Because very little aerodynamic innovation has taken place with wheels compared to the top surface and underbody. Vehicle aerodynamics can be complex, particularly around the wheels and wheel wells. Only recently has development focused on the wheel region of the vehicle.
Want to know about drag? Consider that the primary sources of drag in an automobile are broken down into about 50% from the top surface, 25% from the underbody and 25% from wheels and wheel arches. This means that a quarter of the aerodynamic drag on your vehicle has barely been optimized.
This diagram explains the main sources of aerodynamic drag in an average passenger car:
Potentially yes, but all performance improvements are highly dependent on specific vehicle configuration. It's possible certain configurations could actually increase drag. Generally speaking though, the faster you drive with AEROcaps, the larger the drag reduction impact. In straight line wind tunnel testing (called onset flow), AEROcaps reduced the drag force by about 30% compared to round valve caps.
If they are out in the open airflow (motorcycle wheel) the benefit usually increases. Essentially, the more exposed the tire valve cap to airflow, the greater the benefit from using AEROcaps.
To fully understand how AEROcaps will effect your specific vehicle configuration, full drag analysis of a spinning wheel and moving vehicle would be necessary. In straight line wind tunnel testing, AEROcaps reduced the drag force by about 30% compared to round valve caps. The potential exists to reduce drag on rotating wheels, but the exact benefit will depend on many factors such as your specific vehicle configuration, weather conditions, other vehicle aerodynamic effects, etc.
They are well aware of wheel aerodynamics. However, due to mostly economic and practical constraints there have been simpler areas to focus on to improve performance until recently.
The expense of manufacturing both driver side and passenger side wheels (optimized to rotate counter-clockwise on the driver's side, clockwise on the passenger's side) is a significant barrier to optimizing wheel aerodynamics.
Other practical reasons wheel aerodynamics have not been pursued has been the need to rotate tires from drivers side to passenger side for even tire wear, as well as the difficulty of developing solutions in a rotating environment.
While there are no large automakers currently manufacturing aerodynamic valve caps, some large automakers have recently entered the market with aerodynamic wheels as an option that improve range.
One example is from a leading electric car manufacturer Tesla Motors on their Model S.
The aerodynamic wheels are a $2500 option:
It's also critical to note that the 3 % range increase for the Model S was measured at an average speed defined by the EPA of around 48MPH, At higher speeds often encountered on interstate highways, the benefit of aerodynamics increase. For example, at 85 MPH it is possible the aerodynamic Model S wheel would have a 5% range improvement, compared to the driving 85 MPH with the standard wheels.
If you were on a closed circuit racetrack,at 140 MPH, the improvement could be even greater. AEROAPP devices will similarly experience a greater benefit at higher operating speeds.
"Tesla Motors" and "Model S" are trademarks of Tesla Motors, Inc.
Yes. Mercedes-Benz has recently finished work on their 2012 B Class automobile (not available in the USA). It is a highly aerodynamic vehicle, and part of the reason for excellent aerodynamics is due to optimizing aerodynamics around the wheel. The device is called a "serrated wheel arch spoiler."
The diagram below shows how the device blocks airflow from underneath the car from entering into the wheel arch.
Aerodynamic Wheel Well Device by Mercedes-Benz
"Mercedes-Benz" and "B Class" are trademarks of Daimler AG.