THINGS YOU NEED TO KNOW…

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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.

Springs pre­vail

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

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