Capstone Project: Human Powered Vehicle Design and Machining
I worked as a Drivetrain Lead on a large team of 14 mechanical engineering students to design and build a human-powered recumbent trike as our school’s first entry into an ASME recumbent vehicle competition, scheduled for April 2020. No physical competition was held due to the COVID-19 pandemic. It included the following general components and designs:
•Steel pipe frame
•Carbon fiber fairing
•Ackermann steering geometry, similar to those found on most cars
•11 speed drivetrain with chain management system and adjustable boom for riders of different heights
Since this was my university’s first year for such a team, our team didn’t have a previous trike to improve and learn from. I visited local recumbent bike shops to ride trikes with different features, iterated through Solidworks designs, performed preliminary drivetrain gear ratio calculations, and prototyped new mating part mechanisms in a machine shop to develop our ideas.
Final Solidworks assembly view of frame, drivetrain, chain management system, and fairing
Me trying out different recumbents in the early, early, early project days! We used a design matrix to quantify specific design choices eventually settled on a tadpole-style (2 front wheels, 1 rear) design similar to the the configuration in the photo to simplify handling and vehicle stability.
As the drivetrain lead, I was active in the trial-and-error process of designing and prototyping parts for mating mechanisms for moving drivetrain components. This included custom steel parts to attach front and rear wheels, custom rear derailleur hangers, a chain management system, and an adjustable front clamp and boom to telescope the front chainring to different positions to accommodate different rider leg lengths.
Before settling on specific drivetrain design choices, we derived rough gear ratio and power transfer calculations considering rider weight, vehicle weight, competition hill grades, drag, rolling resistance, and estimated drivetrain efficiency to settle on feasible driving/driven cog sizes on the drivetrain. Rough formulas are shown below, in case you'd like to relive your physics and early fluid mechanics phases.
(Video) Prototype of quick-release dropout mating plates for clamping rear wheel
Prototypes like the rear dropout mating plates above involved learning machining processes like operating a waterjet and steel TIG welding. I learned manufacturing practices of basic welding processes for metals that I now use to apply to develop and sustain laser and ultrasonic welding processes for polypropylene consumable medical devices at Talis. Although large-scale plastics manufacturing and molding involves design considerations for repeatability and part consistency, this iterative prototyping and manufacturing aspect of the project allowed me to translate welding skills and knowledge to welding soft materials at my current company.
Other prototypes I developed include a chain management passive pulley system to control and hold an 11-foot chain spanning the length of the vehicle. The chain management system included a chain gobbler mechanism to take up any chain slack when telescoping the front chainring towards or away from the seat to accommodate tall and short riders on the team. The chain gobbler is shown in the video below as the support system for the crossed chains near the front of the turning cranks. It consists of two pulleys supporting the chain, taking up chain slack if the chainring and cranks are pushed closer towards the rider to shorten the reach. A separate telescoping chainring mechanism was created by milling steel pipe and welding extruded steel pins to catch at different boom positions and clamp into place. The video and photo below shows both of these systems.
(Video) Chain management system consisting of a chain gobbler at the front end of the frame and supporting pulley wheels in the midsection of the frame.
Chain gobbler system attached to frame
Simple twist-pin locking mechanism prototype for telescoping front chainring towards or away from the rider
In-progress steel frame. Has a attachment points welded for a rear wheel (back) and supports for a roll cage (top) and seat.
Near-complete trike with frame, wheels, seat, telescoping chainring pipe attached.
Completing this project gave me the opportunity to execute both classroom manufacturing knowledge and hands-on iterative thinking during this project. Although Covid-19 cut this specific project's manufacturing process short, about 90% of the drivetrain and trike was completed. I gained skills to become comfortable in the machine shop, worked heavily through the engineering design process with my teammates, and produced a product I’m proud of despite the circumstances. We finished 9th of 36 total teams.
