But there's always some trade-off. True, the V-twins were more powerful than the singles they replaced, but they still shook, and the narrower the angle between the cylinders the worse they did. The peculiar firing impulses caused by fitting both rods disto a common crankpin also meant the engine felt uneven. The solution was to install large, heavy flywheels, which quelled the vibration and kept the engine turning smoothly between the power strokes. It also gave it a nice, relaxed feel, particularly at moderate rpm. Of course the heavy flywheels prevented the engine from turning the type of rpm you needed to make big horsepower, but because most designs utilized side valves and were undersquare, they didn't flow enough air at high rpm to make twisting them much above 3500 rpm worthwhile. Horsepower wasn't really an issue, especially in the U.S., where wideopen spaces and long empty roads made the slow-turning and dead-reliable (if modestly powered) V-twin the perfect motorcycle engine.