Child Safety

Let's assume that most buyers are responsible and are only secondarily concerned about pedestrian safety -- their top priority is safeguarding their friends and loved ones in the vehicle.  Thus the meat of the Euro NCAP ratings is the adult and passenger ratings.  Here's how various models stack up:
  1. Volvo V60 PHEV --176
  2. Nissan Leaf ------ 172
  3. Toyota Prius ----- 170
  4. Renault` ZOE ---- 169
  5. BMW i3 Coupe --- 167
  6. Chevy Volt --------163
  7. Peugeot iOn* ----- 151

* A rebadged Mitsubishi Motors Comp. (TYO:7211) sold under Peugeot SA's (EPA:UG) Peugeot and Citroen brands
^ Volvo AB (STO:VOLV-A) (STO:VOLV-B)
` Renault SA (EPA:RNO)

Clearly the BMW was a bit behind, but it was close enough that it's worth digging into what the crash tests actually said.  Let's first compare the Volt, the Leaf, and the BMW i3 Coupe in the child safety category:

BMW v. Volt v. Leaf

So clearly the i3 Coupe and Leaf score better in child safety than the Volt due to actual crash performance.  Both the i3 Coupe and the Leaf score the full 40 points, indicating more or less near-perfect performance inc crashes, easy installation, and sufficient instrucitons.  But the EU regulators award Nissan two more percentage points -- what gives?

The regulators write:

A disabling system is available as an option for the front passenger airbag, allowing a rearward-facing child restraint to be used in that seating position. However, the information provided to the driver regarding the status of the airbag is not sufficiently clear. The risks of using a rearward-facing seat in the front passenger seat without first disabling the airbag are clearly indicated in the car.

In other words the only "safety" flaw in terms of child safety in the i3 Coupe is it doesn't have an airbag status indicator, even if you can turn of the airbag.  So that must be where the BMW gets docked.  Actually no; regulators write for the Leaf:

The passenger airbag can be disabled to allow a rearward facing child restraint to be used in that seating position. However, information provided to the driver about the status of the airbag is not sufficiently clear. Permanently attached labels clearly warn of the dangers of using a rearward facing child seat in that seat without first having disabled the airbag.

At first I guessed from the descriptions that EU regulators didn't like that customers could remove the label regarding the airbag disable seat.

But looking at the top performer (the Volvo V60 PHEV) it gets the same score as the Leaf -- despite performing a fraction below the top rating in one of the two major crash tests:
Volvo V60
And the regulators write:

The passenger airbag can be disabled by means of a switch. However, information provided to the driver regarding the status of the airbag is not sufficiently clear. The dangers of using a rearward-facing restraint in that seat without first disabling the airbag are clearly indicated on labels in the car.

So so much for the "permanently attached" hypothesis.  As far as I can tell, it appears that EU regulators after concluding that the Leaf and i3 Coupe were equivalent in crash tests decided to arbitrarily assign more points to Nissan and less to BMW.  And both BMW and Nissan should be irritated by Volvo's rating, when the actual science tests showed the Volvo was the inferior member of the trio.

Had points actually logically followed the performance, the ratings become:

  1. Volvo V60 PHEV --173
  2. Nissan Leaf ------ 172
  3. Toyota Prius ----- 170
  4. BMW i3 Coupe --- 169
  5. Renault` ZOE ---- 169
  6. Chevy Volt --------163
  7. Peugeot iOn* ----- 151
Given the problematic nature of the child crash ratings, let's move on to the adult crash test ratings.

IX. Europe EV Crash Tests -- Adult Safety

Here's the adult crash test results for the first (Volvo V60 PHEV) and second (Nisan Leaf) placed peformers, plus the i3 Coupe, and the Volt:

NCAP ratings

Compared the Volvo V60 PHEV, which is significantly better than the rest in terms of rear impact whiplash, the rest are in a virtual tie (with the Leaf/Volt at 2.9 and the i3 Coupe at a 2.8).

The Frontal impact is where things get interesting.  On a six part model the Volvo V60 is again ahead, scoring at worst  6 "adequate" ratings (on three parts of driver and passenger dummies, a piece).  The BMW appears the next best -- it has only one marginal part (the driver's leg) and two adequates.  It actually receives more "Good" (the best rating) than the Volvo V60.

By contrast the Chevy Volt received 4 marginals and 4 adequates, while the Leaf received 3 marginals and 1 adequate.  So they should get worse ratings than the BMW right?  Well, the Volt does.  But mysteriously the Leaf is awarded a tie (13.6 points) with the Chevrolet Volt.  I'd expect something more along the lines of a 12.5 for the Leaf and a 13.5 for the i3 Coupe.

The i3 Coupe reportedly had "weak" protection for the driver's chest when it came to side pole impacts.  This is a relatively rare type of crash, but let's just leave that rating as is.

X. Conclusions From Review of the EU Crash Data Reports

With the adjustments based on the number of "marginal" ratings in the frontal impact rating we get:

  1. Volvo V60 PHEV --173
  2. Nissan Leaf ------ 171
  3. BMW i3 Coupe --- 170
  4. Toyota Prius ----- 170
  5. Renault` ZOE ---- 169
  6. Chevy Volt --------163
  7. Peugeot iOn* ----- 151
Now, I admit there's a flaw you might have noticed in my methodology.  I only tweaked the Volvo, Nissan, BMW, and Chevy ratings.  The Toyota, Renault, and Peugeot might similar shift.  But at the very least based on the publicly available data on the four models we examined, the i3 Coupe looks to have perhaps unfairly received the short end of the stick.

That's not to say the i3 Coupe is perfect from the information we've seen thus far, safety-test wise.  It appears to have a weakness to pole crashes. 


But also be aware that the EU pole test is performed by carrying the car at 18 mph towards a rigid pole, which is struck at a 90 degree angle [source].  The U.S. crash test is arguably far more relevant to the most common kinds of pole collisions, angling the car at 75 degrees [source]. 

At worst I'd guess the i3 Coupe will get one four-star rating in either the side or frontal impact NHTSA tests.  It's almost certain to score a five star score on the rollover tests.  So if I had to make an educated guess, I'll guess that it will earn a five star rating, but not the "perfect" five star rating the Model S received.


But ultimately, regulators decision to rough up the i3 Coupe in the rankings also demonstrates the shortcomings of current safety tests for battery vehicles
.
Road Debris
Road debris can be deadly to a poorly protected EV like the Model S. [Image Source: HBarber]

Road debris collisions are arguably at least as common as pole collisions.  With lithium ion batteries, this typically irritating form of accident can become a serious safety risk.  Compared to the air-cooled Leaf or the alumnium-armored Model S, the i3 Coupe would likely score far better if battery safety were considered.

Unfortunately, crash safety test regulators don't currently think it's worth considering the safety of a highly flammable lithium ion battery pack in their testing.


Comments     Threshold


This article is over a month old, voting and posting comments is disabled

Issue
By Keeir on 2/3/2014 2:53:10 PM , Rating: 2
In describing a punch out test, you refer to a shear strength allowable as a "tensile" strength allowable. This is not true. Tensile Strength refers to a straight pull.

Based on the comments that it should have taken ~21 ksi of shear stress, it appears likely that Tesla is using a form of 5 series or 6 series (likely 6061) which is significantly less capable than other forms of aluminum alloys such as 7 series, (40 ksi+ shear ultimate).

quote:
And that's not to mention that even stronger aluminum alloys have lower fatigue strength, meaning over time it will be weakened to a greater degree from repeated non-penetrating blows.


Sigh. Not true. Unless your running over debries at a rate of several times a day, your unlikely to stress most aluminum enough to have reduced impact resistance. Corrosion or unrepaired damage sure. But not due to fatigue damage.

quote:
While the i3 Coupe doesn't really need to be as damage resistant due to its substantial higher clearance than the Model S, it goes the extra mile, where Tesla chose the cheap route. Namely, BMW chose to put a quarter-inch thick high-strength steel armor plate over its batteries.


No.... Tesla's aluminum was likely more expensive than BMW's steel. The issue BMW has is that aluminum + certain types of Carbon Fiber do not mix well in a corrosive enviroment. BMW didn't have the option of using aluminum, but titatium. BMW chose the "cheaper" route, which is also safer/heavier, and probably is the correct choice.

The issue isn't the thickness of the plate. In a typical airliner, aluminum skins are in the range of .032" thick. Even a wide-body like an A340, B747, etc has skin below .050" thick. This plate in the Tesla was .25" thick. Aluminum is very soft (rockwell hardness) and prone to "gouge" damage. Aviation spent years developing methods to protect aircraft aluminum from impact, incluing cleaning runways and automatic air turnbacks/emergency landings. If the item hitting the aluminum plate is a hard material with an inclined plane, that object will slice through the aluminum regardless of the aluminums "strength" value. Even low strength plastics can wear away at aluminum. This is a lesson learned that wouldn't show up in FEM modeling or standard analysis.

Tesla needs to slap on (using glue and a few cherri rivets) a thin sheet of a hard material. Either stainless or titatium. Once the intitial bite of the object is blunted, the aluminum is strong enough to absorb quite a bit of damage.

For instance, consider that "25 tons". That's roughly equal to 10 model Ss being applied to a 3" diameter circle. Or 5,307 lbs/sqin. The same impact force on most floor plates would lead to complete penetration into the engine/cockpit area.

As more months go by without additional incidents, it becomes more possible that it was a string of bad luck, not a critical design flaw. Something to definately improve for the future, but given that the 800lbs is probably a decent estimate for the additional wieght, I doubt Tesla will be moving to full steel solution. It would add an extra ~50 dollars of energy cost a year, or fleet wide cost of millions just to avoid a few accidents without injuries in a year.




RE: Issue
By Mint on 2/4/2014 2:52:02 PM , Rating: 2
You brought up some very good points, like distinguishing the difference between shear strength and tensile strength. This subject matter is obviously well beyond Jason's expertise.

Regarding your solution, pure aluminum is very soft, but alloys are much less so. I seriously doubt gouge resistance is a problem, or there would be more problems from general rocks and more common but less severe impacts.

Tesla's floorpan is far safer than that of regular ICEs, which generally user 16-gauge steel (about one quarter the thickness of Tesla's plate) or thinner. This is what happens when metal debris hits a regular car's floor:
http://www.youtube.com/watch?v=nJUWXRWK4xs

I'd rather have to pull over and deal with a slowly developing fire than have a debris impale my leg.

Oh yeah, Jason, this is the dumbest thing I've ever seen you write:
quote:
But even if you make that argument, it seems foolish to trade safety for performance in a mass market vehicle.

Yeah, let's all ride in 10-ton tanks to avoid this "foolish" tradeoff.


RE: Issue
By Keeir on 2/4/2014 7:16:39 PM , Rating: 2
quote:
Regarding your solution, pure aluminum is very soft, but alloys are much less so. I seriously doubt gouge resistance is a problem, or there would be more problems from general rocks and more common but less severe impacts.


No... even alloys are "soft"

6061-T4 (likely the alloy Tesla is using) has a Brinell Hardness value of 65.

Most steels have hardness values in the 100s on the same scale, with relatively cheap steels capable of 500.

Titaniums also tend to be in the hundreds range.

Even 7175-T6 (a very strong alumium alloy at 2-3 times stronger than 6061-T4) has a hardness value only 150, far below steel and titanium.

The "solution" is pulled straight from the aviation industry, which has dealt with impact/gouge damage to aluminum structural members for 75+ years. Pretty sure if the peice of structural that keeps an aircraft in the sky flying at 500+ mph didn't need additional protection, it wouldn't get it.


RE: Issue
By Mint on 2/7/2014 12:39:26 PM , Rating: 3
quote:
Pretty sure if the peice of structural that keeps an aircraft in the sky flying at 500+ mph didn't need additional protection, it wouldn't get it.
Gouge damage to a .05" skin on an aircraft carrying hundreds of passengers at 600mph is orders of magnitude more relevant than gouge damage to a Tesla's .250" floorpan.

None of the Model S fires would have been saved by your solution. A thin steel skin would not add anything to puncture resistance from a levered trailer hitch, or an airborne car hitting a concrete fence.

Hardness is not the relevant metric here. Batteries get damaged from plate deformation. The factors there are yield strength (tensile and shear), second moment of area, thickness, cavity dimensions, etc.

If we get spontaneous fires in future from rocks smacking and wearing throught the underbody, then you will be right, but otherwise your definition of "soft" is irrelevant. Just because steel is harder than 6061-T4 doesn't mean the latter is soft, just like glass having Brinell hardness of ~1500 doesn't make steel soft.


RE: Issue
By Bubbacub on 2/18/2014 11:22:45 AM , Rating: 2
in terms of resistance of the surface of the underbody to plastic deformation hardness is exactly the metric that needs to be assessed.

tensile strength is not really the issue.

agree with other comments on this thread that the article reads like a bmw advert.

p.s. no accusing anyone of taking dodgy payments - i'm sure Jason did this without any cash from bmw, it is just Jason's way to write such blindingly biased articles (in order to generate clicks) that they look like adverts for the subject material.


RE: Issue
By Mint on 2/26/2014 4:36:17 PM , Rating: 2
Hardness is a material property, irrespective of how that material is shaped.

Deformation depends on a whole bunch of engineering decisions, like thickness, support beams, etc.


RE: Issue
By Keeir on 2/28/2014 9:45:04 PM , Rating: 2
I'll try once more.

So far Tesla has 3 incidents of fire.

1.) Mexican Wierdness
2.) Steel Trailer Hitch at 70 mph. Pack penetration?
3.) Seattle Road Debris.

Now, clearly in situation 1 and 2 we might be dealing with gross deflection. Very possible. Situation 3 seams unlikely to be gross deflection. There is also the possiblity that situation 2 was not gross deflection either.

In the case of gross deflection, typically aluminum is only ~30% weaker than steel. Thus a .25" thick 6061 plate resists deflection similiar to a .17" of stainless steel/etc. This on the surface seems adequate even for situation 2.

The issue here is that when flat plates are hit to deflection, the shear load must be transfered to the boundary stiffeness/reaction points.

In the case of a sharp/jagged peice of steel hitting alumium, the initial "cut" into the aluminum significantly reduces the aluminum's ability to transfer this shear load. While the gross section might be fully capable, this reduced section might not be. In the case of a tool steel/construction steel type trailer, you might easly get 50% penetration into the soft aluminum plate, resulting in significantly less ability to resist shear at that location. (As well as increase local defection of loads that can be resisted)

As I have been saying, adding a thin strip of hard material will prevent this reduction in capability of the underlying aluminum. Thus increasing real world resistance to impact/road debris with relatively low wieght penalty. (Moving to a .17" steel plate would add ~250-500 lbs of curb wieght)

Telsa seems to have gone the route of increasing distance from road. Thereby lowering impact number of likely the force of impacts. However, this will increase Aerodynamic drag resulting in lowered range. This is an easier solution since its just software, but its only a matter of time until there is another situation 3... which should be avoidable. (Situation 1 and 2 are acceptable. Hitting a trailer at 70 mph should not be a design consideration)


RE: Issue
By ramuman on 2/20/2014 8:07:04 AM , Rating: 2
So this article:

- Starts off with an uninformed discussion on material strength
- Goes into a random diatribe about EV power trains
- Talks about EU safety standards with the Tesla not even having been tested there
- Finishes off by calling the Tesla potentially deadly when not one person in a Model S has even been seriously injured, much less killed

Gotcha, thanks Jason.


Let's see...
By okashira on 2/4/2014 11:45:45 AM , Rating: 4
Let's review.

Interesting article.

Model S...

NHTSA
------
Model S: Highest rated car ever tested. Highest score.
i3: Not rated

Real World
----------
Model S: Still no passenger deaths, with enough miles to make it statistically significant enough to say it might really be the safest car on the road.
i3: No data

Dailytech
---------
i3 is clearly the safer car.

Well... actually, that's not interesting at all. I need to stay off this website. Damn computer jockeys who know nothing about engineering, design or insurance.




RE: Let's see...
By TheDoc9 on 2/5/2014 11:17:00 AM , Rating: 2
Whats so funny is that I seem to remember dailytech being so excited and cheering on Tesla when the model S was first rolled out.

Now I'm wondering if someone is on the wait list for an i3, or perhaps even given one haha.


RE: Let's see...
By Flunk on 2/6/2014 10:06:17 AM , Rating: 2
It is pretty fishy, it makes you wonder if this is a sponsored story.


RE: Let's see...
By Guspaz on 2/7/2014 10:43:45 AM , Rating: 2
It's pretty obvious. I mean, it's actually got things in it that are blatantly false, like this face-palm statement:

"The vehicle informed the users that they shouldn't drive, but they did anyways -- and it let them."

It's completely untrue. In all the cases, the car told the drivers to pull over as soon as it detected damage, they drivers did so and exited the car safely. They did not keep driving, and the car didn't "let" them; it does shut down eventually. But it seems the author thinks that shutting down a car completely in the middle of a freeway is a better idea than the car trying to buy enough time for the driver to safely exit the vehicle.


Road debris
By lemonadesoda on 2/6/2014 4:07:38 PM , Rating: 2
A very interesting article highlighting the vulnerability of the batteries. Runway debris took out Concorde and many lives. Road debris is everywhere, and a car design needs to take that into consideration




RE: Road debris
By jimbojimbo on 2/19/2014 12:29:27 PM , Rating: 2
So you're saying they should design a car for any possibility right? There are more street shootings than Tesla battery fires so every car manufacturer should install bullet proof glass in all their cars!! Pass some laws dammit!
By the way I don't want road debris shooting through the bottom of my car and ripping my leg off. Should EVERY car get the same steel armor protection then?


wtf did I just read?
By zodiacfml on 2/14/2014 6:03:52 AM , Rating: 2
the article is all over the place.... I don't know what's the subject/point in all these?

I think Tesla has no problem with the armor/shielding (considering the protection used by gas/diesel tanks), what they should work is detection of the problem and/or disabling the battery when there's an intrusion.




RE: wtf did I just read?
By zodiacfml on 2/14/2014 6:10:13 AM , Rating: 2
ok, I overlooked the title since I would have never thought they were competing in the EV space.


By Philippine Mango on 2/3/2014 3:11:37 PM , Rating: 2
Because Tesla's design allows them to upgrade the battery pack and whatnot to a better design, when the battery packs get a higher energy density, potentially saving weight, they can always upgrade the armor to something heavier/thicker if they so choose. BMW doesn't have nearly as much flexibility in this regard as it's going to be a long while before that BMW gets a 200+ mile range.




Death by Requirements
By arthursranch on 2/15/2014 4:49:19 PM , Rating: 2
Let's not make a requirement that EV's be safer from death-by-fire than gasoline/fossilly-fueled vehicles. Currently, an EV battery packs energy at about 88 watt-hours per kilogram (Chevy Volt, and only 63% of that is usable). The USDoE and others are targeting 200 Wh/kg, meaning that future batteries will be more densely packed with energy, more capable, cheaper (probably), and more likely to catch fire.

Car fires currently account for thousands of deaths annually. Let's make the EV requirement for safety at the same level as gasoline, perhaps in deaths per vehicle kilometer.

Or let's make fossilly-fueled cars perfectly safe from fires.

Japan currently puts such severe requirements on EV batteries (including unlikely penetrations) that they may lose leadership when new and more energetic batteries appear. Same for hydrogen. In the unlikely event of hydrogen-fueled cars or fuel cell cars, lets not crush innovation by excessive requirements.

Arthur at Arthur's Ranch




Odd Article
By josh_b on 2/24/2014 4:31:15 PM , Rating: 2
This article seems purpose-written. The most common types of accidents are those tested by the NCAP, NHSTA and IIHS. In the EU tests, an i3 earned only four stars. The Model S got the highest score the NHSTA has ever seen.

http://www.autoblog.com/2013/11/27/bmw-i3-misses-t...

In essence there can be a discussion about the Model S battery's safety but there should be little question that in the most common types of accidents the Model S is safer than an i3.




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