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The logical way to add extra displacement to an engine then, is to add cylinders. Doubling up on the cylinders, means a twin can easily outperform a single without a huge weight penalty or width penalty in transverse V format. Twins have an enduring legacy; this configuration has managed to excel in the top echelons of racing for the last 100 or so years. The traditional transverse crank, V-twin layout normally springs to mind when thinking of twins. Most manufacturers have succumbed to the lure of a V-twin and Ducati, Aprilia, KTM plus a host of others have made the V-twin their signature engine. As already mentioned, a transverse V-twin makes for a narrow engine, but to fully exploit the balancing benefits that a V-twin holds over a single, a 90º V angle is required.
Only a 90º angle between a V-twin’s cylinders is suitable for engine primary balance. Primary balance refers to cancelling out all the forces arising from an engine’s pistons and crank counter weights. So complete primary balance is impossible with a single cylinder engine
although the crank weights can mostly cancel out the piston forces at top and bottom dead centre, there’s nothing to cancel out the crank weights when they are at 90º and 270º. But it’s a trick a 90º V-twin is able to pull off because when the crank weights are at 90º and 270º to one piston, those same heavy counterweights now cancel the TDC and BDC forces of the other piston. Thus balancing the two pistons and smoothing out vibrations which would otherwise fatigue the rider and fatigue the bike itself. And a motorcycle equipped with an engine that is no longer trying to thump itself to death, can be made that much lighter.
Light and smooth, a 90º V-twin may be, but unluckily, a large capacity, 90º V-twin motor can be difficult to shoe-horn lengthways within a sports bike’s wheelbase. Some manufacturers put a high value on bike weight distribution, like Aprilia for example. They decided a 60º V-twin was the perfect engine for their all new 1998 RSV1000R Mille.
The resultant lack of engine balance necessitated the use of two balance shafts to keep the engine smooth running, but the longitudinal compactness compared to a 90º V allowed Aprilia to keep more weight towards the front of the bike and still retain a sensible swingarm to wheelbase ratio. All three factors are fundamental to a sorted, quick steering chassis.