The liquid-cooling of engines has been a trend for more than a hundred years but, unlike advancements in other areas (suspension, fueling, and lighting, for example, where the technological boneyard starts at the last model year), air-cooled engines have shown a remarkable longevity. Nowhere is this dichotomy more prevalent than in American V-twins. That's in part because there are benefits to air-cooling. Foremost, simplicity. Air-cooled engines require fewer parts, both for manufacturers to build and for consumers to fix. Unique to motorcycles, another advantage can be aesthetic. Because motorcycle engines often sit in plain sight, the look of cylinders bristling with delicate cooling fins is, understandably, too cool for some to resist.