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The Auto-Bike Gear Selecting System 

Stefano Mazza and Patrick Gall, Department of Mechanical Engineering, University of Ottawa

 

 

First year

 

 

The following year

 

Description

                This report outlines the design steps and operation of the Auto-Bike bolt-on electronic gear selecting package.

Reasons for System

This system is meant to create a more user friendly interface with the bicycle. There are electronic group sets available for bicycles, however they are very expensive and are only compatible with certain frames types. The Auto-Bike bolt-on package will be able to mount to almost any bicycle. Electronic gear selecting is not only more accurate, but also puts less pressure on the cyclist to monitor and maintain proper chain lines and gear settings.

Design Goals

                The following points are design features that were selected for this project. They outline a specific application for this system.

·         The system must remain general enough to be able to bolt onto any bicycle.

·         The system should display to the user which gear the bicycle is currently in.

Using the System

                The system is switched on simply by plugging in the batteries located on the main mount, on the down tube of the bicycle. The system will always initialize thinking that it is in the highest gear, i.e. the gear with the fewest teeth. For this reason it is important to shift the bicycle to this gear before switching it off. In the case of the demonstration bicycle, it is an 8 speed bicycle, and so the system starts in 8th gear, and should be returned to 8th gear before shut down.

                User input is done via three buttons located on the handle bars. In the normal operating mode, signified by the unlit warning LED, the ‘+’ and ‘-’ buttons will control the gear selecting. Once pressed, either button will cause a gear change. Keystrokes during a change are ignored. If a button is pressed and the warning LED flashes, then the shift requested in not possible since there is no gear in that direction. On the demonstration bike for instance, while in 8th gear, pressing ‘+’ will result in the warning LED flashing as there is no 9th gear. The same will occur in 1st except when ‘-’ is pressed.

Set mode is entered by pressing the ‘Set’ button on the interface. Set mode is signified by the warning LED being constantly lit. In this mode, the ‘+’ and ‘-’ buttons will simply move the step motor in the direction specified for as long as they are held. This can be used to fine tune a gear during riding, or to first setup the system on the bicycle. Exiting set mode is simply done by pressing the ‘Set’ button again.

 Discussion of Construction

                The entire system has two main output streams and one input stream. The input comes in the form of 3 buttons actuated by the user to perform specific tasks. The output streams go to the step motor and to the screen.

Figure 1: The general outline of the gear selector.

Mechanical System

Lead Screw

Figure 2: A model of the lead screw.

                The lead screw transforms the rotational input of the step motor to a linear translation of the cable. The step motor’s shaft is retained into the lead screw coupler by a single M4 set screw. The lead screw itself uses a M6 threaded rod that rotates on ball bearings and is threaded into a sliding block that holds the cable. The sliding block fits into a square piece of aluminum tube which absorbs the torque generated by the step motor while still allowing the block to slide.

                An approximation for the torque required to pull the cable can be found using the following equation:

Where, f is the friction coefficient of the material, W is the load carried (the cable tension in this case), is the mean diameter of the thread, L is the Lead (which is equal to the thread pitch in this case), is the contact angle of the thread (which is 60 degrees as this is an ISO thread). The above equation ignores the friction of the ball bearings and only considers the friction from thread contact; however the friction of the ball bearings is very low to begin with, thus it can be neglected. Therefore the torque can be found as follows:

                It is important to note that the cable tension (W) is a variable between different derailleurs. It is therefore possible that, when implemented on some other bicycles, the system would require a greater or lower torque to accomplish the gear changing.

The Step Motor

                The step motor used on the demonstration bike was from a common printer. Very little information could be found pertaining to its operating voltages and maximum torque. Future versions of this system will utilize specifically built step motors. Therefore, more accurate predictions and calculations can be made.

Derailleur Modifications

                The derailleur had to be modified to allow the step motor to be increased in leverage over the derailleur return spring. This was accomplished by increasing the length of the lever arms used to actuate the derailleur. Doing this would of course increase the cable travel required to get to the next gear. The system was designed with a copious amount of cable to travel. Under no circumstances, on any bike, should the cable run out.