Most of this is coming from an aeronautics perspective, but it applies to our bikes as well. Drag is influenced by a lot more than just surface area. There are three main types of drag, only one of which affects our bike: Parasite Drag (the other are Induced Drag, produced by lift pulling backwards; and High Speed Drag, produced by the shock wave at Mach speeds). Under Parasite Drag we have five sub-types, which I'll use the definitions from
Flight Theory for Pilots 4th Ed.:
- Skin Friction Drag is caused by the viscous friction within the boundary layer. The total area of the aircraft skin that is exposed to the airstream will be affected by this type of drag. Skin smoothness also greatly affects this drag. Flush head rivets. waxed and polished surfaces, and removal of aluminum oxide help reduce skin friction drag.
- Form Drag is the type of parasite drag that is influenced by the shape or form of the aircraft. Streamlining the fuselage, engine nacelles, pods, and external stores helps reduce form drag.
- Interference Drag is caused by the interference of the boundary layers from different parts of the airplane. If the drag of two component parts of an airplane are measured individually and then the parts are assembled, the drag of the assembly will be greater than the drag of the parts. The boundary layer interference is the reason for this. Smooth fairings at surface junctions reduce this type of drag.
- Leakage Drag is caused by differential pressure inside and outside the aircraft. Air flowing from a higher pressure inside the the fuselage through a crack or door seal will create an airstream that impinges on the airflow around the aircraft and creates drag. Door and window sills are sealed with masking tape before starting an air race to lessen this drag.
- Profile Drag is of particular interest to helicopter pilots. Profile drag is the drag of the moving rotors, and it develops anytime the rotors are in motion. So this drag can exist even if the aircraft is not in motion or developing lift.
All of these types of drag affect our bikes (and cars) in some form. Leakage drag is almost nil since there isn't much place for higher pressure air to reside inside the bike's fairings. I'd say Profile Drag is the second least concern since the only place we have rotating parts moving faster into the relative wind than another part is our wheels, but our wheels are definitely more exposed than most cars.
Skin Friction Drag is the one you mentioned is less than a car's, of course. However, for the same area on the skin of our bikes we also have screws and other fasteners that stick out making each square foot that less efficient than a car's. You also have the rider whose gear or clothing is rarely a smooth surface, adding to this drag. Leather is still grainy and porous at the microscopic level, and synthetics? Well, we can easily see that isn't not a smooth surface without the microscope.
I expect Interference Drag is the greatest factor on a bike than on a car. We have most of our engine, all its screws and sharp junctions exposed to trap air and disrupt smooth airflow around us. We also have our bodies out there which create junctures with the bike that definitely disrupt smooth airflow. All of this is typically hidden within the body of a car. We also have a radiator that is exposed heavily to the wind. While it's great for cooling efficiency, it crates a lot of drag on its own.
Drag is really a lot more complex than just how much surface area is exposed; it's also what kind of surface is exposed, the angles at which is is exposed, and anything that disrupts the smooth shape. Bikes just have a less overall aerodynamic shape than most cars by the way they're designed, mostly because bikes with everything covered inside just don't appeal to the majority of riders.
). If anyone else members this article and can link it, it was interesting.
