Tesla Model 3: center underbody

I joined Tesla in January of 2013 as an engineer on the Body Structures team. This team is responsible for the design and development of a car’s body structure, sometimes referred to as the body-in-white. This structure serves as the “skeleton” of the vehicle, a strong frame to carry all of the vehicle’s systems and provide a number of performance characteristics. The design of the body structure is driven by crash performance, global stiffness, cost, and manufacturing requirements, among many other factors. I worked first on Model S cost- and mass-down studies, before leading the design of the right-hand drive variant for the body team; I took lead on the design of the center underbody system for Model X; and I was lead again on the center underbody system for Model 3. On this page, I seek to highlight my work on the Model 3.

On the Tesla Model 3, I was again responsible for the design of the center underbody of the body structure. The scope of ownership was very similar to that of Model X: side sill members, seat crossmembers, toeboard area, floor panel, various brackets.

The Model 3 structure, however, was not an evolution of the Model S or Model X designs, but a new clean-sheet design. Model 3 is a smaller vehicle than its older siblings, and its construction is primarily steel. Compare this to the Model S and Model X, which are composed or large aluminum castings and extruded profiles. The stamped steel construction used in the Model 3 is generally better-suited for a lower-priced, high-volume vehicle; steel offers much higher ultimate strength capabilities than aluminum, a factor that is important in the center underbody design.

Like with the Model X, the driving crash load case for the components I designed was the rigid pole side impact test, part of FMVSS 214. In this test, the vehicle strikes a rigid pole sideways. The 10-inch (254mm) pole is positioned in line with the head of the driver, and the vehicle moves at 20mph (32 kph) at a 15-degree angle into the pole. Injury values are calculated and combined with door opening requirements to calculate a performance score.

This test is particularly challenging for electric vehicles with large batteries, because these battery packs are typically mounted beneath the occupant cabin, and close to the side of the car where the pole strikes. The interaction between the pole and battery pack is extremely important, and varies with different battery architectures; at Tesla, it was crucial that the battery cells receive no damage during the crash event. Our design approach was to establish two distinct zones, much like any other crash case: first, the deformable zone, composed of parts designed to crush and absorb the kinetic energy of the pole; and a rigid backup structure zone, composed of extremely strong parts that would not yield, but instead function as a backstop to enable the deformable zone to do its job.

The illustration above shows the components of the system. The primary element of the deformable zone is the aluminum extruded profile, which runs the length of the sill / rocker section. Bonded in place, this part is carefully designed to absorb the maximum amount of energy possible in the very small crush space available. At the outset of the program we studied a variety of alternative materials and geometries, but the longitudinal extrusion proved to deliver the best balance of energy absorption, mass, cost, and manufacturing complexity for our system. Supporting this part is a series of cross-car steel beams. These crossmembers are the foundation of the backup structure. Made of ultra-high strength Martensitic steel, these crossmembers are roll-formed from flat sheet and laser-welded along their length to form a closed section. The crossmembers are remote laser-welded to the floor panel, stitched across their entire length to help dissipate impact load into the rest of the body structure. The development of these roll-formed members, in a grade of material beyond anything that Tesla or our manufacturing partner had achieved before, required a long process of iteration and design development. Similarly, the remote laser-welding operation required multiple iterations and a novel fixture design to succeed.

While the side pole crash load case was a major factor in shaping the design of the center underbody, there were other important design factors. Model 3 features deep integration of the battery pack structure with the body structure, employing a number of bolted joints in the cabin that helped distribute front impact load from the front rails into the battery pack longitudinal members. The challenging new small-overlap crash load case influenced the design of the side sill inner and outer, as well as the aluminum insert inside. The center underbody also provided fixing points for the seats, interior console, lower dash carrier, and a host of wiring harnesses, HVAC ducts, cooling lines, and brake lines.

The net result of our work is a 5-star overall NHTSA rating with an extremely low probability of occupant injury. Our side pole strategy was validated through testing and the Model 3 delivers not only a 5-star rating in this test, but also the lowest intrusion ever measured in a production vehicle. This outstanding result enables the vehicle to carry more battery cells closer to the edge of the car, helping deliver a midsize premium sedan capable of up to 353 miles of range.

The Model 3 is my proudest achievement so far. I’m honored to have helped create the most compelling EV on the market today, one that brought real-world range and Tesla driving performance to an affordable price point. My wife and I liked it so much, we bought one for ourselves. It’s been an absolute pleasure for the last three years.