desmo

Desmo is a shorthand term for desmodromic valve actuation, a method used in internal combustion engines to control the opening and closing of intake and exhaust valves without relying on conventional valve springs. In a desmodromic system, both the opening and closing of each valve are mechanically driven, providing positive control over valve motion at all engine speeds.

The desmodromic mechanism typically uses a pair of cam-driven pushrods or rocker arms that actuate separate paths for opening and closing the valve. The design aims to avoid valve float at high RPMs, a phenomenon where springs can no longer close the valve promptly, potentially leading to loss of performance or mechanical interference. The name desmodromic comes from Greek roots desmos, meaning bound or bound together, and dromos, meaning running or course, highlighting the idea of valves being positively controlled rather than sprung closed.

Historically, the concept predates modern motorcycles but was popularized in production by Ducati, an Italian manufacturer renowned for its use of desmodromic valves in many of its motorcycles. The approach has since been developed into various configurations, including multi-valve engines and Ducati’s branded evolutions such as Desmoquattro, which featured four desmodromic valves per cylinder. While associated most closely with high-performance and racing engines, desmodromic systems have been adopted in other applications where precise valve control at high speed is desired.

Advantages of desmodromic actuation include precise, consistent valve timing without reliance on spring strength, improved high-RPM control, and reduced valve float. Disadvantages include added mechanical complexity, higher manufacturing and maintenance costs, and potential increased wear on cam-followers and linkages.