Increasing downforce while reducing drag?

[Hermie]

I'm more interested in control than straight MPGs. More point weight on the tires is a very good thing to me.

Lowering the car would also give it a lower center of gravity, and thus better cornering, but there are no lowering springs made for a Tempo. The best I have is a set of variable-rate cargo/control coils from Moog. Can't afford to have them installed right now, though, but I'm going to pair them with new Gabriel Ultra struts when I can. They're the only high-pressure rated struts for a Tempo.

No one really makes good parts for my car.

If I can reduce lift at the same time as reducing drag, that's a mod I'll do. If it creates lift, I'm not going to.


On VGs... Here's my rear roofline, which most likely causes a great deal of separation.. I'll find out later, though, when I do a tuft test. That black strip is raised somewhat, and doesn't allow a smooth flow onto the rear glass.


http://i3.photobucket.com/albums/y93...y/DSCF0908.jpg

http://i3.photobucket.com/albums/y93...y/DSCF0909.jpg

http://i3.photobucket.com/albums/y93...y/DSCF0910.jpg

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[Deezler]

Two things I've learned:

- There's just no way flow is going to be attached all the way down your rear window and trunk lid. Tuft test it and see. Smooth curve into to the rear glass or not, the angle is just too steep.

- There are not "massive" savings in Cd to be had simply using vortex generators. To really get the gains you are looking for you need to think about making a kammback.

[2003protege]

Originally Posted by Deezler

- There are not "massive" savings in Cd to be had simply using vortex generators. To really get the gains you are looking for you need to think about making a kammback.

This is key. A kammback will get you significant gains, and a boat tail would take you as far as you could with the flow off of the sides and top of your car and would help smooth out the air from underneath after being subjected to the undercarriage.

[Hermie]

I'm not doing anything THAT extreme... Plus I don't have the materials. And the gains are in drag reduction, not lift reduction.

I'd like my car to still look somewhat normal, with low-profile mods.

[Hermie]

Did the tuft testing, and tomorrow I'll have the pleasure of removing tape residue...

Testing done at a max of 55 MPH.

There's a HUGE area where it loses touch with the window and about half the trunk, but it does reconnect. Probably at around 90, it'll be so large that it won't reconnect.

There's a smaller area where the air disconnects (where the flat plastic trim is), but surprisingly the air reconnects after that.

I'm going to look into some vortex generators to help the flow, and install an M3-style lip spoiler (that's 3/4" tall). That should help the rear end out a lot.

Then, when I find the material and money, I'll smooth out the underside with a partial pan and rear diffuser.

I also found out, much to my relief, the hood vents do in fact vent, and don't take any air in. ^^ Front lift reduction FTW!

[2000mc]

climb under your car and see if your rear bumper cover is a big parachute? not to say it would be pretty, but i think there is a number cars that would have less drag and less lift if the rear bumper cover was completely removed. but you wouldn't have to remove it, maybe just see if the cover extends well past the impact bar, and consider if it would be too much of an eyesore if the cover was cut off at that level.

a thought on the VG's... if you could get air attached down the back, perhaps you could reduce drag, but wouldnt the car then have more lift being generated across the rear glass and trunk than if air detached near the roof line?

[robchalmers]

DON'T ADD TOO MUCH TO THE REAR !!! adding a diffuser and a duck tail to the trunk lid may kill the lift at the rear but the front will still be lifting and may feel worse as a result

[Hermie]

Obviously, I'd balance out the front. I've already got hood vents, which reduces front end lift, and I'll also be adding a front air dam, angled forward, to further reduce front end lift, as well as side skirts to create a low-pressure zone under the car.

VGs have been shown to increase downforce when paired with spoilers, because it increases the pressure of the air flowing over them. Without a spoiler, I do believe it will add to lift. I got some info on that from a Porsche 914 racing forum. The 914 has a flatter rear end than mine, so if it plants the back end on those, it should definitely help for me!

Also, the rear bumper isn't exactly smooth. I'll take a picture soon.

[MadisonMPG]

"The TT has 148 pounds of lift at the rear axle at 125 mph without the spoiler, but only a 53-pound lift when the spoiler is fitted."

Twice the speed, four times the drag/lift/down force. So you will be doing 60 or so with a Tempo on the highway. (hopefully) Your car is not shaped like the TT and will not generate that much lift. But assuming it does, at 60mph you will have 36ish pounds of lift. Which probably makes your 1,000 pound rear end weigh 964 pounds. Not exactly something to be concerned about. Is your car even rear wheel drive?

[2000mc]

Originally Posted by Hermie

I'm actually more concerned about stability at highway speeds......

if your car has a problem at speed, have you considered that it may not be an aerodynamic problem, but a mechanical one? ever had the alignment checked? toe and especially rear toe could cause this feeling of instability.