Crr ~= 0.0102, about right?

[bwilson4web]

Using the roll-down test spreadsheet and data gathered last summer, I was able to load up the spreadsheet for Cd and Crr. However, I already had the Cd, 0.29, from earlier Toyota sources. So all I had to seek was Crr and that came in about 0.0102. Is this pretty much what other folks are seeing?

The spreadsheet pretty much asks for about 60-70 seconds of data but I only had 40 seconds. As a rough approximation, it seems to fit the higher speed parts close enough.

Yes, I'll go back and gather some fresh data next weekend with longer roll-down intervals. Just I thought I'd ask if 0.0102 seems about right.

Thanks,
Bob Wilson

[Daox]

That sounds about right. From the figures I've seen Crr goes from ~.07 to ~.12. Unfortunately, I can't recall where I've seen those numbers.

[wyatt]

Wikipedia has numbers for Crr, and I have seen at least one guy's numbers from a coast down test on instructables. You can see how he did his test here, and use his spreadsheet if you like. According to co_driver, you can include Cv (coefficient of viscous [skin] drag) also, and just approximate it at 1 Newton per Meter/second. You can see his equation in this thread if you like. Like I said, you have to approximate the skin drag, since the solver function will blow the skin drag number way out of proportion.

[MetroMPG]

The author of that instructable estimated his '92 Metro with 145/r12 (I think) tires inflated to 45 psi: Crr = .0106