DEVELOPMENT

Working plan:

The design of the swingarm started with a full, rough, triangular steel body, just to have a preliminary idea of the qualitative stress distribution.

As we expected, from this picture it can be noted that in the model there are large "unloaded" regions of the part that can be removed.

The next step was to develop a structure with the excess material removed and that considered the positions of the geometrical constraints with the given measures, such as the wheel hub, pivot, and the two attachments to the suspension components. An initial design of the two terminal plates (made of aluminum) was also performed.

Afterwards, the structure was modified using hollow-section cylindrical elements. The tubular parts were designed with the "sweep" function in the surface design module of CATIA V5, using circles sweeped along a curved trajectories. After some refinements, such as the cutting at the intersection of the sweeped surfaces, the model  was imported in the "Part design" module and the surfaces were converted into solid parts with constant thickness.

The tips of the tubes, where they are linked through the holes to the aluminum plates, are full solid elements, actually inserted in the tube and welded to it.

In this model the space occupied by the wheel and the shock-absorber (in the two extreme configurations) was also taken into account

The FEM analysis pointed out, that the most stressed and deformed regions of the part are located on the two tubes on which the shock-absorber and the pro-link leverage are attached. For this reason, in the next model, the length of these elements was shortened in order to reduce the maximum stress and deformation.

Another solution to increase the rigidity is to introduce lateral stiffening elements between the longitudinal bars.

The following model adopted some solutions to increase the rigidity, such as:

- the two lateral stiffening bars

- the two central elements reduced in length.

- thickness of the bars was slightly increased

The attchments struts for shock-absorber and pro-link were designed considering the dimensions and size of a Uniball attachment system.

In this rendering the swingarm (composed by the tubular steel truss and the two aluminium plates), the wheel and the size of sprockets and transmission chain are shown:

The FEM analysis of this structure showed that the maximum stresses and deformations were much smaller than the previous model. The maximum deformation was reduced from 0.8mm to 0.35mm, and the maximum stress (Von Mises) from 450 to 160MPa

Designing the attachment of the pro-link and the shock-absorber, also the volume of these parts and the dimension of the uniball connections were considered. These data were given from group 9B.

The shock-absorber with pro-link assembly:

Subsequently, the final design of the swingarm was started.  The model was modified with the additon of a higher couple of tubes and a slightly curved element for the pro-link attachment point.

Two lateral stiffening rods per side were also added in this model. This kind of solution is used also on other motorbikes, for instance several motorbikes produced by the Bimota company, although the suspension system configuration is totally different.

See the swingarm page for the final design

An alternative proposal was a swingarm in which the pivot attachment is not a full-length element but it has space in the middle for the engine. In this configuration, adopted by some motorbikes, the swingarm can be connected both to the engine (inside) and to the chassis (attached outside of the swingarm). The main disadvantage of this solution is that the swingarm tubes are very close to the transmission chain, so a different shape (less stiff) is required or the chain have to be repositioned in order to avoid the contact with the swingarm. The chain is in traction near the driving sprocket, more tolerance on the clearance of the chain is needed in corresponce to the lowest tube.

In the following picture it can be clearly seen that the pivot element is replaced by two short lateral cylinders: the internal sides are attached to the engine while the outer ones are connected to the chassis via the coaxial pivot.