flatplane

A flatplane is a type of crankshaft used in some internal combustion engines, most notably in V-twin motorcycle engines produced by brands like Moto Guzzi and BMW. Unlike a crossplane crankshaft, which has crankpins offset at 90-degree intervals, a flatplane crankshaft has all crankpins located in the same plane. This means that the pistons move in pairs.

This configuration results in a distinctive engine firing order and exhaust note, often described as a "rumble" or "burble." The primary advantage of a flatplane crankshaft is its simplicity and lower manufacturing cost compared to a crossplane design. It also tends to be lighter, which can contribute to a more responsive engine. However, flatplane engines are inherently less balanced than crossplane engines. This imbalance can lead to increased vibration, especially at higher engine speeds. To mitigate this, manufacturers often employ counterweights on the crankshaft and balance shafts within the engine.

The firing order of a flatplane V-twin is typically uneven, which contributes to its unique sound and torque delivery characteristics. While not as smooth as some other engine configurations, the flatplane design offers a characterful and engaging power delivery that is appreciated by many enthusiasts. Its application is primarily found in motorcycles where the associated vibrations are considered part of the machine's character, and the cost and weight benefits are advantageous.