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Old 01-04-2009, 10:25 AM   #41
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Don't see how tire pressure changes tire circumference. Yes, more of the tire is on the ground and the height is lower, but the circumference of the tire doesn't change. The tires do not "stretch" when inflated. Lower air pressure means more tire is in contact with the road surface, creating more resistance, taking more energy to be moved, generating more heat.
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Old 01-04-2009, 10:57 AM   #42
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Don't see how tire pressure changes tire circumference. Yes, more of the tire is on the ground and the height is lower, but the circumference of the tire doesn't change. The tires do not "stretch" when inflated. Lower air pressure means more tire is in contact with the road surface, creating more resistance, taking more energy to be moved, generating more heat.
I don't feel like it, but an inquisitive character can always do empirical determination: Set tire pressure to 10 psi. Measure diameter. Increase pressure in 5 psi increments. Graph and publish results.

The contact area does definitely change with pressure. Most tire mfg. have photos of such area on their web site.
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Old 01-04-2009, 11:06 AM   #43
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I don't feel like it, but an inquisitive character can always do empirical determination: Set tire pressure to 10 psi. Measure diameter. Increase pressure in 5 psi increments. Graph and publish results.

The contact area does definitely change with pressure. Most tire mfg. have photos of such area on their web site.

Yeah, confused me at first too. The circumference is only directly related to the diameter of a CIRCLE. A flat or flatter tire isn’t completely round but the circumference is the same. It may be that a tire does stretch its circumference when it’s over inflated. I know that tires are said to “grow” at high speeds.
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Old 01-04-2009, 11:10 AM   #44
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Yeah, confused me at first too. The circumference is only directly related to the diameter of a CIRCLE. A flat or flatter tire isn’t completely round but the circumference is the same. It may be that a tire does stretch its circumference when it’s over inflated. I know that tires are said to “grow” at high speeds.
Ok. Jack up the wheel, so tire is not weight bearing.
Rinse, repeat pressure tests from 20 psi.
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Old 01-05-2009, 10:28 AM   #45
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Ok. Jack up the wheel, so tire is not weight bearing.
Rinse, repeat pressure tests from 20 psi.
Maybe we should transfer this post to the But what will I DO all day?!? "What did you do today?" thread...
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Old 01-05-2009, 10:33 AM   #46
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Thanks guys, and thanks, Honey. The tires are ordered and on their way.

My only regret is that I didn't think to name this thread:
Time to Retire
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Old 01-05-2009, 10:52 AM   #47
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Don't see how tire pressure changes tire circumference. Yes, more of the tire is on the ground and the height is lower, but the circumference of the tire doesn't change.
True, the circumference may not change, but the effective radius changes. For one revolution of the tire on the left, the car will go farther than for the tire on the right.

tires.jpg
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Old 01-05-2009, 01:08 PM   #48
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Ok. Jack up the wheel, so tire is not weight bearing.
Rinse, repeat pressure tests from 20 psi.
See T-Al's excellent diagram above. The weight-bearing part of it is essential to the discussion. I doubt you will see much difference with no weight on it.

Since we drive our cars with the wheels on the ground, that is the only measurement that matters anyhow. Measuring center of hub to ground at the low and high pressures will give the delta in radius. Pi * R* R (edit - correction) 2*Pi*R will give the effective circumference using the hub to ground radius. I'm not motivated enough to do it though.

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Old 01-05-2009, 01:20 PM   #49
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See T-Al's excellent diagram above. The weight-bearing part of it is essential to the discussion. I doubt you will see much difference with no weight on it.

Since we drive our cars with the wheels on the ground, that is the only measurement that matters anyhow. Measuring center of hub to ground at the low and high pressures will give the delta in radius. Pi * R* R will give the effective circumference using the hub to ground radius. I'm not motivated enough to do it though.

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I am not sure that this is relevant. Unless we find a way to break traction, the ciircumference is what matters, not the radius. The formula given above is incorrect even for an abstract perfect circle. However, to model a tire in use on an automobile we need to use circumference as physically measured around the tire. We can visualize it accurately as a bulldozer track. Absent traction breaks, it seems clear that an inch of tire must traverse an inch of pavement.

ha
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Old 01-05-2009, 02:04 PM   #50
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See T-Al's excellent diagram above. The weight-bearing part of it is essential to the discussion. I doubt you will see much difference with no weight on it.

Since we drive our cars with the wheels on the ground, that is the only measurement that matters anyhow. Measuring center of hub to ground at the low and high pressures will give the delta in radius. Pi * R* R will give the effective circumference using the hub to ground radius. I'm not motivated enough to do it though.

-ERD50
Isn't the circumference 2 * pi * R? Or are we talking about different measurements? Pi R^2 is area.
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Old 01-05-2009, 02:34 PM   #51
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Isn't the circumference 2 * pi * R? Or are we talking about different measurements? Pi R^2 is area.
If it's a circle.

Take the extreme case. You let almost all the air out of the tire so that you are almost riding on the rim. Much of the tire will be flopping around like Bob Dole on an island without a pharmacy.

Traction is broken when the hub gets closer to the ground.
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Old 01-05-2009, 02:36 PM   #52
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If it's a circle.

Take the extreme case. You let almost all the air out of the tire so that you are almost riding on the rim. Much of the tire will be flopping around like Bob Dole on an island without a pharmacy.

Traction is broken when the hub gets closer to the ground.
Now, that is an extreme case!

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Old 01-05-2009, 02:46 PM   #53
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Isn't the circumference 2 * pi * R? Or are we talking about different measurements? Pi R^2 is area.
Thanks, I edited my original. Yes, we want circumference. Darn it, I *knew* the difference, went through both in my mind, but when it came to typing it I still did the wrong one. I'm getting old


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If it's a circle.

Take the extreme case. You let almost all the air out of the tire so that you are almost riding on the rim. Much of the tire will be flopping around like Bob Dole on an island without a pharmacy.

Traction is broken when the hub gets closer to the ground.
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Now, that is an extreme case!

ha
Yes, it's extreme, but it demonstrates the point. The effective radius is really the point from the hub to the ground, and that should be used to determine the effective rolling circumference. Or, to eliminate other variables in how that tire might move around - mark it and the pavement with a chalk line, push the car one revolution, and measure the distance.

I'll get right on it .

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Old 01-05-2009, 02:57 PM   #54
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I think SteveL and HaHa are right--we care about circumference, not radius/diameter. To test this, you could put a chalk mark on the tire, inflate to 45 PSI and measure the ground distance covered by exactly one revolution (chalk mark at 6:00 to chalk mark at 6:00). Then deflate to 20 PSI and do it again. I'm fairly sure the length of the lines on the ground will be virtually the same regardless of tire pressure (those steel belts aren't getting any longer and shorter with tire pressure, I'll bet). If this is correct, the speedometer/odometer accuarcy won't vary due to tire pressure.

I think higher tire pressures increase gas mileage by reducing the amount of flex and distortion of the tire as it rolls. This flexing causes internal friction within the tire rubber (just as a paper clip heats up as you rapidly and repeatedly bend it at the same spot) and also increased "scuffing" of the tire rubber against the pavement as it squashes out to the sides.
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Old 01-05-2009, 03:41 PM   #55
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Or, to eliminate other variables in how that tire might move around - mark it and the pavement with a chalk line, push the car one revolution, and measure the distance.
This I buy.

Ha
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Old 01-05-2009, 03:45 PM   #56
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I think higher tire pressures increase gas mileage by reducing the amount of flex and distortion of the tire as it rolls. This flexing causes internal friction within the tire rubber (just as a paper clip heats up as you rapidly and repeatedly bend it at the same spot) and also increased "scuffing" of the tire rubber against the pavement as it squashes out to the sides.
Absolutely. Another effect is that due to the larger road footprint of a correctly inflated vs. an overinflated tire, there is more friction. Thus more rolling resitance (possibly bad) and more stopping and cornering adhesion (definitely good).

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Old 01-05-2009, 09:29 PM   #57
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This discussion is getting way over the top, so I hesitate to add to the discussion, but the circumference will in fact grow with more pressure. Many cars have low tire pressure systems that compare revs per mile from the wheel speed sensors so they can "tell" when a tire is low on pressure without measuring the pressure directly. The wheel speed sensors were not put there for tire pressure monitoring, they are part of the anti-lock brake system. See the indirect TPMS discussion in the link below

Tire-pressure monitoring system - Wikipedia, the free encyclopedia
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Old 01-05-2009, 10:04 PM   #58
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... but the circumference will in fact grow with more pressure.
Quite possibly will, but no one denied that. We only said that the circumferance must mbe measured, not calculated from the "radius" - which in fact is not a true radius since a tire going down the road is a distorted cirlcle, not a perfect circle.

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Old 01-05-2009, 10:15 PM   #59
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Ha, here is the comment I referred to (and I think there are others)....I knew I shudda kept my big mouth shut.

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Don't see how tire pressure changes tire circumference. Yes, more of the tire is on the ground and the height is lower, but the circumference of the tire doesn't change. The tires do not "stretch" when inflated. Lower air pressure means more tire is in contact with the road surface, creating more resistance, taking more energy to be moved, generating more heat.
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Old 01-05-2009, 10:32 PM   #60
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Ha, here is the comment I referred to (and I think there are others)....I knew I shudda kept my big mouth shut.
I see. Do you have data about this stretch? It seems that a steel belted tire inflated to normal recommended pressures shouldn't stretch much, say within the range of approved pressures.

I admit I have zero experience measuring tire circumferance, and I have definitely seen light racing tires on a bicycle stretch, but I never considered if or how much an auto tire might stretch at normal pressures.

Ha
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