Testing wheel skirts A-B-A

[Blacktree]

Kurzer has a good point. It seems like you're driving at the same throttle level, rather than the same speed. That, in itself, introduces more variables into the equation.

I think you should wait to draw conclusions until you've found a methodology that will yield accurate results.

[California98Civic]

But if I drive at a steady 15.3 throttle position, speed should vary in a regular way--everything else being controlled as effectively as possible. Therefore, if I cover the same terrain at a higher speed and in less time as a result of a mod my gal/hr. should be lower and my MPG higher. Seems logical to me. What's the flaw, exactly?

[Krayzie]

i think your gal/hr would still stay the same because you throttle position is at the same point. Your MPG would also stay the same but you get to your destination faster. Drive at the same speed and i think you will see an increase in your MPG

[California98Civic]

Originally Posted by Krayzie

i think your gal/hr would still stay the same because you throttle position is at the same point. Your MPG would also stay the same but you get to your destination faster. Drive at the same speed and i think you will see an increase in your MPG

I do think you are correct that a steady speed would make differences more apparent. I just can't see the steady throttle making the numbers necessarily the same. If I set a steady fuel consumption rate (15.3 throttle) and achieve the same distance (18 miles) in less time, I should see higher MPG. But maybe the differences in fuel economy would not be as apparent with steady throttle as they would be if I held a steady speed. Does that make sense?

So a modification of my test method: on a shorter route (a 5.2 mile stretch of freeway I'll drive in each direction) the ground is flatter still and I'll be able to hold a steady speed. I'll do that. Thanks.

[kurzer]

it isn´t so easy. higher revs at same throttle, are sucking more air into the engine and more fuel. i´m curios to see the results with constant speed.

[Blacktree]

Originally Posted by California98Civic

I do think you are correct that a steady speed would make differences more apparent. I just can't see the steady throttle making the numbers necessarily the same. If I set a steady fuel consumption rate (15.3 throttle) and achieve the same distance (18 miles) in less time, I should see higher MPG. But maybe the differences in fuel economy would not be as apparent with steady throttle as they would be if I held a steady speed. Does that make sense?

So a modification of my test method: on a shorter route (a 5.2 mile stretch of freeway I'll drive in each direction) the ground is flatter still and I'll be able to hold a steady speed. I'll do that. Thanks.

Just FYI, steady throttle does not necessarily mean steady fuel consumption. There are many other variables in the fuel consumption equation.

BTW, did you monitor vehicle speed during the test? If the wheel skirts allowed a slightly higher speed at the same throttle position, then you have less aero drag.

[PaleMelanesian]

There's no time factor in MPG, only miles and gallons. You need to aim for equal speed so the aerodynamic load is equalized. Otherwise the skirted run might be faster, but has a quadratically higher drag load.

I'm puzzled by the result. Can you do a coast-down to verify the drag difference? It won't give hard MPG numbers, but it will show if the problem is in the test or in the mod itself.

[California98Civic]

No I understand clearly. Thanks for sticky with me on the explanation. BTW, I used the steady throttle method and wrote about it in my WAI test last summer and I don't recall any such objections. I might need to do that test again with steady speed instead of throttle, no? As for this test, since I have already redesigned the skirts, making them hug the body more closely, leaving minimal clearance with the wheels, I can't do a coast down tests on the same design, but I can do coast downs with the new design. Wouldn't I need some pretty high speeds, though? Could I get reasonable results from a coast down between say 55mph to 30mph using specific points on the road for reference?

[ConnClark]

The best coast down tests are done by coasting down a hill from a chalk starting line. That way you can always have the exact same energy to deal with for each test.

[Frank Lee]

I ran skirts over a year. Results didn't show up in tank-to-tank records. Skirts are now off. I prefer to be able to check and air up the rear tires without the hassle.