Does reducing ground clearance REALLY reduce drag?

[SteveP]

Originally Posted by cbergeron

He once asked his students to determine the volume of a lightbulb and they scribed formulas on their notepads for hours upon hours on end. Eventually a solution was reached on paper. To wit - Edison took a lightbulb, unscrewed it and poured water into it until it was full. He then, poured the bulb full of water into a flask and measured the volume of a lightbulb - not by measure, but in practice (experience); and in a matter of seconds.

Ah, but the only reason that approach worked is that the lightbulb whose volume was to be measured already existed! Suppose, on the other hand, it would take $1000 and six weeks to make the lightbulb whose volume you cared about--mightn't it be worth a a few hours of scratchpad time to calculate it in advance, especially if the volume was critical to some other aspect of the project (like fitting (volume-wise) in some sort of enclosure)? Suppose that first bulb turned out to be too big? Now you have to spend another $1000 and six weeks to make a smaller one to measure. [I know, you could build a *model* of a lightbulb -- cheaper and faster-- and measure that, but I hope my point is clear.]

On the other hand, Edison was forced to find filament material by trial and error--the theory of what makes a good filament didn't exist then (and maybe not even now, for all I know). Sometimes it pays to do some research and calculatin'. Sometimes it doesn't. Most of the time you have to do both.

Remember, this discussion started when the OP *did* the experiment (with the air dam). It didn't (appear to) work. OK, what next? Decide that "air dams don't work"? Look into the measurement methods that were used in case those were faulty? Give up? Figure out what the variables are and see which ones might be worth adjusting to get better results? Try things purely at random and hope that with luck something proves beneficial? How long would it take to run, say, a dozen experiments, making sure weather conditions (temp, wind, humidity) were nearly identical for each experiment?

Experiments always *look* easy--until you dig into the measurement issues, the number of variables that are involved, the time to build the test objects, and so on. It always *looks* like it will only take an experiment or two to decide the issue--but it usually takes a lot more simply because the early results are often contradictory, or at odds with current understanding, and so on. Take the air dam that started this thread--ignoring measurement issues, we could: change the clearance between the air dam and the road, say 0.5" per test. We could put a "chin" of various dimensions on it. We could narrow its height in the center but leave it "tall" in front of the wheels, and do that experiment 0.5" at a time. And, of course, we could change the ride height. But what if it's the case that it takes a combination of things to make the air dam effective--now we have to test combinations of changes and that really increases the number of experiments we'd have to do.

Heck, if someone *gave* me a wind tunnel (i.e., accurate testing was relatively cheap), I *still* would read the literature before I started testing cars. And I'm saying that as someone who unfortunately has to read the darned SAE papers on crummy microfiche at a library that's 30 miles away. (Yes, the irony is there: I'm burning gas in order to hopefully save some in the future )

That said, I'm not against experimentation. Really

--Steve

P.S. LostCause: I'm not overlooking your comment--please let us know what you find out.

P.P.S. I don't want to sound too discouraging or anything--what we're trying to achieve is *hard*. Aero mods (generally) aren't silver bullets where one mod will buy us gobs of efficiency. Rather, it's a series of small mods which, when added up, buy us a significant improvement. Speaking as someone who's done a bit of calculation, we're talking about changes which individually are on the order of a few percent. Those are tough to measure outside of a lab setting.

[trebuchet03]

I agree with steve on that...

Consider the seemingly simple section of a plane where the trailing edge of the wing intersects the fuselage. A HUGE amount of money and time was spend designing that geometry. Intersecting flow regions are a no no, but in a case like this - it's unavoidable.

Besides, I really enjoy learning this stuff! When someone like trebuchet03 reports an Re of 184 and that seems to me to be an odd number, I figure I'm about to learn something--after all he's got access to this cool software and testing apparatus, so he probably knows a thing or two that I don't.

That makes me happy Seriously though, I'm still scratching my head a little... Intuitively, something seems off - but, we're given no reason to believe otherwise. We know for flow testing that it stays laminar, we know it's foil like... And we arrived at the shape via an evolutionary approach (start with one model - fix/test "bad" areas for each subsequent model until you're tired). I don't think the software is calculating Re# like an equation so much as it determines Re# based on how laminar or how turbulent it is (as Re# is just a psuedo relative comparison).

P.P.S. I don't want to sound too discouraging or anything--what we're trying to achieve is *hard*. Aero mods (generally) aren't silver bullets where one mod will buy us gobs of efficiency. Rather, it's a series of small mods which, when added up, buy us a significant improvement. Speaking as someone who's done a bit of calculation, we're talking about changes which individually are on the order of a few percent. Those are tough to measure outside of a lab setting.

And that is absolutely correct. I wouldn't even say a few percent - some are fractions. Another thing to remember is that it's not anything individually - it's the whole package. One feature/modification you add might not play well with another - and negate one (or both) of their potential benefit. But by the same token, one modification could make another work even better... Two steps away from a catch 22

[fabrio.]

about this interesting topic, I have a pdf document (downloadable at http://www.fisita2006.com/pdf/abstracts/F2006M078.pdf).

I have some difficults to understand the document and in particulary the diagram on figure 2.

In he, appear several parameters that do not understand:

Q3 or Q2 ..it is difficult to read...what refer?

thanks

[Randy_the_Hack]

So here is where I stand. I drive an aerodynamic-as-a-concrete-block 3/4 ton Chevy pickup. I've determined that I can lower the truck 3-4" for about $70 plus and alignment. I know from practical experience that my fellow truck owners who install lift kits suffer in mpg, so it seems intuitive that lowering the truck should improve mileage.

Clean underbody? HA! And I don't see any practical way to put belly pans on this thing either, what with the tranny, exhaust, driveshaft, and other sundry stuff hanging down. I figure this truck of mine is the epitome of the "aerodynamic wreck".

So... is there a chance I'm going to even notice any mileage improvements if I lower my truck, or is this going to be one of those minuscule improvements that won't by itself even be noticed?

While I enjoy the esoteric discussions out here as much as the rest of the community, I'm looking for practical information and advice that will help me improve the efficiency of my existing vehicles - an '04 Chevy Silverado (see my garage), and my wife's '07 Chevy HHR (another brick on wheels, IMHO).

[RH77]

Originally Posted by Randy_the_Hack

While I enjoy the esoteric discussions out here as much as the rest of the community, I'm looking for practical information and advice that will help me improve the efficiency of my existing vehicles - an '04 Chevy Silverado (see my garage), and my wife's '07 Chevy HHR (another brick on wheels, IMHO).

I've driven both. The HHR, of course, has some potential with the 2.2L - I4. I'd apply some aero, feedback, underhood, and tire techniques to that one.

The Silverado. Depending on your uses, a tapered bed cap like Big_Dave's design can help Cd. The rest is going to have to be technique or serious swaps: engine and/or transmission. Maybe tire choice and ride-height as well. There's no magic bullet here. It may come down to technique only. The best advice is to limit driving it, when you can.

RH77

[LostCause]

My most practical piece of advice would be to buy a ScanGauge.

The biggest benefit will be from changes in driving habit. It costs nothing and can probably net you several MPG (which should be huge when normal is in the low teens).

A ScanGauge will also allow you to determine the practicality of mods you perform. Most here don't drive large pickup trucks, so techniques can only transfer so far. Your application of mainstream mods will be relatively unique (BigDave and some others drive large trucks), so cost/benefit analysis will have to be a personal task.

You can determine whether the ScanGauge is cost effective by calculating how much you drive/pay @ your current mileage. If you perform the same calculation with a hypothetical mileage you expect the ScanGauge to bring, you can see savings as a function of time. Whenever $150 has been "saved," the ScanGauge will be paid off.

- LostCause

[Randy_the_Hack]

Originally Posted by RH77

I've driven both. The HHR, of course, has some potential with the 2.2L - I4. I'd apply some aero, feedback, underhood, and tire techniques to that one.

The Silverado. Depending on your uses, a tapered bed cap like Big_Dave's design can help Cd. The rest is going to have to be technique or serious swaps: engine and/or transmission. Maybe tire choice and ride-height as well. There's no magic bullet here. It may come down to technique only. The best advice is to limit driving it, when you can.

Does anyone have a link to anything for Big_Dave? Can't find anything...

Yeah, the Silverado is my daily drive, and no, I can't walk to work or ride my bike (for now)... it's about 18 miles one way. The engine is a Duramax diesel w/Allison tranny, and I wouldn't trade it for anything. I'm getting ~22 mpg on the highway, which is where I'm trying to gain an aero advantage, hence the questions about lowering the truck. I like the idea of a tapered bed cap, but I'm not gonna shell out $1800 for one (Bondo's target price), which means I'll probably try to fab one myself. Haven't seen Big Dave's yet.

As for tires, the stock tires I have are about as optimal as one can have for this big truck. They're not even close to an aggressive design. After 1700 miles on the highway on cruise control (Fla and back), I'm convinced that I require an aero mod to improve things.

The HHR is a 2.4 Ecotec w/auto transmission - currently doesn't have running boards. I'm thinking about adding them though, as these would clean up the airflow (I think) around the lower edges of the car as it transitions from the front wheel wells to the rear wheel wells. It does have a clean underbelly, and the ride height is 3-5", IIRC. But there's no chance to alter the body design here without significant, unrealistic fabrication (IMHO). Other than possibly the running boards, I suspect my best gains are under the hood. If anyone has any suggestions on how to clean up the aero profile of an HHR, I'm all ears...

[tasdrouille]

You could go with propane injection in the Duramax, but tune it for economy instead of power.

[Randy_the_Hack]

Originally Posted by tasdrouille

You could go with propane injection in the Duramax, but tune it for economy instead of power.

Thanks for the suggestion. I am looking at something similar...

Actually planning water/meth injection for economy reasons. 'Pane injection, IMHO, is simply swapping one fuel for the other, which under current pricing is virtually identical in price unless you buy in bulk (which I can't/won't do). It is a good idea, though. One guy I know of is getting about 67 mpg diesel when using 'pane... but he's burning 4 gallons of 'pane for every gallon of diesel.

Water/meth is simple windshield washer fluid, and one gallon will last 2-3 tanks of fuel (or so I've been told), and costs between $.90 - $1.65. The water/meth system runs about $420.

[RH77]

3 mistakes on my part... 2.4L Ecotec (upped from Cobalt 2.2 platform); forgot the truck was a Diesel, and here's the link to the Bed Cover. I think Dave has something similar.

It's a Ford F-350 7.3L Manual, but check out Big Dave's garage entry for lowering and aero mods. He's getting 24-25 mpg.

I've heard that chipping for economy is popular too (with or without injection)...

RH77