My first coast-down test: results, analysis, your input

[daqcivic]

I finally did a proper coast-down test last weekend. I had 2 goals: calculate my Cd (or CdA) and compare my front undertray with an air dam. I haven't bothered to do the math to find Cd yet, but finding improvements is more important than having a hard number to measure drag anyway.

Here's what I did:
I went out to Lower River Rd/Erwin Reiger Memorial Hwy outside of Vancouver, WA, which is about as lonely, flat, and straight for several miles as I can find anywhere in the area. Fortunately the wind was minimal, between 6-13 mph according to the nearest weather station.

I took awhile finding a good segment where I could do runs in both directions. I found one about the length it took to coast from 70-45 mph, with visual cues marking both ends. I practiced a few times, figuring out how to use my camera to record the speedo while shifting and not crashing the car. Then I did 3 runs in both directions, getting up to a cruise at 70 mph and then coasting to below 45. This was with my grill block, full undertray and front tire deflectors. I then taped on lawn edging to my bumper to create a front lip extending low enough to match the car's average ground clearance. No sense in removing the front undertray, since the dam extends far below its front edge. With the front lip in place, I did another set of 3 runs in both directions.

Here are the results:


As you can see, while I started coasting at 70 mph I didn't begin measuring time until 65 mph, in order to isolate driver inconsistency. And while I went down to 45 mph, at that point mechanical drag is too much of a factor to measure aero, so the last 2 columns measure the total time from 65-55 and 65-50 mph. The air dam appeared to help, though only by about as much as the variations in identical runs. You scientists can tell me if that means the difference is "statistically significant."

What is interesting is that there is really no significant difference in the B to A runs, suggesting a wind factor that affected the lip more than the front undertray. Does that seem right? Does it make sense according to aero principles? Does it suggest that only Also, the longer coast times of the B-A runs is consistent with the wind direction; it was mostly a cross wind, but slightly against the A-B direction.

Another question: are the variations of .4-5 seconds in identical runs normal? I would think that minor wind gusts and the tires tracking in slightly different places on the road could account for that.

Conclusion: It would seem that the front lip lowers the stagnation point and thus reduces the mount of air going underneath, lowering drag more than does the smooth undertray. So now I want to do a proper, permanent front lip. Also, I am thinking of removing the front undertray and replacing it with one that attaches loosely to the lip, so that it is flat. This will mean the front edge of the undertray will be hanging several inches below the factory undertray and bumper, so I will have to make sure it does not flap around but can also flex when scraping driveways. How much of a factor (for good or evil) the tire deflectors play at this point is another question, one I want to do another test to answer before making the lip and new undertray. Anyway, they will have to be redesigned too.

[aerohead]

daq,does your car have a manual or automatic trans? There is a reason I ask.

[Daox]

Nice testing!

Do you have any pictures of your lip? If its just on the bottom of the bumper, its not going to lower the stagnation point. However, it will divert air from going under the 'aerodynamically dirty' underside of the car, and that would be what I think is happening.

[MetroMPG]

Originally Posted by aerohead

daq,does your car have a manual or automatic trans? There is a reason I ask.

His garage entry says manual. Now I'm curious why you asked.

[daqcivic]

Yes, it's a manual, and I see why that would matter. I guess I'm a little confused about the stagnation point comment. If less air is going under, it's going somewhere else, which would mean over, mostly, and somewhat to the sides. Even if you have a flat face and you increase its height by adding to the lower portion, the stagnation point is effectively lowered because the midpoint is lowered, is it not?

[Daox]

My guess would be that more is going to the sides. However, I see your reasoning and agree that you probably have moved the stagnation point a little.

[3-Wheeler]

How did you measure the elapsed time?

I have been collecting coast down data for about two months now, and I'm starting to reach the conclusion, that the GPS is not accurate enough to really measure the deceleration properly.

I also noticed that the coast down data at the Instructables looks a little fishy after really getting into the physics and math of coast down tests.

If you take the time to really process the data, and calculate your "change in velocity" or dV/dT (delta-V over delta-T), then you will notice that the rate of deceleration at high air drag speeds, will give you constant change in deceleration over time. In otherwords, the slope of the rate change will be linear.

And as the wind effect tapers off at lower speeds, the effect of Crr will start to predominate, as this effect is also quite linear. One can prove this by calculating the total kinetic energy of the moving vehicle and then start subtracting out the effects of air drag, Crr, and wind.

The kinetic energy of the moving vehicle is dependent upon the square of the velocity, AND the air drag at high speeds is also, as luck would have it, also affected by the square of the velocity. These two effects cancel each other and we are left with a linear rate of deceleration at high speeds. (The Instructables shows this rate changing; i.e. not linear). Once we get to the point were air drag is no longer much of a factor, then Crr starts to take over, and it's rate of change regarding velocity is linear, not a squared function as air drag is.

It gets a little complicated, but I am still collecting data, and will hopefully report my findings later this spring, after I can compensate for barometric and temperature effects.

There is a graph of deceleration over time included below. This graph is in Log/Log format, and the reason which I will not try to explain here, but is quite useful for certain applications.



Hope this helps, Jim.

[daqcivic]

I measure elapsed time by shooting movies of the speedo with my camera. It records at 30 fps and I find the frame that looks right on target, though the variation could be as much as 5 frames, or .17 seconds. I don't know if that's better than what you figure for GPS, I don't have one.

Thanks for sharing your lessons on the rate of change in velocity. I'm a bit overwhelmed with trying to figure out Cd. I should be able to do the computations fine once I get into the formula, but picking good atmosphere and Crr figures is really tricky, and finding perfectly flat pavement and no wind for testing is near impossible.

Since you've been doing this for a little while, with a lot of attention to detail it would seem, does my procedure seem adequate to you, or would you have some suggestions for me?

[3-Wheeler]

Originally Posted by daqcivic

...Since you've been doing this for a little while, with a lot of attention to detail it would seem, does my procedure seem adequate to you, or would you have some suggestions for me?...

Yes, the video camera method seems like it should work better than my particular GPS model. And the scan rate of 30 fps is also excellent.

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About the only thing that the GPS can tell you is if the road is reasonably flat or not. It can give readings within 10 feet or so, and usually better.

However, driving through thick over head trees and such can block the signal enough, that now the data appears much more erratic and inconsistent.

I also think that the cheaper units like I have appear to move the elevation about 20 feet from one day to the next, and this is traveling down the same exact road in the same exact location. The GPS has been stabilized for at least one mile before any meaningful readings are taken. This allows all the satellites in the area to sync up.

And this is my main beef with using the GPS. It seems rather arbitrary in it's output from day to day, and run to run.

__________________________________________________ __________

I am going to look into digital bicycle speedometers, and see if there are any that record speed every few seconds, and store it for later retrieval.

The hardest part about using a video camera, is getting the data back out after each run.

Jim.

[KamperBob]

Originally Posted by 3-Wheeler

Yes, the video camera method seems like it should work better than my particular GPS model. And the scan rate of 30 fps is also excellent.

FWIW, I don't trust the frame rate of any video too precisely. It really depends on the equipment. In my work I have carefully analyzed some footage and found time stamps from individual frames deviated from the average frame rate by a few percent. With proper analysis you can quantify the effect on what you are really trying to measure. Or just take your results with a grain of salt.

Cheers
KB