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)