Range Reality

By Martin Eberhard, Co-founder of Tesla Motors

April 18, 2007

As you may have read in previous blogs, we recently built our first couple of Validation Prototypes (VPs). These cars represent a significant step forward toward production as they implement numerous corrections to issues discovered during safety and durability testing of the Evaluation Prototypes (EPs), are built from hard-tooled components for all body panels, include production headlights, taillights, and interior components (including much more comfortable seats), and many other subtle changes.

Along the way, we made several design decisions that improve the safety and durability of the car, understanding that there would be tradeoffs. The two most significant decisions were:

We made many design changes to the car, as a result of testing, to make it more durable and safe. As Murphy would have it, most changes added weight, increasing the car’s weight by several hundred pounds.
We deliberately chose lithium ion cells with a slightly lower capacity than the largest cells available, because these smaller (and more mature) cells have better long-term durability and higher tolerance for abuse.

VP1 in San Francisco (EP1 in the background)

We recently performed our first actual driving range tests with a Tesla Roadster that incorporated these design changes (and many others!) on an EPA-compliant dynamometer. Based on the results of these tests, we now anticipate that the range of the Tesla Roadster will still be greater than 200 miles, but will not meet our original target of 250 miles. You may have noticed the change on our homepage and in our specs.

Last week, we sent letters to our Tesla Roadster customers, as they deserve to know about this specification change first. They have asked us many probing questions, and I hope that my answers satisfy them. Many of their questions center on the gain in weight. I have explained many of the tradeoffs we have made – generally trading additional weight for increased safety or durability – the result of our safety and durability tests on the EP cars.

Here’s my response to customer questions about weight, as posted in the Owners Area, a private area of our website for Tesla Roadster owners:

There is no single 100-pound (or even 50-pound) addition. What I get from Engineering is a long spreadsheet of incremental weight. Some examples:

SAE shake and vibration testing caused us to stiffen up (and add a fair bit of weight to) several parts inside the Energy Storage System (or battery pack), damping out resonant frequencies.
Durability testing caused us to improve (and add weight to) the mounting system for the modules within the ESS, and to strengthen the brackets that mount the ESS to the chassis.
Durability testing also caused us to increase the strength of one of the front suspension brackets on each side.
We broke the top motor mount on two EP cars doing demonstration "hole shots," necessitating changing from magnesium to aluminum, and increasing the bracket size as well. (Did any of you notice a funny clunk in the car late in the evening at our launch last July?)
Hard driving convinced us to change the motor end housings from magnesium to aluminum as well - under severe testing, we had trouble with the bearings spinning in their mounts.
Hard driving in the desert caused us to increase the size of the coolant pump. Several heat sinks (both in the PEM and on the motor) got a bit bigger (and heavier) too.
We got very conservative when we redesigned the transmission because we knew we had to get it right the first time - no opportunities to strengthen it later.
Upon strong urging from our new transmission supplier, we also changed to an electro-hydraulic shifting mechanism (instead of a purely-electrical one), because it is far more reliable and durable and because it shifts much more quickly. Needless to say, it is a bit heavier too.
On our first try, the side intrusion test failed, largely because we lowered the door sill height. The redesigned door beam is, naturally, stronger and heavier.
NVH (Noise, Vibration, and Harshness) testing caused us to add damping material to solve various noise issues, particularly the sound of the A/C compressor.
Our original electric door latches were simply unreliable. We changed to a much better latch from a different (American!) vendor that is (as you already guessed) heavier.
The original stereo did not sound like it belonged in a sportscar of this caliber (okay, it sounded like heck), so we redesigned it to take a 12-liter subwoofer and higher-end speakers.
Because the car got heavier, we had to increase the strength (and therefore weight) of the suspension and brakes.

This and twenty other lesser items add up to a couple hundred pounds. Each is a relatively small item; each is totally justifiable and even necessary. The result is extra mass, the result of long, hard safety and durability testing.

Maybe I was a bit naive expecting to hold the line on mass. Those of us at Tesla Motors who have a long automotive experience say that fixes to problems discovered at this stage of the program always add mass.

The upside of all this is that the Tesla Roadster will be a much more reliable car for having added this mass.

At more than 200 miles, the Tesla Roadster will still have the highest range of any production EV in history by a large margin, and we will continue working hard to deliver even better range in the coming months.

My original premise was that the Tesla Roadster’s range is high enough that you would not have to worry about charging during a typical day, even if you have a long commute, take the car out for dinner and chores, or even take the scenic route home. Once home, you plug it in – just like you would your cell phone – and by the time you’re ready for another day, your Tesla Roadster is fully charged and ready to go. I believe that this premise is still intact with a range above 200 miles.

With the benefit of extensive testing of our EPs, I am confident that we will achieve a final range above 200 miles. Now that we have completed cars and have a deeper understanding of the EPA’s testing methodology, our future range estimates will be based on empirical testing, not simply modeled estimates. We continue to drive engineering improvements to increase range, and explore options that would allow customers to choose between increased range and enhanced performance. There are still some unknowns and variables that will become known as we develop and test our VP cars, so I have chosen to communicate a floor of 200 miles and strive for upward revisions in the future.

We are holding the line on 4-second 0-60 mph acceleration, largely because the extra mass has been offset by improvements in the drivetrain. We won’t know exact numbers until we have real transmissions this summer, but the team remains confident. I drove VP1 home last night, and as I rocketed around the curves of Skyline Boulevard, I could not keep the smile off my face. The Tesla Roadster is still the quickest and most fun car I have ever driven. The way it hugs the corners, the way it pulls out of a corner is simply without compare.

Tesla Motors has come a lot closer to shipping a DOT-compliant and roadworthy performance electric sportscar than anyone has ever before. One thing for sure – the cold, hard reality of actual test results on a fully-equipped car destined for production is a lot tougher than estimates, simulations, projections, and every manner of vaporware about non-production (and even nonexistent!) cars.

Obviously, writing a blog like this is not the most fun part of my job, and I have challenged our team to give me some good driving range news that I can announce later this year. We will see – they are an amazing team.

“ For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled. ” – Richard Feynman.