Junior Design Project
Since 2022, I’ve been a member of UCSB’s FSAE team, Gaucho Racing. Gaucho Racing builds an electric race car for the annual FSAE competition. I worked on the design and manufacturing of powertrain components such as the drivetrain and cooling loop.
Problem and Proposed Solution
Manually shifting gears on a bike requires user attention to operate at an efficient and comfortable gear. Some bicyclists don’t know which gears to select while riding, so an automatic bike shifter is a solution to this problem.
Concept
The automatic bike shifter selects gear ratios by detecting wheel speed with a hall effect sensor. Gear ratios are selected by an Arduino to minimize the change in user leg effort at different speeds. The gears are shifted by actuating a motor to adjust the chain derailleur position. The motor mount is designed to mount on standard bottle cage holes, so the product is able to be used on any bike.
Analysis
A motor is used to actuate the derailleur wire via a pulley to pull a metal cable. The torque of the motor must be high enough to overcome the tension of the derailleur wire. The tension of the wire was found to be 100 N, so a 12 kg⋅cm worm gear motor with a 12 mm pulley was selected (1.3 FOS). To actuate all 7 gears, the motor rotates through 187 degrees at 20 RPM. This draws a peak current of 1.3 A with an 11.1 V, 1.3 aH Lipo battery (15.6 W). This allows for about 19500 shifts with full charge. Formlabs Tough 2000 resin was used for the pulley, which undergoes a max stress of 35 MPa with a 2.5 FOS.
