Mythbusters Part 2: The Tesla Roadster is not a Converted Lotus Elise

Darryl Siry, Vice President of Sales, Marketing, and Service, 3. März 2008

One of the more common misconceptions about the Tesla Roadster is that it is an electrified Lotus Elise. This has been an interesting problem to deal with because the Tesla is vastly different from an Elise and it is important that people are aware of this. In fact, we recently counted how many parts the two cars shared and the total number was under 7% by parts count. If you were to analyze it by parts value, the number would be even smaller.

So you could say that the Tesla is similar to a Lotus Elise, except it has a totally different drivetrain , body panels, aluminum tub, rear sub-frame, brakes, ABS system, HVAC and rear suspension. The Tesla also neglects to carry over the gas tank, emissions equipment and exhaust. If you were to try to convert an Elise to a Tesla and started throwing away parts that aren’t carried over what you would basically be left with a windshield, dashboard (complete with airbags!), front wishbones and a removable soft top.

For comparison, Lamborghini cars share upwards of ten percent of their parts content with Audi cars. I can only guess what the number is between Ford cars and their high end Aston Martin and Jaguar brands.

At the same time, the Elise is a great car in its own right for what it was built to do. Several engineering breakthroughs that Lotus first implemented in the Elise, in particular the use of aluminum extrusions in the chassis that are bonded (as opposed to welded), have since been adopted by several carmakers and brands, including Aston Martin and Jaguar (note: I am not aware of any formal relationship between Lotus and those two automakers.) It was the fact that this approach yields a lightweight, rigid chassis and that the up-front tooling costs of extrusions are significantly cheaper than stampings that led Tesla to connect with Lotus about adopting a similar approach for the Roadster.

Early in Tesla’s history, after the company had focused on developing core battery technology, the time came to think about how to approach building a fully integrated EV car. As a startup, there were potential advantages to identifying an existing platform that had already been tested for US safety regulations and which could donate most non-EV parts. As with most things, however, the devil is in the details.

For starters, placing a 1,000 pound battery in the midsection of the car changes everything about the behavior of the structures that were originally engineered for the ICE powered Elise. In order to accommodate the added weight of the battery pack, the aluminum tub chassis had to be redesigned to a Tesla specification, increasing strength and using the battery pack as a stressed member to increase rigidity. Since we had decided to redesign the chassis, we also chose to redesign the side rails so that they would sit a few inches lower than those in the Elise, greatly improving ingress and egress. The chassis is produced for Tesla and does not start its life as an Elise chassis.

To accommodate the battery pack, motor and transmission, Tesla designed a new rear subframe. In order to handle the increased weight in the rear, new rear wishbones were designed. An added effect of the new rear subframe and suspension was a wheelbase about 2 inches longer than the Elise. The added weight, longer wheelbase, and desire for a different type of driving feel for our customer required a redesign of the suspension to achieve our ride and handling goals.

A challenging area of development for an EV that is not always obvious to the average person is the 12V accessories (fans, headlights, AC, heaters, etc.) All of these things have evolved over the last 100 years to run off the internal combustion engine. In an EV where there is no ICE and the main battery is running at about 400 volts, sourcing, designing and integrating these systems is no small feat. GM recently told of unforeseen problems they ran into in the development of the Volt related to 12V accessories that they say has driven the likely cost of the car up $5K to $35,000. Pretty much every car on the market uses a heat exchanger to transfer heat from engine coolant to generate cabin heat. That’s why the car has to warm up before it blows hot air. Every car on the market uses a belt driven compressor for the air conditioning. Every gas car also uses a belt driven alternator to generate 12V DC to run all of the other accessories. None of these things apply for the Tesla Roadster or any other EV, so they need to be designed specially for the car. I also left out power brakes, which normally rely on the vacuum from an ICE to generate boost (on an EV you need an electrically driven vacuum pump.)

All along, Tesla was developing its own signature styling for the Roadster. Once completed, every single panel on the Roadster was unique. The only exterior parts that are carryover are the rear-view mirrors, which Tesla carried over to avoid expensive development and safety testing costs. The headlights and taillights were designed to be unique to Tesla as well because they are so important to the character of the car.

As for materials: body panels (except for the bumpers) are made entirely of carbon fiber to minimize weight. By way of comparison, the next cheapest car that uses carbon fiber for body panels is the Mercedes McLaren SLR, which will set you back a cool $500,000.

So what remains as carryover from Lotus? The most noticeable pieces are the windshield and the dashboard, including the steering wheel. The fact that these pieces are so visible to the driver, passenger or gawker might partially explain the reason why people sometimes think the cars are closer cousins than they are. The reason we carried over these parts of the car was to avoid the onerous development and safety costs associated with passenger safety and airbag testing of a new design. Lotus had already successfully designed and tested the interior passenger compartment with the Elise and carrying it over made financial sense.

That concludes part 2 of the Mythbusters series. I’d love to hear your thoughts on other myths to address in future installments.