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Which batteries for the roadster

Hi. Autocar is misreporting the issue. Elon didn't say 100 kWh won't fit into the Model 3. He said more than 75 kWh won't fit. Check out his tweet here. In other words, the contradiction is stronger than you are presenting. As you have pointed out, the issue is that the Model 3 and the Roadster 2 seem to be around the same size. Yet one has enough space for 200 kWh and the other can only fit 75 kWh. How is that possible? It would have been a lot better if you had asked the same question and left it there instead of making an incorrect statement about the Roadster 2. The mistake you are making is that you are assuming the Model 3 statement was correct. Therefore you are questioning the Roadster 2 statement. It is the other way around. The Model 3 statement was not correct at all.







 

bolosky

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May 5, 2009
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Wouldn't the roadster perform better with a smaller and thus lighter battery? I don't see people using it for long road trips and adding a ton of mileage to an expensive car.
No. The bigger battery increases the power that the battery can produce. That's almost certainly the reason for it, and the extra range (and effective battery life) is just a nice bonus.
 
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The whole 200kwh pack is an elon pipe dream imho. It is aspirational. You could stuff that much in there and have a terrible car with great 1/4 mile stats though. He had to have been hoping for some kind of doubling of energy density that still hasn’t happened.

But to do it with current batteries would result in like a 6,000lbs car with a high cg
 
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Understandably many are skeptical of the weight of the 200 kWh pack, just based on how bad some overly heavy cars feel on the road. But I think Tesla will make good on it. Here is why.

First it's the feeling. We believe that we feel the mass of a car when we drive it. But actually we don't. What we do feel is the ratio between mass and the stiffness of springs, bushings, tire walls, tire friction, motor power, braking force and things like that. We also feel mass distribution as polar motion of inertia, but we can't feel absolute mass. There aren't any thoroughbred heavy sports cars that are built as optimized as the lighter brethren. That partly stems from the old days when sports cars were mostly made from standard car parts. It made economical sense to optimize most components and sub assemblies for the mass market cars, i.e. passenger cars. Sports car divisions had to a large degree to make do with engines, transmissions, tires and many suspension parts as they were. In that context it was of course true that a lighter car would generally perform better.

Colin Chapman pretty much cemented the idea with his motto "add lightness" Lotus Philosophy - Lotus Cars Official Website - For the Drivers. His team's Team Lotus - Wikipedia multi decade domination in racing made many take his world as gospel and incorporate it into their own design philosophies.

Tesla is not necessarily going to use only existing components for of its roadster. Nor is it going to follow convention. As the most vertically integrated car manufacturer on earth, they have more freedom to reason from first principles. Principle one is to always look at each problem in the limit.

What are the limits for mass? -The lower limit is the payload. In the Roadster case one to four people plus luggage. In the lower limit the car won't handle well because the car has zero mass and wouldn't move, the passengers would sit still on the ground. Moving up from the limit, what if the car weighed one kg? Probably wouldn't handle well because its tiny wheels would be overwhelmed by the weight of the passengers.

What happens at the upper limit? Infinite car mass makes the payload fraction tend to zero. This is ideal for performance since 100% of the car is used for propulsion and grip while 0 mass is wasted on payload. This is why Saturn V weighed 3000 tons with only a few tons of payload. Unfortunately if the car had infinite mass, Earth would be sucked into the it and we would all die. Another max mass constraint comes from the fact that that the car needs to be small enough to fit on the road, which in turn limits the tire contact patch. There is a disproportionality between coefficient of friction and reaction force. Here is an explanation of that Tyre dynamics - Racecar Engineering. With that in mind, one way to approach the problem would be to set the maximum possible contact patch, tire compound, road surface and temperature to fixed and draw the curve for friction coefficient as a function of mass.
Then we set the payload to fixed and draw a curve of performance as a function of mass of the car.

At some point these two curves cross at an ideal mass. It stands to reason that the crossover point is higher than the best ICE cars, because the lower energy density of batteries vs ICE skews the curve. Is the crossover point high enough to accommodate 200 kWh? Maybe, maybe not.

(Of course there are other constraints like center of mass, but this was just to show that lower mass doesn’t per definition equal a better sports car)
 
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In order for the 2023 Z06 to perform well with it's mass of 3,400lbs and power of ~670hp it needs 275mm front tires and 345mm rear tires, along with 380mm brake rotors.

If Tesla builds a ~6,000lbs roadster with ~1000+ horsepower the amount of tire and brake needed for it to perform well is going to be problematic in a few ways. Like if you work outwards from the length you need for nice double wishbone suspension and then add all the tire you will end up with a really wide car! And the pack is going to have to extend vertically so you won't get the same low center of gravity as on current cars.

You can certainly build such a car and put down some insane drag strip numbers but its going to be a weird sports car.
 
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In order for the 2023 Z06 to perform well with it's mass of 3,400lbs and power of ~670hp it needs 275mm front tires and 345mm rear tires, along with 380mm brake rotors.

If Tesla builds a ~6,000lbs roadster with ~1000+ horsepower the amount of tire and brake needed for it to perform well is going to be problematic in a few ways. Like if you work outwards from the length you need for nice double wishbone suspension and then add all the tire you will end up with a really wide car! And the pack is going to have to extend vertically so you won't get the same low center of gravity as on current cars.

You can certainly build such a car and put down some insane drag strip numbers but its going to be a weird sports car.
Good points. As you say CoG will be higher. At 100mm ground clearance and 2 layers 4680 with 10 mm pack structure under bottom layer and 10mm between layers between we get CoG of 195 mm for the pack, measured from the ground. Is that bad? Does that pack have to be double stacked all the way? Would be cool to see a rendering of a structural 200kWh 4680 for a roadster.
For sure it could be a weird sports car. Does it matter?
How do we know that it doesn't get 600 mm tires? Unlikely, but. Or perhaps it will get a super wide range of dynamic ground effect to compensate for narrower tires? Maybe it gets no brake rotors at all, to save weight? A resistor to dump extra braking energy not accepted by the battery could weigh a fraction of a brake rotor.
We are narrowing ourselves too much if we just disregard a 200kWh pack by making analogies with existing sports cars.
 
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Good points. As you say CoG will be higher. At 100mm ground clearance and 2 layers 4680 with 10 mm pack structure under bottom layer and 10mm between layers between we get CoG of 195 mm for the pack, measured from the ground. Is that bad? Does that pack have to be double stacked all the way? Would be cool to see a rendering of a structural 200kWh 4680 for a roadster.
For sure it could be a weird sports car. Does it matter?
How do we know that it doesn't get 600 mm tires? Unlikely, but. Or perhaps it will get a super wide range of dynamic ground effect to compensate for narrower tires? Maybe it gets no brake rotors at all, to save weight? A resistor to dump extra braking energy not accepted by the battery could weigh a fraction of a brake rotor.
We are narrowing ourselves too much if we just disregard a 200kWh pack by making analogies with existing sports cars.

You are really reaching for complicated explanations when a simple one of "Elon was speaking aspirationally and with a bit too much optimism this one time" works fine =)
 
You are really reaching for complicated explanations when a simple one of "Elon was speaking aspirationally and with a bit too much optimism this one time" works fine =)
Not reaching for anything. Just saying don't assume the big battery is wrong even if it doesn't fit into an old recipe ;). You may very will be correct, will be exciting to see the car either way.
 

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