Stefano Mazza and Patrick Gall, Department of Mechanical Engineering,
University of Ottawa
The following year
This report outlines the design
steps and operation of the Auto-Bike bolt-on electronic gear
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.
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
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.
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.
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.
The general outline of the gear selector.
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 motors 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
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.
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.