TT, I believe that the tank tread analogy was used to illustrate to me the effect of the “length” of the tread on the rotational speed of the wheel. It did the trick. The “light came on” when I realized that the tread of the flat tire doesn’t travel in a circle. It travels an erratic path, just as a fanbelt does. For a given and constant speed of a fanbelt the pulleys will each travel at a different rotational speed.
In short, the length of the path that the tread of the tire travels is what determines the rotational speed of the wheel, rather than the “rolling radius”.
Now that we all agree, what the heck do we agree?? Does a low tire have a shorter circumference or not. All this mathematical yack yack has confused me.
Next tidbit. Steel belts - which are actually layers of parallel steel wire embedded in rubber and set at an angle. There are usually a pair so there’s one angled each direction (which in theory cancels out the directional effect - but in reality doesn’t quite do that)
That means that the steel belt CAN expand with additional pressure. The process is called “pantographing”. Imagine a square mesh of parallel wires where the points of overlap are pinned together. If opposing corners are pulled (to make the diagonal longer), then the width shrinks. That’s pantographing.
So not only is our OP wrong, he hasn’t accurately described the way a tire works. If he had, he would have realized his error.
Yes, Elly, that’s exactly what we agree on. The main change, the original disagreement, is that we now agree that the total langth of the tread will not shrink below that of the unexpanded tire body. And that using the distance from the axis to the tarmac is an improper way to determine the speed at which thw wheel will turn.
All explanations here and elsewhere on the www sound like Abbot and Costello scripts. I may, for my own satisfaction, mark the front and rear tire on my truck and deflate one of them until the rim is 3 inches closer to the pavement and driving ahead 10 feet to compare the difference in the location of the marks.
If I may suggest, the front and rear tires will give different results, as load on the tire may also a factor in the rolling circumference - so be sure to take that into account.
Also, you may want to use inflation pressure rather than the distance from the ground. I would discourage anyone from using less than 50% of the placard pressure for this kind of test.
If you over inflate a tire it wears in the middle of the tread, if you under inflate it it wears on both edges of the tread. This is because the air pressure changes the shape of the tire. More pressure pushes the center of the tire away from the rim increasing the diameter.
Grossly under inflating the tire greatly decreases the effective diameter due to the decreased distance between the rim and the road and the tire has to go through a lot of wrinkling to not slip on the rim and the friction gets the tire so hot it either blows out or catches on fire.
Oldtimer, your second paragraph perfectly describes my original argument. After pondering the input of others, I realized that it is the length of the tread body that determines the speed at which the wheel rotates, not the distance from the axis to the ground. There’s no question that a great amount of heat can form as the tread squirms on the pavement and the sidewalls flex and bend to accomodate the constantly changing distance from the rim to the ground and its effect on the shape of the tire, but the speed of the wheel is determined by the tread. The wheel does not attempt to pull the tire carcass at a different speed than dictated by the tread “length”. The tread “length” is the “master”, the wheel is the “slave”.
While I agree 100% with your first paragraph, I’ve come to consider your second paragraph (which I originally agreed with) as being a misinterprettaion of how the wheel and tire are actually functioning at very low presssure.
It would seem that the logical conclusion is the correct conclusion. Reducing the distance from the center of the wheel to the ground by partially deflating the tire will result in a shorter rolling circumference. With a 2 1/2 inch reduction from center to ground there was an 8 inch reduction in linear travel through 3 revolutions of the wheel.
Correction, that was a 1 1/2 inch reduction in the loaded radius. It was dropped from 13 1/2 to 12 inches. I don’t seem to have the technical skills to edit posts.
Thiis is my opinion also. And I am 99% sure of it!!
Rod Knox3:10PMReport It would seem that the logical conclusion is the correct conclusion. Reducing the distance from the center of the wheel to the ground by partially deflating the tire will result in a shorter rolling circumference. With a 2 1/2 inch reduction from center to ground there was an 8 inch reduction in linear travel through 3 revolutions of the wheel.
Rod Knox3:13PMReport Correction, that was a 1 1/2 inch reduction in the loaded radius. It was dropped from 13 1/2 to 12 inches
First - so we are clear here - you can NOT determine either the rolling circunference, nor the change in rolling circumference by measuring the distance from the ground to the axle.
Rod: I’m not sure what you are saying - did you get 8" difference for 3 revoltuions - that is 8/3 = 2.67" per revolution? And was the difference in static load radius 1.5".
If so the math doesn’t work for pi* D.
Oldtimer said: “If you over inflate a tire it wears in the middle of the tread, if you under inflate it it wears on both edges of the tread. This is because the air pressure changes the shape of the tire. More pressure pushes the center of the tire away from the rim increasing the diameter.”
Be careful here, because while that is true, the affect inflation pressure has on how even tire wear is small compared to other things. It is quite possible to mis-diagnose uneven wear as a pressure issue - and vice versa.
However, the “loaded radius” is not what determines the wheel speed. The perimeter length of the tread body does. It helps to recognize that a given spot on the tread of a very low tire does not travel in a circle, therefore the radius is an inappropriate variable to use to calculate the wheel’s speed. A given spot of the tread will travel a flat line for a distance and then an arc length. The total of the flat line (call it a cord if you’d like) plus the arc length is what determines the wheel speed, not the “loaded radius”.
