BMW Patents Lightweight Frame

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BMW has filed patent drawings on a new design that takes the idea of using the engine as a stressed member a few steps further. (BMW/)

The idea of using the engine as a structural part of a motorcycle’s chassis is far from novel, but BMW has applied for patent protection for a design that takes it a step further than we’ve seen before. It features a half-frame that bolts directly to the engine via huge bolts that also attach the cylinder head to the block to make for a design that’s lighter and more compact than a conventional chassis.

Clearly shown in the patent illustrations, the design features a relatively conventional transverse engine, but instead of wrapping a steel or aluminum structure around it to connect the steering head to the swingarm pivot there’s a simple, cast-alloy front chassis attached directly to the engine. It provides a direct connection to the steering head and allows the swingarm to pivot directly on the gearbox, with the engine doubling as the main frame. Ducati’s Panigale models, initially in V-twin form and in their current V-4 iteration, adopt a similar design, with the engines doing double-duty as the main structure of the bikes. We’ve also seen half-frames from other brands in the past, however, BMW takes the idea a step forward by using the cylinder-head bolts as the means to attach the frame to the bike.

Another view of the frame and how it bolts to the cylinder head via large bolts (20). (BMW/)

The patent images show the benefits very clearly, with the cylinders and half-frame forming a straight line to take stresses from one end of the bike to the other. As well as reducing the bike’s parts count and weight, the design is intended to minimize width—often the Achilles’ heel of bikes with transverse-mounted inline engines. Width is an area where V-twins and V-4s usually have an advantage, offering a side benefit in terms of aerodynamics, at least partly making up for the complexity they introduce in terms of intake and exhaust routing and the extra components that come with a second cylinder head and valvetrain. BMW’s design could bring the best of both worlds, creating a narrow bike that also has the lower cost of an inline engine.

BMW’s patent also hints at another element that could reduce width, making specific reference to using the new chassis design around a three- or four-cylinder engine. While the firm already has a footing in four-cylinder engines, like the S 1000 RR’s, a triple would be narrower. Although there have been three-cylinder BMWs before, notably the K75, it’s been decades since a production triple has sat in the BMW motorcycle range.

Luca Cadalora (pictured) and Jeremy McWilliams both tested a BMW inline-triple intended for MotoGP. It never progressed past the prototype phase. (BMW/)

The company has looked at three-cylinder bikes more recently though. Perhaps most publicly, it’s a layout the company pursued for its aborted MotoGP project in the early 2000s, when many new manufacturers were tempted toward the series by the then-new four-stroke regulations. At the time, BMW had a successful F1 racing project, using a 3,000cc V-10 engine. Three cylinders from such an engine came very close to the 990cc MotoGP capacity limit of the era, so that’s the direction BMW took. While a bike was built and evaluated by Luca Cadalora and Jeremy McWilliams, the project was dropped.

Aprilia also attempted to make an F1-derived triple work in MotoGP. It was a beast. <i>BMW</i>

The decision was the right one. Aprilia took the “three cylinders from an F1 engine” route to create its legendarily evil RS Cube, using a Cosworth-developed triple that looked perfect on paper but proved nearly unrideable once it hit the track. BMW’s prototype appears to have suffered similar problems—years later, in 2018, McWilliams tweeted that it was “the most untamed engine I’ve ever ridden”—but it provided the company with a knowledge base that aided the development of the S 1000 RR.

Three-cylinder BMW sportbike rumors have persisted since then with suggestions of a smaller machine—perhaps an S 675 RR—emerging online on a regular basis with little substance behind them. This patent, with its specific mention of a three-cylinder engine, might just be the first genuine evidence that such a machine has really been considered.

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