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Discussion Starter #1
This is a heated debate on some other forums, but I'd like to see what you guys come up with....

Does the effective diameter/radius of a tire and thus it's revolutions per mile change with respect to tire pressure. For instance, if I air down to 10psi and affect a change in the tire shape, do I effectively have a smaller tire in gearing and distance calculations?
 

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ljschnel said:
Yes, but very minimally so.
How do you figure?

When you air down to 10psi the actual circumference of the tire (the tread) doesn't change.

The only way airing down would make the circumference change is if the act of airing up the tire stretches the tire. If you've ever seen a cut up tire, especially an MT, you'll likely believe that 30psi does not measurably stretch the tire.
 

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Discussion Starter #4
360° of angular displacement will equate to a linear displacement equal to the circumference of the tire. Reducing the air pressure will change the shape of the tire, but not the circumference. IMO, airing down will not cause mileage errors, assuming that you maintain a flat contact patch. As long as you are rolling on the tread (tread face of the tire is in full contact with the ground), then each revolution of the wheel will move the vehicle the distance equal to the circumference of the tire.
 

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flyingwil said:
360° of angular displacement will equate to a linear displacement equal to the circumference of the tire. Reducing the air pressure will change the shape of the tire, but not the circumference. IMO, airing down will not cause mileage errors, assuming that you maintain a flat contact patch. As long as you are rolling on the tread (tread face of the tire is in full contact with the ground), then each revolution of the wheel will move the vehicle the distance equal to the circumference of the tire.

~zing~ ...an easy way to tell is to mark your tire with a sharpie. ..then drive it one complete revolution at 30 psi and mark the distance traveled.

next, air down to 5 psi and drive one complete revolution and mark distance travled.
 

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flyingwil said:
This is a heated debate on some other forums, but I'd like to see what you guys come up with....

Does the effective diameter/radius of a tire and thus it's revolutions per mile change with respect to tire pressure. For instance, if I air down to 10psi and affect a change in the tire shape, do I effectively have a smaller tire in gearing and distance calculations?
Though I haven't verified it myself, my understanding is that if you run an automatic locker in the rear, you need to make sure that not only are the rear tires the same diameter, but that they are inflated to the same pressure to keep the locker from acting strangely. The latter "requirement" would evidently be due to an operational circumference difference between the two tires if they are not inflated the same. Perhaps someone with real-world experience with this can weigh in.
 

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I would say that an VERY underinflated tire will actually have to rotate faster to keep the same mph as a properly inflated tire. I believe this because if you take the radius of the tire (from center of hub straight down to the ground) when its properly inflated it is more than when it is VERY underflated. This means that even though your circumference of your tire tread doesn't change the actual circumference that is touching the ground is smaller.

clear as mud?
 

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I think it would be how much you aired down, how fast the tire is spinning and what the construction of the tire was like. If you go slow enough that the centripetal force and tire construction of the spinning tire does not off set the change in radius due to airing down, yes.
May folks already know that airing a tire up to higher pressures increases gas mileage but doing so wears the tires out faster so there really is not economic advantage to do so. Especially when things like safety and ride quality are also factored in.
 

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It will cause a difference in RPM; just air down one rear tire and drive it with a locker. The locker will raise hell because one wheel is traveling farther than the other.
 

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Rebuilt98 said:
Use the little geometry trick that "most" of us learned in 2nd or 3rd grade and do the pi*D or 2pir trick and see how much it has change.
I guess in your 2nd grade they forgot to tell you that those formulas only work for circles. Even at 40psi your tire will not be round where it contacts the ground.

If you're going to come around calling people stupid you should at least be smart.
 

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sjvicker said:
I would say that an VERY underinflated tire will actually have to rotate faster to keep the same mph as a properly inflated tire. I believe this because if you take the radius of the tire (from center of hub straight down to the ground) when its properly inflated it is more than when it is VERY underflated. This means that even though your circumference of your tire tread doesn't change the actual circumference that is touching the ground is smaller.

clear as mud?
Bingo!
 

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For the purposes of this discussion, no, the tire is not round. When considering your argument for 2*Pi*r the tire is absolutely not round. Let's do the math:

Take for example my 35" tire. Assume for discussion that I have inflated my tire to 40psi and it is a nearly perfect approximation of round. The circumference of the tire is, as we know from 2nd grade, 35 * Pi. This is approximately 110"

Now let's take my 35" tire and air it down to 10psi. Based on experience the tire is probably now around 31" tall when resting on the ground. 31 * Pi is approximately 97". Did the tire actually shrink along its circumference from 110" to 97"?

Tour assertion above is that "look, the diameter drops so of course the circumference went away." You are effectively arguing that the tire stretches along its circumference by 13" when it is aired up from 10psi.

The tire is not round, especially when aired down.

The only way to win this argument is to show that the tire does or does not stretch along its circumference when it is aired up or down. Considering that the amount of stretch we could expect here is probably on the order of 1/1000 of an inch this is probably next to impossible for one of us to prove empirically. Further, neither of us have the experience to throw out plausible theories.

However, while we're talking about thousandths of an inch consider what happens with temperature change. With a simple change in temperature from say 50 degrees to 90 degrees the tire will indeed expand some small amount, again probably on the order of thousands of an inch.

In the end its important to note that no one gives a fuck about thousands of an inch: we're not building the damn space shuttle.
 

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Rebuilt98 said:
It's traveling in a generally circular path...no?
Not when the tire is in contact with the ground. An aired down tire especially. On a 35" tire the contact patch is what, 8" long? The point on the tire is traveling in a straighter line for those 8" and as such the point you have chosen on the tire does not travel in a circle.
 

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Of course it changes.. .look at your speedo vs a gps... my speedo reads much differntly at 10psi that my normal 24...

Even still think about rolling diameter vs diameter. My 35 BFG M/T's are 34.7 tall, but only have a rolling diameter of 33.7", at propper pressure at 45mph...

When you let air out of a tire you effectivally change the rolling radius, changing the circumferance...while its not a huge amount the tread still does buckle and change shape... not a huge amount, but enough to change the speedo reading, and rev/mile...

Arent there some tire pressure moniter systems that compare rev's of each tire? I remember reading about that, but could be wrong....
 

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the tire that is underinflated will spin faster imo.
this is how i see it ok, lets say you have a 35 in tire, the distance and the bottom of the rim is lets say, 10 when inflated to 40psi. when at 25 psi, it is at 8 inches lets say acting like a 31 inch tire(8+8+15=31) because the top portion on the tire toes not play a role until moving. which will make that axle shaft spin faster to keep up with that 35 inch tire.also like bigbadbob said earlier 13 inch circumference difference between the two tires. no where did that go? same tire? well alot of it goes to your contact patch. which probably doubled it resulting in drag and more friction. less mpg
 

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Look at it this way, say you have a 33" tire on a 15" rim. If you take it to extremes and take all the air out of the tires and load the truck down with so much weight that the tires will flatten completely out. I predict the wheel will spin like the tire is 15" high or in effect like the tire is completely gone.
 
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