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Old 10-26-2009, 11:07 PM   #10 (permalink)
Ernie Rogers
Ernie Rogers
 
Join Date: Feb 2008
Location: Pleasant Grove, Utah
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Here's air density versus temperature

Hello, Neil,

I'm sorry about leaving you hanging on the measurement (versus guessing) of rolling resistance--you can measure it with the same roll-down test you use for air drag, just a little different. Alternatively, if you can find a slope where the car just barely rolls out of gear at low speed, then the rolling resistance coefficient is equal to the tangent of the angle, which is also the road grade. (A grade of 1% means a rolling resistance coefficient of 0.01.)

Rolling resistance of tires increases as the rubber gets cold.

Now, about air density--

At sea level and 59 degrees F, the density is 1.225 kilograms per cubic meter. For another temperature (T), the density changes in this way:

density = 1.225 x (492 + 59) / (492 + T deg F)

For example, at 100 deg F, the density is--

density = 1.225 x (492 + 59) / (492 + 100) = 1.14 kg /cu.m.

If you are far from sea level, then you need to put in the right number for your elevation instead of 1.225.

Or, cheat and call the airport weather station. They have the correct density number for the airplanes to use.

Ernie Rogers



Quote:
Originally Posted by NeilBlanchard View Post
Hi,

I did a second coast down test today, and I'm having problems with the spreadsheet from the Instructables web page: the air temperature today is a lot different than when I did the first coast down. The only value for this is by altitude, but shouldn't there be one for temperature?

Also, my velocities at the end of the test were considerably lower than last time -- either my tires are very much worse (they are probably better, since they are nearly worn out) or I have a drag in a rear brake (a bearing had to be replaced) -- you can sometimes hear it dragging.

Another part of my problem was that I could not do the test as long as the 70 seconds on 4 out of the 6 coast down tests.

This method is pretty flawed, IMO -- you have to guess at the tire rolling resistance, and the air density bugs me, too.
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