Where the Rubber Meets the Road

Andrew Simpson, Vehicle Systems Engineer, September 24, 2007

When I last posted to this blog in May, the news was a mixed bag. Martin (Founder Martin Eberhard) had just officially revised our range expectations down from 250 miles to greater than 200 miles, but we remained committed to holding the line on 4 second 0-60 mph acceleration and delivering the Tesla Roadster with the performance, handling, looks, and safety of a world-class electric sports car. Now it is September and in the last three weeks we have completed performance and range validation testing of Validation Prototype 1 (VP1 aka “the green car”) in order to verify our Tesla Roadster performance claims. I’m extremely pleased to say that the results are in and our hard work has really paid off!

VP1’s History

VP1 arrived in San Carlos in March and has since been our primary developmental platform. It’s always the first car to receive the latest and greatest upgrades from our engineering teams. For our validation tests, VP1 was upgraded to production intent – a configuration to match the equipment levels we intend for the production Roadster. This list of upgrades has included:

  • Improved brakes plus our production tire/suspension tunings that minimize driveline friction without compromising roadholding or safety
  • Design improvements in motor and Power Electronics Module (PEM) cooling to increase thermal headroom and reduce the need for active cooling
  • Latest-generation firmware algorithms for optimized performance and efficiency
  • A production-intent, 2-speed, electrically-actuated-manual-shift transmission
  • A brand new ESS (battery pack), baseline tested and confirmed against our production specification
  • Ballast (weight) to bring the car to production curb weight, since not all cosmetic/accessory items were included in the VP1 upgrade.

VP1 after it rolled off the production line in the UK

I then drove VP1 more than 1,000 miles in representative real-world conditions to wear-in the new components, prior to testing, as required by federal procedures for range certification. These miles included impatient commuting in the suburbs, sustained aggressive driving on the highways, and (most fun of all :) ) sporty driving in the hills around San Francisco. Once the mileage accumulation was complete, we were ready to put VP1 through its paces...

Performance Testing

For our performance tests we took VP1 to the NASA Ames Research Center (Moffett Field) in Sunnyvale Calif., where we were granted temporary access to a 1.5 mile runway. The runway had a gentle slope and there was a fresh uphill breeze blowing on the day, so we completed runs in both directions to average the results. Speed data was logged in the Roadster’s onboard computers as well as using standalone instruments for added confidence.

The 0-60 mph runs were rapid with gut-wrenching acceleration from the surge of electric torque. I shifted to first, punched the throttle, and the car rocketed easily to 60 mph in less than 4 seconds. I had mistakenly left traction control enabled so there were no squealing tires nor smell of burning rubber, but had I switched traction control off, we might have reached 60mph even faster (albeit more dramatically!)

The top speed runs were the most-exciting. Starting from both ends of the runway, I shifted to second and floored it. Again, the car surged forward with seamless acceleration and then settled as the speedometer nudged 130 mph. I held that speed for about half a mile, and it felt amazingly fast but also incredibly smooth and stable. My colleagues Aaron Platshon (product marketing) and Philip Luk (vehicle engineer) were there in support and later described how the Roadster emerged from the shimmering haze and tore past them at 130 mph leaving a cloud of dust in its wake. Aaron had hoped to showcase his amateur video skills by capturing the top speed runs on tape, but all you see is a green blur and hear him say “oh my – I could not keep up with that at all!”

Even though we have confirmed the Roadster’s top speed capabilities, driving at sustained 130 mph speeds does subject our drive motor to extreme mechanical and thermal stresses. We have therefore decided to electronically limit the production Roadster’s top speed to 125 mph in order to maintain long-term reliability in the powertrain.

Range Testing

For an overview of our official range test procedures, see my previous blog. We used Moffett Field again for our coastdown testing before we packed up VP1 and headed to the chassis dynamometer facility at Automotive Testing and Development Services in Ontario, Calif. The range tests were significantly lengthier and a bit less fun than our performance tests, but the results are equally impressive!

EPA Test Miles Recharge Energy
City cycle 252 30
Highway cycle 236 32
Combined result 245 31

Our EPA combined range of 245 miles is the result of an intensive engineering effort to maximize efficiency and provide the highest-possible range without compromising the Roadster’s other key attributes. We are now in the process of certifying these results so you can expect to see them on our window-sticker as soon as production starts.

VP1 undergoes range testing on the dynamometer

The EPA range was a key benchmark for the Tesla Roadster, but more telling was the range in real world conditions. In recent months we used VP cars to demonstrate a variety of real-world drives that fully-discharge the battery. These trips highlight how the mileage will vary based on driving style and other factors, but also demonstrate that real-world range of >200 miles can be achieved repeatedly. (If you live in Northern California, you may be familiar with some of these locations.)

  • 267 miles of conservative urban driving in the suburbs around San Carlos (a best-case scenario)
  • 230 miles from North Lake Tahoe to San Carlos with two occupants plus luggage
  • 227 miles of highway cruising on the I-5 freeway, south of Stockton
  • 222 miles including sporty driving from San Carlos to Santa Cruz via the hilly Skyline Blvd, and highway cruising from Santa Cruz to San Carlos via Hwy 1, San Francisco, and US 101
  • 213, 209 and 203 miles of highway cruising with A/C using I-280, Hwy 85 and US 101
  • 186 miles of aggressive driving on I-280 and the round trip from Woodside to San Gregorio Beach via Hwy 84
  • 165 miles of impatient commuting, aggressive stops and starts, high speeds, and air conditioning on from Saratoga Gap to San Carlos via Hwy 9, Hwy 85 and I-280 (a worst-case scenario)

Note also that the improved efficiency of the Tesla Roadster has provided other benefits – recharge time-per-mile and electricity cost-per-mile have been reduced, while the overall battery lifetime mileage has been improved.

Overall, we are incredibly proud to have developed the highest-range production electric vehicle in history with a result that is very close to our original claim -- all while retaining the performance of a world-class sports car. I’ve been fortunate enough to spend a lot of time driving the Tesla Roadster and have no doubt that people will be amazed by the incredible driving experience offered by this car. But you don’t have to take my word for it – just ask the customers who have already driven one.