ASK THE MXPERTS: WHAT'S SO SPECIAL ABOUT A DESMO ENGINE?

11/09/2024 10:00 AM

THE LUCY AND DESMO SHOW
Dear MXperts,
Can you explain what makes a desmo engine so highly regarded when no one can tell me what makes a desmo engine different from any other four-stroke?
Back in the 1950s, Fabio Taglioni, the world’s greatest motorcycle engineer, pioneered the desmo valve train. It was one of the most groundbreaking inventions of the time. Prior to the 1950s, four-stroke engines could not rev above 10,000 rpm. Why not? The valve springs of that era were not strong enough to withstand high rpm and broke as engine rpm soared.

Most modern four-stroke motocross bikes rely on increased rpm to make horsepower, but the valve spring issue stymied the effort. Just as bad as valve-spring breakage was valve float. “Valve float” referred to the valve springs not being able to keep up with the cam lobes, so the valves floated somewhere in the middle of their opening curve. Manufacturers of the 1950s tried every cure they could think of to keep the valves from breaking or floating, but no matter what they tried, they couldn’t get above 12,000 rpm. That’s when Fabio Taglioni introduced the desmo valve train and broke the rpm barrier.

 “Desmo” is short for “desmodromic,” and it refers to the way that a four-stroke valve inside of an engine is controlled. Every four-stroke motorcycle engine’s valves are opened by the camshaft lobe and closed by the spring pressure of the metal valve spring. It is the dominant system because it works, is inexpensive to build and is simple to work on. A desmodromic valve train eliminates broken springs and valve float by eliminating the spring. The valves are opened via the cam lobe and closed by an additional cam lobe that operates an L-shaped lever that positively pulls the valve closed.

It is simple because one cam lobe opens the valve and a second cam lobe closes the valve. Desmodromic valves were the rage in the 1950s. In fact, the famous straight-eight, W196, Mercedes-Benz Formula 1 cars had desmo valve trains. Ducati’s first desmo engine was the 1956 125cc Grand Prix road race bike.

Today, only Ducati still races with the desmodromic system, largely because the valve-spring issues have been eliminated by pneumatic valves that work off air pressure to close the valves. Pneumatic valves make it possible to shorten the valve’s open time and increases lift, which enables the engine to rev as high as 20,000 rpm.If and when the Ducati 450MX comes with its highly developed desmo system, the 450MX engine’s valves will offer short-duration, higher-lift cam profiles that will produce a torque band that is very wide. But, it won’t be the first desmodromic production motocross bike sold in the United States, because that honor belongs to the 1971 Ducati 450 R/T Desmo—a one-year model that suffered from frame issues that kept its desmodromic powerplant from living up to its potential.

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