Ducati Desmodromic Valves Explained

Unless you ride a two-stroke or an electric motorcycle, your motorcycle’s engine has poppet valves that control the flow of air and gas into the combustion chamber and the flow of exhaust gases moving out. All four-stroke engines use a cam to push the valves open, often via rocker arms as on this example, and all motorcycles use strong springs to force the valves closed. All motorcycles, except Ducatis.

Ducatis use desmodromic valve control, called desmo for short, and the difference is in how the valves are closed. Instead of relying on springs to push the valves back against their seats, desmo heads have separate, complementary cam lobes and forked, L-shaped rocker arms that rest against collars on the valve stems and act to pull the valves closed. So whether opening or closing, the valves’ motion is positively controlled at all times and follows the cam timing exactly. In fact, the word “desmodromic” is a combination of the Greek words “desmo,” which means controlled, and “dromic,” which means course.

Why does precise valve control matter? It’s how you make good power, for one, but it’s also how you ensure your engine doesn’t barf its guts out the exhaust pipe. And that was actually fairly common back in the ’40s and ’50s when race engines were beginning to rev really high. The metallurgy of the day wasn’t great, and valve springs would often break or fail to return the valves to their seats fast enough, resulting in a loss of power or a grenaded engine if the valve hit the piston. And that’s definitely not a good way to win races.

So, in 1956, Ducati, under the guidance of the great Fabio Taglioni, decided to sidestep the valve-spring problem altogether by putting a desmodromic valvetrain in its 125 Grand Prix racer. The system kept the valves under control at high revs, Ducati won a bunch of races, and today, all Ducati’s use desmodromic timing, from the Scrambler lineup on up to Ducati’s MotoGP bikes.

Ducati and desmo may be synonymous, but Ducati didn’t invent the technology. Norton actually implemented it before Ducati, and Mercedes and other car manufacturers gave desmo a try too, but Ducati is the only company to put desmodromic timing into mass production and it’s become a hallmark of the brand. Another common misconception is that there are no springs whatsoever in a desmo head. There aren’t coil stacks under each valve as on a conventional head, but there are fairly substantial hairpin springs on the closing arms that help the valves seal and make sure the closing collars stay in place.

So the big benefit of desmodromic timing it that it frees you from the weaknesses of conventional valve springs. The thing is, metallurgy and technology have improved a lot. Spring-valve engines can rev to 15,000 rpm or more without floating a poppet, and when was the last time you heard of a valve spring breaking? The truth is, the problem that prompted Ducati to start using desmo doesn’t exist anymore, and the system has, historically, presented some problems of its own.

For example, Ducati’s are famous for having frequent valve-service intervals. These Italian bikes are more difficult and time-consuming to service since they have twice as many shims to check and change—a shim on the opening arm and the closing arm of each valve—plus there are two cam belts that need to be replaced and tensioned whenever you check the valves. And, assuming you’re taking your Duc to a Ducati dealer, you’re probably going to pay a higher hourly rate for the work.

Older Ducatis and newer heritage models that use the older air-cooled engines probably deserve the reputation for being high maintenance, but Ducati’s modern liquid-cooled bikes actually have service intervals on par with other manufacturers. Slick synthetic motor oils, new-tech Kevlar cam belts, the use of lighter reciprocating parts, harder low-friction coatings, and better heat dissipation, among other things, have allowed the service intervals to be pushed out to 15,000 miles for the 1200 engines and 18,000 miles for the 1260 motors like that used in the Multistrada. So while valve service may still be trickier on a Ducati, at least you don’t have to do it any more frequently than on other sportbikes.

In the end, the reality is that with the materials and technology available today, desmodromic timing doesn’t have any major advantages. Yet at the same time its disadvantages are fewer than ever before, and that’s thanks to the persistence of Ducati’s engineers. Ducati doesn’t have any reason to use desmodromic timing other than it wants to, and that’s because desmo is the make’s tradition and the company is damn proud of it, as it should be. It was an innovative solution at the time, and desmo is still a viable technology that’s been proven at the highest level of competition. And, in a marketplace dominated by spring-valve engines, Ducati’s motors are unique. And now you know why.