World Su­per­bike’s rules were in­tended to put twins and fours on a level play­ing field – but no­body counted on Du­cati’s fab­u­lous desmod­romic valve sys­tem


How Du­cati’s clever desmod­romic valves gave them the WSB edge

World Su­per­bike was con­ceived as a road-based for­mula in which four-cylin­der bikes were lim­ited to 750cc and twins could be 1000cc. Many said this gave Du­cati’s 851cc V-twin a mas­sive and un­fair ad­van­tage over bikes like Honda’s RC30 748cc V4, and was handed out to en­sure a com­pet­i­tive Ital­ian con­tender. Is it true? Let’s grab a metaphor­i­cal back of an en­ve­lope (the en­gi­neer’s great­est tool) and do some sums…

Lim­its to en­gine per­for­mance are many and var­ied, but firstoff the re­al­ity is en­gines are air con­sum­ing de­vices. How much power they make is pro­por­tional to how much air they can flow. Both Du­cati’s 851 and Honda’s RC30 were four-valve en­gines, and the 851’s bore was 92mm while the RC30 had 70mm. As­sum­ing valve throats the same pro­por­tion of their bore di­am­e­ters, then on a per cylin­der ba­sis the Du­cati could flow about 73% more air (re­mem­ber, we are talk­ing ar­eas here). How­ever, the Honda had twice as many cylin­ders, and so might be ex­pected to flow about 15% more air than the 851. Pretty much in-line with the dif­fer­ence in swept vol­umes.

How­ever, smaller cylin­ders can be ex­pected to pro­duce more power than larger ones for other rea­sons. Among these is the abil­ity to rev harder due to lower me­chan­i­cal stresses, and the fact the flame does not have to travel as far to com­plete com­bus­tion (so knock is less likely). Larger cylin­ders waste less heat, but the to­tal amount of heat lost to the pis­ton, put­ting it un­der stress, is larger. To get an idea for the me­chan­i­cal stress a pis­ton is un­der (and, let’s face it, the pis­ton has the hard­est time in an en­gine) we can use the ‘mean pis­ton speed’, which is es­sen­tially a prod­uct of twice the stroke and the en­gine speed. For a high-per­for­mance four-stroke of the late ’80s this was in the re­gion of 20m/s, and at max­i­mum power the pro­duc­tion Du­cati’s mean pis­ton speed was 20.5m/s. How­ever, for the Honda it was only 17.8 m/s, so the V4 could rev harder be­fore hit­ting a me­chan­i­cal limit and be tuned to give more within it.

So far, one would prob­a­bly back the en­gine with more cylin­ders to have the ad­van­tage, so there are sound engi­neer­ing rea­sons for let­ting twins be larger. Though the dis­crep­ancy be­comes smaller as the ra­tio of the num­ber of cylin­ders re­duces, it is clear that the twin’s engi­neers will have to use what­ever tools they can to make a dif­fer­ence. In the case of Du­cati, the ace up their sleeve was desmod­romic valves.

Rewind slightly. Mean pis­ton speed is ac­tu­ally a fairly blunt tool to com­pare en­gines with. Given one can make the pis­ton and crank train sur­vive, and that the amount of air an en­gine can pump through it­self es­sen­tially gov­erns per­for­mance, we need to look at other fac­tors af­fect­ing air flow. En­gines need to have some mech­a­nism valv­ing the com­bus­tion cham­bers to let mix­ture in and ex­haust gases out – a func­tion which for most four-strokes is pro­vided by a pop­pet valve driven by a cam. So just con­sid­er­ing throat ar­eas won’t re­ally do, at least un­til the pop­pet valve is suf­fi­ciently clear of the seat that the ‘cur­tain area’ (the cylin­dri­cal sur­face formed by the edge of the valve head and the valve seat) ex­ceeds throat area. Do the maths and this is at a valve lift of just over about 25% of throat di­am­e­ter.

Then it gets a bit com­pli­cated. Heav­ier valves need heav­ier springs, and the maths en­sur­ing the spring is kept in con­tact with the cam mean that higher en­gine speeds re­quire longer cam pro­files, which can start to break into com­pres­sion and ex­pan­sion phases (for in­take and ex­haust cams re­spec­tively) – cut­ting effective com­pres­sion and ex­pan­sion. Valve springs are more of a real limit to per­for­mance than you’d think.

How do you cir­cum­vent the prob­lem of the valve spring, given the need to sell road bikes to ho­molo­gate rac­ers? One ap­proach would be to have many more smaller valves, which weigh less and so have to lift less far un­til cur­tain area equals throat area. This is what drove Honda to five-cylin­der 125cc and six-cylin­der 250cc en­gines in the 1960s, when valve spring de­sign and ma­te­ri­als were much less ad­vanced. Fabio Taglioni at Du­cati was aware of an­other ap­proach, how­ever – pos­i­tive valve con­trol, or ‘desmod­romic’ valve gear (com­ing from the Greek mean­ing ‘cap­tive run­ning’).

Taglioni wasn’t the first to suc­cess­fully use desmodromics – for ex­am­ple, Mercedes had been pretty dom­i­nant in Grand Prix rac­ing in the 1950s with their sys­tem – but he is the only one to productionise it. To the ca­sual ob­server his method, where the spring is deleted and one cam-and-rocker sys­tem is used to open the valve be­fore an­other set closes it, might seem an ex­trav­a­gant in­dul­gence when the crude old valve spring could do the job in­stead. But elim­i­nat­ing the spring brings sev­eral ad­van­tages. First is that heav­ier valves can be con­trolled more ac­cu­rately up to higher speeds, and any mar­gin can be used to fa­cil­i­tate a more ag­gres­sive lift pro­file. This gives the re­quired ‘time area’ of the sys­tem – ef­fec­tively its breath­ing ca­pac­ity – in a shorter time pe­riod. Be­cause the


pro­file is now gov­erned by ma­te­rial stresses in the rocker sys­tems, and not a spring, faster open­ing and clos­ing can be pro­vided, in­creas­ing air flow and re­duc­ing the pro­file length – to the ben­e­fit of real com­pres­sion and ex­pan­sion.

Se­condly, the en­ergy for clos­ing the valves is not pro­vided while open­ing them, as has to be the case with a spring. The re­sult is a softer in­put (or ‘stab’) torque pro­file into the cam drive sys­tem, which can lead to lower fric­tion. For proof, look how Du­cati can use tim­ing belts, and are the only com­pany cur­rently of­fer­ing hy­draulic cam phas­ing (vari­able tim­ing).

Fi­nally, be­cause there is no spring seat, the head height can be lower for a given port shape. With a V-twin this is very im­por­tant. And Mas­simo Bordi, in forc­ing the Du­cati Qu­at­trovalv­ole en­gine into ex­is­tence, hadn’t wasted any op­por­tu­nity here – he’d con­sulted with Cos­worth, at the time one of prob­a­bly only two en­gine con­sul­tan­cies who re­ally un­der­stood any­thing about four-valve en­gine port de­sign. He can’t be ac­cused of squan­der­ing the op­por­tu­nity.

Put this to­gether and you can see the 851 and RC30 should have been evenly matched. Du­cati’s mar­ket­ing ge­nius was not to go straight to full litre ca­pac­ity, thrash ev­ery­one and in­voke the wrath of the rule mak­ers. Du­cati won the first WSB round in 1988, and though Honda won the ti­tle in ’88 and ’89, the Ital­ians were just get­ting started. Grad­ual ca­pac­ity in­creases kept them on par with fours which had to be tuned harder and harder to com­pete. The 851 be­gat 888, 916 and 996, riders in­clud­ing Roche, Polen, Bayliss and Fog­a­rty fin­ished the job off, and Du­cati be­came the dom­i­nant WSB force. In the end Honda went twin-cylin­der, but does any­one re­mem­ber that? Aprilia never won the cham­pi­onship with a twin. And Du­cati built a leg­end for per­for­mance while sell­ing more bikes as they in­creased the ca­pac­ity in­cre­men­tally. Frankly, this was ge­nius.

Du­cati even con­sulted to Fer­rari to ap­ply desmo to the 1990 3.5-litre V12 F1 mo­tor. Put ‘Testa mo­tore desmod­romico F1 Fer­rari’ into Youtube and try not to be mas­sively im­pressed by the fact that the en­gine shown had five valves per cylin­der and thus log­i­cally must have had 120 rock­ers…

It was desmodromics that gave Du­cati their WSB ad­van­tage, rather than ad­di­tional ca­pac­ity. And, af­ter 62 years us­ing the sys­tem, they now un­der­stand bet­ter than most what it can be used for. In to­day’s Mo­togp they are be­lieved to have the most pow­er­ful en­gine – yet it is the only one not us­ing air valve springs, an ex­pen­sive and frankly non-road-viable tech­nol­ogy. Du­cati use the same tech­nol­ogy that’s in their road bikes to win races and cham­pi­onships; if you want to buy an en­gine em­body­ing Mo­togp, buy a Du­cati with desmo valves.

As a me­chan­i­cal en­gi­neer, my guilty plea­sure is that I re­ally like desmod­romic valve con­trol. And it is World Su­per­bike that re­ally gave it a plat­form on which to shine.


Who is Jamie Turner?

Clas­sic Bike’s ex­pert is Pro­fes­sor of En­gines and En­ergy Sys­tems for the De­part­ment of Me­chan­i­cal Engi­neer­ing at the Univer­sity of Bath. A firm two-stroke fan, Jamie de­vel­oped ro­tary en­gines for Nor­ton, worked at Cos­worth and Lo­tus Engi­neer­ing, and has lots of let­ters af­ter his name (M.eng., PH.D., C.eng., F.I.MECH.E.)

Fabio Taglioni (left) and Mas­simo Bordi with the pro­to­type (in 748cc En­durance guise), Au­gust 1986

Du­cati’s pro­to­type eight-valve ‘desmo­quat­tro’ V-twin was based on a Pan­tah bot­tom end

Desmod­romic camshafts have a unique ap­pear­ance Four-valve head – this is a pro­to­type, from a time when fac­to­ries weren’t so guarded

Honda’s RC30 en­gine had gear-driven dou­ble over­head cams, ti­ta­nium con­rods (a road bike first) and a slip­per clutch. Ex­otic? Yes. But note those reg­u­lar valve springs...

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