A bike rack on top of a car or truck is not the best option for vehicle aerodynamics. This is because it adds unneeded wind resistance to the vehicle.
For buses, cars, and trucks, bike racks are typically mounted on the back bumper or back door. This reduces wind resistance because it does not extend above the roof of the vehicle like a roof-mounted bike rack does.
Holding a bike on the roof of the car is not the best option for aerodynamics
Hanging a bike from the rear will have negative effects on airflow around the car. It will reduce drag but also force air to flow upwards, making turbulence more likely.
There are no official guidelines for this type of thing because there are too many variations in vehicles for this to be possible.
Aerodynamics is the study of air and wind, and how they interact with objects. Traffic engineers need to take into account the aerodynamic qualities of vehicles when designing new roads and intersections.
We can use a bike rack on top or behind a car to transport our bikes. The difference in airflow around each setup depends on whether the rack points up or down.
The vehicle’s front will be less aerodynamic with a bike rack at the back, but it will be more aerodynamic with a bike rack at the front.
Although both techniques are useful, the benefit of the ‘wheelie’ is limited because it creates more frontal ‘drag’ which will also move you back and swing wider. Hitch mounted bike racks cause the bike to be covered by the vehicle’s footprint, so they can experience a significant amount of aero-induced fatigue. If you’re trying to decide between the two options, don’t forget to consider how difficult it is to load a bike on top of your car. This is something some people who are forgetful or in a hurry may be prone to do.
You could even find yourself in the awkward position of forgetting the bike in your car if you drive through or into a parking garage ( And you forego the ease of placing a bike on a rack because bikes stand there and not hang. It’s much easier to secure them that way. Remember: if you make biking an easy task, then people will use their bikes more often. I use my bike rack on a regular basis to access where I have the option to park the car for free and then take the bike into the city to avoid traffic.