BMW F800 engine balancer

By Kevin Ash

Pictures: BMW Press

Smaller manufacturers such as BMW don’t come up with new engines as often as the Japanese Big Four, but when they do there’s often a feast of technical innovations to feed on. Sure enough, the F800 motor offers some rich pickings for tech-heads, in particular its innovative counterbalance system. This is the mechanism it uses to kill the devastating vibration you’d normally expect from a relatively high revving, big capacity parallel twin.

Click on image for galleryParallel twins offer plenty of advantages to engine designers, in particular their very compact design and the fact both cylinders share camshafts and their drives. Arranging the intake and airbox is very straightforward too. This makes them cheaper and much easier to package within a motorcycle chassis than V-twins, and they also have a highly centralised mass which helps handling, but their big disadvantage is vibration.

The conventional solution is to use rotating balance shafts, or as they’re correctly known, Lanchester shafts, after Dr Frederick Lanchester who invented them in the early part of the last century. But these are quite costly – often two are needed, driven from the crank to rotate in opposite directions, and they sap significant power. More pragmatically, the patent was obtained by Mitsubishi in 1976 and most designs require licence payments.

So the F800 instead uses a third ‘slave’ conrod to drive a bobweight positioned beneath the engine. One end of the conrod attaches to a crankpin in the centre of the crank, 180 degrees away from the two cylinder crankpins. The other end is connected to a horizontal arm which pivots on a point behind the gearbox, and the bobweight is on the end of this arm close to the conrod connection. As a result, the bobweight moves up when the pistons move down, and vice versa, balancing them throughout the engine’s cycle.

In fact although the pistons’ movement is perfectly linear up and down the bores, the bobweight’s movement isn’t. BMW has used a long locating arm which means it’s nearly so, but in fact the weight moves through a large-radius arc. Some of this counterbalances the oscillating forces of the conrods, but in practice it means the engine can’t be perfectly smooth. Even so, it brings vibration down to acceptable levels despite the motor being a fully stressed frame component allowing every vibe and tingle to get through to the rider. This system uses less power than Lanchester shafts too.

The system is similar to that used by Ducati on its Supermono race bike, but because the conrod-driven bobweight on the Supermono is at 90 degrees to the single cylinder (so the engine mimics a 90-degree V-twin), and the F800’s is opposite the cylinders, BMW says no patents are infringed. Yamaha’s T-Max parallel twin also uses a slave conrod opposite the cylinders, but this drives a dummy counterbalancing piston, so that’s different too. Even so, BMW is not the first with this idea, and I know of at least one other engine manufacturer which is studying the F800 system very closely with a lawyer peering over its shoulder…

* BMW F800 GS test
* BMW F650 GS test

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