[What's That?] Different from 180-degree or 270-degree! What are the benefits of BMW F450GS's unique "135-degree crossplane crankshaft"?

BMW has newly designed a parallel twin-cylinder engine for the F450GS, featuring a "135-degree phase crankshaft" as its most distinctive characteristic, unique among production motorcycles. How does this differ from the 270-degree or 180-degree cranks, which are currently mainstream for parallel twins, and what innovations does it bring? This article will provide a thorough explanation.

The 135-Degree Phase Crankshaft Delivers a Unique Sound and Feel

In recent years, the 270-degree phase crankshaft has become the standard for parallel twin-cylinder engines, providing a pulsing feel and torque similar to a 90-degree V-twin. It is widely adopted in models such as the Honda Africa Twin/NC750X, Yamaha MT-07 series, Suzuki GSX-8 series, BMW F900/800 series, and Triumph Bonneville series. It is particularly dominant in the middleweight class (400-800cc).

Amidst this trend, BMW has introduced a completely new parallel twin-cylinder engine with a 135-degree phase crankshaft in the F450GS. This marks the first time a 135-degree crank has been used in a production motorcycle.

The new F450GS, with domestic deliveries starting in September. Its lightweight and compact 178kg body, 48PS power, and price range starting from 961,000 yen are attractive.

The new F450GS features a 420cc DOHC parallel twin-cylinder engine with a 135-degree crank. The design is by BMW, and production is handled by TVS in India.

Rival comparison around 400cc (from BMW materials). The 135-degree crank is unique, and the power is equivalent to or greater than its rivals.

First, what is a "phase crankshaft"? In a multi-cylinder engine, it refers to a crankshaft where the position of the crankpin is offset (shifted) on the crankshaft. This allows for changes in the ignition timing of each cylinder, significantly altering the engine's pulsing feel and the sensation of the tire pushing off the road (traction).

The part of the crankshaft where the connecting rod attaches is the crankpin. The F450GS pursues a feeling different from both 270-degree and 180-degree cranks by offsetting this at a 135-degree phase angle.

The biggest advantage of the 135-degree crank is its unique sound and feel.

When cylinder #1 is at its top dead center (TDC), the crankpin of cylinder #2 is offset by 135 degrees. Cylinder #2 ignites a short interval (225 degrees) after cylinder #1, and then there is a relatively long wait (495 degrees) until the next ignition timing.

Ignition timing of the 135-degree crank. Cylinder #2 ignites 225 degrees after cylinder #1, and then cylinder #1 ignites again after 495 degrees. If expressed with onomatopoeia, it might be like "Dodo... Dodo..."

In contrast, a 270-degree crank, with its crankpin offset by 270 degrees, ignites cylinder #2 270 degrees after cylinder #1. Cylinder #1 then ignites again after 450 degrees of crankshaft rotation. Onomatopoeia might be "Dodd... Dodd..." (Diagram is from the Honda NC700S/X launch. Same for below.)

In other words, the 135-degree crank generates torque continuously over a shorter period than the 270-degree crank, with longer intervals between torque pulses, resulting in more irregular ignition timing. This produces a deep, rough sound and makes it easier to grip the rear tire's traction, enhancing controllability on off-road or slippery surfaces.

BMW describes the sound of the 135-degree crank as a "unique off-beat exhaust note." They also state, "It has a tone intermediate between a 90-degree V-twin (e.g., Ducati) and a 45-degree V-twin (e.g., Harley-Davidson), but it doesn't perfectly match either. It gives a more 'irregular' and rough impression than a 270-degree crank."

Combines the Strengths of 270-degree and 180-degree Cranks

The 135-degree crank offers the low-to-mid-range pulsing feel similar to a 270-degree crank, while also providing the smooth acceleration into higher RPMs characteristic of a 180-degree crank.

The 270-degree crank is known for its "dokodoko" (rumbling) V-twin-like torque feel across the entire rev range. The combustion interval is the same as a 90-degree V-twin engine. Parallel twin-cylinder engines with 270-degree cranks were originally conceived to achieve the pulsing feel and traction performance of a 90-degree V-twin.

The 180-degree crank is common in sporty models, adopted by bikes like the Kawasaki Ninja 650/Z650RS, Ninja 250, Yamaha YZF-R25/R3, Honda CBR250RR, and CBR400R/NX400. With the crankpin offset by 180 degrees, the two pistons move up and down alternately, canceling out primary vibrations (vertical vibrations) and allowing the engine to rev smoothly to high RPMs.

A 180-degree crank with a 180-degree offset crankpin ignites cylinder #2 180 degrees after cylinder #1 ignites. It ignites again every 720 degrees.

The 135-degree crank has one ignition interval of "225 degrees," which is close to the 270-degree crank's "270 degrees," providing a pulsing feel similar to the 270-degree crank at low RPMs. The other interval, "495 degrees," is close to the 180-degree crank's "540 degrees," resulting in smooth acceleration and a high-frequency sound at high RPMs, similar to the 180-degree crank. It truly offers the best of both worlds from the 270-degree and 180-degree cranks.

The impressions also mention a pulsing feel close to a 270-degree crank and a rotational feel reminiscent of a 180-degree crank. https://news.webike.net/motorcycle/527664/

Incidentally, a 360-degree crank with a 360-degree offset crankpin (which can also be considered a 0-degree offset) features even firing at every 360 degrees (one revolution) of the crankshaft, providing stable torque and pulsing from low speeds. Current models include the Kawasaki W800, Yamaha TMAX, and the recently discontinued Harley X350/X500.

The Charm of a Single Balancer Allowing for a Lighter Engine

The 135-degree crank allows for a simplified balancer, which is also an advantage for a lighter engine. While the 270-degree crank's benefit is its ability to cancel out inertial torque (torque generated by piston reciprocation that is not intended by the rider), it may require multiple heavy balancers to suppress the engine's rocking couple vibrations (rotational vibrations).

In contrast, the 135-degree crank can offset unpleasant primary vibrations with a simple, single-balancer structure. This allows for a lighter engine design. The engine unit of the F450GS weighs a remarkably light 41.5kg.

Vibrations from crank offset are canceled by a simple gear-driven single balancer. It suppresses unpleasant vibrations while retaining a pleasant pulsing feel.

There Was Once a V-Twin with a "135-Degree Firing Interval"!

While the 135-degree crank parallel twin is a first for the F450GS, the "135-degree firing interval" itself was put into practical use in Honda V-twin engines in the 1980s.

This was achieved by combining a 45-degree V-twin with a 90-degree phase crank, used in models like the XLV750R. Theoretically, the piston movement and firing interval are the same as a 135-degree crank.

The Honda XLV750R (debuted in 1983) was a large off-road model developed for the Paris-Dakar Rally. The Pacific Coast (debuted in 1989), a premium cruiser, also featured a similar engine.

The purpose of its adoption was to theoretically eliminate primary vibrations without the need for a balancer shaft. However, it was too smooth and lacked popularity, leading to its discontinuation. The intention behind it was completely different from BMW's current approach.

If We Were to Point Out Any Disadvantages...

If we were to point out any disadvantages of the 135-degree crank, it would be that the 270-degree crank has better tenacity at extremely low RPMs. The 135-degree crank has a shorter interval between one ignition (0 to 225 degrees) compared to the 270-degree crank (0 to 270 degrees). While the 135-degree crank is still manageable at very low RPMs where it might feel like stalling, the 270-degree crank would likely hold on a bit longer.

Furthermore, the selection of aftermarket mufflers is not expected to increase immediately. The global twin-cylinder market is dominated by 270-degree cranks. As the 135-degree crank is currently exclusive to the F450GS, there is limited development know-how, and it is highly probable that it will take time for aftermarket custom mufflers to be released, or the selection may be scarce.

However, the 135-degree crank offers numerous advantages that far outweigh these drawbacks. It combines the strengths of both the 270-degree and 180-degree cranks, possessing truly crossover merits. We look forward to seeing its future development and the potential for derivative models.

※ Click here for a scoop article on the future development of the BMW F450GS!

BMW also released a "315-degree crank" in 2012! The Husqvarna Nuda 900/R at the time featured this, with an engine base derived from the parallel twin engine used in the F650/800GS. (Photo from Webike Community)

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