Shafts. Meaning Crankshafts.

di Giorgio Mulliri

Shafts. Meaning Crankshafts.

In previous articles I had compared crankshaft positions of the Honda HRC 213 RCV (the current victorious motorbike) with respect to the Ducati Desmosedici GP, briefly mentioning some of Ing. Fabio Fazi's statements taken from his book "La progettazione della motocicletta" regarding a "golden rule" relating the distance of the crankshaft from the front wheel axis. 

A golden rule experimented by a Japanese manufacturer on a multitude of different models which is basic in obtaining good driving characteristics, having a tolerance of just a few tens of millimetres in order to avoid problems. I wish to share these statements with you, associating them to Ducati's understeer problem.

In order not to breach any copyrights I wrote to the author (Fabio Fazi, currently technical director of the world superbike championship) in order to a kid any problems with both Motocorse (which hosts me) and the author/editor. This was his answer.

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 Fabio Fazi
Currently technical director of the World Superbike Championship. Has always been passionate about motorcycle frames, obtained a degree in mechanical engineering in 1977 with a thesis in "deformations in a motorcycle frame."
Started his career as frame designer at Honda Italia and subsequently Bimota in the winning years of the World 350 Championship with Jon Eckerold.
He has been freelancing since 1981, collaborating with Morbidelli (the aluminum monocoque frame of the 500GP bike is his), Gilera Corse, Ducati, MBA, BYRD (Yamaha Belgarda Corse), Benelli. In his latter years as a frame design engineer he was the technical responsible at Renault for the development of the tilting three-wheeled Ublo, and he designed the frames of BMW's new scooters. Since 1982 he is a member of the Technical Commission of FMI, so he was involved in the technical regulations of the superbike, superstock and supersport categories.




This is his statement.

One must also consider an effect linked to the distance between the crankshaft axis and the front wheel axis, determined by dynamic forces generated by the rotation of the crankshaft. This effect manifests itself also in rectilinear motion and is therefore independent of gyroscopic forces.
If the crankshaft position is too far back with respect to the front wheel, understeer will manifest itself and consequently, the tyre will tend to overheat. Conversely, if the position is too far forward, the front end will be overloaded and under acceleration the front forks will be unnaturally loaded and won't optimally filter out asperity in the road.
These effects have been verified over the years by a Japanese manufacturer that has checked them on many dozens of different motorcycle models. This gave rise to a "golden rule" which imposes on designers to position the crankshaft at a given distance from the front wheel which can vary in a small range (a few tens of millimetres).
Obviously there is a different "magic number" for every motorcycle category (the rule is valid both for scooters as well as motoGP bikes).
_


And here is the comparison between HRC and Ducati GP regarding crankshaft position by overlaying two relatively flat pictures with compatible perspectives.  

The comparison is not precise to the millimetre but is reasonably credible. Using as scaling reference the front wheel diameter and as measurement reference the front wheel axis, it is evident that the rear wheel positions do not coincide, confirming the different wheelbases.

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More specifically.

It is evident that Ducati's crankshaft lies approximately 5-8cm farther back than the Honda's. Curiously, Yamaha's '09 M1 has its crankshaft position exactly in the same position of the HRC motorbike (I haven't found compatible pictures of a more recent M1).

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It would appear that the 2014 M1 does not have the crankshaft in the same position whereas the 2013 crankshaft position lies somewhat between HRC's and Ducati's. Is this one of the real reasons why Ducati suffers from understeer?

There are all the elements to conclude that this crankshaft distance is effectively the problem, although I doubt Ducati doesn't know this rule. I rather tend to believe they chose a setting which privileges traction.

Since the GP12 arrived, development has been focused on weight distribution, by moving both the real petrol tank and the rider's weight farther forward via shortened false petrol tanks. Moving to the Lab motorbike the dimensions apparently remained very similar, focusing rather on different chassis stiffness. Until now only small steps forward have been achieved, never totally resolving the issue.



A new motorbike will be on track in October (as stated by Pirro to Cereghini) whereas Gobmeier talks about a new motorbike being ready only in February; at least they will have different solutions to test in view of the 2014 championship. 

Déjàvu: Preziosi too, prepared three different frames for the Valencia 2011 tests, the GP Zero chosen by Barberà and the subsequently modified 12 for the February 2012 Sepang tests. 
Ducati has until now worked flat out like never before on all sectors of the motorbike: frames, electronics and recently also differently designed exhaust systems with a compensator, as seen in the recent Misano tests. 

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I only hope that after all the experience and data acquired on track they now have clearer ideas, even though a malicious thought comes to mind: in hindsight, does the invitation/meeting between Preziosi and Furusawa (not positive) make more sense?

I always believed that it was a means of keeping Rossi... but maybe I'm mistaken...

Forza Ducati.




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