Bamboo Tricycle

Structure

Natural bamboo is structurally similar to a unidirectional fiber composite shaft, meaning it is very strong and stiff along the fiber orientation axis, but has much lower strength and stiffness in torsional and transverse loading. To minimize the risk of the bamboo frame members cracking and/or failing, we developed a 3-dimensional truss design for the frame to minimize the less desirable torsional and transverse loading on the frame members.

Structure Analysis

To evaluate the frame designs, finite element analysis tools were used to simulate various loading conditions. Shown left is a side load scenario that might be seen in the course of a rollover accident. In addition, simulations were performed to cover hard cornering, heavy rider, top load (rollover), and pedal loading scenarios.

Joint Development

To obtain the necessary geometries, ABS jigs manufactured with FDM were used to hold the frame members in place. Structural rigidity and strength for each joint was provided by cobonded carbon fiber/epoxy joints, fabricated by overwrapping CF/epoxy over the jig and frame members with a stacking sequence approximately [0/90]2  and [0/90]4 , depending on design loads for each joint.

Material & Joint Testing

Structural testing was conducted on test joints and unaltered bamboo frame members to evaluate our joint fabrication technique and obtain strength allowables for use in the design phase.

Fairing Motivation

A fairing was needed primarily to minimize drag for the speed portion of the competition. For this purpose, an aerodynamic fairing was designed utilizing a carbon fiber/foam core sandwich construction that minimized its weight.

Fairing Evaluation

CFD simulations were used to evaluate the candidate designs, and compare to the unfaired vehicle. The chosen fairing design cut the predicted drag at 40 mph windspeed in half from 18 lbs unfaired, to 9 lbs with the fairing.

Drivetrain

Typically recumbents are driven by running the chain to the back wheel. In this setup, it very difficult to keep the chain aligned and on the sprockets. In addition, there are significant power losses from the long chain route. For these reasons, we decided to design a front wheel drive system. A typical bicycle chainring and cassette was mated to differential that fed power to the wheels through two articulating driveshafts, resulting in a robust and efficient system.

In the competition, hosted by the University of Florida in Gainsville, we placed second overall including second in the women's drag race, and second in the endurance race.