Is there a chance a aero mod hurts your mpg

[DoctorM]

Originally Posted by SmellyCat

I install rear wheel skirts and ground skirts and a rear spoiler. Is there a chance I've created more drag? What if I seldom drive over 50 mph. Could the front skirt and grill block possibly create more drag at low speeds? Sc

Speaking to wheel skirts -- Look underneath your car at the bottom -- there is a very high probability there is nothing aerodynamic about the underside of the vehicle, and all the air moving through and around your underside components is likely to be very turbulent. Unless your wheel wells are isolated from that turbulence (highly unlikely) wheel wells are not likely to be effective at low and moderate vehicle speeds. And if you have designer wheels with sharp edges, they're not likely to aerodynamic either. Even if you put moonhubs on the outside of the wheels, the other side of the wheel will still be turbulent.

Wheel-well skirts may be effective at high freeway speeds, but I think any gain would be small and difficult to accurately measure.

Aerodynamic fairings around fixed gear aircraft wheels do reduce parasitic drag, but typical aircraft fixed gear aircraft using wheel fairings normally cruise at 100 mph and well upwards of that. Aircraft operating below 100 mph generally don't bother with them - often for reasons other than speed, such as operating out of soft-fields.

In the final analysis, you should test them on and off your vehicle. Realize that you can very easily subconsciously affect the outcome of the test runs with a lighter foot with then on than with them off, which would invalidate the test.

Notwithstanding the psychological aspect, I for one would be very interested in your test results.

[christofoo]

DoctorM,

I'm a little skeptical about your point of view, particularly since it seems to contradict the prevailing reported data:
Rear wheel skirts - EcoModder
Smooth wheel covers - EcoModder

[christofoo]

Originally Posted by SmellyCat

I'm not clear on the box spoiler, is there a link? I was thinking that the front wheels could use some spoiler action. Would doing something to the front wheels like this picture do any good? SC

This shows the example of a box cavity that I was thinking about on p13-16.

http://wwwm.coventry.ac.uk/researchn...%20Vehicle.pdf

(I saw this paper in Vekke's post, who credits Ausias for finding it.)

I don't remember seeing any ecomodder cars with box cavities though. It doesn't seem to have caught on around here.

[Gealii]

considering all the results from wheel skirts even if the air is still affected on the inside it is not on the outside and the airflow smoothly flows over them avg gains are normally very noticeable. at any speed aero helps the faster you go the more they help but there is a wall were it advances your max speed to keep the same mileage as a lower speed without mods. the reason there is parasitic drag on airplanes even with fairings is because the airflow wants to smoothly flow over the bottom being interrupted by another object

[DoctorM]

Originally Posted by Gealii

considering all the results from wheel skirts even if the air is still affected on the inside it is not on the outside and the airflow smoothly flows over them avg gains are normally very noticeable. at any speed aero helps the faster you go the more they help but there is a wall were it advances your max speed to keep the same mileage as a lower speed without mods. the reason there is parasitic drag on airplanes even with fairings is because the airflow wants to smoothly flow over the bottom being interrupted by another object

Every vehicle may be different. Way too much rationalizing - not enough testing.

And as another member has pointed out, even testing has to be taken with a grain of salt or more, because we can inadvertently, subconsciously, affect the test by a subtle lighter foot on the gas pedal with our favored mod on than with it off.

And that knife cuts both ways -- we can adversely affect a test subconsciously (or even on purpose) on mods we don't like or support.

There is no substitute for doing your own testing.