Looking at the Shaking forces analysis page in the same folder as this document, you see shaking forces represented by blue and red arrows. All of the engines with a blue arrow rotating in a perfect circle can have their primary (blue) shaking force eliminated by a simple counterweight opposite the shaking force. The secondary(red) force is much smaller and is usually neglected, but could be reduced by a weighted rotating shaft at 2x the crank rpm. The 90 degree v-twins with 0 degree crank angle are smoother than most engines due to this reason.But they have odd firing cylinders (uneven firing time between 1st and 2nd cylinder). Although that is not bad,an even firing engine sounds smoother. Not as good as conserving space and any of the other 2 cyl designs except the horizontal.

The horizonatlly opposed twin cylinder (like BMW) is obviously the best balanced, as evidenced by the lack of any forces on the diagram. Horizontally opposed engines are easily cooled, but they are the worst at taking upspace or getting in the way of leaning low during cornering. This is not to say that there is no shake force in this engine, there is a small shake force that cannot be represented correctly on any of these diagrams, in the plane shown. It appears that the author has decided to not address those forces, possibly since they aren't addressed by engine designers either.

The 45 degree engine, with 45 degree crank pins would be reasonably well balanced with a couterweight. The 45 degree engine, is probably more compact a design than most. A good tradeoff of characteristics.

The Inline even firing 2 cylinder (last on page) is obviously the worst shaking engine on the page. A counterweight helps this engine the least. By couterbalancing the big inline force with a rotating one, a newshake force is added to the engine when the counterweight is not inline with the cylinder axis. Engine designersof single and 2 cylinder inlines with a with counterwights size down the counterweight, so that the overall shake of the system is reduced from what it would be without a counterweight or with a full counterweight. A counter-rotating balance shaft impoves the balance, but the single cylinder cyl engine is the worst vibrating engine made, followed closely by the even firing 2 cyl inline. The sinle cylinder and inline 2 cyl are themost compact of the 2 cyl or less engine designs.

Next time you hear an odd firing v-twin, or shaky inline 2, you can tell your friends the pros and cons.

FYI, inline 6 cylinder engines and V8's are much smoother running than all of these motorcycle configurations,and better balanced that inline 4's, 5's, and V6's.

This discussion only applies to design, since poor manufacturing process or tolerances can make any engine shake.Also, mass can dampen vibration (heavy engine block), liquid engine mounts, or rubber engine mounts. None ofthis discussion pays any attention to torque pulses either, which are obviosly worse with the lower number of cylinders, and drastically reduced by flywheel mass, clutch damping, and torque converters.