Like almost everyone else, I was blown away by Tesla’s surprise introduction of a second-generation Roadster at the recent semi-truck reveal event.
Zero to sixty in 1.9 seconds, a 620-mile range, gorgeous sleek lines, and a price of $200,000. Mind-blowing indeed.
But a few of the new Roadster’s numbers didn’t add up for me.
First, that 620-mile range, by far the longest of any production electric car. Some critics doubt that such a thing is even remotely possible.
But my response was just the opposite: why isn’t it 800 miles?
Weight and drag
The new Roadster has a massive 200-kWh battery, twice the capacity of my Model S 100D. Yet it has less than twice the range of my car, which is EPA rated at 335 miles.
That means a small two-seat sports car is actually less efficient than my big five-passenger sedan. How can that be?
Assuming a battery weight of 1500-2000 pounds, I’d estimate the Roadster’s weight at 4,000 pounds. That’s about 20 percent less than my 100D.
Besides weight, the other main factor that determines efficiency is aerodynamic drag, which is the product of frontal area and drag coefficient.
The Model S has a drag coefficient of 0.24, which is very low. Based on the Roadster’s similar sleek profile and Tesla’s fanaticism about aerodynamics, I’d presume the Roadster’s Cd is in the same ballpark.
With a similar drag coefficient, the total aero drag of the Roadster compared to the Model S would then be proportional to the frontal areas of the two cars.
My rough eyeball estimate: The Roadster has about 20 percent less frontal area than the Model S—and therefore, 20 percent less total drag.
My question: how can a car that’s 20 percent lighter than a Model S, with 20 percent less drag, be less efficient?
I’m guessing that the tires play a role. The Roadster’s tires will be presumably optimized for dry traction and structural integrity at 250-plus mph, not efficiency.
The high-performance 21-inch tires on the Model S cut its efficiency by 5-12 percent compared to the standard 19-inchers.
Could the extreme track-style tires of the Roadster cut its efficiency by as much as 20-30 percent?
Perhaps some tire experts can weigh in on this topic.
My second question: why such a huge battery?
Elon Musk has often said that there’s no need to have a range of much more than 300 miles, and I agree with him. The Roadster could achieve that with a battery half its size.
My 100D’s range of 335 miles translates into five hours endurance at 65 mph. No way I want to sit in a car longer than that.
Why pay the weight and cost penalties for extra battery capacity that will never be used?
Power, not range
For the Roadster, the answer is probably a matter of instantaneous power delivery, not range.
A battery can discharge its energy only so fast. Even if the motor is capable of 1,000 hp, it can only develop the power that’s delivered to it by the battery. The bigger the battery, the more instantaneous power it can deliver to the motor.
(Tesla sowed some confusion on this front a while back by listing the horsepower of its motors on its online configurator—without explaining that in some cases the battery was not capable of delivering that much power to the motors, essentially “governing” them to a lower max hp. Nowadays the company simply lists performance specs, not power numbers.)
But it seems to me the max power delivery of the standard Model S 100-kWh battery ought be sufficient to get the Roadster 0-60 time under 2 seconds.
If the 100-kWh battery has the power to propel the P100D to 60 mph in 2.28 seconds, (Motor Trend’s number), the 20-percent lighter Roadster should accelerate about 20 percent faster, all else being equal. Theoretically, with the same motors and battery as the P100D, the lighter Roadster should be able to at least flirt with the two-second barrier.
At the very least, it seems to me that a 130-140-kWh battery should be able to easily power the Roadster into sub-2-second territory. So why 200 kWh?
Perhaps the massive 200-kWh battery is required simply to achieve the claimed 250-mph-plus top speed.
Going that fast is really hard. One reason is that the power required to overcome aerodynamic drag goes up with the cube of the airspeed.
Thus it takes 64 times the power to overcome air resistance at 260 mph as it does at 65 mph.
When I’m cruising at 65 mph in my Model S, the power meter shows a consumption of about 20 kW. Assume that 5 kW of that is due to rolling resistance and driveline friction, leaving 15 kW required to overcome aero drag at 65 mph.
Again assuming the Roadster has 20 percent less aero drag than the Model S, we arrive at 12 kW of power required to overcome the Roadster’s aero drag at 65 mph.
Multiply by 64: it thus takes about 768 kW to overcome the Roadster’s guesstimated aero drag at 260 mph. That’s about 1030 hp.
Add 20 percent for rolling resistance and driveline friction, and we arrive at about 1200 hp required for the Roadster to hit 260 mph.
That’s way more than the 750-ish hp that the Model S 100-kWh battery is said to deliver in the P100D.
My guess is that the Roadster was fitted with a 200-kWh battery not to achieve its 620-mph range—nor even the 1.9 second acceleration, which is probably limited by tire coefficient of friction (i.e., traction), not available power.
Here’s my speculation: the Roadster needs the huge 200-kWh battery to supply the power for its off-the-charts top speed—and to maintain that speed for a reasonable length of time without overheating.
Who needs it?
But really, now. What is the point of owning a car with a top speed of 260 mph, other than simple bragging rights?
There is no point. In the U.S., at least, even the Model S’s top speed of 155 mph is entirely useless.
How many Bugatti Veyron owners have actually driven their car at its 250-mph top speed? A number very close to zero, I would imagine.
For them, the whole point of paying $1 million for a Veyron (or $2.7 million for its 261-mph successor, the Chiron) is the prestige of owning a car that can go that fast, and the smug knowledge that no production car is faster. Owning a car like that is all about status and ego. The point is to possess it, not drive it.
The new Roadster will certainly attract a few of this kind of hyper-rich buyer. But for many of them, the new Roadster will have a serious drawback: it’s price is too low. When you buy a car for ego and status, the higher the price, the more status is conferred.
Heck, a moderately successful orthodontist could probably come up with the $200,000 to buy a new Roadster. (Did you notice the throngs of Roadster buyers lining up at the credit-card machines at the reveal event? So gauche.)
If I were Tesla’s marketing chief, I would have priced the Roadster at about $500,000. Still way less than any other hypercar, but pricey enough to scare off the riffraff.
And then, for the riffraff: a Roadster to actually drive: a Lite version with a 100-kWh battery, more efficient tires suitable for everyday driving, a 400-mile range, and a zero-to-60 time of 2.0 seconds.
A half-ton lighter with its smaller battery, Roadster Lite would certainly handle more nimbly. And the lighter weight might even allow it to break the 2-second zero-to-sixty barrier, given the right tires. But for marketing reasons, I’d limit acceleration to 2 seconds and reserve sub-2 territory for the big dogs paying the $500,000 tab for the full-on version.
Top speed of Roadster Lite? To keep it faster than the Model S P100D, let’s say 180 mph, which demands only one-third of the power required for 260 mph. With the lower top speed, Tesla might even be able to get away with direct-drive motors. (I’d bet a lot of money that the 260-mph version has a two-speed transmission for the top end.)
Price: again, a notch above the P100D, say $175,000.
I can already here the cash registers ka-chinging.