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  #46  
Old 04-06-2003, 12:28 AM
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How about looking at this way: if you were to remove one cleat from a bulldozer's track it would obviously cover less ground per revolution of the drive sprocket. Or would it? Actually the drive wheel's radius remains the same and it covers the same amount of ground, whether there are 50 cleats or 100.

Heheheheh....
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  #47  
Old 04-06-2003, 05:55 AM
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All right...that does it!

Someone bring an air compressor to Reading so we can do the test I suggested with Rob! LOL...the only way I will accept that the same steel belting that makes up the circumference of an inflated tire slips, shrinks or vanishes when a tire looses air is if we get together and perform this test. I'll gladly use my car as a guine(?) pig. Like Lee, my mind works in a "classroom environment" mode. I'm not saying that Porter is wrong but until I see it actually happen in the real world I won't be able to put aside my doubts. Maybe there is some 4th dimensional factor occuring that I am not aware of.
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Ebony Mica '92 LS 80K Oct 2002 - Dec 2004: Victim of theft. She served me well.

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  #48  
Old 04-06-2003, 07:46 AM
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We could all save a lot of gas money if we put taller tires on the rear, thereby allowing the car to run downhill most of the time.
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  #49  
Old 04-06-2003, 07:53 AM
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Talking I gotta quit doing this

BTW - Why don't we have to jack a tire up off of the ground in order to correctly adjust the air pressure? When the weight is on it the bottom of the tire flattens - doesn't that raise the pressure?
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  #50  
Old 04-06-2003, 07:55 AM
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Re: All right...that does it!

Quote:
Originally posted by CigarJohnny
Someone bring an air compressor to Reading so we can do the test I suggested with Rob! LOL...the only way I will accept that the same steel belting that makes up the circumference of an inflated tire slips, shrinks or vanishes when a tire looses air is if we get together and perform this test. I'll gladly use my car as a guine(?) pig. Like Lee, my mind works in a "classroom environment" mode. I'm not saying that Porter is wrong but until I see it actually happen in the real world I won't be able to put aside my doubts. Maybe there is some 4th dimensional factor occuring that I am not aware of.

LOL! The 4th dimension is there man, just waiting to suck you in!

OOOH... I just had a thought... I'll be right back with a pic.
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  #51  
Old 04-06-2003, 08:11 AM
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Check it out... this illustrates it better.
Attached Images
File Type: gif sidewall.gif (15.4 KB, 191 views)
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  #52  
Old 04-06-2003, 08:50 AM
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Re: I gotta quit doing this

Quote:
Originally posted by Beav
BTW - Why don't we have to jack a tire up off of the ground in order to correctly adjust the air pressure? When the weight is on it the bottom of the tire flattens - doesn't that raise the pressure?
damnit beavis, cut that out
We don't because the manufacture tells us what the recommended presure is taking into account the weight of the car - which will change the pressure in a tire when applied.

Oh yeah - so how do you check the spare?

And for the record and to put this one to rest - the pressure will change (drop) if you jack up the car off the ground.
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  #53  
Old 04-06-2003, 09:17 AM
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OK, so I did the tire growing by pressure thing (as Lee continues to beat on this dead horse).

My right front tire grew by approximately 7/16" in circumference via changing pressure from 29 to 33 PSI.

test methodology: let pressure down to 29, measured around tire. Added air from compressor to 33 and measured around again. No control or measurement of air temp inside tire, but both the tire and compressor were sitting overnight in my garage(about 74F).

Tire particulars: Fulda Extremo, 225-50X16, Z-rated.

This amounts to 0.5% growth in circumference.

BTW, Assuming all else remained the same, a tread depth difference of 1/4 inch on these tires (front to back) would work out to be almost 1.6 inches difference in circumference (or about 2%).

Maybe if I get re-energized (doubtful, but...) I'll try this with say a range of 15 to 33 PSI and see how much it changes.

For now I have to do some heavy thinking on Beav's latest thoughts. Beav, wouldn't the position of the valve stem matter, i.e., it would balance out if the stem was at the bottom where the sidewall was flexed - wouldn't it? Oh, and assuming I forever after jack up my car to check tire pressure, I would think the stem needs to be in the same position on each tire when I measure - you know like altitude adjustments???
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  #54  
Old 04-06-2003, 09:22 AM
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For the record, 225/45/17 falken azenis - 42psi with car on ground measured with digital tire pressure gauge in 60 degree garage air -41.5 psi with car jacked up. Repeated test and results repeated again.




...I just had to know
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  #55  
Old 04-06-2003, 09:36 AM
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Quote:
Originally posted by immortal_suby
For the record, 225/45/17 falken azenis - 42psi with car on ground measured with digital tire pressure gauge in 60 degree garage air -41.5 psi with car jacked up. Repeated test and results repeated again.




...I just had to know
Just to confirm - I got about a third PSI difference at 33PSI. Same reason
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  #56  
Old 04-06-2003, 09:39 AM
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You guys crack me up... It was good seeing you last night, Matt! And your lovely wife too. Mike was convinced that she thought we were nuts or something. I was like, no way man.. she has one too!
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  #57  
Old 04-06-2003, 10:15 AM
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Quote:
Originally posted by Porter
Check it out... this illustrates it better.
It shows me what a low tire looks like from the cross section but I already knew what that looks like What that does not explain is what happens to all the belt and rubber that goes around the tire. Lee's experiment explains the subtle expansion that you are trying to explain better than your graphics will, however. Apparently there is some stretch in the belts that can occur with a 4 PSI difference between identical tires. Still, for your theory to work, you would have to decrease the radius of the tire in all degrees at once, not just at BDC. For the total circumference of the tire to drop like you say in an example of a tire with a 28" diameter that is running 2" low in air, which would mean a drop in radius from 14" from center of hub to the ground to 12" would mean that the circumference would drop from an inflated 87.92 inches to a deflated 75.36 inches. How do you account for the disappearance of 8.5 inches of steel belts?

Lowered air pressure only means a longer & wider contact patch and less effort for the engine to rotate the tire (leverage advantage od shorter radius). It does not mean that the circumference of the tire decreases. You will still have 87.9 inches (C = 2 x 3.14 x R(14")) of rubber to be travelled in one revolution of the wheel. If that shortened radius you keep basing it on was the only factor then you might be correct but you still have not explained what happens to the other 8.5" of tire circumference. Yes the tire is squashed where it touches the ground but tire still has the same circumference. That is the mystery that you are not explaining.

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Ebony Mica '92 LS 80K Oct 2002 - Dec 2004: Victim of theft. She served me well.

You can tell the lack of craftsmanship by the wrinkles in the duct tape.
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  #58  
Old 04-06-2003, 10:17 AM
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You do realise that for this discussion to properly occur, we will need to be sitting in a bar with some good tunes in the background while we nurse some beers and munch some peanuts.
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Pearl '92 LS-L 179K (Historic 1st 5-speed SVX)
Mods: 5-speed, 4.11's, Group-N motor mounts, dual Magnaflows, cone air filter, Kenwood MP-228 CD/Receiver, white-faced gauges, '97 grill, custom window tinting.

Ebony Mica '92 LS 80K Oct 2002 - Dec 2004: Victim of theft. She served me well.

You can tell the lack of craftsmanship by the wrinkles in the duct tape.
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  #59  
Old 04-06-2003, 10:45 AM
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Yeah, but we'd have to keep our voices down in order to not attract the guys with the big nets and funny looking, long sleeved jackets.
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  #60  
Old 04-06-2003, 10:57 AM
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Quote:
Originally posted by CigarJohnny
It shows me what a low tire looks like from the cross section ...snip.....

That is the mystery that you are not explaining.
I think Trevor came closest to explaining the situation. Or at least it may have turned on a bulb in my head. Trevor, if you're still reading this drivel, let me know. Here goes my explanation....

Your concern (mine too originally) would be valid if all wheels were in locked hubs (that's the part I forgot - they aren't - I need a 'duh' slap). The critical difference for this explanation is that they are allowed to differ in the number of rotations between side to side & front to back.

The tire rotates based on the radius at mid-patch contact. To go all the way around, it still has to use all the belt, but it doesn't have to turn around at the speed that the other tires do. So if one tire is at full pressure, it will only rotate at say 90% of the rotational speed of the half-flat tire because it has a larger effective radius at the contact point. Hence, that's why the differential gets taxed by differing pressures/sizes.

Porter/Trevor, either I'm getting closer, or one of you is gonna have to come shoot me and put me out of this misery
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