Why Tri-Motor??

[alan auerbach]

I understand one motor at one axle, or one at both axles. But I don't get the benefit from three motors.

Presumably, for the 3-motor to have a longer range (and to justify the extra $20k) it must have a beefier battery. For buyers willing to pay for extra range, would it not have been more logical to offer that bigger battery as an option for all three models?

In short, what's the thinking behind offering three motors -- and tying them to a heftier battery?

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[ajdelange]

The TriMotor is probably going to have mass of about 3000 kg and goes from 0 to 26.8 m/s in 2.9 s meaning an average acceleration of 9.24 m/s/s (0.94g!) and it must, therefore, be able to deliver average thrust of 3000*26.8/2.9 = 27724.1 N. With half meter radius wheels (to make the math easy) that means 55,448 kN torque. That's a lot of torque. But split between 3 motors its 18.483 kN per motor. Multiplying the thrust by the speed at 60 mph assuming that acceleration is constant from 0 to 60, we'd have power consumption of 27724*26.8 = 743 kW which is close to 1000 HP or 333 HP per motor.

Compare to the dual motor with a 0 - 60 time of 4.5 sec. (it's not coincidence that the 0 - 60 times are in the same ratio as the number of motors). It is only capable of 0.61 g acceleration and the two motors must deliver 3000*26.8/4.5/0.5/2= 17.867 kN each. Thus it is apparent that the motors can deliver about 18 kN each (their torque limit) and 247 kW each (their power limit). Adding a third motor gives 50% more power than you have with two and it gives 50% more torque. 50% more torque means you can accelerate 50% faster (and have 0 - 60 times about 2/3 smaller) and it means you can get about the same acceleration at low speed from a mass about 1.5 times larger (check the towing specs). IMO the third motor is there to get towing capability at 14,000 pounds. The fact that the torque required to do that also lets you do 0 - 60 in 2.9 sec (that's insane for a truck) is icing on Tesla's cake.

Two motors on the rear confer another benefit and that is greater flexibility in torque vectoring. This confers advantages in all modes of driving but is especially powerful when towing as it can nullify trailer sway without the use of goose necks, fifth wheels or pivot point projecting hitches.

When towing you increase energy consumed per mile driven by a factor of 2 or more so if you are building a truck for towing, which I believe the TriMotor is, it makes sense to put in a bigger gas tank just as one does when designing an ICE truck for towing. But if you put the bigger gas tank in the dual motor you could increase its range too and buy about the same amount as the non towing consumptios are going to be about the same for dual and tri motor configurations. I'm guessing they aren't doing that in order to keep the dual motor cheaper and keep it from competing with the trimotor.

 

[Dids]

Plaid powertrain will have 3 motors. Why not 4? Cooling the battery is probably the limiting factor there.... why not put bigger battery for 2 motors? Batteries weigh alot. I think the trade off is that with 2 motors and a giant battery would be lower towing capacity and lower zero to 60. Yes you gain range but people worry about range more than they should.

 

[alan auerbach]

Thanks again -- useful explanations.

 

[hridge2020]

Because he can..

 

[Miralf]

The TriMotor is probably going to have mass of about 3000 kg and goes from 0 to 26.8 m/s in 2.9 s meaning an average acceleration of 9.24 m/s/s (0.94g!) and it must, therefore, be able to deliver average thrust of 3000*26.8/2.9 = 27724.1 N. With half meter radius wheels (to make the math easy) that means 55,448 kN torque. That's a lot of torque. But split between 3 motors its 18.483 kN per motor. Multiplying the thrust by the speed at 60 mph assuming that acceleration is constant from 0 to 60, we'd have power consumption of 27724*26.8 = 743 kW which is close to 1000 HP or 333 HP per motor.

Compare to the dual motor with a 0 - 60 time of 4.5 sec. (it's not coincidence that the 0 - 60 times are in the same ratio as the number of motors). It is only capable of 0.61 g acceleration and the two motors must deliver 3000*26.8/4.5/0.5/2= 17.867 kN each. Thus it is apparent that the motors can deliver about 18 kN each (their torque limit) and 247 kW each (their power limit). Adding a third motor gives 50% more power than you have with two and it gives 50% more torque. 50% more torque means you can accelerate 50% faster (and have 0 - 60 times about 2/3 smaller) and it means you can get about the same acceleration at low speed from a mass about 1.5 times larger (check the towing specs). IMO the third motor is there to get towing capability at 14,000 pounds. The fact that the torque required to do that also lets you do 0 - 60 in 2.9 sec (that's insane for a truck) is icing on Tesla's cake.

Two motors on the rear confer another benefit and that is greater flexibility in torque vectoring. This confers advantages in all modes of driving but is especially powerful when towing as it can nullify trailer sway without the use of goose necks, fifth wheels or pivot point projecting hitches.

When towing you increase energy consumed per mile drive by a factor of 2 or more so if you are building a truck for towing, which I believe the TriMotor is, it makes sense to put in a bigger gas tank just as one does when designing an ICE truck for towing. But if you put the bigger gas tank in the dual motor you could increase its range too and buy about the same amount as the non towing consumptios are going to be about the same for dual and tri motor configurations. I'm guessing they aren't doing that in order to keep the dual motor cheaper and keep it from competing with the trimotor.

Wow thanks for that very complete explanation

 

[Geo]

Presumably, for the 3-motor to have a longer range (and to justify the extra $20k) it must have a beefier battery. For buyers willing to pay for extra range, would it not have been more logical to offer that bigger battery as an option for all three models?

In short, what's the thinking behind offering three motors -- and tying them to a heftier battery?

The answer provided by ajdelange is excellent regarding the 3rd motor, but to focus on the subject of range, implicit in the question and answer, I think the Cybertruck Truck Guy suggested it first, is the range may be extended for a fraction of the $20 k ( $32 k Australian ).

If the Cybertruck is configured for it, it may be possible to carry a small generator, say of 7kw/h or so and extend the range by approx. 34 miles, per hour of charging. Thereby turning it into an impromptu Hybrid !
For the cost of less than $ 600 U.S to buy a generator, you can easily gain more than an extra 200 miles of range, the Tri Motor model provides. And you weight penalty will be far less as well.
Generator = approx 90 kg vs Extra Battery + Motor = approx 450 kg

Note, someone has previously I believe reasonably calculated the approximate weight and battery capacity of the 3 Cybertruck models.
Counter intuitive to the thought of heavy 3mm thick Stainless Steel Exterior, the CT is relatively light, he worked out the following
(if memory serves me correct).

Single Motor : 5150 lbs, 80 Kw/h battery
Double Motor : 5300 lbs, 100 Kw/h
Triple Motor : 6100 lbs, 200 Kw/h

 

[Rootinboy]

Will the generator produce clean power since it’s not an inverted generator? I’ve seen several YouTube posts that show you can trick the Tesla by jumping the ground... not sure how that would work on the 240v outlet on the 7kw generator?

 

[ajdelange]

A rotary mahine is always going to produce reasonably sinusoidal current (much cleaner than a modified sine inverter) but the car's charger doesn't care as it rectifies the AC and then chops it up anyway making it very rich in harmonics (but not of 50 or 60 Hz).

For safety reasons the car charger must be able to detect 120 V between the hots and earth. In most portable generators the neutral and earth are isolated. So if you want to charge from a generator, which Tesla recommends you do not do, be sure there are 120V between the earthing (rounded) pin and the two hots. If not, make up an adapter which joins earth to neutral (the rounded pin to the pin opposite it on the plug or receptacle).

 

[Bankercobb]

Plaid powertrain will have 3 motors. Why not 4? Cooling the battery is probably the limiting factor there.... why not put bigger battery for 2 motors? Batteries weigh alot. I think the trade off is that with 2 motors and a giant battery would be lower towing capacity and lower zero to 60. Yes you gain range but people worry about range more than they should.

Worry about it more than we should? Do you have a tesla and have you driven extended distance? My Tesla takes 39% charge to get to the nearest charger... When I had to move and had no place to charge. I had to drive over an hour away to charge and have barely any distance left to do errands and house shop... yes range anxiety exists for good reason.

 

[Dids]

Worry about it more than we should? Do you have a tesla and have you driven extended distance? My Tesla takes 39% charge to get to the nearest charger... When I had to move and had no place to charge. I had to drive over an hour away to charge and have barely any distance left to do errands and house shop... yes range anxiety exists for good reason.

Seems like what you really need is more chargers? Most ice vehicles have average 300-400 mile range before refill. The reason no one cares is that they can quickly refill it almost anywhere.

 

[ajdelange]

Clearly you are in unusual circumstances such as a remote military installation where power is at premium to the extent that the facility commander won't allow BEV charging. You mention "house shopping". That implies that you have a house. Houses have electric outlets from which, if at very slow rate, one can charge. Unless, of course, the homeowners' association or the base won't allow cords trailing across lawns. Whatever your situation is it is very unusual. Apartment and condo dwellers (and people in military housing) have challenges for sure but in general this is not that much of a problem hence range anxiety has dropped way down the list of things people worry about when buying a BEV. Price is doubtless still No. 1 but I think charging speed is probably No. 2 at this point with service at No. 3 and range No. 4.

PS: I do have a Tesla I have driven longish distances (hundreds of miles).

 

[alan auerbach]

Because he can..

Also, I'm sure he can use the money.

But to add a motor and FSD for an extra $30k? Seems kinda high.

 

[Bankercobb]

Clearly you are in unusual circumstances such as a remote military installation where power is at premium to the extent that the facility commander won't allow BEV charging. You mention "house shopping". That implies that you have a house. Houses have electric outlets from which, if at very slow rate, one can charge. Unless, of course, the homeowners' association or the base won't allow cords trailing across lawns. Whatever your situation is it is very unusual. Apartment and condo dwellers (and people in military housing) have challenges for sure but in general this is not that much of a problem hence range anxiety has dropped way down the list of things people worry about when buying a BEV. Price is doubtless still No. 1 but I think charging speed is probably No. 2 at this point with service at No. 3 and range No. 4.

PS: I do have a Tesla I have driven longish distances (hundreds of miles).

Military correct. Had a house while on recruiting duty. In my encounters its charger location, speed, cost, ... service hasn't been mentioned by anyone that has shown interest. Us military like nice cars.