eli5; Why are inline-6 engines “perfectly balanced”
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For engines to be “inherently balanced”, you need to have all the primary forces and secondary forces balanced out. Primary forces are the inertial forces caused by the pistons reaching the top and bottom of their travel within the cylinder. These are balanced out by having pistons on corresponding but opposite ends of the engine move in tandem with each other. As one goes up, the other goes up with it. Secondary forces are the forces associated with the piston traveling faster in the top half of its movement than in the bottom half. This is balanced out by having a mixture of pistons in all ranges of their movement: top, middle, and bottom.
Of the inline engines, the inline-6 is the only one that is commonly used that has both of these forces inherently balanced. Other engine configurations in the inline group might have inherent primary balance (inline-4) or inherent secondary balance (inline-3), but not both. If you don’t have both, then you need to engineer some other solution such as heavier counterweights on the crankshaft or other rotating balances in the engine. Otherwise, you have a lot of engine vibration which increases wear and reduces engine efficiency.
If you want to learn more, the YouTube channel Driving 4 Answers has a great video series on engine balance. [Here’s one of his videos.](https://www.youtube.com/watch?v=82rxavW0A3c)
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