Rolling resistance of Tires?

Is an objective (empirical) measurement of rolling resistance available? All I know is the column in Consumer Reports assigning one of five icons,
ranging from all-black to all-red.
CR rated the Michelin MXV4 Plus Energy all-red (best), along with one Continental tire, and they recommended the MXV4+ for long tread life. Our new Prius was fitted with Goodyear Integrity OEM tires, not tested.
Any recommendations for a low rolling-resistance tire besides the MXV4+? I guess there's nothing wrong with the Goodyear Integrity, but it gets a much lower rating on Tirerack.com.
CR's top-rated tire in the Nov 2007 test was the Dunlap SP Sport 5000, only $85, but apparently not available in the correct 185/65-15 size for the non-Touring Prius.
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Here is one source of rolling resistance measurements: http://onlinepubs.trb.org/onlinepubs/sr/SR286Rolling_Resistance_Data.pdf
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Ray O
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Ray O <rokigawaATtristarassociatesDOTcom> wrote:

Thanks a lot! Lower is better, eh? Although sizes are slightly different, it looks like the Michelin MXV4 Plus Energy has lower rolling resistance than the Goodyear Integrity. And much longer tread life!
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You're welcome! Yes, lower rolling resistance is better, at least from a fuel economy perspective.
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Ray O <rokigawaATtristarassociatesDOTcom> wrote:

Good point. Based on Consumer Reports rankings, low rolling resistance is a tradeoff for quick dry-pavement braking. Snow and wet-road braking seem more related to tread design (I don't know for sure).
Do you have any idea why larger tires have lower rolling resistance? Small-radius tires have always been associated with economy cars, so I've always assumed they are more fuel efficient. Guess not.
The Prius Touring model might get better mileage than the regular model, because it has 16" tires, and the RRC (rolling resistance coefficient) is .00889 for the Goodyear Integrity 225/70R16, 0.00993 for the 215/70R15. That's 12% higher, although sizes are not as supplied on the Prius.
According to this, here are tires that in 2002 had RRC of .009 and less: http://www.trb.org/publications/sr/SR286Rolling_Resistance_Data.pdf
0.0062 Bridgestone B381 185/70R14 0.0078 Continental Ameri-G4S WS 235/75R15 0.0081 Goodyear Invicta GL 235/75R15 0.0083 Continental Contact CH95 205/55R16 0.0088 Uniroyal Tiger Paw AWP 185/70R14 0.0090 Michelin Energy MXV4+ 205/55R16
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interesting, what are the units of the rolling resistance coef?
Mark
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The publication defines rolling resistance coefficient (RRC) as "the value of rolling resistance force divided by wheel load".
They say that rolling resistance increases with load, so the RRC remains constant-- as load increases, so does rolling resistance. Thus no units are needed. Hope that helps.
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For the most part, everything about a tire's design is a compromise. Softer tread give better traction but poorer tread life. Deeper grooves provide better wet and snow traction but cause more heat buildup and tire noise. Skinnier tires provide better traction in wet conditions and cost less but poorer traction in dry conditions. Stiffer sidewalls provide better cornering but a rougher ride. A lower aspect ratio provides better handling but a rougher ride and less ability to withstand damage from potholes and curbs.

"Economy" cars tend to have smaller radius tires because they cost less. The effect of a wheel and tire's diameter is much the same as with gears. A small drive gear had an easier time turning a larger gear or tire than it does turning a smaller gear or tire but the drive gear has to rotate more times to travel the same distance.
Although I might know more about tires than the average person on the street, I do not consider myself an expert on tires, but I would imagine that 2 of the biggest factors that affect rolling resistance is sidewall stiffness and tread belt stiffness. If the sidewall and tread are soft and squirm or flex more, then more energy is used to overcome the flex and squirm than with a stiffer tire. If you think about the steel wheels on a train, there is little or no rolling resistance and they have a long life but they have poor traction and no cushioning. A train's traction is provided by the sheer weight of the train and the cushionins is provided by massive springs.
I think the reason that larger tires tend to have lower rolling resistance is that people are willing to spend more money on larger tires so the tire makes spend more money on R&D and materials and construction on larger tires than on smaller tires. I can't think of a physical reason why a larger diameter tire would have lower rolling resistance than a smaller tire with the same construction. I'm guessing that a wider tread width tire might have lower rolling resistance because the belts across the tread are stiffer than with a narrower tire, or because the additional traction provided by the wider tread means that the tire grabs with less sidewall flexing.

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"Ray O" <rokigawaATtristarassociatesDOTcom> wrote in message

I was thinking that a larger tire would deform less at the contact patch and that would probably be reflected in reduced rolling resistance.
The other tradeoff is that a larger tire will probably have more mass and the inertia of the tire itself will affect your fuel economy and brake life - but I'm not sure if that would be a measurable effect or not.

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Could be. But I just noticed that 2 of the top 6 (in 2002) were 14" tires.

It's possible that the Continental Contact CH95 is similar to the ContiPremiereContact tested by Consumer Reports, because that tire had among the lowest rolling resistance. However it was lower rated for noise and ice/snow braking than the Michelin Energy MXV4 Plus. Consumer Reports did not test other tires in the above list.
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Speaking of measurable effect, how big of a difference in mpg are we talking about here between the worst and best rolling resistance tire ? Did they measure that?
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Yes, the op. cit. paper discusses this. There is approximately a 2% improvement in mileage for each 10% reduction in RRC.
Wind resistance is a much larger factor in fuel economy, at 55 mph causing approximately 1/3 of total system drag, IIRC.
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