This image of the early Desmosedici...
This image of the early Desmosedici clearly shows the chassis structure, with the frame consisting of a steel-tube trellis connecting the cylinder banks to the steering head, while the swingarm pivots directly in the crankcase.
Big things were expected of Ducati’s MotoGP team for 2011. The much-publicized signing of nine-time world champion Valentino Rossi, and the resurgence of Nicky Hayden late in 2010, pointed to a promising year. But the team has moved from bad fortune to worse as the season winds down. Much of the blame has been pinned (mostly by the media) on the Desmosedici’s carbon frame providing little front-end feedback to its riders, but switching from the Ducati’s stressed-engine design to a more conventional beam-type frame is not a simple task.
The Desmosedici is unique in MotoGP in that the V-4 engine is part of its chassis structure. The bike is an engine with a front subframe bolted to the cylinder heads and supporting the front end, and the swingarm mounted to the rear of the engine. The original Desmosedici, built for the 2003 season, used this configuration with a typical Ducati steel-tube trellis forming the front portion of the frame. In 2009 this was changed to a carbon fiber box-like structure, doubling as the airbox. Before the 2011 season began, the company changed aspects of the chassis — fork tube diameter, triple clamps, and the frame itself — to add flexibility and enhance rider feedback, all in an effort to cure the front-end trouble that seemingly only Casey Stoner could deal with.
Since the early ‘90s, chassis designers have understood that a frame needs to flex in certain directions, acting as suspension when a bike is cornering and giving the rider a feel for what is happening at the tire/pavement interface. The idea can be explained by the analogy of that creaky old bookshelf in your college dorm, stacked with books from top to bottom. An old bookshelf will sway alarmingly from side to side and twist from top to bottom, yet still support shelves stacked with books. If we were building a frame based on the bookshelf analogy, we would want some sway (lateral flex) and twist (torsional flex) to absorb bumps but little sag when loaded with books (vertical flex).
The Japanese manufacturers have accomplished this by using frames with tall, thin spars running from the swingarm pivot to the steering head and long, thin mounts to the front of the engine — a tall bookshelf with deep, thin sides. The long frame spars allow tolerance in both design and construction, as the bending forces have more material to act on, just as it would be easier to build a tall bookshelf if you really wanted one that swayed and twisted a certain amount. From various interviews, it appears that Ducati engineers encountered difficulties in the Desmosedici’s short-bookcase steel-tube frame, as they had to sacrifice too much vertical rigidity to obtain the lateral and torsional flex desired. Hence the move to carbon fiber, a material well understood from car and bicycle use. By utilizing the directional characteristics of the fibers themselves, even the short Desmosedici frame could be built with the desired levels of flexibility or stiffness in each direction.
When the GP11 was not providing the feedback that Valentino Rossi desired, the questions arose. Is it the frame or something else in the chassis? Is it too stiff? Too flexible? Or is it the material itself? This is where things unraveled. One difficulty is that a carbon fiber part such as the Ducati’s frame has a lead time measured in weeks if not months, so experimenting with flexibility is a slow process. At Aragon, Rossi’s bikes had front frame sections made from aluminum. One long-term benefit of switching to aluminum is that new frames can be built in days rather than weeks, opening up more potential for changes in the future.
Shortly after Aragon, it was rumored that Rossi had ridden the GP12 Desmosedici at Jerez with an aluminum beam frame, similar to that used by the Japanese. Again, this is a huge change as it most likely requires a complete redesign of the engine mounting points — simply adding aluminum beams from swingarm pivot to engine mount only stiffens an already too-stiff area. If this is the route Ducati takes, it would mean that using the engine as a fully stressed member, as the company has done since the Desmocedici’s inception, does not provide enough flex or feel, no matter what the material.
What must add further frustration to the company’s troubles is the current rules package for MotoGP. In past years, the tire company would have been involved and produced specific tires to suit the Ducati. Now, the bike must be designed and evolve to suit the tires rather than the other way around. The limit of six engines for a season adds further headaches when the engine is part of the frame. This is how Rossi ended up starting from pit lane in Aragon, as the new frame required different engine mounts and hence new engines, putting him over the limit for the year. With every new engine after number six, the rider must start once from pit lane with a 10-second penalty. Finally, the limit on testing days has further curtailed development. To skirt any penalties arising from having Rossi or Hayden test the GP11, they have been testing any changes on the GP12 1000cc model and then making those changes on the GP11.
That, of course, cuts into the team’s allotted 2012 testing dates, putting Rossi and Hayden potentially even further behind if the path the team has chosen turns out to be wrong. SR