TPI was able to get the required structural strength by employing carbon fiber selectively, reserving it to reinforce areas carrying the heaviest loads. “We use carbon fiber in those places where it can essentially buy its way onto the vehicle,” Altman says. Overall, carbon fiber makes up less than 10 percent of the bus body’s composite material reinforcement; the remainder is glass fiber.

TPI chose vinyl ester resins for similar reasons. “When we look at epoxies, they are fantastic, but when you cure them you have to elevate the temperature so you have to heat the molds. That’s an added expense,” he continues.

The company uses vacuum-assisted resin transfer molding (VARTM) to produce a composite sandwich, which provides the necessary stiffness for the monocoque body. Some metallic fittings, such as threaded fittings and tapping plates, are incorporated into the body during the manufacturing process. The bus is molded in two parts – upper and lower – and then bonded together. There are a few small composite trim pieces, like fairings, that workers have to add later, but the number of parts is a fraction of what it would be on a metal bus.

Once the completed body is sent to the Proterra bus production factory, the manufacturing line flows much faster because there is less work to be done. “They don’t have to do all the welding and grinding and manufacturing, and they have a really simple interfacing to attach the body to the drive train,” Altman adds. Proterra saves time and reduces its overhead because there’s less manufacturing space needed for the monocoque body.

Altman believes that demand for composite bus bodies will continue to grow as cities turn to electric buses to reduce pollution and cut costs. According to Proterra, battery-electric vehicles have the lowest operational lifecycle (12-year) cost when compared to diesel, compressed natural gas or diesel-hybrid buses. That could be one reason why Proterra says that sales of battery-powered electric buses now claim a 10 percent share of sales for the total transit market.

There are still hurdles to wider adoption of composite materials for electric bus bodies. One is the specialization that different bus customers demand. “Every transit authority likes to get their buses differently – the seat configuration, the way their hatches open. This is a big challenge for bus manufacturers, and many of these configuration items can flow down to us,” says Altman. “Composite bus body manufacturers want to have a standard build, but that’s difficult to do if each customer wants a high level of customization.” TPI is continuing to collaborate with Proterra to enhance the bus design to better manage the end customer’s desired flexibility.