Posted by Bob Dold on Monday, October 9, 2006 8:50 PM
To get an idea of the aero drag on the trike with a rider I built a FloWorks model of it to study the drag loads and flowline around the trike. My first model is a crude model withhalf of the trike and rider modeled to take advantage of symmetry to allow the model to run faster. The picture below shows the simplified trike model used in the analysis.
For this analysis I used an external air flow speed of 25 MPH, ground effects and spinning effects of the wheels and pedals were ignored. From this run, the solution converged after 44 iterations at about 2.4# for half the trike, or 4.8# total. The plot below shows the pressure on the rider and trike at 25 MPH:
To compute the Cd of the rider and trike, the drag is divided by 1/2 the density times the frontal area times the velocity squared.
Cd = Drag / (.5 * density * A * V^2)
Published values for the Cd of a traditional bike and rider vary from .70 to 1.1, a fully enclosed trike with an aero shell similar to the GoOne would be around .150. A recumbent trike should be somewhere inbetween these values, probably on the order of .50 to .70. From the model the frontal area is 5.58ft^2, filling in the rest of the equation yields a Cd value of .53 – right about where it should be. To study further reducing the drag, a fairing may be added to the simulation to see how much it helps reduce the trike’s drag.
Flow trajectories at trike centerline: