Coastdown test for an aeromodded 1998 Ford Escort ZX2: Determining Cd & Crr

[BabyDiesel]

Originally Posted by freebeard

Since you are thinking about a boattail, take a look at http://ecomodder.com/forum/showthrea...tml#post481536. He's shedding parts on the highway. BamZipPow has also run aground lately. I'm not a big fan of coroplast and duct tape.

Look for a build quality that would pass tech inspection at a race track. I just got a good part for an airdam on my car. It's the 'towel rack' bumper overrider from the American spec 67-down Beetle front bumper (cut down), and it will hang upside-down under the later bumper with tabs on the bumper brackets. I've got steel sheet for the diverter.

My goal is to get Prius-like mpg, starting with a 0.40 Cd. But first it's off to the Woodburn Drag Strip for the NW Bugrun on Saturday. Oops, that's today.

I have seen the carnage of the boat tails recently. Pretty saddening to see their hard work get destroyed.

I don't exactly like coroplast and duct tape either. But for my first boat tail build, I plan to use cardboard and duct tape to keep cost down and its ease of working with. If the results are worth pursuing, then I will go back with aluminum, fiberglass and paint and make it perty

Funny you bring up race tracks and drag strips. I have been wanting to get the ZX2 to the drag strip for quite some time! I think it will perform very well considering the Cd is much lower than stock. Good luck today if you are racing freebeard!

[BabyDiesel]

I know graphs do not help you much Cd, but it is all I can do for the time being

Here is a comparison of coast-down speeds with my current 0.269/0.01 combo versus 0.33/0.012. The numbers are a stock Zx2's Cd and a hypothetical increase in RR due to placecard air pressure.

The pink line is the hypothetical coast-down during a duration of testing a stock ZX2 in the same conditions as the original test. The dots are the data points for my coast-down test where I came up with Cd= 0.269, Crr= 0.01



Note the incredibly high Sum of Error^2. This shows the amount of deviation from my data points and it is quite a lot!

[oldtamiyaphile]

Worth noting that this test only works if your speed signal is accurate. Dashboard speedo's are typically 5% off, but the OBD data will usually match GPS, however, unlike GPS the OBD doesn't lag as much and is probably the best way to go

Good to see people gathering and posting their data.

[oldtamiyaphile]

Originally Posted by BabyDiesel

Note the incredibly high Sum of Error^2. This shows the amount of deviation from my data points and it is quite a lot!

I think you need to adjust your CRR, so that the dots at the end of the graph better match the model. You'll probably then have to adjust CD slightly.

[BabyDiesel]

Good point, OldT! The speedometer is very well calibrated. I compared the Speedo to the GPS and it is spot on

That last graph was a comparison between stock Cd and Crr vs. My coast down test. The line is stock, the dots are modified

[BabyDiesel]

While driving a few minutes ago, I thought of something that needed to be added to this thread:

When I deduced the numbers down and got the RRC for my Blacklion tires, they seemed pretty good since they had a value of 0.010. This is right on the line between LRR and normal tires.

Then I remembered - The tires were pumped up to 70 psi for the test!

So! That will throw the numbers off if tire pressure is lower than I tested. I typically run them around 60 psi, so that would mean the RRC is higher than my test. Maybe not now, since my tires have worn down some and "loosened up".

Hmm... makes me wonder what the RRC is if they are at placecard psi (35 psi).

Actually, I think I will test the RRC between placecard psi and hypermile psi eventually. It will be good to have numbers behind what we already know is a benefit.

All in all, the test concludes that I need LRRs... asap!