Parallel-twin engines are a staple in the motorcycle world. Nearly all motorcycle manufacturers have a range of parallel-twin-equipped machines meant to target the mid-size segment across all disciplines of motorcycling. Given the massive popularity of this compact, cost-effective engine configuration, it’s pretty interesting to see just how much this engine has evolved in just a matter of years.
As is the case with all of FortNine’s videos, their latest video is thoroughly informational, well-researched, and excellently executed. It tackles the subject of the beloved parallel-twin engine, more specifically, the sudden surge in popularity of the so-called “crossplane” parallel-twin, characterized by the use of a 270-degree crankshaft. In the video, Ryan goes into great detail and dives into some really deep math when it comes to explaining the ins and outs of parallel-twin engines. Indeed, the first parallel-twin engine found in a production motorcycle dates all the way back to 1894, in the Hildebrand & Wolfmüller motorcycle that made use of a straight twin – also referred to as in-line twin or parallel-twin – engine.
Since then, a lot has changed when it comes to the technology found in motorcycles, but not much when it comes to the engine layout per se. Nearly all twins of the past few decades either had a 180-degree configuration or a 360-degree configuration. To put that into perspective, the entirety of Honda’s CB500 range, as well as Kawasaki’s 250, 300, 400, 500, and 650 range make use of 180-degree parallel-twin engines. Conversely, bikes like the earlier Triumph Bonnevilles and Thruxtons, as well as the BMW F 800 range of bikes all made use of 360-degree parallel-twin engines.
When it comes to the way the engines actually behave in the real world, Ryan makes a very interesting point. 360-degree twins feel very substantial and beefy, thanks to their sound and torque, they make a bike feel powerful and responsive. On the other hand, a flat-plane crank or a 180-degree crankshaft makes an engine want to rev up faster, hence providing more usable power.
In essence, a 180-degree parallel-twin sees the two pistons moving up and down at opposite intervals – i.e., when one piston is at top-dead-center, the other piston is at bottom-dead-center. On the other hand, a 360-degree parallel-twin sees both pistons move up and down simultaneously, hence giving the engine the mechanical balance of a big single.
With ever-tightening restrictions when it comes to emissions, manufacturers had to get creative in eking out more performance from their engines. Ryan uses two popular engines as an example, both of which from Honda. On the one hand, we have the CBR1000RR with a 998cc four-cylinder, and on the other hand we have the previous generation Africa Twin with a 998cc parallel-twin with a crossplane crank.
After doing the math, Ryan determined that the four-cylinder engine has a total of 942 millimeters of piston ring crevices spread across all four cylinders. This space is where unburnt hydrocarbons can be left behind, resulting in more harmful emissions. As for the parallel-twin, this number is reduced to 578 millimeters – 39-percent less than the four-banger. This, Ryan explains, is the reason why superbikes – like the now defunct YZF-R6 – are being replaced by punchy twin-powered bikes – like the YZF-R7 and Aprilia RS 660.
Ryan talks a lot about primary and secondary forces, and this is where the modern-day 270-degree parallel-twin enters the picture. Ryan explains that secondary forces don’t really play a big role when it comes to smaller engines, but with the bigger parallel-twin engines that have joined the game in recent years, secondary forces become a much bigger thing. In a 270-degree twin, when one piston is at top-dead-center, the other piston is still halfway to bottom-dead-center, hence providing excellent secondary balance.
All it takes is one ride of a modern-day parallel-twin bike to feel the difference. From personal experience, my Yamaha MT-07 is much smoother than my Kawasaki Ninja 650, contrary to what their exhaust notes may suggest.
At the end of the video, Ryan makes one interesting final point. Referencing the work of engineer Phil Irving, he points out that an engine with a theoretical perfect secondary balance isn’t in fact a 270-degree crankshaft, but rather, a 285-degree crankshaft. Why, you may ask? Because math. Now, I’m no math genius, so I left all the fancy number work to Ryan, and he pointed out that a piston moves fastest when it’s 75-degrees after top-dead-center, and not 90 degrees. So if we deduct 75 degrees from 360 degrees – a full rotation of the crankshaft, we get 285 – not 270. Consider my mind officially blown.
Luckily, for perfectionists looking for a bike that has the closest thing to perfect secondary balance, one may need not look far. In fact, KTM’s been making 285-degree parallel-twins for quite some time now – ever since the introduction of the LC8-C back in 2017. So there you have it. We definitely recommend watching FortNine’s video from start to finish as Ryan goes into great detail about the math, history, engineering, and production of the parallel-twin engine – an engine that lies at the very heart of the modern-day motorcycle industry.
