Some days ago, Stoner and Melandri tested a Ducati GP9 prototype, the new weapon that Ducati is developing for 2009 MotoGP World Championship.
The most interesting feature of this prototype is its new carbon fiber chassis. In principle it may seem odd that Ducati engineers have decided to build the chassis of their new bike using carbon fiber. Obviously, any Motogp fabric wouldn’t change the material of their chassis, moving from steel to carbon fiber without having very precise reasons to do it.
The Ducati chassis is a small structure created to join the headstock to the engine. In such a small structure, the structural efficiency (the relationship between stiffness and weight) that is possible to achieve is very similar independently of the used material (steel, aluminum or carbon fiber). In my opinion the reason behind this material change is neither related with stiffness nor with a weight reduction.
During the last years we have seen that the engineering departments have been playing with the stiffness value of their chassis and swingarms. Current trend is to design structural parts to allow certain deformation when the bike is inclined, in this way the chassis absorbs part of the energy generated when the bike goes over bumps, because in this situation of maximum lean angle the suspensions can not work properly. We could say that engineers want the chassis to work as a second suspension system when the bike is at full inclination. But this idea has a problem because we all know that a good suspension system needs two elements, one beeing elastic and another one with damping properties. In a normal suspension the elasting element is the spring and the damping element is the hydraulic system. If we want our chassis to act as a secondary suspension system, we can use the chassis as an elastic element, but we also need a damping element to dissipate the energy in order to minimize the oscillations.
In current chassis, this damping element almost doesn’t exist, and this is one the sources of one of the biggest problems that technicians can find in nowadays racing motorbikes: chattering. I said that the damping element “almost” doesn’t exist because every material has some damping, called “intrinsic” or “passive” damping. If we compare the intrinsic damping of aluminum, steel and carbon fiber, we will find that aluminum has the smallest intrinsic damping. Steel (which is the material that Ducati has been using till now in their chassis) has an intrinsic damping bigger than aluminum. And finally, carbon fiber has an intrinsic damping greater than steel. Saying it with other words, carbon fiber is a material that has a natural tendency to damp vibrations.
Moreover, a carbon fiber chassis gives to the designer more possibilities to improve even more its damping characteristics. The own nature of the chassis, built using fiber layers, allows to insert other materials between the layers. For example “viscoelastic materials”, which are materials that have both elastic and viscous properties when are deformed, presenting a great vibration damping capacity.
At the beginning of the eighties the chassis weren’t stiff enough and during some years designers were looking to increase stiffness. In a few years they arrived to a point in which the chassis were too much stiff. Then the objective was to find the optimum stiffness values. Currently we are going into a new stage in which dynamic factors are playing a more important role (stiffness is a static concept).
If you are interested in this subject, here you have some interesting texts to read:
In chapter 6 of the book “Motorcycle handling and chassis design” by Tony Foale, you will find a detailed explanation about why it is more necessary to increase lateral damping than lateral deflection in the chassis.
Here you have an interesting link to an article that compares in a very simple way damping properties of steel, aluminum and carbon fiber.
Finally, in latest issue (#25) of Cycle News magazine, you will find a very interesting article about the Ducati GP9, written by Neal Spalding. He analizes other aspects of this motorbike, as well as the damping factor I have commented in this post.
[tags]ducati gp9, damping, carbon fiber, chassis, viscoelastic materials, chattering[/tags]