Design Questions about Tesla Semi

[Olle]

I am curious about some of the design choices of the Semi from a first principles‘ lens. So now I am going to nerd out and list questions and what I think the answers are. What do you think are these the answers or do you have another interpretation?

1) Why are there continuous frame rails instead of a load bearing battery pack? As any engineer will tell you, a large tube (e.g. structural pack) is much much stronger than two small C sections by mass, so this choice added mass.
-answer: Tesla is working on a structural 4680-pack and doesn‘t want to expend energy on a temporary structural 2170 pack. So went with the proven design of old school frame rails that the other semis have. This might be why it turned out a little heavier than planned and not listing tractor weight yet.

2) Why did they go from four M3-motors to there Plaid motors? and lost half of the coveted anti jack knifing torque vectoring that was previously advertised? And possibly increasing cost if 3 plaid motors would cost more than 4 M3 ditto?
-answer: I think it’s a consequence of point 1. With the chassis turning out a little heavier than planned they needed to hunt through the truck for other weight savings. Side benefit; with one motor and gearbox instead of two powering cruise, there is a tiny bit of lowered energy consumption, enabling the shedding of a few lbs of batteries. The exciting point is that if they keep this configuration when the structural pack comes, the truck will be even more badass. Maybe that’s where the 650 mile statement comes from?

3) Why 1000 V charging?
Answer: -thinner cables and fewer FETs save a few pounds. not much bit all little savings add up. Then you have thinner charge cables as a side benefit.

4) No AC charging. This is my guess. Haven’t seen the spec yet.
Resons: -with a battery 10 times the size of a Tesla car, think of your most powerful your AC outlet And divide your charge speed by 10. That’s one. The other is mass and cost of onboard charger.

5) No 400V DC Charging This is my guess too.
Reasons: Mass of DC-DC converter would defeat the purpose of going with 1000 V, which was to save a tiny bit of mass. Cost: if a customer had a route through a 400 V SC that was already truck sized (highly unlikely) would be cheaper to install a 1000 V stall there rather than putting a dc-dc converter in all the trucks. Likely will never be an issue bec there are almost no truck SC. So they’ll build 1000 V truck SC.

6) why did they lose the aero side skirts?
answer: -to have less Prius vibe and look more bad ass

 

[MP3Mike]

1) Why are there continuous frame rails instead of a load bearing battery pack? As any engineer will tell you, a large tube (e.g. structural pack) is much much stronger than two small C sections by mass, so this choice added mass.
-answer: Tesla is working on a structural 4680-pack and doesn‘t want to expend energy on a temporary structural 2170 pack. So went with the proven design of old school frame rails that the other semis have. This might be why it turned out a little heavier than planned and not listing tractor weight yet.

I doubt it. Where would you put the motors/gearboxes/etc. if you only had a structural pack in place of the C channel frame rails? (There would be no room for them.)

5) No 400V DC Charging This is my guess too.
Reasons: Mass of DC-DC converter would defeat the purpose of going with 1000 V, which was to save a tiny bit of mass. Cost: if a customer had a route through a 400 V SC that was already truck sized (highly unlikely) would be cheaper to install a 1000 V stall there rather than putting a dc-dc converter in all the trucks. Likely will never be an issue bec there are almost no truck SC. So they’ll build 1000 V truck SC.

No, I'm sure 400v charging is likely available, and support probably doesn't add that much weight. The main point of the 1000v architecture is to allow Megawatt+ charging while still having a charging cable that a human can handle.

 

[Olle]

I doubt it. Where would you put the motors/gearboxes/etc. if you only had a structural pack in place of the C channel frame rails? (There would be no room for them.)

only the mid section will have structural pack. I’m thinking the rear is finished as is. Might they eventually go with cast rear frame once they hit volume? It’s hard to find anything weaker per mass than open C-channel. But I understand why the incumbents use C channel. It enables them to move stuff along the frame and drill wherever they wish for customization

No, I'm sure 400v charging is likely available, and support probably doesn't add that much weight.

what would the point be?

The main point of the 1000v architecture is to allow Megawatt+ charging while still having a charging cable

That’s a good point.

 

[scottf200]

2) Why did they go from four M3-motors to there Plaid motors? and lost half of the coveted anti jack knifing torque vectoring that was previously advertised? And possibly increasing cost if 3 plaid motors would cost more than 4 M3 ditto?
-answer: I think it’s a consequence of point 1. With the chassis turning out a little heavier than planned they needed to hunt through the truck for other weight savings. Side benefit; with one motor and gearbox instead of two powering cruise, there is a tiny bit of lowered energy consumption, enabling the shedding of a few lbs of batteries. The exciting point is that if they keep this configuration when the structural pack comes, the truck will be even more badass. Maybe that’s where the 650 mile statement comes from?

a) Tesla has the software tuning and computer modeling designed for the tri-motor set on the Model S Plaid.
b) The single Plaid motor had enough power (with right gear ratio) to move a fully loaded semi-truck at highway speeds at an efficient Wh/mile. The combined 2 motors for the M3 must have taken more kW than the 1 Plaid.

Perhaps there is a nice table or graph somewhere that whose the power and efficiencies of the Plaid vs TM3 motors (ignoring the gearing).

I only found the Model S/X comparisons.

 

[TSemi]

2) Why did they go from four M3-motors to there Plaid motors?

The carbon-wrapped Plaid motors weren't available when the Semi was introduced. The way they have it now set up with just one motor for cruising I doubt was possible with one M3 motor. I'm also guessing the Tesla motor engineers may have tweaked the Plaid motors a bit for the Tesla Semi application. They said just one Plaid motor is more powerful than a diesel engine used in class 8 tractors(guessing a 500HP engine). Also said 3x the power(alluding to the 3 motors in the Semi) of a diesel tractor.

and lost half of the coveted anti jack knifing torque vectoring that was previously advertised?

I dunno, maybe just two motors are sufficient?

And possibly increasing cost if 3 plaid motors would cost more than 4 M3 ditto?

How much more would be a good question. I suspect the engineers at Tesla figured the benefits of using 3 Plaid motors overcome any cost increase versus using 4 M3 motors.

 

[Cal1]

The previously advertised part is something I’m very interested in. Just how much of the original claims/goals were achieved? No mention of anything except that they did hit the 500 mile range fully loaded (which is a great achievement!). NOTHING about the anti jack knifing, tesla caravans, gas vs electric savings, etc. God it’s been so long that I don’t really remember everything but I bet potential truck buyers have it all memorized. Alas this was only a delivery event and really about Pepsi getting their trucks. I’d be pissed if someone used delivery of my car to overshadow my event. So maybe it will be covered in the 20022 Q4 reports.

 

[scottf200]

The previously advertised part is something I’m very interested in. Just how much of the original claims/goals were achieved? No mention of anything except that they did hit the 500 mile range fully loaded (which is a great achievement!). NOTHING about the anti jack knifing, tesla caravans, gas vs electric savings, etc. God it’s been so long that I don’t really remember everything but I bet potential truck buyers have it all memorized. Alas this was only a delivery event and really about Pepsi getting their trucks. I’d be pissed if someone used delivery of my car to overshadow my event. So maybe it will be covered in the 20022 Q4 reports.

FYI on the 500 miles.

Where things got strange is when O’Connell, according to the article, talked about the range of the Tesla Semi:

PepsiCo’s new Semis can haul Frito-Lay food products for around 425 miles (684 km), but for heavier loads of sodas, the trucks will do shorter trips of around 100 miles (160 km), O’Connell said.

Via: Pepsi talks Tesla Semi; exec makes strange comments about the electric truck's range

 

[MP3Mike]

FYI on the 500 miles.

Via: Pepsi talks Tesla Semi; exec makes strange comments about the electric truck's range

Of course Electrek left off this sentence from the original article: Exclusive: PepsiCo to roll out 100 Tesla Semis in 2023, exec says

PepsiCo then will also use the Semis to haul beverages in the "400 to 500 mile range as well," O'Connell said.

 

[Olle]

Of course Electrek left off this sentence from the original article: Exclusive: PepsiCo to roll out 100 Tesla Semis in 2023, exec says

Yup, I noticed that too. not an accident that sentence fell off. Electrek are masters of clickbait/fake news. The scary part is people take their word as gospel. The majority of comments under the electrek article were essentially egging each other on in saying that Tesla is a fraud, Told ya it was only going a 100 miles and the like.

 

[Rocky_H]

It's not all Electrek's fault though. It's prevalent mental failure among a lot of the haters. Pepsi gives the statement in the form of:

"Here's how we are CHOOSING to use them."

And people take that as, "HA! See? It's all that the trucks are capable of."

No, that's not what it said. If Pepsi wants to be overcautious at first, they can. If I don't choose to drive over 100 mph with my car because I want to be safe, it doesn't explicitly mean the car can't do it.

 

[mongo]

I am curious about some of the design choices of the Semi from a first principles‘ lens. So now I am going to nerd out and list questions and what I think the answers are. What do you think are these the answers or do you have another interpretation?

1) Why are there continuous frame rails instead of a load bearing battery pack? As any engineer will tell you, a large tube (e.g. structural pack) is much much stronger than two small C sections by mass, so this choice added mass.
-answer: Tesla is working on a structural 4680-pack and doesn‘t want to expend energy on a temporary structural 2170 pack. So went with the proven design of old school frame rails that the other semis have. This might be why it turned out a little heavier than planned and not listing tractor weight yet.

2) Why did they go from four M3-motors to there Plaid motors? and lost half of the coveted anti jack knifing torque vectoring that was previously advertised? And possibly increasing cost if 3 plaid motors would cost more than 4 M3 ditto?
-answer: I think it’s a consequence of point 1. With the chassis turning out a little heavier than planned they needed to hunt through the truck for other weight savings. Side benefit; with one motor and gearbox instead of two powering cruise, there is a tiny bit of lowered energy consumption, enabling the shedding of a few lbs of batteries. The exciting point is that if they keep this configuration when the structural pack comes, the truck will be even more badass. Maybe that’s where the 650 mile statement comes from?

3) Why 1000 V charging?
Answer: -thinner cables and fewer FETs save a few pounds. not much bit all little savings add up. Then you have thinner charge cables as a side benefit.

4) No AC charging. This is my guess. Haven’t seen the spec yet.
Resons: -with a battery 10 times the size of a Tesla car, think of your most powerful your AC outlet And divide your charge speed by 10. That’s one. The other is mass and cost of onboard charger.

5) No 400V DC Charging This is my guess too.
Reasons: Mass of DC-DC converter would defeat the purpose of going with 1000 V, which was to save a tiny bit of mass. Cost: if a customer had a route through a 400 V SC that was already truck sized (highly unlikely) would be cheaper to install a 1000 V stall there rather than putting a dc-dc converter in all the trucks. Likely will never be an issue bec there are almost no truck SC. So they’ll build 1000 V truck SC.

6) why did they lose the aero side skirts?
answer: -to have less Prius vibe and look more bad ass

1) the loads are primarily in the rear axle structure. Anything forward of that is only pack/ cab support and induced moment from the king pin vertical offset vs acceleration and hoizontal offset vs dual axle centerline. Front axle static load is only 12k pounds max.
As opposed to a diesel that needs to react drive torque.
2) with ABS/ ETC and an open diff, they can still torque vector with a single motor.
3) much thinner cables. If voltage is doubled they can have 1/4 the cross section for the same power loss per unit length.
4) shrugg, it would be slow, but they could add on a couple charge modules.
5) could split the pack for compatibility, or use modules from #4
6) also lost the rear wings