Writer seeking examples of tested DIY aero mods for book

[Frank Lee]

Originally Posted by JulianEdgar

...

Coast-down testing is a good example:

I did an aero paper in college wherein I made several aero add-ons to a Rabbit and attempted to quantify their effects via coast-down testing. I so wanted them to work but had to be wary of test bias. In the end I had to admit the efficacy of the devices (air dam, wedge nose, skirts, etc.) didn't rise above the noise of test variations.

That is why I haven't made solid claims for any mods; I know my testing is crap.

[freebeard]

I think the best location for successful coast-down testing would be the H. B. Van Duzer Forest State Scenic Corridor

Originally Posted by Wikipedia

H. B. Van Duzer Forest State Scenic Corridor is a 12-mile (19 km) scenic driving route along Route 18 in Lincoln, Tillamook, and Polk counties in the U.S. state of Oregon that passes through a forested corridor. The Van Duzer Corridor stretches from northwestern Polk County to Lincoln City, passing through the Northern Oregon Coast Range.

We're talking 80-120ft tall trees right up to the shoulders of a 2-lane highway with lots of elevation changes.

[aerohead]

Originally Posted by Frank Lee

I did an aero paper in college wherein I made several aero add-ons to a Rabbit and attempted to quantify their effects via coast-down testing. I so wanted them to work but had to be wary of test bias. In the end I had to admit the efficacy of the devices (air dam, wedge nose, skirts, etc.) didn't rise above the noise of test variations.

That is why I haven't made solid claims for any mods; I know my testing is crap.

Same for me,which led to paying CAR and DRIVER to do it.

[aerohead]

Originally Posted by freebeard

I think the best location for successful coast-down testing would be the H. B. Van Duzer Forest State Scenic Corridor



We're talking 80-120ft tall trees right up to the shoulders of a 2-lane highway with lots of elevation changes.

OOOOOOOOOOOOOOh,that would be a no-no!

[freebeard]

Why, the elevation changes?

Bidirectional passes on 'level' ground would cancel out effects of wind. Multiple downhill passes take twice as long but wouldn't the gravity assist act as a multiplier for the effect your trying to measure? A vehicle that maintains a steady coasting speed will be more aerodynamic than one that loses speed, and less aerodynamic (plus RR) than one that gains speed on the test section.

I've been wrong before.

[aerohead]

Originally Posted by freebeard

Why, the elevation changes?

Bidirectional passes on 'level' ground would cancel out effects of wind. Multiple downhill passes take twice as long but wouldn't the gravity assist act as a multiplier for the effect your trying to measure? A vehicle that maintains a steady coasting speed will be more aerodynamic than one that loses speed, and less aerodynamic (plus RR) than one that gains speed on the test section.

I've been wrong before.

The low-load/high-load whammie of up and down elevation throws the engine's BSFC map around so much you'd never know what to attribute to what.MPGs would be all over the place.
With the SAE testing protocol,the only significant variable is a change in drag,so there's no additional noise to filter out of the scatter-plot, to discern that drag signal.

[JulianEdgar]

Originally Posted by freebeard

Why, the elevation changes?

Bidirectional passes on 'level' ground would cancel out effects of wind. Multiple downhill passes take twice as long but wouldn't the gravity assist act as a multiplier for the effect your trying to measure? A vehicle that maintains a steady coasting speed will be more aerodynamic than one that loses speed, and less aerodynamic (plus RR) than one that gains speed on the test section.

I've been wrong before.

I describe in the book using a constant gradient, long downhill slope to do comparative speed testing in different drag configurations. Using a 10Hz updating GPS digital speedo, I got results that were:

1. consistent in the one drag configuration
2. made sense in the speed variation versus the different drag configurations

[JulianEdgar]

Originally Posted by aerohead

Same for me,which led to paying CAR and DRIVER to do it.

Just a point re testing. Some on-road testing can be spectacularly accurate and repeatable eg measuring lift and downforce, measuring surface body pressures. (ie don't throw baby out with bathwater)

https://www.youtube.com/watch?v=YrSDRB9Hnvg

[aerohead]

Originally Posted by JulianEdgar

Just a point re testing. Some on-road testing can be spectacularly accurate and repeatable eg measuring lift and downforce, measuring surface body pressures. (ie don't throw baby out with bathwater)

Absolutely!
Even with purely mechanical cable linkages and chart recorders,accurate lift measurements were conducted in the 1960s.
Scanivalves with either Magnehelics or U-Tube manometers have been successfully used to capture static pressure profiles around a body during road testing.
I should have prefaced my remarks with the caveat,that in my own personal situation,I neither had a safe venue,nor instrumentation of high enough sampling rate or resolution,to conduct coastdown measurements which would satisfy the SAE standard procedures and protocols,and deferred to the technical staff of CAR and DRIVER and their equipment at the Chrysler Proving Grounds at East Chelsea,Michigan.
The car was measured an a 4-load cell balance for accurate mass measurement for the calculations.
We had our own scientific-grade weather equipment with us.
Chrysler has a long,straight,level straightaway,perfect for the necessary back-to-back reverse-direction runs.
The Daytron-Messteknik optical 5th-wheel introduced no rolling resistance of it's own,and was state-of-the-art in accuracy at the time.
The surrounding trees offered wind protection,had there been any.
And CAR and DRIVER had their own software program for data reduction,to tease out each road horsepower for drag analysis.
Plus,the 8-mile oval,offered an absolute terminal velocity for comparison.
Between a record at World of Speed at Bonneville,the Chrysler Proving Grounds,and Glenn Scharf's help at the the GM Aero. Lab,we got a good agreement on drag.Cd 0.235 for the modified 1984 CRX HF.
And CAR and DRIVER cut me a little break on their fee.

[gumby79]

Sounds like the grades (2) on my test rout 6%x3mi and 6%x12mi should be adequate to achieve a test with a reasonable margin of error.
The grades are long enough to achieve terminal velocity at gravity horsepower in neutral 75 to 95miles an hour depending on local weather conditions and configuration of the vehicle. Take note 95mph is 2.5mph faster than my engine will spin in gear(2500rpm).
This gravity drop testing is what I use to determine that the air dam was not beneficial to my configuration prior to the wind tunnel(lost speed on terminal velocity).
Gumby, stay flexible