The objective of this thesis is to study and design a bicycle frame that incorporates a type of physical suspension to limit the transfer of vibrations from the ground to the rider. The operation of this system is based on the way the frame is designed in combination with the material used.

Existing suspension systems are complex and consist of many individual components, which increases the cost and weight of the bike.

The current system will be differentiated by its simplicity, which serves the main purpose of the suspension: the absorption of vibrations to provide more comfortable, fast and safe driving.

BRAINSTORMING

The image below illustrates some preliminary sketches. It should be noted that the two wheels, the bottom bracket, the seat and the part that supports the steering system steering represented the main constraints for the development of these designs.

DESIGN CONCEPTS

After the study and evaluation that took place in all the sketches, the creation and composition of three basic geometries emerges. During this process, the plans that satisfy the maximum basic criteria such as the ability to oscillate the upper part, the uniform form, the open curves and the simplicity in the structure of the frame were selected for further development and improvement. At the same time, the key points that are a constraint in the design were maintained, where the seat and the rear wheel hubs, the middle friction and the front wheel steering system are. A common feature in all three proposals is the proportions in the design, because they are based approximately on the main measurements of existing geometry of intermediate size. These basic measurements consist of the wheelbase, wheel diameter, "stack" and "reach".In all the drawings, the cross-sectional shape of the frame is a rectangle and sufficiently thick to ensure that the movement performed by the rider's feet during the pedalling is not hindered.

The main source of inspiration for the creation of the first geometry (pic on the middle) was the U-shape, rotated counterclockwise by ninety degrees. This makes it possible to remove the seat tube, thus allowing a large amount of vertical oscillation of the entire upper part of the frame, which has a single support point on the "Head Tube". The U-shape has been suitably modified to maintain open curves and has been adapted to the key points required to complete the bicycle composition, which stems from the combination of the frame with the bike components.

The main feature of geometry B (pic on the left) is the continuous single line that follows the trunk of the skeleton, while maintaining the intermediate space. In this way the frame is combined with the components of the bike. It has a similar structure to the classic geometry of bicycles, with the main difference being the removal of the seat tube, in order to allow the vertical displacement of the upper part caused by the weight of the rider. The peculiarity of this design is the two turns that form the S shape on the back triangle of the frame. This results in a different aesthetic from the usual ones, while functionally transferring and focusing on this point the physical suspension.

The main source of inspiration for the creation of the third design proposal (pic on the right) was the S-shape, properly designed to harmoniously connect the frame with the components of the bike. This is a fairly experimental geometry, with many open curves, which in contrast to the previous ones gives weight and emphasis to the seat tube that supports the seat and the upper part of the frame.

ANALYZES

With the completion of the design of the three geometers and the testing of the mechanical behavior of three types of wood to be used, the study of the strength and displacement of each design was followed separately using the modeling and analysis software for simulating the frame, Creo Parametric. The tests performed include all possible combinations resulting from the key factors that affect the strength and displacement of the frame in the vertical direction. Factors taken into account in this experiment are the geometry, material and thickness of the frame. 

The data of the geometry of the frame, the properties of each material, the boundary conditions and the load exerted by the weight of the rider were introduced in the software, in order to study the deformations and stresses that develop in all parts of the frame. The present study was based on the loading model applied by Maestrelli and Falsini and defines six different cases of loading during cycling. The experiment examines only the case where the rider is seated and a vertical force (2400N) is applied to the seat post, where it is caused by an abnormality of the ground.

It should be noted that this study does not aim to compare the results for optimal selection, but to build a basis for further study and development of bicycle frames with built-in physical suspension.

The figure above shows the three-dimensional model of geometry B used in the analyzes. The model features the vertical force (orange arrows) exerted on the seat post, the puncture points that are the "Head Tube", the bottom bracket and the rear wheel axle, in order to focus the analysis on the behavior of the upper part of the frame and the material (yellow card) with the values of the properties carried out by the tests in the laboratory. The thickness measurement has been configured in order to facilitate and quickly adjust the price to the requirements of each case.

The above images are two indicative examples of the distribution of stresses and deformation of geometry B. It is observed that the stresses are mainly focused on the upper part of the seat stays, while they are more widespread than the trends of the first geometry. Also, in the case of deformation, it is observed that the maximum displacement is located in the S shape of the rear triangle. Thus, as in the first geometry, the point on the seat post has been used for the same reason.

Table shows all the results from all the possible combinations that result from the factors that affect the strength and displacement of the geometry B in the vertical direction. Also, the last right column shows the results of the point used to measure the displacement of the seat. The prices of green cells are closer to the desired results without exceeding the limits of material failure and respectively the predetermined maximum displacement. 

CONCLUSIONS OF ALL ANALYZES

From the static analyzes it was shown that in the first geometry high stresses and displacements develop and in order to lead to safe spraying we have to greatly increase the thickness of more than 25 mm even if we use the nerve. 

Comparing the second and third geometry we observe that for all materials and for a thickness of 15 mm the maximum stress is around 50 MPa and the displacement from 11 to 27 mm. For geometry C, the stresses for the same thickness are quite large at 125 MPa and the displacements from 8 to 20 mm.

We note that the second geometry offers greater safety and satisfactory displacement to provide good shock absorption. In terms of weight, geometry B shows 40% more weight than geometry C for the same thickness. However, if we wanted to ensure durability, the thickness of the geometry C had to be increased, with the end result that the two geometries had the same weight.

Based on the above, the choice of geometry B was made. In relation to the materials and due to the different stiffness, the beech shows the largest displacement and the oak the smallest. All three materials, in terms of weight and durability, can potentially be selected as frame materials. The final choice could be made by a detailed study of construction costs, material costs and actual mechanical behavior from the study of frame prototypes.

3D MODEL PHOTO REALISM

Below is an overview of the prevailing geometry along with the components of the bicycle.