Steering

 

 

WHEELS

 

Before either the frame or steering column could be accurately sized, the dimensions of the wheels had to be confirmed, as this would allow other aspects of the design to progress. Market research identified the predominant wheel diameters on commercially available folding bicycles were within the range of 16in to 20in. Therefore the trade-off lay between the smallest folded volume; the improved handling and turning circle; and the relatively high power and speed delivered by a bicycle with larger wheels. It was eventually agreed that reducing the folded capacity was not only in line with the requirements of the CDIO initiative, but also met the increased portability standard which was a key selling point in SynopTech’s commercial model. Therefore, the decision was made to design the bicycle with 16in wheels.

 

 

 

HANDLEBARS AND UPPER STEERING COLUMN

 

The chosen folding concept, involved a set of handlebars that were free to rotate 90o about the top of the steering column, and could be locked into the steering position with a spring-loaded bearing. After a translation of 90o, so that the handlebars were aligned with the top tube, a standard locking hinge, located just above the headset, will enable the entire top section to drop down in front of the wheel.

This fold can be made prior to splitting the main frame, and therefore the handlebars would be contained between the front and rear wheels when fully folded. The detailed analyses required for this design include the forces translated through the steering column and hinge via the handlebars, and identifying suitable materials and thicknesses for the appropriate members. The rotational mechanism was key to the compact folding volume of the bicycle, but foremost it had to be designed for safety, as a failure in the locking mechanism could result in the rider’s loss of steering control.

Moreover, the component that is used to lock the rotation of the handlebars, must also be responsible for transmitting the turning motion to the steering column, and so the design has to be significantly robust. The conceived solution involved the use of a spring loaded bolt attached to the underside of the handlebars.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The handlebar tube was to be directly welded to a small diameter tube intersecting perpendicularly at the centre of the bars to form a T-section. This smaller tube would then be inserted concentrically, inside the larger tube that is the upper portion of the steering column. The tolerance between the upper steering column and insertion tube will be maintained tightly to ensure a smooth rotation of the handlebar T-section within the steering column. A series of holes would then be drilled within both concentric tubes to accommodate a spring loaded bolt. In order to attach the spring loaded bolt to the handlebars, a deviation from traditional bicycle shapes is required. A hollow tube of ovular cross-section was sourced that had a flat section with two round edges. This enabled the spring loaded bolt to be mounted directly to the handlebars, but also maintained the curved surfaces that are important ergonomic features when gripping the bars. Ultimately, this bolting method should satisfy the prototype fold; however it is likely that the spring loaded bolt would be contained and concealed within the handlebar tube for aesthetic reasons when preparing the business plan and a commercially available model. Two oblong holes were removed from the underside of the handlebars permitted a 20% weight reduction.    

 

A full ANSYS analysis was performed on the upper steering assembly to determine the thickness requirements for the steering column tube. It was discovered that this member had to have a greater thickness than any of the main frame components as it was bearing a significant compressive load. The maximum equivalent stress was measured just below 50MPa at the intersection of the bolt and the upper steering column. This can be readily explained as the bolt is transmitting all moments from the horizontal handlebars to the vertical column, in addition to rotational loads when turning. While this stress is well below the yield stress of mild steel at 370MPa, it was still decided to use as large a bolt as possible in the spring loaded mechanism.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

FRONT FORK

 

The second objective of the steering design, focused largely on the fork that holds the front wheel in position; and once again a controlled convergence matrix was generated to narrow the number of options considered. For simplicity it was decided to procure a standard 16in fork as a complete component, and then modify the length and type of lower steering column to suit the handlebar fold. The standardisation of lower steering column would also allow for an easier assembly of the bearings within the headset, as these are specifically designed to fit over industry sizes. Finally, the steering hinge assembly was to be mounted on top of the lower steering column. This was connected using another insertion tube welded to the lower face of the hinge that would slot inside the lower steering column. A bolt would then be inserted to secure the orientation of the hinge. The upper steering column would then be directly welded to the top face of the hinge, thus completing the steering assembly.