Cybertruck discussion for engineers

Cybertruck discussion for engineers

After the reveal, @sbeggs (who is in aerospace) and I had the same thought: why not use titanium for the exoskeleton? It's much stronger and lighter than steel, highly corrosion resistant, eco-friendly, and attractive. One of the main drawbacks of titanium - difficulty of stamping into curved shapes - has been eliminated by the origami design. It could have looked quite striking unpainted. Think of the Bilbao Museum, for example.

A quick google search on the topic turned up an interview with Elon from over a year ago in which he said of the pickup, "it's gonna have a lot of titanium." However, Ti was not mentioned in the reveal.

Here is what I think happened. The genesis of the design was partly based on the desire to use titanium for its lightweight strength. That suggested the use of flat polyhedrons for the entire exoskeleton. When they wanted the price of truck to start at less than $40k, staff probably convinced Elon to switch to stainless steel to hit the price point.

Stainless steel is the obvious lower cost alternate, but would not be a simple task to work out all the engineering late in the game - especially the appearance. DeLorean managed to make it look good, but they weren't working with 30x ballistics grade SS as the structural load bearing material. It can discolor with welding, for example. I'm guessing that they made this switch pretty close to the reveal and were still experimenting with alloys, fasteners, annealing processes, etc. That's why the truck looked the way it did at the reveal, with color mismatched body panels and a blotchy finish. The promo pix that show a lighter, more uniform silver color might be based on a titanium prototype.

I think Tesla will just need a little more time to get the skin right, along with the glass. The final product should look a lot better. Finish will be uniform, and steel frees them to tweak the design if they want. FWIW, I would have paid a pretty decent premium to get an actual titanium truck... and with all the reservations coming in on the higher end of the price spectrum... might they reconsider?

bruryan | November 24, 2019

think origami only with stainless. much less body shaping costs, no paint room=the low cost.
cost = sales

Magic 8 Ball | November 24, 2019

Deloreans got "blotchy" with age. I suspect the thing is completely handmade with lots of hands on everything. This is a functional prototype, not a concept car. I suspect they will use titanium where it makes sense but SS is a great choice for body skin. Titanium does not crumple well and it can splinter.

The exoskeleton concept is intriguing but I would like to see details on how strength of outer skin structure is transferred through the doors to the rest of cabin. You get a lot of "strength" in a pane with contours, embosses, beads, etc. Panels on CT look to have have at least three fold. Holding tolerances will require, I am sure, the parts to still be die stamped and formed. They may still use titanium in other parts of the truck but outer body shell, I doubt it.

Magic 8 Ball | November 24, 2019

I am also interested in how the panels attach. I wonder if they are going to be bonded to sub structure with adhesive.

lbowroom | November 24, 2019

I wonder about crumple zones and bumpers.

jamilworm | November 24, 2019

@OP - I think you may be on to something. SS can definitely look a lot better than the truck at the reveal did, so maybe it was a late design change. Ti isn't really stronger than steel, it is just much lighter. Making the switch over to SS would have been pretty straightforward if the material strength was similar.

If they did switch (back) to Ti then it would reduce weight a lot, which would make it easier to achieve the performance specs.

jamilworm | November 24, 2019

Another benefit of using SS is that (depending on the type) it can be welded to steel, so if they want internal structure to be steel it would be easy to combine with the SS shell.

Hal Fisher | November 24, 2019

Who is going to wrap it? Look at a prius (almost same sharp angles. Imagine that in stainless, yuck!
But a wrap or paint will be nice.
A quick graphic:

Hal Fisher | November 24, 2019

I meant who ‘isn’t ‘ going to wrap it.

Ross1 | November 24, 2019

The solution was due to the problem of getting rivets

TickTock | November 24, 2019

Wrap? Definitely not me. I have nothing against folks adding color but you are giving up one of the big benefits of the exoskeleton. Being immune to tossed up rocks and Arizona pinstripes is a big plus.

Bighorn | November 24, 2019

Couple tweets about forming bends vs stamping.

Magic 8 Ball | November 24, 2019

Wow, that really clears a lot up. The material is so thick and strong it cannot be stamped. Now I really wonder how it all works in terms of absorbing energy on impact and how these panels are constructed and fastened. Somebody brought it up before and they definitely are doing a first principles ground up body design. It will be fun to see how or if it evolves.

sbeggs | November 24, 2019

Good points and interesting thread!

Ross1 | November 24, 2019

SpaceX made the transition from titanium to stainless recently too, this year.

MAB1980 | November 24, 2019

“After the reveal, @sbeggs (who is in aerospace) and I had the same thought: why not use titanium for the exoskeleton? It's much stronger and lighter than steel, highly corrosion resistant, eco-friendly, and attractive”

Cost seems the most likely reason. Also, a correction:
Titanium is not much stronger than steel. Steel is in fact stronger than Titanium. Titanium’s specific strength is somewhat better, but for this application it is likely not worth the cost.

steveishere | November 24, 2019

I am no material engineer but I believe it has to do with the decision that Starship's outer shell will be using Stainless Steel as oppose to Titanium. Focusing research on one material will certain drive down cost for both SpaceX and Tesla.

I won't worry too much about looks; look matters, especially for 3/S/X customers. But I suspect general truck drivers are practical people who care more about specs, usability, and costs than looks.

SamO | November 24, 2019

Question: What came first, decision to switch from carbon fibre to stainless steel on Starship or using stainless steel for Cybertruck?

Answer: Starship steel decision came first. We were going to use titanium skins for Cybertruck, but cold-rolled 30X stainless is much stronger.

walnotr | November 24, 2019

A quick google search gives SS cost at about $1.00/pound. Titanium comes in around $30.00/pound.

howard | November 24, 2019

Let's hope the panels are well thought out for replacement purposes. Any type of damage is going to require complete panel replacement for it to be restored to new. The issue is the new panels are going to look different than the adjoining older panels. So will there need to be a chemical and/or mechanical process for the complete exterior to restore it to an even new look over the entire surface after repairs? Or body shops may just say what color do you want it to be after we fill and rework the damaged area.

I can see this as being the most popular vehicle in history for exterior treatments, wraps, powder coating etc. etc..

Given how much insurance is for a Tesla now how much more will it be for the truck and its "space-age exterior"?

SamO | November 24, 2019

More FUD and imbecilic suppositions from Hbomb.

Magic 8 Ball | November 24, 2019

If the panels are flat they will be simple to ship, store, transport, etc. Replacement panels will most likely be cheaper than trying to do grind and fill body work. The cosmetic treatment is a non-issue. Re-brushing Delorean panels is/was done easily.

Madatgascar | November 24, 2019

To clarify, when I said titanium was stronger than steel, I means on a per unit weight basis. Steel strengths are all over the place, and strength needs to be balanced against ductility - not sure if the selected SS would be stronger than titanium.

Here's a picture where you can see the difference between the reveal truck and what I presume to be the Ti prototype:

bruryan | November 24, 2019
MAB1980 | November 24, 2019

“ Steel strengths are all over the place”

Definitely. Most common stainless alloys tend to be stronger than most common Ti alloys (6Al4V is actually the only one which comes to mind, but I have an aerospace-centric view).

Either way, I’m still holding my initial theory: if steel and titanium were traded, cost was certainly a factor, and steel wins that trade for pretty much every application where weight isn’t a primary concern.

Madatgascar | November 24, 2019

@MAB1980, agreed - but what about from an aesthetics standpoint? I have to think they can improve on what they showed at the reveal. The front quarter panel was silvery, the doors were gray, the rear quarter panel was bluish, and everything was kind of mottled. What was up with that? (Link above.)

I understand the costs of titanium can come down substantially based on new refining processes. It is now finding architectural applications where stainless would have been used in the past. I want what they used in the video where it pulls the F150 up a hill. If they had that truck, why wouldn't they show it at the reveal?

Madatgascar | November 24, 2019

@SamO's link seems to confirm they were planning to use titanium for the cybertruck's skins. So I was right!

I totally get the decision to use stainless steel for the Starship. Strength per $, it wins hands down, and when you're blasting something into outer space it doesn't matter too much what it looks like. For something I'll park in front of my house, I'd be curious to know just what that $ premium was. It could not have been too much if Tesla carried it forward for a while. If they get the SS to look like a DeLorean, it won't matter, but again that was cladding, not structural steel. If the final product looks like what they showed at the reveal, I'm going to hope they reconsider.

SamO | November 24, 2019

Q: Ultimately, what was the tipping point for Steel over titanium?

A: Higher hardness, higher strength & doesn’t cost crazy money for ~3mm skins

Madatgascar | November 24, 2019

@SamO, my question is: when was that decision made? Probably late, and didn't give the team enough time to sort the aesthetics.

I trust they will get there in time. | November 24, 2019

@Madatgascar - Does it matter? The current design appears to be a huge success.

Madatgascar | November 24, 2019

@TeslaTap - did you see the photo I linked above? Matters to me!

sbeggs | November 24, 2019

That's exactly what I would write, unless weight savings are the critical goal, stainless steel's strength, lower cost, and, apparently for Elon, ballistic shielding quality, win the trade off study. 30X cold rolled=dense, fine grain structure. Forged for Mordor! | November 24, 2019

@Madatgascar - Yep. I suspect they are the same vehicle. Totally different lighting. Using a projected photo made in outdoor lighting to the truck with totally different indoor LED lighting - they are going to look different.

Magic 8 Ball | November 24, 2019

"..Higher hardness, higher strength & doesn’t cost crazy money for ~3mm skins"

It the skins are ~3mm they can easily be stamped even in high strength stainless. The presses in Fremont are huge but maybe not big enough for the new material.

Madatgascar | November 24, 2019

@TT, sunlight is much more unforgiving. There are a whole series of high resolution outdoor photos where the finish appears white-gold and very uniform. The reveal truck is obviously a different material. Look how different the rear quarter panel looks - think that would not show up in direct sunlight?

Magic 8 Ball | November 24, 2019

Lighting is everything. Huge difference between direct lighting and indirect.

Passion2Fly | November 24, 2019

I was also reading that titanium is very hard to weld, it doesn’t like oxygen. It makes the welding crack... steel is much easier to work with... another reason in favor of SS... especially for body work...

Hal Fisher | November 24, 2019

I have a 97 Z28 that has plastic door and fender skins. Never a problem with them and no door dings. Pretty sure they would pass the sledgehammer but not the bullet test. They have an internal bar for crash protection. Not sure why other cars don’t use it? Maybe expense?

hughbie | November 24, 2019

Interesting discussion. As a retired firefighter, the safety / rescue factors that I wonder about: 1) crumple zones, and energy dissipation in extreme deceleration (high speed impacts); 2) ability to get through the safety glass to access trapped occupants; and 3) if current hydraulic tools (Jaws of Life) will be sufficient to make access. Will special tools be needed, or is the Cybertruck so tough that we just pry away whatever vehicle hit it, and the truck occupants walk out unharmed?

Madatgascar | November 25, 2019

@hughbie, excellent questions. I'm sure Tesla will have addressed the crumple zones. Stronger skin does not necessarily short circuit crumple zones - regular sheet metal is not a significant part of energy dissipation, and those side panels can be designed to allow buckling at a specified impact. So they can still have good front and rear crumple zones, and for side impact I'll take the extra skin toughness any day.

Emergency access issues will be harder to solve. Tesla should address this before they bring it to market. They will have solutions, but it probably requires special training for first responders.

Iwantmy3 | November 25, 2019

I don't think the design was restricted by the thought of using Titanium. This is the kind of styling that Elon has been teasing since the Semi truck reveal. The benefits of SS are obvious. Higher strength (by volume), easier form-ability, easier weld-ability, and vastly lower cost.

As far as aesthetics, how long will it be until some mod comes out polishing the whole thing to a mirror finish?

gballant4570 | November 25, 2019

Price point is the elephant in the room here. As always, price drives engineering.The extra weight apparently lost the price evaluation. The use of the same material as the Starship skin won.

Managers will groan reading an engineering discussion thread - and complain that it will only lead to an expensive product that is never quite finished. As a person who moved from engineering into management, I understand both perspectives. But being retired, I revert to the inner engineer as a default position. Engineering discussions certainly have a lot of value.

Different paint options (both before and after sale point) and after sale wraps will give Cybertruck owners a lot of options other than the SS exterior.

MAB1980 | November 25, 2019

“As far as aesthetics, how long will it be until some mod comes out polishing the whole thing to a mirror finish?”

I pity the pour soul who drives behind that on a sunny day.

haywood.ed | November 25, 2019

As I understand it, the decision to use Unibody instead of Body-on-Frame construction drove the shape. The lack of a D pillar makes it difficult to build a Unibody truck torsionally rigid enough to tow a heavy load. Hence the "sail" configuration rather than a traditional 3 box truck. Chevy Avalanche and Honda Ridgeline had the same issue, and note the similarities in their design. Cybertruck has a much greater towing capacity, so the sail had to be much larger and extend all the way to the rear.

The problem is that such designs, while fine for recreational vehicles, are not functional as work trucks. The traditional box bed design, while not attractive, is highly functional and versatile. Sail sides are not conductive to loading materials or configuring to carry to equipment.

IMO they missed a big opportunity. Working guys drive a lot of daily miles in their work trucks, and they spend a lot of money on gas. Most of their travel is local, not road trips, so range is not a huge performance spec. An EV truck suitable for daily work use could be a compelling value proposition at that price point. Sex it up with stainless steel body and watch it fly off the shelves.

walnotr | November 25, 2019

Another point to consider with titanium is the welding requirements. Titanium must be welded in an oxygen free environment making spot welds problematic. TIG welding is a specialized form of welding requiring highly skilled welders to achieve consistent results. More added costs.

gballant4570 | November 25, 2019

haywood.ed, humans, while somewhat delusional, are certainly adaptable. They will use this great tool.

Madatgascar | November 25, 2019

@Iwantmy3, you can't polish this material.

@gballant, you can't paint this material.

A wrap will be the best bet. It looks great for the post-apocalypse, but unless Elon knows something we don't know, I hope not to be there in 3 years.

Magic 8 Ball | November 25, 2019

@Madtgascar Why do you think you cannot polish or paint this type of SS?

WW_spb | November 25, 2019

I don't believe CyberTruck is a work truck just like Honda Ridgeline is not a real work truck either. But it won't stop people from buying one bc of the versatility. It's capable, gas free, maintenance free, cheap to own, looks like nothing else on the road. It will sell to Truck enthusiasts and Tesla fan's like hot cakes.

SamO | November 25, 2019

However, you can powder coat stainless steel. Normally you don’t need to, because stainless steel doesn’t rust. But it is very easy.

SamO | November 25, 2019

However, you can powder coat stainless steel. Normally you don’t need to, because stainless steel doesn’t rust. But it is very easy.