EX­PERT’S GUIDE TO... WHY DU­CATI DOES DESMO

Clever valve sys­tem of­fers ex­tra con­trol

Motorcycle News (UK) - - Garage - Si Martin

Si has a TEC Diploma in mo­tor­cy­cle Engi­neer­ing, Mer­ton Col­lege and has 30 years of ex­pe­ri­ence build­ing, mod­i­fy­ing and re­pair­ing mo­tor­cy­cles from Su­per­monos to CBX1000 Café rac­ers.

Du­cati and desmod­romic valve sys­tems go to­gether like chips and may­on­naise – for­eign and a lit­tle bit ex­otic – but what is it and why has one Ital­ian fac­tory em­braced tech­nol­ogy that ev­ery­one else has ig­nored? Four-stroke en­gines use pop­pet valves to con­trol the pas­sage of the air-fuel mix­ture into and the ex­haust gas out of the cylin­der. The valves are in­vari­ably opened by a camshaft, ei­ther di­rectly act­ing on the valve stem or via a rocker lever. Con­ven­tion­ally the valve is closed by a spring or springs, usu­ally in the form of com­pressed coils push­ing up against a col­lar at­tached to the valve. Al­ter­na­tively, a sec­ond set of cams and rock­ers can pull the valve closed – as cham­pi­oned by Du­cati’s leg­endary en­gi­neer Fabio Taglioni – al­though the firm aren’t the only com­pany to have dab­bled in desmod­romics. The name it­self comes from ‘desmo’, the Greek for knot or link and ‘dromic’ from dro­mous, the Greek for a road or course.

Du­cati’s rocker arms of­fer pre­cise valve con­trol

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