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I’m sure I’m not the only one captivated by new bikes promising to be the fastest, the most powerful, the most innovative, or the most whatever. There’s nothing wrong with checking out the new stuff, but there’s a downside: Some interesting developments that might deserve attention inevitably get relegated to the shadows. In my case, I’ve been neglecting parallel twins for years, despite the fact that I grew up on them. Parallel twins tend to disappear in the shadows behind 200-hp superbikes and six-cylinder tourers.
I was recently doing research for the design of a midsize, mid-performance bike using belt final drive. This research led me to BMW’s F800 line, powered by a parallel twin with a 360-degree crank, dual overhead cams with finger followers, and belt drive. But what really captured my attention was the balancer. I saw significant innovation hiding in the details of these less-than-super bikes.
The F800 engine uses a balancing mechanism that’s basically a weight hanging from a connecting rod bolted to a third crankshaft journal, one located between and 180 degrees opposite the two big end journals. This balance weight moves in the opposite direction from the pistons, going down as the pistons rise. The balance weight pivots back and under the transmission, so while its movement is not straight up and down like the pistons, the long arc defined by that pivot makes it pretty close.
This balancer arrangement is superficially similar to that of the Ducati Supermono, which used a V-twin crank and second connecting rod to drive a balance weight composed of a short pivoting arm—but that balanced a single, not a twin. It’s more similar to the Yamaha T-Max maxiscooter. Like the F800, this uses a third connecting rod between the laydown parallel-twin’s two cylinder rods, but this rod faces straight back at the transmission and drives a heavy “slave” piston sliding in its own bore as a balance weight.
Another parallel-twin balancing act I considered was Triumph’s dual counter-rotating balance shafts—one ahead of and one behind the crankshaft—as employed in the retro Bonneville twin. This is what I would have called the “standard” solution before undertaking this reconsideration. The final option is the mechanism employed on the Yamaha FZ-07. This parallel twin uses a single rotating balance shaft; perhaps a second shaft isn’t needed because balancing duties for this “crossplane concept” engine, with its 270-degree crank and uneven firing interval, are quite different from the more typical, 180-degree parallel twin.
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BMW’s clever counter-balancing solution; a crank weight, pivoting aft, moves up and down opposite the pistons.
So here we have four balancing schemes for four parallel twins in four very different bikes. Why go to all the trouble if it takes such a range of mechanisms to smooth these shaky parallel-twin engines? It’s all about packaging. As power demands rise and noise regulations get more restrictive, components like airboxes and mufflers get bigger. Electronic systems proliferate as buyers demand more features. This all has to go into the same space, and the compact parallel twins are an attractive compromise, not much bigger than a single, but with more power and—assuming proper balancing—more smoothness.
Is there one “best” way to balance a parallel twin? BMW’s reciprocating balancer is probably the most efficient in terms of energy use, while Triumph’s dual gear-driven balance shafts are likely the least efficient. The balance shaft solutions probably add less weight overall than the reciprocating versions, however. Suffice it to say that a parallel twin must be smoothed, and just how that’s done might not be the most important issue.
For me, one thing is clear: I had some catching up to do. In the future I’ll be giving parallel twins the attention they deserve.
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