Remember Suzuki's Recursion concept? The undoubted star of the 2013 Tokyo Motor Show, it appeared to point the way toward a turbocharged future for the firm's production bikes thanks to an innovative forced-induction parallel twin.

Now, more than five years on from that initial unveiling, it appears that Suzuki’s engineers are still working on the project, though it’s changed almost beyond recognition from that original concept bike.

Recursion concept
Suzuki’s Recursion concept from the 2013 Tokyo Motor Show.Suzuki

Over the last few years there have been several indications that Suzuki is deadly serious about its turbo bike project. After the Recursion concept's initial 2013 showing, the firm followed up by revealing its XE7 engine at the 2015 Tokyo Show. Plonked on the Suzuki's stand with little explanation, the XE7 was far more developed than the original Recursion's engine. Whereas the Recursion's mocked-up motor was a 588cc SOHC design, intended to make 100 hp, the XE7 was clearly a real piece of engineering with intent for mass production. Featuring a DOHC cylinder head and a much more compact turbo and intercooler arrangement than the Recursion, it appeared to be a higher-performance design. The XE7 name hinted at a 700cc capacity, and the adoption of a twin-cam head pointed at a higher-revving design intended for much more performance.

Since then, there’s been a steady stream of patents emerging from Suzuki’s R&D department, all showing the XE7 engine fitted to a revamped Recursion-style sportbike.

Whereas the original Recursion concept featured an aluminum beam frame, almost all of Suzuki’s patents around the bike since 2015 have shown it with a steel trellis-style chassis. The reason behind the switch is similar to Kawasaki’s thinking when it adopted a trellis for the H2; it’s a relatively lightweight and easily modified design that, importantly, allows space for the additional intake pipework and cooling hoses needed for a forced-induction engine to snake between the trellises.

However, the latest Suzuki patent application, which appears after dozens that show the trellis-style frame, reveals that a second chassis is also under consideration—either for the production Recursion or for a spin-off model sharing the same turbocharged engine.

new model
Could this be the latest iteration of the Recursion or a new model born from the concept?US Patent Office

As you can see in these images, the new layout is a traditional-looking full duplex frame, with two steel rails running over the engine and another pair cradling it from below. Given that it’s a layout that’s been popular ever since the debut of the Norton Featherbed frame, you might wonder what Suzuki can hope to patent, and the answer lies in its relationship with the unique turbocharged XE7 engine that it holds.

Look carefully, and you’ll see that the airbox above the engine and the turbocharger hanging in front of the cylinders are both attached directly to the chassis via short pipes. That’s because Suzuki’s idea is to make the duplex frame double as pipework for the turbo system, cleaning up the look of the bike and saving on components and weight.

Patent drawing
Patent drawings show Suzuki using the frame as pipework for the turbocharging system.US Patent Office

Air enters via the airbox on top of the engine, which houses the air filter. From there it runs through a short hose (labeled #42 on the patent drawings) into the frame’s left-hand downtube. Inside the frame tube, the air runs down to the turbo, entering it via another short, flexible hose (#44).

Once compressed, the air makes a return journey, through another flex hose (#52) and into the right-hand frame downtube, where it travels back up to the intercooler, mounted alongside the intake airbox and attached to the frame via hose #54. The intercooler is bolted to a pressurized plenum chamber that feeds the engine’s throttle bodies directly below.

Airbox, engine, and turbo locations
Airbox, engine, and turbo locations can be easily seen in this patent drawing.US Patent Office

To make the system work, Suzuki’s patent explains how the relevant sections of the bike’s frame are sealed off from the rest of the chassis to ensure they are gas-tight. It’s a simple idea, and one that helps remove a significant amount of pipework from the turbocharged engine.

Suzuki may have developed the idea as a replacement for the trellis chassis that’s been seen in earlier Recursion patent documents, and the sketches show it clad in Recursion-style bodywork, but it’s more likely that the design is intended for a potential naked or retro-styled bike bearing the same engine. Not only does the duplex frame design better suit a retro or neo-retro bike, but by ducting the turbocharger’s gas flow through the chassis tubes it allows for a cleaner style for a naked or half-faired machine.

turbocharged engine
A front view of Suzuki’s turbocharged engine and frame.US Patent Office

The question remains as to when Suzuki will actually reveal the production turbo bike. Although development is clearly still underway—it’s been the subject of more patent applications than any other forthcoming Suzuki model during the last few years—there still isn’t a firm date for its launch. Some insiders say it could be more than two more years before a showroom version is ready.

Later this year, the biennial Tokyo Motor Show takes place once more, and surely it’s got to be a likely place for Suzuki to reveal more information about the long-running Recursion project. But don’t expect to be riding a boosted Suzuki production bike in 2020 or 2021.