Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - View Single Post - My first coast-down test: results, analysis, your input

daqcivic · 02-17-2010, 05:46 PM

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.

02-17-2010, 05:46 PM	#1 (permalink)
daqcivic EcoModding Lurker Join Date: Nov 2008 Location: Portland, OR Posts: 53 Daq Civic - '92 Honda Civic DX sedan 90 day: 42.54 mpg (US) The Hardbody - '91 Nissan D21 (pickup) The Hardbody - '91 Nissan D21 (pickup) Thanks: 1 Thanked 7 Times in 5 Posts	My first coast-down test: results, analysis, your input 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.