Author Topic: Tire pressure  (Read 13832 times)

birdman

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Re: Tire pressure
« Reply #50 on: September 07, 2011, 08:22:25 PM »
You might want to double check your FBD on that one.


FBD?

Well, prove me wrong.  Find me an FEA analysis or a stress measurement that shows compressive stress or strain on the sidewalls of a tire between the contact patch and the rim (in the carcass, not the tread blocks) and I'll believe you and admit I was wrong.  Until then, I'll simply re-read my vehicle dynamics books, And retake all 8 years of engineering. 

And no, thats not tongue in cheek.  It is one of the most common (and very wrong) misconceptions that the tire pushes upwards on the rim, rather than pulling it up.

geronimotwo

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Re: Tire pressure
« Reply #51 on: September 07, 2011, 08:31:06 PM »
That was lightened up :).  You got me thinking, so I shared my thoughts, I was wondering myself why they use nitrogen even when cost is no option (e.g. F1)

in the aviation world nitrogen is used as it is DRY, and will not cause corrosion.

« Last Edit: September 07, 2011, 08:40:46 PM by geronimotwo »
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seeker_two

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Re: Tire pressure
« Reply #52 on: September 07, 2011, 08:45:17 PM »
You got me thinking, so I shared my thoughts, I was wondering myself why they use nitrogen even when cost is no option (e.g. F1)

It was a good thought....but I thought xenon & krypton would work since it's larger atomically than nitrogen....


....learn something new everyday....


....how about radon?....it's atomically larger than nitrogen....and it'll really make your rims shine.....  =D
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birdman

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Re: Tire pressure
« Reply #53 on: September 07, 2011, 08:57:28 PM »
It was a good thought....but I thought xenon & krypton would work since it's larger atomically than nitrogen....


....learn something new everyday....


....how about radon?....it's atomically larger than nitrogen....and it'll really make your rims shine.....  =D

Well, it's 50% heavier than sf6, and the 3.8 day half-life would mean you would have to clean out a kilogram of highly radioactive solid polonium-218 every few day (so your pressure would drop rapidly)

You would have to find some way to dissipate the 10-15 megawatts of decay heat per tire (equillibrium temperature of pure 30psi radon contained in something is about 5000 degK or hotter...

Oh, and it would take several times the entire radon quantity on earth to fill a tire...if you could even get it in one place.   Oh, and it would cost several hundred billion dollars

But it would glow...with the color and intensity of 30,000 HID headlights :)

seeker_two

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Tuco

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Re: Tire pressure
« Reply #55 on: September 07, 2011, 10:20:21 PM »
If your vehicle is AWD stick to the medallion and stick to OEM tire size, religiously.

Support this arbitrary blanket statement with fact or personal experience please.
« Last Edit: September 07, 2011, 11:04:17 PM by Tuco »
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drewtam

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Re: Tire pressure
« Reply #56 on: September 07, 2011, 10:25:17 PM »
As an engineer, I thought you would have caught the reference... FBD = Free Body Diagram.

The bead around the rim is not welded, bonded, or mechanically interlocked.
The internal stresses cannot put a tension load on such a joint.

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birdman

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Re: Tire pressure
« Reply #57 on: September 08, 2011, 07:41:13 AM »
Ah.  I see what you are saying, but I didn't say it pulled up on the top of the rim, it pulls up (like a sling) on the bottom of the rim, so the tensile loading of the bead and sidewall apply a compressive stress to the bottom of the rim.  Apologies for not including that part, I had thought I put that in.

birdman

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Re: Tire pressure
« Reply #58 on: September 08, 2011, 07:45:10 AM »
Caveat: run flat tires do have stiffer sidewalls that become compressive load bearing when the tire loses air pressure, in that case, the loading is compressive and pushes upward on the rim.  But even those are highly limited in loading--further evidence that in tires, the tensile stress is the dominant load path.  Also, when inflated, the hoop stress on the tire is greater than any residual compressive stresses applied by the sidewall between the rim and ground, so under inflated conditions, the carcass is under purely tensile loading.

Other run-flat (foam filled or inner ring) tires also operate in a compression loading mode when not inflated.