Measure the drag coefficient of your car

[AndrewJ]

Originally Posted by RH77

I would like a rough estimate using the modified spreadsheet, but I have 2 problems...

I'm sure most of the Kansas City folks around can attest:

1. The winds are RARELY calm, and 2
2. Finding a level road that's long enough, is rare

Does anyone else have the same problem?

Yep. I can definitely attest to having those problems.

My coastdown location goes around Humboldt bay, so it's exactly sealevel the whole way, but very windy.

As for any of you who are more inland, try finding a road through some river-bottom farmland. That's about as flat as you're likely to find.

[dremd]

How long of a road is needed (approximately)?

[Daox]

I'd say 1/4 to 1/2 mile stretch would be ideal since you want to be coasting for as long as possible at a high speed.

Has anyone else used Fabrio's modified sheet? I originally thought that temp (air density) would play a huge role and was wondering why it wasn't originally included in the spreadsheet. Where do we get that info? Anybody know any websites that publish something we can use for local areas?

[IWillTry]

Just wanted to point out that air density was included in the original spreadsheet as a constant that the user could change. Fabrio's modified sheet simply adds a calculation of the air density based on more measurable quantities (temp, pressure, humidity, etc). Your best bet is probably to measure these directly at the test site using a mini weather station rather than rely on information for your larger region.

[Old Tele man]

...be sure to alternate DIRECTIONS when doing each coast-down test, and do so as soon as possible, so that wind speed and wind direction haven't changed, as they can do at morning and late afternoon.

...the idea is to do opposite-direction tests so that what you gain/loose going oneway, you loose/gain going the other way, so that the 'atmospherics' and 'road-grade' changes will effectively "cancel" each other out to yield a better (less influenced) answer.

[NeilBlanchard]

Hi,

I've done a coast down test on my xA, and yes finding a flat straight road is the hard part. It does not have to be level, as long as it is a low slope -- it just can't have a dip or rise in it.

The other major piece of guesswork is the tire rolling resistance.

My best estimate based on 0.011 Crr (which is on the low end of "normal tires on smooth roads"), is 0.27Cd. (The stock rating is 0.31, though my car is a RS 2.0 that has a far more open grill than the standard xA.)

BTW, my average terminal speed after 70 seconds (starting at 70kph/43.5mph) is 27kph/16.7mph.

[Edit: At the time I did the coast down test I had the grill blocks, fog light covers, zigzag tape on A-pillars, top of the windshield, backs of mirrors, and trailing edge around the hatch, I had the rear wheel skirts, and the underpan under the hatch area, and the mini-Kamm panels.

Since doing the coast down test, I added the front wheel skirts, the video mirrors (which also reduces the frontal area), and I have lost the rear underpan. I hope to rerun the coast down test before I have to replace my tires...]

[Daox]

Great stuff Neil. Can you list your aeromods so we know what you've all done?

[NeilBlanchard]

Hi,

Thanks Doax! I added the items I had modded to the post above.

How can one measure the rolling resistance of their tires as they are inflated on their car?

[PA32R]

Originally Posted by NeilBlanchard

Hi,

Thanks Doax! I added the items I had modded to the post above.

How can one measure the rolling resistance of their tires as they are inflated on their car?

I don't see how to measure ONLY the rolling resistance, but you could measure the non-aerodynamic resistance just by using a force gauge to push or pull the car at VERY low speed (barely moving) in neutral. Aero drag would be negligible so if your car weighs, say, 2400 pounds and it takes 36 pounds to push it, your overall "mu" (Crr) would be 0.015. Obviously, this includes tires plus friction in everything behind the transmission. You'd want to get it to a steady (SLOW) speed so the kinetic energies (linear and rotational) are constant. Also, though a dimensional analysis shows that, to first order effects, tire rolling resistance depends only on down force (weight plus aerodynamic) and not obviously on speed, unfortunately aerodynamic down force depends on speed.

You can come reasonably close to the weight of the car with an accurate tire pressure gauge, two pieces of paper, and a machinist's scale. I think that's been covered in Ecomodder so, unless there's a demand, I won't waste time by repeating the method. The key here is the accuracy of the gauge - the typical pocket gauge may be off by a wide margin.