Swedish scooter maker curves sheet metal with sustainability in mind

Stilride founders Jonas Nyvang and Tue Beijer stand next to the company's electric scooter. The entire chassis comprises just six thin pieces of curved stainless steel. Images: Stilride

Look closely at a stainless steel chassis of an electric scooter prototype from Stilride, a Swedish startup, and you’ll see a sleek design that comprises just a handful of sheet metal components. Traditional chassis designs incorporate dozens of sheet metal, machined, and plastic parts. At this writing, Stilride is using just nine.

The company makes this happen with a unique part forming method. Components begin as very thin blanks. Arranged and formed the way they are, those nine pieces of sheet metal end up being many times stronger than a typical scooter chassis, according to the company, adding that the design is about 50 times stronger than certain tubular chassis.

The forms don't have traditional bend angles or even bump-formed radii, which could have been created on a press brake, panel bender, or folding machine. They instead comprise complex curves creating a combination of convex and concave surfaces—the kind of form that perhaps could have been made by a dedicated, very expensive stamping die (though the complex geometries might present some challenges). In this case, however, there is no stamping die or dedicated tool of any kind.

Tue Beijer, Stilride's co-founder and chief technology officer, said the idea for a new sheet metal forming process came from one of the oldest design processes on the planet—origami. Fold heavy-stock paper in a certain way, and you feel how certain folds make a design more rigid. "When you design curves into your shapes, you need fewer operations [to create the part]. And from a design aspect, the curved shape creates strength. The shape becomes interlocked."

A formed sheet metal part can be extraordinarily strong, elegant, and efficient, but can it be manufactured at scale? According to Beijer, Stilride has found a way.

"When you fold over a curve," he said, "things start to happen."

Stilride's scooter design looks as if it came from a metal artist's studio, and if it really had, it would be a curiosity but not much more; the design might be strong and sleek, but it couldn't be manufactured at scale.

Beijer and his friend and company co-founder Jonas Nyvang have different plans. In the fall of 2022, the company oversaw the implementation of a unique automated cell in which steel sheets are fed to robots with special grippers that form each piece over curves. The robot's motion determines the final geometry. This is the latest iteration of what the company calls the Stilfold sheet metal forming method. "Bending or manipulating one of the surfaces, either the concave or convex, will affect the other surface," Beijer said. "Locking a system of curved, folded surfaces, [almost] like a beam, will therefore create a strong structure."

The forming happens in a robotic cell (because of pending patents, the company couldn't release further system details). The complexity arises in the software—unfolding of the formed shape to create the desired blank size for laser cutting, and the programming that instructs the robot exactly how to move to create the curved form in a repeatable fashion.

In an emailed statement, the company described the process this way: "First, designers use the Stilfold software to simulate folding and unfolding their desired structure from a metal sheet, so they can create folding guidelines and program the robotic arms to carry out the folding process. During the folding process, steel sheets are fed one by one through a robotic cell. Robotic arms with special fittings fold each piece over curves to create the desired structure."

The Stilfold process curves sheet metal in a way that limits the number of pieces an assembly requires, increasing strength at the same time.

"Stilride has developed the Stilfold process. We have mainly been aiding with our core competency around production and steel fabrication."

That was Mikael Norlén, CEO at Brantheim, a custom fabricator in Tyresö, Sweden, that began working with Stilride while participating in a research project funded by the Swedish Steel Association and the Swedish Innovation Agency. The fabricator has a full complement of sheet metal cutting, bending, and welding technologies, including robotic laser welding.

"Stilride has tested [its curve forming] method on several sized sheets, and we have tried it on metal plates from 0.5 to 3 mm [thick]," Norlén stated in an email. "[The process] enables complex shapes and forms … and together with Stilride, we are exploring its limits in terms of material thickness, grade, and yield strength."

Beijer added that welding sometimes occurs after folding, but other times it occurs before folding. "Some parts are welded when still flat, then folded into the final shape."

The process has come a long way over the past two years. The founders began origami-style using cardboard paper—something that's still foundational to the company's design process. Even today, design software augments the paper prototype, but it doesn't replace it.

"These prototypes help us map out the curve folding process cheaply and effectively," Beijer said, "before we test [the design] out on far more expensive sheet metal."

Once prototyping for the scooter chassis moved on to sheet metal, designers used laser-cut blanks with perforated fold lines, to be bent by hand. A succession of bent lines in multiple directions were meant to emulate the final curved structure—again, much like origami.

Then came work on the Stilfold process itself. According to Beijer, the robotic setup is unique, but it's also somewhat simple. The core of curve forming resides in software that folds and unfolds (or curves and uncurves, since the sheet metal isn't being folded on a bend line), accounting for process variability like springback. The company also is considering certain technologies that would provide in-process monitoring to ensure the metal is being formed as anticipated. At the same time, the latest chassis designs use tab-and-slot and similar self-fixturing techniques.

Curve-forming "bends" metal in multiple directions, so the mating edges are curved as well. Still, the self-fixturing designs help produce tight fit-ups for welding. In fact, the latest iterations of the scooter have laser-welded joints. At the same time, the number of weld joints and assembly steps overall have dropped with each chassis iteration. The first prototype had more than 15 sheet metal parts; today it's down to a half dozen.

The founders launched Stilride in early 2020 with several goals in mind, and all relate to environmental sustainability. First, they wanted to design and manufacture an electric scooter with a novel, simple, lightweight design. During a Zoom call in late December, Beijer lifted the stainless steel scooter chassis off a nearby table.

Stilride relies on cardboard paper, origami-style mock-ups. Design software complements and augments, but does not replace, origami-style prototyping.

"This is stainless steel, so it's not the lightest material," he said, "but it's still only 8.9 kilos [less than 20 lbs.]." He added that a conventional scooter chassis might be 120 lbs. and use a dozen or more different types of materials, including sheet metal, rubber, and plastic. And yes, on a per-part basis, plastic can be less expensive than sheet metal and perhaps help reduce weight for some products—but at what price? Any product that uses a host of different parts and materials will require a longer and usually more complicated supply chain that can be bad for the environment and, as the pandemic proved, very costly for business.

Stilride has taken a different approach. Instead of striving to reduce the cost of each part, designers aim to use fewer parts. The more readily those parts can be made, ideally from nearby suppliers, the more resilient and environmentally sustainable the supply chain can be.

This dovetails into the founders’ second goal: to create a process that could scale up using a decentralized, distributed manufacturing model.

"Small shops really hold the economy up," Beijer said.

Especially since the pandemic, the entire supply chain builds inventory buffers to account for uncertainties, like certain parts that can be obtained only from certain suppliers. Even Stilride hasn't been immune to supply chain disruption, especially for the components inside the scooter chassis. But this hasn't been the case for sheet metal. Material prices have varied, of course, but the material is available from local mills and distributors, not from a mill or factory halfway around the planet.

The distributed manufacturing model "can be one of the greatest things that contribute to a low carbon footprint," Beijer said. "Instead of building a megafactory and producing everything in China, we want to empower the small workshops that already exist. We can just send them the [Stilfold] code, and they can produce near the end customer."

He added that, under this model, various custom fab shops equipped with the right technology would be certified to perform the Stilfold process. The arrangement also could simplify the supply chain and even circumvent tariffs and other trade barriers. Product and process design would occur in Sweden, but manufacturing would occur in the country of consumption.

At this writing, the company's electric scooter is awaiting regulatory approval in the EU, and the founders hope to start shipping products this year. At the same time, other OEMs have expressed interest in the Stilfold process, including electric vehicle maker Polestar, which has partnered with Stilride to produce a climate-neutral car by 2030, without carbon offsetting. According to a Stilride press release, Stilride will incorporate its sheet metal forming expertise into Polestar's body-in-white design.

Thomas Ingenlath, CEO of Polestar, added the following in a statement: "The real potential of electric cars will be fulfilled when we as an industry can say goodbye to not just tailpipe emissions, but also production-related emissions."

Besides the environmental implications, curve folding might help fabricators add even more value than they already do. Consider an assembly with numerous parts of different materials. Most of the cost might be in the material itself, not in the added value from fab shops cutting, bending, welding, and assembling components.

When product designs evolve, thanks to the way the sheet metal is formed (or curved), fewer parts do the work of many. At this point, the equation changes. Material could be pricier, but because the amount being used drops, so does its cost. At the same time, the value fabricators provide skyrockets. Stilride's founders have big dreams, and if those dreams become reality, less truly will mean more.