About
Tandem Trike Project
Image 1: Lance riding the second trike iteration with push bar helpers
Team Chase is a group of athletes, dreamers, and tinkerers, that was created in memory of Chase Boyd to exemplify that a physical disability doesn't have to deter from achieving greatness, and aims to help those with physical disabilities pursue their athletic desires. Lance, a quadriplegic athlete a part of Team Chase, has challenged himself by competing in several intensive biking marathons including the 620 mile CAF Million Dollar Challenge and the Silver State 508 mile race. The team approached the Mechanical and Aerospace Engineering department at UC San Diego, looking for a new cutting-edge design for a tandem tricycle Lance uses to participate in these sorts of events.
Background
Image 2: Above- First Trike Iteration
Below- Second Trike Iteration
With two previous tandem trike iterations under their belt as can be seen in Image 2, Team Chase had a set of specific issues to tackle and an idea of where they wanted to go next. The main objectives of the trike however,shifted a few weeks into the planning process with Team Chase expanding their horizons for an even longer and extraneous ride. With a longer ride, the focus shifted from reducing weight to having a 2x factor of safety to ensure the trike could withstand such an event. The main objectives now were: high stability and maneuverability with no tilting when turning, minimize torsional vibration along the axis of the trike, structurally strong to resist ultimate and fatigue loading, and to keep the front pedal boom breaking. They also desired for the length to decrease to about 210 cm, over a 30 cm decrease from the first iterations, and for the weight to be about 27kg. The expected final deliverable is a tandem tricycle frame that is ready to integrate attachments such as wheel, gears and pedals.
Objectives
Design
Set-Up:
The masses of Lance and the pilot were placed on the carbon tube as 72.57 kg and 90.72 kg respectively. A force of 1334.5 N was also placed on the carbon tube to account for Lance's pedaling force. The front assembly main tube was loaded with an axial force of 1334.47 N and an equal normal force in the opposite direction. These forces gave us insight on trike displacement. On the right head tube, a 222.411 N moment was added as a worst-case scenario to test for stress in the instance of an uneven force from a cyclist pushing on the push bar.
Image 4: 3-bar Support: "Almost" Final Design
The material properties used for the titanium were slated for worst case scenario, using the annealed properties rather than the actual cold-worked stress-relieved properties. The carbon was iterated to test 10 different possible lay-ups.
Using SolidWorks, the team ran Finite Element Analysis (FEA) and applied loads and forces onto the trike to test the design and realize its failures. From this analysis, the "almost final design" seen in Image 4, was iterated to adjust for areas experiencing the most amount of stress.
Image 3: Final Design Solution
The final design, features a carbon fiber main axle with titanium front and back ends. The body, remaining
low at about ~46 cm (18") from the ground, will decrease the center of gravity of the riders and the tilting
of the frame; while the carbon fiber axle will better support torsional loads and the titanium will ensure
toughness. This design has also achieved significant weight and length decrease, measuring to 204.93 cm
hub to hub and 11.34 kg without any attachments, exceeding the desires of Team Chase. The UCSD team
also took the comfort of Lance as a priority when designing the shape and positioning of the seats and boom.
Taking this consideration is actually what led to such a great decrease in the length. In the second iteration, the
seats and boom were adjustable to for Lance to find where he's comfortable. However, through further investigation,
it was revealed that once his position is found, adjustments are no longer made. In designing a test rig to find
Lance's optimal positioning, the team was able to remove this level of modularity to create a more static design
tailored to Lance. The seat frames will attach to the trike frame by being epoxied to the carbon and welded to the
titanium, and will reduce vibration felt by the riders with a mesh seat. The push bar, in the past only located in the
back, now has arms that swivel into place on Lance's boom, to shift the forces from the push to the center of the frame.
It was also angled to increase the distance between Lance and riders coming to help push up inclines.
Analysis
Image 5: Stress Analysis of 4-bar Support
The most amount of stress seen on the trike was centered on the front titanium assembly where the arms for supporting the legs, meet the main titanium body. To reduce the stress seen here, a 4th support bar was added and tested. The 4-bar support reduced the stress seen in the 3-bar support by 44%, experiencing a maximum stress of 337.7 MPa (seen in Image 5). This value falls far below the yield strength of annealed ATI 3-2.5 Grade 9 Titanium Alloy, 586.1 MPa, and the fatigue life of the titanium, ~485 MPa. The largest stress area occurs at the joints directly under the cross-bar welds, but the maximum stress is so far below the yield strength of the titanium even under worst case conditions that the likelihood of this member to fail is very low. With this design, the max displacement seen is 6.46 mm at the center of the carbon tube.
Results:
Meet Team Trike!
From left to right (Bottom row) : Nicolas Delfino, Auggie (Lance's dog), John Page (sponsor)
From left to right (Top row): Angelica Saavedra, Jose Doval (sponsor), Sam Eva, Ved Vakharia, Sean Roque
Not pictured: Oyvind Sanden, Lance Weir