For example, let's take Honda's V4 engine of the early 1980s. Depending on the type of motorcycle, the manufacturer had two crankshafts: the crankpins of the first one were at 360 degrees, while those of the second one were at 180 degrees. It should be noted that the connecting rods of the pistons are mounted in pairs on the crankpins. With a 360-degree crankshaft, the piston and connecting rod assembly go up and down in parallel, producing vibrations as intense as on a single-cylinder engine. On the other hand, with a 180-degree crankshaft, the piston and connecting rod assembly operates against the opposite crankpin, which improves balance.
|
180-degree crankshaft |
The Honda VFR800, meanwhile, uses a 90-degree four-cylinder engine. The crankshaft incorporates four pins (one for each cylinder) at an angle of 90 degrees. The resulting cyclical regularity leads to an extremely smooth, almost vibration-free operation. Motorcycle engines using horizontally-opposed cylinders are among the smoothest-running mills out there.
V-shaped engines with a single crankpin offer increased crankshaft rigidity and produce less vibrations. Others (like Honda and Yamaha) use offset crankpins at 52-, 70- or 76-degree angles. These engines are mounted on flexible rubber bushings to solve vibration transfers from the engine to the frame and rider.
An inline four-cylinder engine with a 180-degree crankshaft offers perfect balance of the primary vibrations, though secondary vibrations are still present. A simple solution is to add rubber dampers. Some manufacturers, like Kawasaki and Honda, also use a balancing shaft to prevent vibrations. Certain types of motorcycles, especially sporty models, don't have one; vibrations can thus be quite strong at specific rev spots.