THINGS YOU NEED TO KNOW…
What’s the big idea?
At high engine speeds springs may fail to maintain control of the valve allowing it to ‘float’ off its seat, causing a loss of power or engine damage in extreme cases. Historically, springs were prone to fracturing. With motorcycle engines running ever more radical engine speeds, it was perhaps inevitable that desmodromics should get the call up and perhaps equally inevitable that it would be an Italian that did it.
Who’s the brains?
In 1956 the grandaddy of Ducati V-twins, Fabio Taglioni, designed a ‘desmodromic’ 125cc Grand Prix motorcycle. It lapped the entire field in its first race! Taglioni was quoted as stating that the desmodromic advantage goes beyond simply eliminating the spring, saying: “The specific purpose of the desmodromic system is to force the valves to comply with the timing diagram as consistently as possible. In this way, any lost energy is negligible, the performance curves are more uniform and dependability is better.”
He is referring to the valves’ ability to follow the cam profile which becomes problematic once more radical opening and closing velocities are required – a situation that was compounded by poor springs. By positively closing the valve with rocker arms, accurate control is assured and more radical valve motion is possible.
Springs don’t sap bhp
There is a misconception, held by many Ducatisti, that desmodromics save the energy required to compress the valve springs. In practice the springs return a significant proportion of the energy to the engine by pushing back against the cam as it rotates.
It’s a complex business
Desmodromic systems are very tricky to develop and manufacture, requiring a precise system that closes the valve without hammering it into the valve seat.
Production bikes still require a light closing spring to ensure a good valve seal at low engine speeds for ease of starting and inevitably servicing costs are greater.
The extra rockers add inertia and there are frictional losses due to the action of the rockers against the cams and valves. Both these contribute to power losses, particularly at higher engine speeds.
Inevitably, spring technology caught up to meet the requirements of engine development. Progressively wound springs and double or triple concentric sets are used to prevent spring resonance and valve float, advances in metallurgy pretty much eradicated breakages and computeraided design has helped optimise cam profiles and valve train design to ensure efficiency and reliability.
Unique to Ducati
No other motorcycle manufacturer has risen to the challenge of making the benefits of desmodromics justify the added complexity. They must often wonder if they should when Ducati consistently produce the most powerful big-capacity V-twins and Motogp bikes on the planet!
Taglioni debuted desmo valves in the Tribalero