The NC700X isn’t just another two- cylinder motorcycle. It represents a paradigm shift from the horsepower-centered design of sportbikes to the utility- and economy-centered design of a new class of machines that, in the current economic climate, may have broader appeal.
Sportbikes needed high rpm to generate the class-leading horsepower that once translated to substantial sales. To supply air at such revs, big valves can only fit into very large bores. And to achieve high revs at survivable piston accelerations, short strokes were essential.
Bang! The world changed in 2008 to one in which gasoline soared to $4 a gallon and jobs became scarce.
In an engine, friction loss rises as the square of rpm, so the first thing you do to improve fuel economy is to bring the revs down. The NC700X gives its peak power at 6140 rpm, not the usual 12,000-15,000 of sportbikes. Yes, you give away that “class-leading horsepower,” but in return, you get a torquey, easy-to-ride engine whose wide pulling power is ideal for first-time motorcyclists who think they might like to try the convenience and adventure of two wheels.
Furthermore, the bigger the bore, the greater the heat-loss area of piston crown and cylinder head. So, to boost economy, you do what the car-makers have done: You switch from big bores that are 1½ times the stroke to bores that are a little smaller than the stroke. The NC700X’s bore and stroke are 73 by 80mm, giving it only 58 percent of the heat-loss area of a same-displacement four-cylinder sport engine.
Piston rings are responsible for most of engine friction, so let’s compare the inches of piston-ring seal (the sum of piston circumferences). For the NC, it’s 18 inches, but for our hypothetical 669cc sportbike Four, it would be 33.6 inches, or 87 percent more. Bearing loads are also proportional to the weights of reciprocating parts such as pistons and connecting rods. So, these are scaled to the NC’s lower revs. It all works together to cut fuel consumption.
