Is there a chance a aero mod hurts your mpg
 

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

It mostly all depends on the specific vehicle. If you could post some pictures then it could make it much easier to take an educated guess at it. As far as rear spoilers, some help, some hurt mpg, again it all comes down to what spoiler on what car.

I'm guessing that as long as your rear wheel skirts don't add a ton of frontal area, they will be beneficial.

more drag?
 

For the mods you've mentioned,the rear spoiler would be the biggest candidate for a potential drag increase.The spoiler's upward inclination angle would be the most critical factor.
As far as speed goes,if a mod is providing a drag reducing function,it's going to be that way over the entire speed range of the vehicle.
At 50 mph,your rolling resistance would almost be as critical to mpg as aero.At higher velocities the aero drag would dominate.

Normally spoilers are a bad thing for aero. We decided that unless your spoiler is sitting pretty flat against the trunk lid remove it. Now spoiler is not to be confused with kammbachs a good amount of people get spoilers and kammbachs confused on for example suvs there called spoilers but in reality they are kamms

Ok here are the pics. The front skirt is bending under the car. Bad?
The rear wheel covers, Seem ok but a racer guy said it creates turbulance, The spoiler, x-cookie pan, looks like its ok, or is it? SC

The air dam looks fine

The rear wheel skirt looks like it could be more flush with the body of the car.

As far as your "spoiler" i would slope it downwards slightly to make it more of a kammbach, from the view from the picture it just looks like you extended the top to be even longer, a slight downward slope will allow for airflow to be attached partially downwards forming the airflow behind the car.

an other issue is I scored those low profile tires and rims at the pull a part for 30 bucks each, would you think them awesome rims an tires are laying a hurtin on my mpg? Sc

it depends I believe i read somewhere that the lower profile tires make the tire heavier more metal less rubber/air, but has been said that if you do strictly highway can be better for more rolling momentum (harder to get moving). I personally stay away from them because if you get low profiled tires that adds to the price of the tire

As far as the air damn and wheel skirts I would bet those are improvements ober the factory setup, the rear spoiler, as Gealli said it would be more beneficial if you could slope it downwards a bit.

Perhaps someone should let him know what angle (in degrees) he should bent it to?

I would, but I don't know the exact numbers ;)

I think it is max. 12°.

Mods can increase drag if they aren't right.

Here are my $0.02 on your mods:

• Rear wheel skirts look good except maybe the gaps. I have some skirts with even bigger gaps, my feeling is it's still a net positive, but not ideal.
• I'm not sure anyone knows, certainly I don't know, but I would bet that an air dam bending inward can increase drag. If the air is not stopped by the dam, but continues under the car, then the result is a larger number of messy interactions. A camel with one hump is better than a camel with two humps. A stiffer dam that is angled forwards if possible, is probably better aerodynamically. Not only does a dam need to stop the air, the air needs to end up going over the top or sides of the car - that's the whole point. I'm worried about your air dam, especially if it's not stiff enough.
• Others mentioned bending your Kammback down a bit, I would also say it would work better with some sides, but I'm still guessing.
• I would suggest (partial) grill blocks next. :thumbup:

Unfortunately, I think that you either have to copy tried-and-true designs, or else treat it as a research project:

1. Mockup in cardboard and tuft test, if possible. Differences in audible noise levels also provide clues. A-B-A coast-down or fuel consumption instrumentation tests are better but take more time. CFD would be ideal, but it's expensive and most of us don't have access.
2. Fabricate.
3. Setup a definitive test to verify efficacy, if A-B-A wasn't done in step 1. It's fine to group mods together, more especially if you have confidence you know what each one does. If the magnitude of the improvement is large enough then tank logs or a known road trip and weather monitoring may work. Statistics can help.
4. If the mod didn't work as expected, go back to step 1.

(EDIT: Actually, even if you pick a tried-and-true design, but if your car has a different starting shape that might affect the mod, it may still be a research project.)

mods
 

*I think (as others) that skirts are okay.Taping/sealing the leading edges would optimize them.
*I recently pushed the nose of my truck forward,with a flexible lower urethane rubber airdam which lays back just a few degrees which forces the air to travel up and around,and shearing off the airstream at the bottom 6-inches with a hard edge.I'm recording the highest-ever mpg with this nose configuration.You'll see many manufacturers doing it today.
*A square-back body,as on your car does not produce lift.And the base pressure of the wake is fed from a perimeter separation line of near uniform pressure.Since your spoiler is 'open',the wake underneath is still receiving pressures telegraphed from the side edges ahead of the spoiler.
The only way to reduce the drag of the low base pressure is to move the whole back of the car further rearward with a boat tail elongation which will simultaneously reduce the wake size and increase the base pressure behind the new 'transom.'

I think a box cavity is also an option, with less effectiveness but less fabrication.

Great info, Some wire can pull front air dam forward. When I screwed it on, it was more outward but the air force appears have bent it inward.

The rear skirts need some bending, the new rims are wider than the originals and high speed turns make the wheels grind the skirts and has pushed it out. The previous tires were Diff sizes. 195 on the left and 215 on the right, My rear differential thought I was always turning to the right.

It has a grill block also.

The rear spoiler has enough metal over lapped that I could bend some sides out, but that will ruin the cool round cookie pan edge... Deal breaker. I'll have to survey my junk pile for a suitable side for the spoiler.

I'll angle the rear spoiler and run the super volvo down my test
hill. , SC

Oh!, one more thing, There is a 15 watt solar panel on the roof, would it be better if it was near the front windshield or toward the back of the car?

cavity
 

Yes,a number of open box designs have been proved out in tunnels.There is a bit of loss in performance,but as foe ease of fabrication and weight-savings,you could do a lot worse!
It's just important that all 4-sides of the tail be enclosed by the box.

PV panel
 

Actually,if you lost the cookie pan,and integrated the PV panel into the top of the rear spoiler/box it would be best.If it would fit.

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

i wouldn't for as far as the front wheels there are other mods that would help more

- front wheel skirts
- smooth wheel covers
- front wheel arch gap filler
- tire spats

You want to try to keep frontal area down to a minimum so the very limited area to put block would be very small compared to performing the above mods located on our 65+ efficiency mods list with the corresponding links. The least effective out of those 4 would be the tire spats which would be as you are saying just below the fender blocking farther below the car directing air around the tires

All right, bent the spoiler down and added sides, lets see what happens.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.

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

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.

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.