Aero improvements....what's really practical?
 

Hi everyone....I'm kind of new here and still trying to get a grip on things, so please bear with me. I'd also like to clarify that automotive background is in the performance field, so my approach regarding aerodynamics has always been "performance 1st"....but I'm willing to change. ;)
I've been sifting through the mountain of info here and I continue to come up with the same question: What's really practical when it comes to aero mods? And where should we draw the line between the potential sacrifice of safety for overall efficiency?
For example: Boat-tailing....I'm beginning to understand the design benefits. Please correct me if I'm wrong here, but doesn't the shape of the boattail induce neutral flow characteristics to achieve efficiency? As opposed to a drag inducing, mildly spoiled rear end which has the potential to create usable downforce.
I'd like to cite an example from auto racing history. During the 1950's some auto mfgrs designed race cars with neutral aero efficiency(ie:Jaguar C-type, Alfa Romeo Discovolante(sp?)). Both vehicles were rather competitive in thier respective classes, but were almost completely void of downforce and lacked the outright handling necessary to be dominant. And later designs from these mfgrs often swayed away from the neutral lines in exchange for more aggressive surfaces with downforce/lateral stability benefits.
I've always been under the assumption that you don't have to drive a racecar to benefit from aero mods. If I were driving @ 75mph(legally) on an open highway in 40mph crosswinds or 55mph through twisting roads on a mountain pass, I'd much rather have a small rear spoiler than a large boattail.
There are more obvious mods that could widely be considered less practical(ie:wiper/ext.mirror delete).
What are some genuine opinions regarding this subject?

Thanx, SL8Brick

I think its a matter of what you're trying to achieve with your aero project.

My mechanic buddy drives a car nearly identical to mine however with an intercooled turbo, and larger turbo + i/c to be installed soon. When he saw my front air dam extension he immediately wanted to mold a copy in fiberglass HOWEVER he wanted me to give it a forward-sloping front surface to create downforce.

However creating downforce from aero flow will tax some of your forward motion energy. It's not free. So my version of the air dam has a near-vertical front, and his version will have a slanted front. He does need the downforce since he takes 75+ mph curves fast enough to lift the inside front wheel.

I don't drive my car hard enough to need the extra downforce. I have heavy duty sway bars and 205 tires vs. the oem 185's so that's enough for the handling and traction I need.

Volvo 240s reportedly have a c/d of about .40 which is very high. My 240 really didn't "glide" (coast) until I got the air dam + forward belly pan done. It would just start slowing down seriously, even on downgrades on 65 mph limited access state roads. With the dam + pan done it glides much better, and on a steep enough hill will maintain or gain speed. It's all a matter of what your priorities are.

Same here with the air dam. My purpose was to direct air from under the vehicle, which it does rather nicely as well as around the front tires.

One note for you to file away. With the side skirts, I lose almost no efficiency in heavy cross winds.

A boattail will have lower drag. If lift becomes a problem, then you might have to think about priorities.

Since the boattail extends from the rear, it will tend to point a car into a crosswind, which might actually help self-correct.

That's about all I have to say about that. :-)

Thanks Forrest. 'course I'm an excellent driver. ;)

I agree wholeheartedly, Bill.


And that's kind of my point.....lift can almost always be a problem. Even in everyday driving situations.

Take for example the Audi TT(2000-2006): When the 2000my TTs first hit the US streets in 1999, the rear spoiler was optional and Audi even offered a OE roofrack set-up for the coupes. When the drivers of the spoilerless TTs began complaining in mass about eratic handling charachteristics(especially during high speed cornering), Audi issued a recall which offered a free spoiler to any spoilerless TT owner(Audi also had a waiver to sign for any owner that rejected the spoiler). Simultaniously, the spoiler became standard equipment on all later production models. Realizing the sensative aerodynamics of the car, Audi also discontinued all OE roofrack components for the TT coupes(much to the shegrin of many customers).

I wonder how much of an issue this is for people who are driving for fuel economy (ie. who tend not to drive fast).

TT drivers losing it at autobahn speeds (whether or not they're actually on an autobahn) don't really ring alarm bells on the issue of aero mods for economy, personally.

Point taken.....thanx for the reply!

Most production cars produce lift at speed, not downforce. Cars like Ferrari, the TT, Corvette, etc are the exception.

Regarding the boattail, I think you could design a boattail with neutral lift characteristics or with positive or negative if desired. The primary way the tail works is by reducing the cars turbulent wake.

Downforce does not have to come from a F1 type wing though. F1 racers take the extra drag because they also have lots of power and the higher cornering speeds are to their benefit while they do not care as much about FE. Diffusers, splitters, and similar devices are used in many other race cars to generate downforce. Spoilers like on the back of NASCARs can also be used to tuned to reduce drag while also adding downforce.

FWIW, my butt-o-meter says that my LeBaron feels more solid on the highway with a gille block and filling the gap between the body and the bottom of the bumper cover. My FE is also better than ever.