Recently, the collaboration between TPI and Proterra has produced an integrated monocoque composite electric transit bus. “In a typical bus or truck, there’s a chassis and the body is placed on top of that chassis,” explains Todd Altman, TPI’s senior director of strategic markets. “With the monocoque design for the bus, we are integrating the chassis and body, similar to a uni-body car design.” The single structure is more effective than two separate structures in meeting performance requirements.

The Proterra monocoque body is purpose-built, designed from the ground up to be an electric vehicle. Altman says that’s an important distinction, given the experience of many automakers and electric bus manufacturers that have tried with limited success to adapt their legacy designs for internal combustion vehicles to electric vehicles. “They take their existing platform and try to put in the batteries wherever they can. That does not yield the best solution from a variety of standpoints,” Altman says.

For example, many electric buses have batteries mounted in the rear or on the roof of the vehicle. But for Proterra, TPI was able to mount the batteries underneath the bus.

“If you’re putting a lot of weight on a vehicle structure, you want that weight to be as low as possible both from a performance standpoint and from a safety standpoint,” Altman says. He notes that many manufacturers of electric buses and cars are now going back to the drawing board to develop more efficient, purpose-built designs for their vehicles.

“When people come to a composite manufacturer, they may say, ‘Build this part or this small subsystem,’ and you’re in a box. In these cases, you are often pushed into limited design flexibility. That can result in utilizing more of a higher-cost material like carbon fiber than you would have if you could build with sandwich construction and develop a design that’s more optimal,” Altman adds. “That’s where purpose-built really comes in; you can design a solution that doesn’t have the same number of tradeoffs that you would have if you tried to make the body from something else.”

TPI has signed a five-year agreement with Proterra to produce up to 3,350 composite bus bodies in TPI’s factories in Iowa and Rhode Island.

Customization Needed

Designing the Catalyst bus body required TPI and Proterra to continually balance the pluses and minuses of all the different materials so that they could hit cost targets while achieving optimal performance. Altman notes that TPI’s experience with producing massive wind blades almost 200 feet long and weighing 25,000 pounds has made it relatively easy for them to produce 40-foot bus bodies that weigh between 6,000 and 10,000 pounds.