Main Frame

 

This is the largest section of the bicycle and is usually also the heaviest. It was important to design the shape and the size of the top tube and seat tube in order to make the bicycle not only aesthetically appealing but also sufficiently stiff and lightweight. The top tube was created in Autodesk Inventor 2016, as shown in

Figure 9. To accommodate a rider of medium height, 1.65m to 1.75m, the length of the tube was set to 0.54m based on sizing tables within D. G. Wilson’s Bicycling Science Textbook.

 

In the majority of traditional bicycles, the crankshaft is attached via a circular tube welded directly across the bottom of the seat tube. However, as the chosen concept required the seat post to drop entirely through the seat tube, attaching the crank shaft in this location was not possible. Instead, the tubular section through which it was to be connected, had to be offset from the seat tube, and this demanded a cantilevered housing for support and to attach it to the rest of the frame. For this reason the bottom bracket housing was one of the key components of the bike that enabled the seat post insertion to minimise folded volume.

 

To this end, the smallest available crankshaft hub was sourced and a bottom bracket shell was purchased. This shell was machined to include the specific threads required by the crankshaft, and the intention was to punch the shell directly into the bracket, to avoid issues with the commercially specific thread.

 

Despite the best efforts to reduce the size of the bracket, the fixed shell dimensions ensured that the bracket was a heavy and substantial structure, which introduced difficulty when attempting to reduce the weight. Another setback in this respect was with the material and chosen manufacture process, as this component had to be machined from a solid block of mild steel. Ideally, a shell structure would have been designed from steel plates; however, given the requirement to be accurate with the angles and the limited resources within the labs, it was decided to proceed with the single block approach. It was for this reason that more angled sections were removed from the bracket to minimise as much mass as possible.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

REAR TRIANGLE

 

This section connects the rear wheel to the seat tube and has to withstand some of the largest loads as it supports the rider’s weight. As is customary for most small framed bicycles, it was decided that this would be formed of thin tubes, using finite element analysis in order to reach ideal sizes, and thicknesses.

 

The rear triangle design was created in PTC Creo Parametric 3.0 with dimensions set in accordance with the main frame and assuming a 16in rear wheel. In this case, the bottom tubes (chain stays) are straight while the upper tubes (seat stays) have a more curved design to ensure that the attached dropouts are parallel to the rear wheel axle. The side-on view highlights that the traditional bicycle triangular design is kept, as this is proven to maintain stiffness within the structure. It is understood that all angles used in the design have an associated accuracy that may not be replicable in the manufacture phase, particularly for welded members.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

HINGE MECHANISM

 

This is one of the most important components of any folding bicycle and can often provide a distinguishing characteristic to the model. Within the bicycle, two hinges were required, one on the steering column and another on the top tube. In the interests of consistency, it was decided that both of these fixtures should be identical in their operation.

 

Ultimately, what was desired from the hinge assembly was a component capable of effectively transferring the loads, with an easy and simple to use mechanism. For this reason a simple pivot levered latch was investigated, as it was understood that these could be purchased to withstand heavy duty loads. In terms of mounting the latch, it was decided to make use of angled mild steel sections on either side of the hinge. On one face, only a small section of the angle would be welded, allowing the catch to be fixed perpendicular to the hinge. Likewise, on the other face, a larger section of the angle would be positioned to accommodate the lever. Heavy duty hinges were also sourced that could withstand loads up to 30kg.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

FINITE ELEMENT ANALYSIS

 

A detailed Finite Element Analysis of the bicycle structure was then completed to ascertain the stresses that the bike frame would experience, and to use as a decision tool for choosing grades of steel tubing. ANSYS Workbench 16.0 was used to complete the FEA of the bicycle frame.

 

There were four key areas of the bicycle frame to consider. The first is the attachment of the handlebar to the mainframe through the headset, which is assumed to be the inside area of the head tube. This area will either be under forces from the moment arm of the handlebars or will be a constraint from the wheel and front fork. The second is the area of the frame in the inner area of the seat tube which holds the seat post. This area was used to simulate the forces created by a person sitting on the seat, and having their weight transferred through the seat post, onto the seat tube. The third is the area where the crank arm of the bicycle will go through, which is the circular inner area of the bottom bracket. This area was used to apply forces which occur due to the pedalling motion of the person riding the bike. The final area of interest when setting up the model was the connection from the rear dropout to the frame. This was used as a fixed support area as there should be no movement of that part with respect to the frame due to it being directly connected to the ground through the wheel.

 

The analysis consisted of simulating a person of weight 91kg, sitting on the bicycle seat with their hands on the handlebars and feet on the pedals.

 

The results from the analysis showed the greatest stress concentration was in the connections between tubing. This is understandable, as it is the point where the material is at its smallest thickness and the change in direction acts as a stress raiser. It was initially thought the greatest stress would occur in the connection between the stays and the dropouts, but due to the dropouts being of a higher grade steel than the tubing, they experienced smaller stresses.

In Figure 15, the total deformation of the model is shown. What can be seen is that the highest amount of deformation occurs in the bottom bracket. This could be a cause of the flex in the bicycle rather than the bottom bracket itself bending.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The final point of note, is the location of highest stress in the system, as this will give information as to whether the bicycle can withstand these forces, and if it fits into the chosen factor of safety. The highest stress in the model is approximately 440MPa, which is lower than the Ultimate Yield Strength of Reynold’s Tube Supplier’s weakest steel (Grade 525) with a value of 600MPa [17]. This would leave a Factor of Safety of 0.73 for the model, on the basis that the lowest grade steel tubing is bought and used in the bicycle, and this was deemed acceptable.