Does reducing ground clearance REALLY reduce drag?

[Randy_the_Hack]

Originally Posted by Frank Lee

Some guy over on Gassavers went through the same process- added skirts, grille block, bed cover, etc. to his truck. Finally realized, after getting no improvements, that commuting in his big truck was stupid. He got rid of it in favor of a small car.

Yeah, FWIW, I don't expect to see too much WRT changes in fuel mileage for my commute, though the majority of it is on the highway at highway speeds. My hope is to improve things out on the highway when I'm going 200-800 miles or more.

At this point, with 2 kids in college, 4 car loans all of the insurance, it's kinda tough to justify yet another set of wheels... though if I came across an '88 Metro I might consider it...

[cbergeron]

Here's a link to the writeup I did about how my Aspire paid for itself and is continuing to do so:

http://aspire.dashpc.com/freecar.php

[DifferentPointofView]

I can find $500 or less econo cars all day long. Do the math, they pay for themselves.

Some of us ain't so lucky I've been looking for a while now for something within an hour away and have turned up no such luck. doesn't help that I live in a village.

[Randy_the_Hack]

Originally Posted by DifferentPointofView

Some of us ain't so lucky I've been looking for a while now for something within an hour away and have turned up no such luck. doesn't help that I live in a village.

x2... for that price, you can buy early 80's Lincolns, mid '70's 1 ton vans, or 'bout a hundred cars that have been wrecked or don't start or haven't been started in 5-10 years... or any combination of the above.

Some people have all the luck...

[RH77]

Originally Posted by Frank Lee

I can find $500 or less econo cars all day long. Do the math, they pay for themselves.

Originally Posted by Randy_the_Hack

x2... for that price, you can buy early 80's Lincolns, mid '70's 1 ton vans, or 'bout a hundred cars that have been wrecked or don't start or haven't been started in 5-10 years... or any combination of the above.

Some people have all the luck...

I agree that this may be possible in some locales, but I'll have to go with x2 here as well.

Again, still possible in many areas.

RH77

[Christopher Jordan]

Originally Posted by cbergeron

Here's a link to the writeup I did about how my Aspire paid for itself ...

Very well written! Gas was over $3. and my Aspire was on a used car lot. I was upset about just filling my old car for about $40.+ . When I saw the Aspire- the size sold it- AND it had a full tank

It was low and had aftermarket spinner hubcaps. Removed the caps then I also removed the back seat/mounts/seatbelts/carpet; it sits VERY high in back now. Flat aftermarket hubcaps ($10.-Craigslist) were more original-appearing. More aerodynamics later.

The A/C is terrific! Great to read your positive article. It was such a simple mod that I had time to finish adding the Velokit to my TerraTrike (former dragon) Gas is over $4. in most places around here. I love the 40 MPG!

countersTrike

[trebuchet03]

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).

Practical is all determined case by case. As you said, your truck is an aero "wreck" What works for my car probably won't work for your vehicle.

Having a basic understanding of the concept allows you to make a stab by candle light (rather than in the dark ). Short of telling everyone to make their car a foil shape - there is no practical information that applies across the board (this thread is proof of that ).

Well - there's one exception... Hit the books - get learned I personally do research on streamlined shapes - that aero stuff doesn't apply to the features you've described on your truck :/

[cbergeron]

Getting back on topic...

Another advantage of lowering a car is that the front fender well gaps are smaller with the wheel sitting higher up in them. This keeps some of the wind that the tires are blowing forwards from getting into the air on the sides of the car. You can see that on a rainy day the front wheels throwing water forward and out of the fender wells.

This is equivalent to compressing a larger volume of air into a smaller space. The net result is that air compression has to take place. Compressing air hinders thermofluid dynamics. It's really obvious if you think about it; it just requires a new way of looking at things that have long been considered 'normal' or 'de facto'.

That's just my opinion, of course...

[trebuchet03]

Originally Posted by cbergeron

Getting back on topic...



This is equivalent to compressing a larger volume of air into a smaller space. The net result is that air compression has to take place. Compressing air hinders thermofluid dynamics. It's really obvious if you think about it; it just requires a new way of looking at things that have long been considered 'normal' or 'de facto'.

That's just my opinion, of course...

I highly doubt there's any compression.... Flow compressibility really isn't considered until you reach mach numbers ~.3 or so (which is above 200mph - this number is partially arbitrary).

The easiest way to test this is with a manometer....

[Otto]

Lowering a car or other body in motion increases the drag at a certain point, because the airflow between the body and the ground is constricted. This is called interference drag.

NACA and its various equivalent research groups studied this in conjunction with biplane wing arrangement, where both wings are smooth and streamlined. What is the optimum gap between wings? Too far apart and they lose the synergistic flap effect, where they produce more lift than the sum of their respective lifts. Too close and the interference drag increases, since the airflow beneath the upper wing conflicts with the flow over the lower wing.

Lowering a vehicle reduces the effective frontal area and drag of the wheels, as noted previously by others, However, it also leaves less room for the underbody airflow to efficiently move, so means more interference drag, and otherwise creates suspension and ground-clearance issues. At some point, the benefit of reduced wheel drag is offset and then reversed by the increase in interference drag.

As a practical matter, it seems to me that wheel fairings should be a priority, along with a smooth undertray at the chin. Such undertray should extend back as far as the transmission of a front-engine car, and help induce and smooth the cooling air as it leaves the engine compartment are rejoins the airstream.

Simple fairings of Coroplast could also be made for the front and rear suspension arms, and to plug the parachute-like gap between the fuel tank and the rear bumper cover. Beyond that, covering the entire underbody would seem to have much diminished return on effort, plus carry the complications of heat from exhaust pipes and mufflers, etc..