Ducati may be one of the most well-known motorcycle brands in the world, but not a lot of folks know about the unique cam technology that all Ducati’s use. What is desmodromic valve timing and why does Ducati use it? We’ll explain in this video from the MC Garage.
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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.
How Do Desmo Valves Work?
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.
What Are The Benefits Of Desmo Valves?
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.
Why Is Ducati Maintenance Expensive?
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.
Read the full breakdown of Ducati's Desmo valve train on Motorcyclist: www.motorcyclistonline.com/du...
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