Redesigning our Wheelchair Tricycle Attachment
Hello there! We're Tara and Linda, both students of mechanical engineering at Olin College of Engineering. Over this past summer, we were in Arusha, Tanzania, collaborating with the team at Kyaro Assistive Tech. The goal of our project was to redesign Kyaro’s Tricycle Wheelchair Attachment to make it more user-friendly and efficient. In the following account, we will share the challenges, discoveries, and real-world impact that defined our journey.
What is the tricycle attachment?
The original Kyaro tricycle attachment in use
Before this summer, Kyaro already sold a tricycle attachment that allows a person to turn their three-wheeled push-rim wheelchair into a hand-powered tricycle. This is a much more efficient way to travel over long distances but has a much bigger footprint than a standard wheelchair. To let people keep using the wheelchair in indoor areas, the tricycle is an attachment that can be put on and taken off.
What did we learn about our users?
At the start of the summer, we had the opportunity to interview tricycle users. Through this process, we learned that tricycle users are typically very active and lead busy lives. One tricycle user we interviewed, Simon, commutes for over an hour by wheelchair to get to and from work each day. Another user we spoke to ran a business out of Kyaro’s active wheelchair with the tricycle attachment. It was great to see the flexibility and opportunities the tricycle attachment could bring to people’s lives.
Simon with his Kyaro Tricycle Attachment
During these interviews, we also learned about some pain points that users were experiencing with the current Kyaro tricycle attachment. One of the users seldom detached the attachment from their wheelchair, because the attachment was heavy and difficult for her to remove. Another user never removes the attachment, instead opting to use the Kyaro tricycle outdoors and a different type of wheelchair indoors.
Early in the summer, we joined a trip to Mwanza, a port city on Lake Victoria about 13 hours from Arusha by bus, to see a tricycle get delivered to a user. The user we met had complaints about the fit of the tricycle attachment. Our interview with him showed us how much the arm lengths of tricycle users varied and the effect this variation had on how ergonomic the attachment felt.
What did we set out to improve?
We identified two crucial aspects of the tricycle that needed refinement – the ease of attaching and detaching it from a wheelchair and the fitting of the attachment to individuals.
To address the first issue, we aimed to design an attachment mechanism that users could connect and disconnect multiple times a day without any help or undue strain.
To address the second, we wanted to develop a system that would ensure our attachment was always tailored to the unique dimensions of the user. Although the Kyaro wheelchair is sized to the user, the tricycle attachment was not. This meant that for some users, the pedals were uncomfortably far away, requiring them to sit forward and use additional power while driving the tricycle. For other users, the pedals were too close, which reduces pedal power and efficiency the same way riding a bicycle with a seat too low does.
What design changes did we make?
To make the tricycle attachment process easier, we incorporated a pivoting joint into the bar that connects the tricycle and the wheelchair. This lets the user secure the attachment onto the wheelchair, pivot the castor wheel off of the ground, and then lock the angle of the joint.
Transitioning from a fully rigid tricycle frame to one with a pivoting system came with many challenges. First, there was a lot of force on the bolts holding the joint in place. We found that the first calculations we performed to estimate how strong each component needed to be were not accurate because they didn’t account for the offset tolerances in the system and fatigue over time.
To combat fatigue, we increased the factor of safety (a metric that describes how strong a part is in relation to the expected force on that part) for each component by making them larger and changing their shapes while maintaining the same pivoting system.
A challenge that we didn’t predict is that because in many Tanzanian workshops drilling precise holes is difficult, the holes drilled are often not straight and are slightly oversized. These oversized holes led to a lot of extra unwanted movement in the system, causing the tricycle to shake up and down, and back and forth.
To address this issue, we first attempted to add an extra fastener perpendicular to the other bolts. However, the force on this top bolt was directly on the threads. After several cycles of applying this force repeatedly, the threads were likely to be destroyed. We did some calculations to determine how much force the threads could handle and decided to switch to a larger-diameter bolt.
We also found we could decrease the amount of force on the top bolt by reducing the unwanted movement of the joint. To do this, we increased the distance between the two bolts holding the joint in place on the side and increased their diameters.
Original design shows a difference in slop of 3.81 degrees.
Final design shows a difference in slop of 1.91 degrees by moving the holes further apart.
What does the final product do?
Our final iteration of the tricycle allows the wheelchair user to attach and detach the tricycle attachment without any extra help, providing the user with much more independence to move between long distances and small spaces as needed. We also ensured that the tricycle is custom sized to the wheelchair user for optimal ergonomics. We researched optimal arm angles for propelling a hand cycle and used that to create a formula that takes the input of the user’s arm length in order to determine where the hand pedals of the tricycle should be installed.
With this new design, we hope to grant more wheelchair users the gift of independence, empowering them to navigate life's path with freedom.