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That would have to mean the steel belts in the tire shrink when the pressure is lowered and the bulge increased.
Where did I say that?
"in order for the tire circumference to change, the belts would need to change in length."
Relatively displacing the belts from their static unloaded position as part of the whole tyre by means of tyre deformation under load will not affect tyre radius at the contact point? If so, why is the rolling radius at the contact patch different to the tyres static unloaded radius?
So here it is: the reason that the roll out on a tire varies from the calculated circumference is that the tire does not flex at the surface of the tire. It flexes at the belts below the surface under the tread. The tread blocks are squeezed together as they go through the contact patch. If it were even possible and you inflated the the tire to the point that the contact patch was a single point, the roll out would increase because then the tread and rubber outside the belts would be affecting roll out. Between vehicle sticker pressure and sidewall max pressure, the belts remain the same length so the roll out remains unchanged. Roll out is also changed little to none as the tire wears.It just so happens that the amount a typical tire is compressed at the bottom is similar to the distance the belts are from the ground. Therefore using the center of the wheel to the ground as the radius in calculating circumference may often give a result similar to the roll out. It is just a fudged number though and often will not give that good of an approximation. Roll out is the only true way to measure a tire.
You didn't. But in order for the tire circumference to change, the belts would need to change in length.
I know that this is a bit of an extreme example, but 4 Wheel and Offroad magazine did an article on "rolling radius" on a trail tire. I believe it was a 38" tire or there about, I'll have to find the mag to be sure. I know that they aired the tires up to the sidewall spec, marked the tire, and drove a full revolution of the tire. They then did it at "trail pressure" and the difference in length was feet, not inches. They re-tested multiple times to confirm it, because the difference was so large, and it was the same every time. The steel belts will not stretch per say when properly inflated, but they will cup in at the contact patch when under inflated. That doesn't mean the tires "grow" when properly inflated, but will shrink somewhat when under inflated. Also, I don't know how much of an issue weather is where everyone else is, but the seasons here have the temperature vary a lot. Last week it was 42* C with the humidity (32* without) and tonight there is a frost warning. Air pressure varies a lot with that much temp change.
I mentioned the cupping earlier in the thread. At low pressure, the sidewalls also drag on the ground hurting mileage. I am referring mainly between door jamb sticker pressure and max sidewall pressure on the tire.
The sidewalls won't "drag". More rubber on the road causing more rolling resistance, yes.
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Topic: Tire Roll out (Read 9745 times)
