You already know crankshaft counterweights exist to manage balance — now let's zoom out and look at how engine configuration itself determines what forces you're fighting in the first place.

Every piston creates two types of vibration: primary imbalance (once per crankshaft revolution) and secondary imbalance (twice per revolution, caused by the non-sinusoidal motion of the connecting rod). Whether these forces cancel out depends entirely on cylinder count and layout.

• Inline-4: Primary forces cancel perfectly — pistons 1&4 move opposite to 2&3. But secondary forces do not cancel. All four pistons reach TDC/BDC at slightly different effective accelerations, producing a buzzy vibration at 2x engine speed. This is why most modern I4s over ~2.0L use a balance shaft — a counter-rotating weighted shaft spinning at 2x crank speed to neutralize that secondary shake. Mitsubishi patented this solution in the 1970s (the "Silent Shaft"), and it's now nearly universal.
• Inline-6: The gold standard. Primary and secondary forces both cancel completely due to symmetric firing order. No balance shafts needed. This is why BMW's straight-sixes feel so smooth — it's physics, not just good marketing. The tradeoff is engine length, which is why many manufacturers moved to V6s for packaging.
• V6 (60°): Inherently has an unbalanced rocking couple — a twisting force because opposing cylinder banks don't share the same plane. Most V6s use a heavy counterweighted crankshaft or a single balance shaft to compensate. A 90° V6 (like GM's 3800 series) is even worse for balance but works well if derived from a V8 architecture.
• Flat/Boxer: Horizontally opposed pistons cancel primary forces beautifully — each piston mirrors its partner. But because the cylinders are offset (not truly coaxial), a rocking couple remains in a flat-4. Subaru's characteristic rumble on older EJ-series engines comes partly from the unequal-length headers, but the underlying vibration signature is real. A flat-6 (like Porsche's) eliminates the couple entirely.

Rule of thumb for natural balance: An engine is perfectly balanced when the cylinder count per bank is a multiple of 3 (for even firing) and the layout is symmetric. Inline-6, V12, and flat-6 all hit this sweet spot. Anything less requires engineering workarounds.

Here's a quick calculation: A balance shaft for an I4 spinning at 6000 RPM turns at 12,000 RPM. At that speed, even a small eccentric mass of ~200g at a 15mm radius generates roughly 2.4 kN of centripetal force (F = m·ω²·r) — enough to counteract the secondary shake, but it also costs 1-2% of engine power in parasitic friction.