Suggested Aerodynamic Mods for a Cobalt

[stevey_frac]

I'm only deflecting air in front of the tire. The tire is already contributing to frontal area, isn't it?

[Daox]

My mistake, you're talking about a horizontal deflector on the underbody. I was thinking a deflector on the side of the car. You're right.

[stevey_frac]

I am planning on putting in an air dam. But it will only extend a few inches, down to the height of the lowest parts of the car.

[aerohead]

Originally Posted by stevey_frac

I just tried to take a picture of my bumper to show you what i mean, however, without context, the picture just just a bunch of crap. Truthfully, I'm kinda disappointed. I don't see any easy way forward to make significant gains. The underbelly of the cobalt could use improvement yes, but it's already pretty good. The wheel wells don't contribute to massive amounts of drag, moonies are expensive ($150 for real moonies), and I think that any attempt I make at wheel skirts are gonna come off hokey. At the moment the only thing i have planned is a switch to synthetic oil, at a slightly thinner viscosity. I might get the moonies if I get some extra work.

Steve,I've had a chance to look under both a Cobalt LS and SS.My impression is that neither of the cars bumper bottoms are low enough to be affected by underbody flow.The LS was so high as to clearly expose under-chassis components.The SS was lower and has an faux-aspirated panel on its underside,however,it is also so high,with respect to the cars platform bottom so as not to be swept by "clean" air.----------- Both of these models could benefit from lowering,by use of a flt-bottomed valence,which would intersect an imaginary line rising from ahead of the rear axle at the platform bottom along a 2.5-degree angle gaining elevation to the rear.Paneling in that span would provide a nice diffuser as well as a rocker panel extension to clean up turbulence behind the rear wheels/tires.

[stevey_frac]

Originally Posted by aerohead

Steve,I've had a chance to look under both a Cobalt LS and SS.My impression is that neither of the cars bumper bottoms are low enough to be affected by underbody flow.The LS was so high as to clearly expose under-chassis components.The SS was lower and has an faux-aspirated panel on its underside,however,it is also so high,with respect to the cars platform bottom so as not to be swept by "clean" air.----------- Both of these models could benefit from lowering,by use of a flt-bottomed valence,which would intersect an imaginary line rising from ahead of the rear axle at the platform bottom along a 2.5-degree angle gaining elevation to the rear.Paneling in that span would provide a nice diffuser as well as a rocker panel extension to clean up turbulence behind the rear wheels/tires.

So, basically, a panel that would extend from the from the rear axle gently upward? Wouldn't such a panel end in the middle of nowhere? (i don't think it would come up to be anywhere near the back bumper... Could I end such a panel abruptly before i got as far back as the bumper and still get gains? I really don't want to do a full boat tail. Thanks for you time. I appreciate you taking a look for me.

[aerohead]

Originally Posted by stevey_frac

So, basically, a panel that would extend from the from the rear axle gently upward? Wouldn't such a panel end in the middle of nowhere? (i don't think it would come up to be anywhere near the back bumper... Could I end such a panel abruptly before i got as far back as the bumper and still get gains? I really don't want to do a full boat tail. Thanks for you time. I appreciate you taking a look for me.

Steve,the idea is to extend the perimeter of the rear bumper,back and sides, straight down,to where it intersects the 2.5-degree angled line advancing up and to the rear from the bottom of the rear axle.That whole region would be enclosed to form the diffuser section of a bellypan and rear valance and dictate to the air,where you're going to allow it to go.In this case,this would be where the big-boys with the billion-dollar windtunnels tell us to make it go.If the panel rises at a steeper angle than 2.5-degrees,the flow will immediately break away,leaving you with just as high of drag as before you began.It's a rule of aerodynamics that cannot be broken.

[stevey_frac]

Ok, i think i see what your saying, if not how i can construct such a beast. The boat tails that i've seen seem to rise much faster then 2.5 degrees. How does that work for them? My working knowledge of aero appears to be limited.

So, the idea scenario here would be to extend a panel down from the bumper, and have it meet with another panel that was maby affixed to the axle to gentle guide the air up into the wake. Then enclose the sides.

Do you think i would see significant gains from something like this, or is there other lower hanging fruit to go after first? Also: Does this only really get gains if I have a bellypan first, and there is laminar air sitting under there already to be guided? I'm not sure what the rules are for reattaching air.

Thanks again for your time!
-Steve

[stevey_frac]

Ok, so i was thinking about this. I think i can build a basic metal frame that attaches to the axle, and the little air foily bit that hangs off the back bumper. V1 would be covered in Coroplast. If it offers gains, V2 would be covered in aluminum sheet metal, and painted either black, or the same color as the car.

For some reason it feels like i'll be increasing the wake though, and that there will be 'more car' that the low pressure 'wake' will be 'pulling' back.

also, if i can get some sharks with frikken 'lazers'...

[Istas]

stevey: I'm not sure at all about this, but I -think- the 2.5 degrees only applies to the semi-closed area under the car. Once you get out to where the boat tail is, I think it can rise so quickly because air can get in from either side around a (hopefully) smoothly rounded edge.

I hope somebody who actually knows what they're talking about can confirm or deny this.

[aerohead]

Originally Posted by stevey_frac

Ok, i think i see what your saying, if not how i can construct such a beast. The boat tails that i've seen seem to rise much faster then 2.5 degrees. How does that work for them? My working knowledge of aero appears to be limited.

So, the idea scenario here would be to extend a panel down from the bumper, and have it meet with another panel that was maby affixed to the axle to gentle guide the air up into the wake. Then enclose the sides.

Do you think i would see significant gains from something like this, or is there other lower hanging fruit to go after first? Also: Does this only really get gains if I have a bellypan first, and there is laminar air sitting under there already to be guided? I'm not sure what the rules are for reattaching air.

Thanks again for your time!
-Steve

With a "fixed" boattail,it may be necessary to violate the rules a bit to allow clearance for driving in the real world of driveways and ramps and such.And also,the "big-boys" may have never tested all the permutations of designs we can come up with.The bottom line for boattails is probably,that whatever is done,the wake area and volume is affected and altered such to produce a net-positive result.------ At Epcot Center,
Orlando,Florida,General Motors has an exhibit including a concept car with movable rear valance.As the car speeds onto the highway,the panel lowers closer to parallel with the roadway ( closer to the 2.5 degrees ) and the raises when slower and more likely to encounter obstacles.---------- Typically,the diffuser works best with a full bellypan ahead of it.At the rear axle it's okay to have an open section to allow for axle travel. All mine do.So you could have spanwise structures ahead of and behind the axle at support the skins,leaving an opening.As for numbers,There are some seminars and mod-data info near the top of the aero forum page where you can see specifics about all kinds of mods.------------- I believe that significant gains only come from rear mods.Nobody is going to take a Saws-All to a late model car,re-work the roofline to accept a $3,000 GTP half-semispherical low drag windshield and true fastback roof,and re-engineer everything associated with that area.