Reddit reviews Fluid Dynamic Drag

If it really matters, you probably need to simulate it with CFD. If it kinda matters, you should read up on fluid dynamic drag and drag reduction. If it doesn't matter much, look at pictures of wheel spats and other aircraft fairings and copy the basic shape.

Material choice is irrelevant to drag, as long as the surface is smooth.

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Lets start of with your requirement of 80 mph. At 80 mph, the rubber that is contacting the surface on the rear tire has to provide a force equal to the drag+drivetrain friction.

The drive train friction is the losses in the friction of everything in between the motor and the wheel, as well as the wheel bearing. Each of these can be approximated as a constant torque that slows a freely spinning wheel down. You can do some research on chain drive efficiencies, do your own experiments with setups by spinning a wheel off the ground, then measuring how long it takes it to slow down, and knowing the mass and the wheel you can calculate its rotational inertia and measure what the friction torque is. However a simpler way is to introduce a safety factor of 10%, meaning that whatever torque you calculate you need to go 80 mph, increase that torque by 10% and caclulate your battery/motor requirements from that.

As far as the drag, it would take me WAY to long to explain drag as it is a complicated phenomenon, however I can suggest a book called Fluid Dynamic Drag where the author basically put a lot of shapes in the wind tunnel and computed the drag on them along with the formulas. The general formula for drag on an aerodynamic body is Drag=(Cd)(area)(1/2)(air density)(speed^2 ). The Cd is the drag coefficient, and the area is the reference area. Each of those varies for the type of drag (again, way too long of an explanation), but in general, you can find the Cd of a bike the same shape as yours, and compute the "effective" area of your bike, meaning that if you were to take a picture of the bike from the front, the area that you can see would be the effective area. This should give you a rough estimate on the drag at speed.

So once you compute the force of the drag, you can compute the torque at rear wheel by multiplying that force by the radius of the wheel. Then add 10% to that torque to compensate for friction. You can also calculate the rpm of the wheel at 80 mph by its circumference.