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Old 04-11-2009, 09:06 AM   #15 (permalink)
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Blue Bomber pulled the following quote out of Ernie's post, then disagreed with it:

Originally Posted by Ernie Rogers View Post

Now, about stiffness. Remember, I said the weight of the car is held up entirely by the air pressure in the tire? Having stiffer rubber in the tire doesn’t have any practical effect in holding up the car. But, the deflection energy is proportional to stiffness. And a part of that energy is not returned when the rubber rebounds. The effect on mileage is plain to see—tires made of stiff rubber have high Crr, and result in lower miles per gallon.

I disagree with Ernie about what is holding the car up, but I think he is correct that rubber stiffness plays little effect on holding up the car.

But I think he is wrong about the effect rubber stiffness has on RR.

1) The steel belts and the polyester ply cords (or whatever they are using) are much more stiff than the surrounding rubber can ever be. Those would play a much larger role in the overall tire stiffness than the rubber itself.

2) Inflation pressure stiffens a tire much more than the tire itself. If you've ever sat on an uninflated tire, you'll know it is easy to deflect the tire. Sit on an inflated tire and there's hardly any movement at all.

That's a double hit for rubber stiffness!

Unfortunately, rolling resistance is a function of something other than stiffness. As I've said before, there is a technology triangle: RR / Traction / Treadwear. And a linear parameter like "stiffness" doesn't quite apply - and I think tread rubber with good hysteretic properties tend to be soft (I am going to check on this, but I am under the impression that good wearing tread rubber tends to be "hard".)
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