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05-23-2009, 01:06 AM   #10 (permalink)
Ernie Rogers
Ernie Rogers

Join Date: Feb 2008
Location: Pleasant Grove, Utah
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Hello, DC,

Yes, my idea was to find a quick and dirty way to get a number for Crr. I know it's not a great idea, just thought I might entice someone to work through the details on it.

So, the tire would have to be mounted and pressurized by a presently undefined procedure, and bounced by itself, not on a car.

It wouldn't be hard to simulate the same deflection as seen when rolling on a car, but I haven't bothered to do the math.

I saw a rule of thumb here somewhere, and failed to write it down (my memory says this, I could be hallucinating). The rule-- "a one psi increase in tire pressure reduces rolling resistance by ___" Did somebody say something like that?

Otherwise, I would propose this approximate rule--

"Tire rolling resistance varies inversely with tire pressure." It should be more valid toward higher pressures.

Now, the question was asked, could we detect the change in rolling resistance, say from a 1 psi, or maybe 0.1 psi, pressure change, in terms of the effect on fuel economy?

I'll plug that into a fuel economy program, say for a Prius, and see--

Tires at 40 psi, Crr = 0.009...........49.15 mpg at 60 mph
Tires at 41 psi, Crr = 0.00878........49.59 mpg at 60 mph.

The difference is 0.44 mpg.

In my experience, this 0.9% change is virtually undetectable in the real world where wind cannot be eliminated from tests. (But I could detect a 2 psi change.) A 0.1 psi pressure difference would be truely insignificant.

Could the difference of-- .009 /.00878 = 1.025-- 2.5% in Crr
be detected with a wheel tire tester? Maybe just barely. Any thoughts from the real world experts here?

Ernie Rogers

Quote:
 Originally Posted by dcb I'm a little lost at the application here. Are we just looking at the crr of the tire? Are we hoping to be able to go into a tire store and locate the best tires by bouncing them? Are we looking to measure the crr of tires on a vehicle? (and what about the rest of the drivetrain?) It is an interesting exercise to bounce a tire, have to get it square on every bounce and growth, temperature, etc. But it does seem more practical to just see how much force it takes to move a vehicle that has just been driven around to warm everything up. Is there an intended audience of the tire bounce test? Sorry if I'm not getting it, just not sure what problem it solves.