Yes 'Exoskeleton'?

[cvalue13]

“How does the skin attach to the structure?” I had to tell them, I have no idea.

Not tell them!

This is where you ask them!

“how could you attach the skin to the BIW structure, to make the skin structural in the operational sense, much less the predominant structural component?”

nobody disagrees that these SS panels/glass will mean the CT can take a sledgehammer to the exterior better than an F15O.

what’s less clear, given that the suspension attaches to the BIW and not any of the panels, is how the panels “hanging” on the BIW perform a meaningful role in bearing operational stresses. On which point you raise a convenient example:

They spoke about being able to see in a normal pickup (a couple drive them) you can see the cab flex left while the truck bed flexes right. One of them then commented “This SOB ain’t going to have any torsional flex.”

in a normal pickup, this torsional flex occurs because the suspension is attached to the frame rails, and when the forces on the suspension cause those frame rails to twist the exterior panels are just going along for the ride, doing nothing to resist those torsion forces (except maybe briefly crumpling)

with the CT, given the suspension is attached to the BIW, the question becomes how exactly do the SS panels similarly not just come along for the ride of the BIWs behavior. To say nothing of whether it’s the panels rather than the BIW could provide the predominant rigidity.

i have my armchair view on all this in the OP of this thread and the earlier discussions that followed.

here just boiling down the crux of the question

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[anionic1]

All that said:

• if Cory’s assumptions that the SS panels are hung just as on the Model Y are correct, then I’d tend to side with him about the CT not having operational structure from the panels (though the doors, hood, and tailgate are each independent, self-contained exoskeletons themselves)

• no matter how strong the panels may be, the CT is only as capable as it’s weakest link; as Cory discusses, heavy haulers tend to have solid axels - esp in rear - for a reason (rather than independent suspension), and tend to have mechanical (rather than air) suspension.

• finally, on another view, even if the panels add operational structure, it’s possible they do so only to make up for the shortcomings of the “endoskeleton” - that is, if the casting/frame joints and structure aren’t terribly capable alone, the panels could be working not so much to ‘amplify’ capability, but instead just to achieve acceptable performance

Afterall, there are 1/2 ton trucks that, if optioned correctly, can achieve materially the same operational capabilities as the CT’s claimed specs. Viewed that way, the strengthening panels wouldn’t be using the exoskeleton to offer previously unknown capability, so much as to allow the Model Y-like construction techniques the boost needed to compete in the first place. Less of a revolution in capabilities, and more of an alternative method to achieve the ~same outcomes.

Less being “actually tough” and instead being “also tough.”

On that view, the novel approach to construction boils down more to the advantages of manufacturing costs for Tesla, and scratch/dent (bullet?) resistance for customers. Not for nothing, as those are good outcomes.

But it would take the sheen off the marketed angle of the exoskeleton. Especially for people who really use their trucks, as opposed to urban posers (like me) - as the “real” truck guys tend to not care as much about scratches/dents (and are doing the shooting first).

Tesla obviously wouldn’t just hang stainless panels and not allocate any structural load to them. Teslas primary structural goal has been how can we remove parts and processes to make fabrication cheaper. An aluminum casting is not the strongest or likely even the most optimized shape for a vehicle frame. You have constraints if the casting shape and things that limit you more than stamping and welding steel would. However you probably can make a lighter completely capable part much faster with a casting. So they use castings. The aesthetic flat stainless panel choice of the CT design is a constraint. So they probably sat in a room and asked themselves how to make it work. They ordered the 9k ton press from idra a long time ago. So they knew they were going to use the casting idea.

All this discussion of they probably had to make up for shortcomings or a certain type of suspension is more typical for towing doesn’t make sense to me. These are insanely good engineers. And we see all sorts of different suspension applications in different vehicles. Semi trucks use air suspension. They had 3 constraints. They were going to use a structural pack, flattish stainless body panels and aluminum castings. Then they engineered it to make it work. Of course the SS body panels carry significant load. They are 3mm stainless. Even the windows carry a significant load. Like another person posted they use advanced software to run structural modeling optimization and analysis through many iterations to come up with the most efficient design that is easy and cost effective to build. To think that they would make extreme efforts to make a structural body pack and not utilize the SS panels would be nuts.

 

[anionic1]

We should all start our own CT specific insurance company. I bet insurance cost are going to be double as insurers use the stainless body as an excuse to charge more.

 

[JBee]

ah this nugget again...how I've missed this!

I'm glad everyone has come to the conclusion that the term "exoskeleton" is only an approximation of the truth, like EM likes to put it. It really deserves it's own descriptor. I can live with that.

But... oh no! I know!

I remain adamant that any of the stainless skins still have to be physically "in the load path" to in any way add additional structure. There is a fine line between a skin being structurally "useful" or "useless". It also depends on the specific load conditions.

Note as mentioned before, there are two very distinct conditions that apply forces to a vehicle structure:

1: Operational loads; which is where the vehicle structure primarily supports the mass of the vehicle and routes those forces, of being pushed down by gravity, to the suspension springs and down to the tyre contact patch, where it interfaces the road. This condition typically doesn't not result in structural failure, as the design allows the suspension to carry all the forces for operation.

2: Crash impact loads; where the impact is typically from the side and not from above like gravity. These loads can however impact on any side, like in a roll over, and typically always result in the compression and at least partial structural failure of the body.

Let me add a drawing to explain:

In a operation load scenario the question is simply what load is applied to the skin at all, given it's location outside of the suspension springs, that carry all the load to the ground?

The result of this understanding, should be that for the purposes "operational structural design", that is to say the day to day operations of the CT, and not for the crash safety aspects, there is little to no use of the SS skins in the above configuration, because there is no load being applied to the skins from somewhere else. You would have to sit on the fender etc to do so to creat a load there for it to carry. Hence the interior structure, being the cabin frame, structural battery and the front and rear casts is the main "operational" load bearing structure, because this routes the loads of the vehicle to the ground via the suspension.

Now in crash safety mode this changes to this for a side impact:

Here the load path and dynamics are significantly different, in that the sideways impacting force is reacting against the lateral wheel traction and against the inertia of the vehicle from it's mass.

In this mode the skin is most definitely load bearing, and as a part of the design will absorb impact energy by intentionally deforming, or distributing forces into things that can deform. The result of this is of course a deformed structure, that can't perform in the operational mode as well anymore.

The point is that from an engineering perspective one needs to carefully differentiate between operational and failure modes. In this case the failure mode "crash deformation" of the "exoskeleton skin" is essential for passenger safety, BUT it is not doing much, if anything in the operational mode, because it experiences no force or load.

Hence my analogy that the CT hybrid structural design is that of a "knight in armour". In that the knight himself has a endoskeleton to carry the mass of almost everything, but the "exoskeleton" armour provides the impact protection.

 

[anionic1]

Can’t quite tell to which this is intended to refer exactly

though I can understand the sentiment, insofar as the reaction I get when running into internet lawyers

that said, I have found that it’s real-world engineers (even civil!) who tend to believe they’re the best internet lawyers, internet doctors, internet husbands, and really … anything.

My engineering BS emphasis was in structural. Almost all structural engineers go through a civil engineering program. It’s likely that most engineers designing the CT are either mechanical or aerospace. I don’t claim to know their structural methods or design but seeing people go in depth in design analysis they clearly don’t know seems odd.

 

[cvalue13]

Tesla obviously wouldn’t just hang stainless panels and not allocate any structural load to them.

Sympathetic that this thread is a lot to take in, I can’t help but think you’ve missunderstood it’s overarching… aspirations.

more or less for reasons similar to what you move recounted, I too believe it’s possible at least some of the SS panels contribute to structural load, based on the notion Tesla has smart people.

but for the curious, they might next ask themselves “and so how might smart people go about doing that?”

Because, with what little we know, there is cast a lot a curiosity if not doubt on how this could be accomplished.

to simply hand wave that “Tesla has smart engineers so I’m sure they’ve done something, somewhere” just isn’t all that … interesting.

and meanwhile, it seems to pass over the somewhat glaring questions that generate the curiosity.

to use an example from a wise (engineer) friend:

take a BMX bicycle frame on its wheels: that’s the notional equivalent of the CT’s BIW with its suspension/wheels attached.​

you drive that bmx off a sick ramp in your driveway, and when it lands, the frame bends.​

If you wanted to avoid that bending happening the next time, and all you had available to you was planar sheets of 3MM SS, where exactly on the outside of that bmx frame are you attaching panels to achieve increased strength?​

the aspiration of the thread, friend, is to point why that’s actually a hard problem to solve - before then trying and be just a little bit curious as to how how the brilliant engineers at Tesla might go about it.

No doubt a form of arm-chair quarterbacking, and I don’t *think* I’ve said anything to mislead people that I am or ever should be a Tesla engineer. That sort of mistake I’d like to think more on the reader than myself.

All this discussion of they probably had to make up for shortcomings or a certain type of suspension is more typical for towing doesn’t make sense to me.

afraid I can’t help you with this one

except to say, here again, you seem to be missing the same sort of point.

generally speaking, these types of suspensions are not good for towing - that’s not a statement about the CT (“and so the CT won’t be good”), it’s a question about the CT (“so how did they make it good in the CT?”)

I too, could “answer” that question by saying “they did it by being smart.”

I’m just double-clicking onwards, towards what I find to be more interesting “answers”

otherwise, if I only need to know that smart people exist to explain everything about the human-made world, I’d be bored.

 

[cvalue13]

I don’t claim to know their structural methods or design but seeing people go in depth in design analysis they clearly don’t know seems odd.

I have degrees in molecular biology (genetics emphasis), analytic philosophy, and law

Which I say only to ask you:

do you never wonder about or discuss with others how hereditary traits works, whether god exists, or if an eye-for-an-eye is a good rule for society?

or maybe more to the point: I presume you didn’t start learning engineering by first knowing it all already?

 

[cvalue13]

ah this nugget again...how I've missed this!

same!

BTW regarding the bmx bicycle with planar steel on the outside but providing structural/operational load:

 

[Jhodgesatmb]

After all the BIW photos of late, as in the above I continue to think:

• listening to Musk’s speech in 2019, it was fair of engineers/designers to interpret “exoskeleton” to mean what Munro et al described at the time

• but, there are other interpretations of “exoskeleton” available, and several such meanings can apply at once (and in different locations of the vehivle)

• for example, since 2019, the only concerted explanation of what Tesla means by “exoskeleton” is found in the patent w/r/t the door and other movable panels, and it describes two alternatives applicable simultaneously:

​

• the movable panel’s skin bear the primary loads carried by the panel (eg the door skin carries the door trim/mechanics)***​

• the movable panels provide armor-like protection of occupants/cargo​

• as for the fixed panels, it still remains possible that the pack, glass, and at least certain of the other fixed SS panels provide material structural/operational strength to the vehicle in various locations of operational importance

• what’s described in the prior two bullets may be as much as Musk/Tesla meant (roughly) in 2019, despite what non-Tesla engineers/designers interpreted and spread about

• and even if and to the degree Tesla’s plans in 2019 changed to date, we can assume those changes have been in the direction of bettering the vehicle on an all-things-considered basis


In any event, once the CT is released and socialized, Musk seems the sort who will be compelled to explain his view on all this.





***though, the recent prototype door (nor tailgate?) hinges do not appear to attach to the SS panel, which seems to draw into question this assertion in the patent (as the SS panels would ‘hang’ on *that* structure)

Elon does not need to explain anything, nor do I expect him to, nor does he or anyone have to be correct in his usage of any term he uses to describe his vehicle, nor does he have to maintain his intent for the outer skin over time. I expect that the CT outer skin will serve the functionality that he described, and you and everyone can take it or leave it but to continue this argument is, to me, a waste of time and energy.

 

[JBee]

more or less for reasons similar to what you move recounted, I too believe it’s possible at least some of the SS panels contribute to structural load, based on the notion Tesla has smart people.

but for the curious, they might next ask themselves “and so how might smart people go about doing that?”

Similar to that white bearded old fella talking about jumping his BMX in his driveway, I too wonder if it is possible to achieve some net operational benefit to using the SS skins to transfer loads to the wheels.

I think there are a few questions here:

A) Is it necessary for the SS skin to also bear operational load, given that it already adds value by:

1. not needing to be painted or treated and saves significant manufacturing cost
2. is hard wearing and useful for the intended "truck" customer
3. offers considerable vehicle exterior corrosion protection in the harshest environments (warranty)
4. offers good impact and crash protection
5. only adds about 8lbs of weight for each front fender, and 21lbs per rear sail fender in comparison to a standard steel panel at 0.7mm thick (so about 60lbs extra all up)
6. is the main structure and replaces the steel framing for all the doors and frunk, resulting in a similar overall door weight, but with less parts etc

B) Of the parts of the SS skin that are attached permanently:

1. Which of those can offer any structural advantage
2. how they might do so

Once again there needs to be a load path, whatever the force is needs to "react" against something that is stopping it, be that inertia, gravity, friction whatever.

Given that the usefulness of SS skin crash protection is nearly uncontested as a "exoskeleton", lets continue with the operational component.

Here we must consider how the forces interact with the primary interface of the vehicle with the ground, that being through the suspension and tyre ground patch.

The suspension spring in itself, being of a air spring type, is capable of absorbing and smoothing significant dynamic loads whilst driving, and could technically equalise the load experienced at every spring, until the suspension runs out of vertical articulation and either touches the bump stops at the top, or the full extension on the droop.

Even though this is "possible", and even desirable for such even load distribution, as a cross linked air suspension setup would have (like on Range Rovers etc), especially for traction on and off road, it's likely that the suspension will behave in such a way that it gradually picks up more load the more the air is compressed on an upward stroke. This in turns mean that the lower suspension arms will carry less load and the higher ones more, until the end stops are reached, where all of the loads are just between two opposite wheels. This condition presents the maximum torsional loads on the vehicle, but, still requires careful analysis of where the load is located and how the system responds to that particular load path.

For example: 5 burly guys in the cabin and 400lbs in the frunk with an empty bed, is not the same as 1 skinny guy in the front and 3000lbs in the truck bed.

There's a good chance that either of the two examples above lead to at least three wheels carrying a load, and not just the diagonally opposite two, because of the balance of the mass being more over the front or the rear of the vehicle. For there to be only two wheels carrying the load, would require the center of mass to be between those two wheels. This is important to consider when examining how frequently these loads will exist, and how the distribution of load affects the structural design. It is "dynamic" after all.

At this point I'd mostly stop doing it my head, draw it in CAD and run it through FEA using a range of variables that describes the expected range extremes. Then you add remove structure to compensate for the undesirable FEA results and hey presto you have a structure!

Now replicating the outside of the CT is one thing, but replicating the BIW structure of the CT, with all the correct ribbing and thicknesses, given the few photos we have? Good luck with that!

So really all we could do at the moment is try to get an approximation of how the skin would help, and for what load conditions it does so. For this we need to define which skin is being assessed, and how the proposed forces interact with said skin.

I'll see if I can find some time to do a CAD line diagram of the CT where we can add some loads to get a better feel for it.

 

[JBee]

Elon does not need to explain anything, nor do I expect him to, nor does he or anyone have to be correct in his usage of any term he uses to describe his vehicle, nor does he have to maintain his intent for the outer skin over time. I expect that the CT outer skin will serve the functionality that he described, and you and everyone can take it or leave it but to continue this argument is, to me, a waste of time and energy.

He doesn't, but we choose to do so!

 

[JBee]

same!

BTW regarding the bmx bicycle with planar steel on the outside but providing structural/operational load:

That thing looks super light weight, and the panel bracing is to stop the head tubes from buckling around the welds.

You could also replace those two plates by using a thicker diameter or wall tube, so the welds can carry more. Or you could move the two horizontal tubes further apart like on a normal bike.

You could also do this if you wanted to have more of a panel design:

But the frame geometry is still the same because it is dictated by the load path, that being the force downwards on the handlebars, pedals and seat, that then go to the wheel hubs. Note how they are all sitting on different points of just two triangles.

--

I have a funny story (well sort of) about that specific part of the bike. In my teens I worked in construction, as a labourer for my older brothers. The first thing I bought with my wages was a Mountain bike, which was my access to freedom. I rode everywhere, often 30-40miles a day after a hard days work (we had 80lbs cement bags back then!), just for the fun of it, and to play arcade games in town with friends. Now one day I was leisurely racing down a footpath (I only know two speeds, flat out and stop), listening to my tunes on my Walkman, sunnies and all, not a care in the world riding my brand new bike, knowing full well I was a poser to any loser kid out there without a bike as good as mine.

Now I'd been down this path many times before, and at certain points on the path there were bollards to stop cars from driving down the footpath. Being oblivious to my surrounding and in the zone, looking down and cranking those pedals around, I happened upon said 5Ft concrete bollard with quite some gusto. The result was instant and a bit dramatic. The front wheel hit the round bollard exactly in the middle, the front wheel crumpled, the head tube bent underneath the bike, flipping me up over the handlbars and smack bang down on the centre of the 5ft high bollard.

To say the incident was uncomfortable would be an understatement, I landed on the pole exactly on the center of mass of my body, being just under my sternum, and due to the shiny painted nature for the bollard, was unable to grab anything on the bollard to get myself down. My feet couldn't reach the floor either. Now after the shock of landing there, with arms and legs flailing in the wind, stuck on a pole, things actually got worse before it got better. My position was such that it was really hard to breath, because all of my weight was on my chest, and my lungs couldn't expand my chest to get air. I couldn't reach my precious bike either to use it as a support to get off the pole I was balancing on. Finally realising I was in trouble, and with no help in sight I gradually managed to shimmy myself sideways with my hands, despite the pain from grazing my chest across the top of the bollard, until I fell off the bollard onto the ground, panting and catching my breath. I laid there for some 10 minutes before getting up with a red patch on my t-shirt, and inspecting my not so beautiful bike. I then walked all the way home in shame and pain, dragging my broken pride and bike behind me on the one good wheel I still had left.

Turns out some decades later I had my first chest x-ray, and on there quite clearly, you could see that I had broken and fractured my sternum on that silly bollard. It's still there to this day in the same spot in the middle of the footpath, I tell my boys the story every time we pass it... It taught me the hard way to look up, breath, and not just concentrate on yourself, no matter how well you think you are doing!

To this day I find it amazing to think that our lungs compress the earth's atmosphere around us to draw breath into our lungs. Physics is awesome! But also the ruler of the environment of our reality, and never to be trifled with or ignored!

 

[cvalue13]

Elon does not need to explain anything, nor do I expect him to, nor does he or anyone have to be correct in his usage of any term he uses to describe his vehicle, nor does he have to maintain his intent for the outer skin over time. I expect that the CT outer skin will serve the functionality that he described, and you and everyone can take it or leave it but to continue this argument is, to me, a waste of time and energy.

the clouds’ ears!

 

[rudedawg78]

I apologize if this was stated already, but Munro's latest video on Twitter about the Cybertruck (7/17) stated that the Cybertruck "would not be classified as an exoskeleton" based on the latest frame photos.

 

[cvalue13]

I apologize if this was stated already, but Munro's latest video on Twitter about the Cybertruck (7/17) stated that the Cybertruck "would not be classified as an exoskeleton" based on the latest frame photos.

Yes thank you - and the central point of this thread has been, in effect, to clarify how we should really interpret Munro’s comments:

“would not be classified as an ‘exoskeleton,’ in the way I had assumed based off extremely limited information … sorry I spent over 2 years spreading my assumptions as fact, confusing many people who took my conjectures as settled categorically”

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