Armed Polite Society
Main Forums => The Roundtable => Topic started by: Hutch on September 05, 2011, 01:53:35 PM
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Sure seems like this would be straightforward, but noooooooooo.....
I have asked 2 different authoritative sorts, and got 2different answers.
Should I inflate the tires on my car to the max pressure shown on the sidewall, or to the pressure shown on the medallion on the door frame of the car? Why?
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Are the tires the original brand and model # that came with the car when new? If so, then use the pressure listed on the door plate. If not, then inflate to the maximum pressure shown on the tire, or a bit less. Most car tires are inflated to 32 to 35 pounds (you didn't say if it was a car or truck tire). The maximum pressure will give a harder ride than the slightly lower pressure.
Adjust pressure when the tires are cold, as the pressure increases as the tires heat up while driving. I hate it when I go to a quick change oil place and one of the "technicians" lets air out of my tires because he says the pressure is too high. It's not too high, it's normal pressure after having been driven for more than 10 miles.
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Sure seems like this would be straightforward, but noooooooooo.....
I have asked 2 different authoritative sorts, and got 2different answers.
Should I inflate the tires on my car to the max pressure shown on the sidewall, or to the pressure shown on the medallion on the door frame of the car? Why?
I read something authoritative a while back that said to go by the medallion. But I forget the reason behind it. I'm thinking it's because the rating on the tire represents the max that the tire should be inflated to under any condition, but the figure on the medallion is the inflation for the condition of that particular vehicle's weight and handling characteristics.
DD
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This is car and not truck? On my F250, the door panel calls for 60F/75R. If I do that when there's no load in the back, it's bounce city, so I keep everything at 60. On my trooper, the door panel is 35F/30R and I use that with good results.
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2007 Camry. Bought used ~ 45kmi. Tire says max load at 50 psi, but medallion reads in the 30's, iirc. That is a mighty wide spread.
Thanks, keep 'em coming.
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The car maker's tire pressure recommendation is made from long-term testing and is the best compromise of ride quality, economy, and wear characteristics. The pressure on the tire sidewall is the tire manufacturer's maximum "do not exceed" pressure and is there strictly for safety reasons.
For economy reasons I typically run my tires at about 10% over the mfg recommendation. Any more, especially on a light-in-the-behind truck, and traction becomes an issue, especially in the wet.
Brad
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Follow the medallion. As stated before, the sidewall number is the maximum allowable pressure...and since temperatures can vary pressure substantially, if you filled to that pressure cold...and had some moisture in the tire from periodic fills with humid warm air, and venting when cold out...you could easily blow the tire the next day if it was hot. Best thing to do is to fill to medallion, and if you really want to push it, fill to medallion, then drive it hard, on a hot day, under heavy load, and check the pressure hot...if it's still substantially below the sidewall...you could go up a bit if you want to get better mileage.
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The pressure on the door is the minimum you should use (assumes original size tire.) The number on the tire sidewall is the maximum. Pick a number you like that's in between. Also, when you get your oil changed it's a good idea to tell them not to mess with your tire pressures, otherwise some will let the air out to get it down to what the door plaque says -- even if you have a totally different size tire.
Don't go over 65# if you have rubber valve stems.
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As stated before, the sidewall number is the maximum allowable pressure
It is the maximum pressure for the load range. The bead seating pressure is the maximum allowable before the sidewall goes. The trick is that tire pressure is dynamic. You measure it at 60F at 1200ft with 25% humidity and it's *this*. Drive all your friends down to the beach at 90F and run over a speedbump too fast and it will be entirely different. If you stay on top of it, that might work. If you inflate it to the 44psi on the sidewall, you may find it dangerously close to blowing out when conditions change. The manufacturer has (with the notable exception of Ford) generally done the math (and hopefully testing) to give you a number that will not allow you to cross the minimum/maximum under normal conditions.
when you get your oil changed it's a good idea to tell them not to mess with your tire pressures
I always asked customers how far they had driven to get to me and only really made an effort to inflate an obviously low tire. If it was cool out and they had just driven around the block to get to me, I'd set it to the manufacturer's recommendation. After more than maybe 5 miles, or if it was a hot afternoon, you can't tell. On a warm afternoon, I could tell a customer what direction their car had been facing while parked by the tire pressures. You could have one side sitting at the specified 32# and the other side at 38# if they had parked it right.
I check mine in the morning while still in the garage, and I'll usually overinflate by less than 10%, but I do exceed the sticker pressure on most vehicles. Except Ford SUVs (which we called "flippers").
I did have a few come in with 12#. I don't know how a tire stays on the rim at that pressure, but they did. Also, it's all different with low-profile tires. They'll pick up the same heat from the road surface, but far less solar gain. They also have a lower volume, so their pressure drops faster.
If you really cared all that much, you'd run the tire over a wet paint stripe at varying pressures to find the largest contact patch that stayed on the tread. That's probably going to be the "best" inflation level. The thing is that there is no magic number. Overinflated tires have lower resistance so you'll improve your mileage. You'll be riding on a thin strip of rubber, so you'll lose handling and tread life. At current prices, halving your tread life might actually be worth it, but do that in the rain and you yourself might live half as long.
ETA: You will need a MAJOR change in tire size (like putting gigantic mud tires on your Jeep) to change the optimum pressure by more than the margin of error on your gauge. If you change the load range of the tire, then you'll throw the pressure to something completely different.
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the max, is the max cold pressure. tires heating up is taken into account in setting them. ;)
now then, there are two ways to find the right pressures for your car and tires. first up we have the math. you take the wet weight of the car, front and back. and the max tire pressure, and max weight and figure out what percentage of the max you need. :laugh:
next up, you fill 'em to the door sticker. no that ain't it. >:D find a place you can drive 10-20 feet or so and stop, 10 to 20 times. chalk the thread from the inside sidewall to the outside, at least one front tire and one back tire. better to do both sides. if the center rubs off faster, pressure is to high. the edges rub off faster, pressure is to low. ;)
oh, and running higher pressures lowers the temps your tires are reaching. and gives you better mileage, but can make the car slide around more. on the other hand you'll be hydroplaning less. [popcorn]
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This discussion has been fun, but let's take it up a notch.
Is there a difference if the mechanic checks the tire pressure when the car is on the lift, with no weight on the tire, versus checking the tire pressure when the car is sitting on the tire? What's the difference? Which is the correct way?
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There is a difference, but your average tire gauge won't detect it. Lowering the car onto its wheels deflects the tire a bit, but not enough to noticeably decrease volume.
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There is a difference, but your average tire gauge won't detect it. Lowering the car onto its wheels deflects the tire a bit, but not enough to noticeably decrease volume.
Then what holds the car up?
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Some years ago when Ford was having problems with Firestone tires on Explorers, the medallion pressure was, IIRC, 26 PSI. The temporary "fix" was to increase pressure to 30 PSI, which is where I had been keeping it all along. Sidewall pressure was 35 PSI.
Never exceed the sidewall pressure!
I normally keep my tires a couple of PSI above the medallion, which theoretically reduces ride comfort but improves mileage. Pressure is still well below the sidewall max. Keep an eye on the wear pattern - if your tread starts showing less depth down the middle, you're probably overinflating.
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When measuring a cold tire, I like to stay a few pounds under the sidewall pressure (35psi = 30-32psi for me)....gives it plenty of room for heat expansion....
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Keep an eye on the wear pattern - if your tread starts showing less depth down the middle, you're probably overinflating.
With the new low-profile tires, the opposite can also be the case. The wide, narrow-sidewall tires need pressure to hold shape, and underinflating can cause the center to wear faster as the tire is deformed by centrifugal force (so I've been told).
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If your vehicle is AWD stick to the medallion and stick to OEM tire size, religiously.
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when you get your oil changed it's a good idea to tell them not to mess with your tire pressures with anything except the oil and filter
FIFY.
I go a few PSI under the max pressure on the tire sidewall- this has never caused a problem.
I yelled at the quickielube monkey once who was letting the air out of my tires to bring them down to 33 PSI when the pressure was at 38 and the sidewalls said 41.
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List of other greivances:
-They mashed the crap out of a K&N filter on the truck when my wife took it in for oil change.
-poured oil into the window washer tank on one of my vehicles.
-Lost an air cleaner nut (vehicle had 2 of them) and didn't tell me about it.
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If your vehicle is AWD stick to the medallion and stick to OEM tire size, religiously.
better buy back the same tires to then, and good luck with that, here's a link to the tires that came on my car (http://www.tirerack.com/tires/tires.jsp?tireMake=Bridgestone&tireModel=Potenza+RE92&partnum=06HR5RE92OWL&vehicleSearch=true&fromCompare1=yes&autoMake=Subaru&autoYear=2000&autoModel=Outback%20Sport&autoModClar=). there's no way in hell i'm paying over four hundred bucks for tires that roll onto the sidewalls at the specced pressures (http://www.tirerack.com/tires/tires.jsp?tireMake=Bridgestone&tireModel=Potenza+RE92&partnum=06HR5RE92OWL&vehicleSearch=true&fromCompare1=yes&autoMake=Subaru&autoYear=2000&autoModel=Outback%20Sport&autoModClar=). it's really easy to find out the right pressures. being lazy is just going to cost you. might even be your life, or worse. having to live with killing some kid. =|
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better buy back the same tires to then, and good luck with that, here's a link to the tires that came on my car (http://www.tirerack.com/tires/tires.jsp?tireMake=Bridgestone&tireModel=Potenza+RE92&partnum=06HR5RE92OWL&vehicleSearch=true&fromCompare1=yes&autoMake=Subaru&autoYear=2000&autoModel=Outback%20Sport&autoModClar=). there's no way in hell i'm paying over four hundred bucks for tires that roll onto the sidewalls at the specced pressures (http://www.tirerack.com/tires/tires.jsp?tireMake=Bridgestone&tireModel=Potenza+RE92&partnum=06HR5RE92OWL&vehicleSearch=true&fromCompare1=yes&autoMake=Subaru&autoYear=2000&autoModel=Outback%20Sport&autoModClar=). it's really easy to find out the right pressures. being lazy is just going to cost you. might even be your life, or worse. having to live with killing some kid. =|
I would probably switch those to 195/65/15's. You can get H-rated tires with A A temperature and traction ratings for about $65 to $75 each.
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205/60R15 isn't really a problem size, i'm running much better/cheaper tires (http://us.coopertire.com/Tires/Passenger/CS4-Touring-H-V.aspx). =) that was just what it came with. and tehy were ok for learning the cars limits* but that was done within 3000 miles. >:D
*they would start screaming like banshees under 15mph in moderate curves and you could push well past that point before they started slipping but they also rolled onto the side walls before they started sliping. [tinfoil] looked almost like bike tires when i was done with 'em. >:D
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The mfg maximum on the sidewall takes into account potential pressure increases due to heating. That's why the rating is given along with a temp (i.e. 44 psi at 70 deg F).
Brad
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a few of us have said that already. ;) have we missed any more ways to say it? :laugh:
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Don't forget you have change them from winter to summer air and vice versa when the seasons change. >:D
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If your vehicle is AWD stick to the medallion and stick to OEM tire size, religiously.
Hail Subaru, Mother of All Wheel Drives =D
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heh, anybody think these would be a good upgrade from a subaru. summer car (http://en.wikipedia.org/wiki/Mercedes-Benz_SLK), >:D winter car (http://en.wikipedia.org/wiki/Mercedes-Benz_C-Class#W203_.282000.E2.80.932007.29). both from a 2001-2005, somehow it seems like they might be more fun to drive. >:D just have to get to a point i can part with a car that's pretty much been a part of me for ~11 years. :laugh: and is still running good. [popcorn]
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Then what holds the car up?
The sidewalls hold the car up.
The tire pressure forces the carcass to maintain correct shape.
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The sidewalls hold the car up.
The tire pressure forces the carcass to maintain correct shape.
Kinda.
The sidewalls and bead PULL the wheel upward. Basically, the weight of the car pushes the rim downward, which pulls down on the top of the tire (think of the bead and sidewall as a rope pulling upward on the bottom of the rim). The ground pushing up on the bottom of the tire results in the top of the tire trying to expand outward (due to the internal pressure), which is restrained by the sidewalls and bead pulling downward. The tire is acting in tension which is distributed by the internal pressure. sidewalls have little compressive strength.
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And don't forget the nitrogen fills because it's like, you know, better than regular old air.
Brad
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And don't forget the nitrogen fills because it's like, you know, better than regular old air.
Brad
I have a theory about that; ya want to hear it? (of course you do) The nitrogen-filled tires hold their pressure longer because oxygen and argon are constantly leaking *in* thru the slightly-permeable rubber sidewalls faster than the nitrogen leaks out. ;)
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or it could just be the n2 not expanding as much and not spreading the rubber thin enough to fit through. :laugh: :angel:
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And don't forget the nitrogen fills because it's like, you know, better than regular old air.
If nitrogen is better, would filling your tire with neon or argon be more better?.... =|
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hydrogen for the win!!! >:D
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hydrogen for the win!!! >:D
Gives a whole new meaning to the term "blowout"...
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I am thinking uranium hexafluoride gas, in case you need to stop and do some roadside fission.
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I am thinking uranium hexafluoride gas, in case you need to stop and do some roadside fission.
That's not a gas except at relatively high temperatures. If you're going for density, how about radon or sulfur hexafluoride?
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I am thinking uranium hexafluoride gas, in case you need to stop and do some roadside fission.
What lure do you prefer, or are you a live bait man?
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If nitrogen is better, would filling your tire with neon or argon be more better?.... =|
Not really. There are several factors at work here. The goal is to reduce pressure changes (constant volume assumed) with temperature changes AND to reduce leakage.
Nitrogen leaks just as easily as air, but it effectively eliminates any water vapor content--which is the largest contributor to pressure changes with a conventional air fill. (leakage IN of air constituents is effectively zero due to the pressure difference). While dry air could be used, it's difficult to dry the air sufficiently, and just more cost effective to use nitrogen.
Leakage is driven mainly by molecular size, and nitrogen (or oxygen) are larger than neon or argon, so noble gas fills are bad from that perspective (though SF6 would be great!)
In terms of pressure changes with temperature, since it's non-adiabatic, all the gases are similar, once you have eliminated any condensible parts (water vapor), so that's a wash.
So in terms of ideal gas fillers, we have nitrogen (advantage low cost and dry) and sf6 (higher cost, but very little leakage), so now it comes down to weight (ignoring cost for now). A 235/50-17 tire has an internal volume of about 47 liters. If pressurized to 30psi (gauge), the gas fill (relative to the air around it, so correcting for buoyancy) would weigh ~110g for nitrogen and close to 820g for sf6-- an unsprung weight increase of 1.5 pounds per tire! For larger tires (285/35-19) the difference is even greater, ~120g vs 950gr...and for high sidewall formula tires, even more.
So there is your result, when cost is a factor, nitrogen gives the best performance as it's totally dry and leaks relatively slowly, and if cost is no option, still nitrogen, as it weighs less than the more leak-proof gasses.
The ideal fill would likely be methane, as its molecular size isn't much smaller than nitrogen, it's unreactive with rubber and metals, doesn't condense, and is lighter than nitrogen (saving 50+g of unsprung weight). Of course, it's flammable, and the methane in a tire if it blew out and ignited would be the same as a pound or more of TNT...so that's probable not good.
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Of course, it's flammable, and the methane in a tire if it blew out and ignited would be the same as a pound or more of TNT...so that's probable not good.
But it is at time hilarious ;)
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Don't forget to change from summer air to winter air before long. [popcorn]
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You can always fill the tires with water for added traction. (add antifreeze or calcium chloride in cold climates) It's amazing how much a tiny farm tractor tire (6.70-15 full lug traction tire) can weigh when it's full of water. Might not be a good idea with a tubeless tire...
"Grab that tire for me out of the back of the truck" Hilarity ensues.
Leakage is driven mainly by molecular size
Don't forget about partial pressures. That's why I said oxygen and argon would leak into a nitrogen-filled tire.
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Don't forget to change from summer air to winter air before long. [popcorn]
But where do I look for winter-grade nitrogen?
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FIFY.
I go a few PSI under the max pressure on the tire sidewall- this has never caused a problem.
I yelled at the quickielube monkey once who was letting the air out of my tires to bring them down to 33 PSI when the pressure was at 38 and the sidewalls said 41.
when running consistently over the vehicle recomended pressure, don't you find that the center of your tread wears out first?
it came to me one day while filling my high psi bike tires that the weight of the vehicle will be nearly exactly the total amount of tire tread on the road (in inches squared) multiplied by psi of the tires.
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You can always fill the tires with water for added traction. (add antifreeze or calcium chloride in cold climates) It's amazing how much a tiny farm tractor tire (6.70-15 full lug traction tire) can weigh when it's full of water. Might not be a good idea with a tubeless tire...
"Grab that tire for me out of the back of the truck" Hilarity ensues.
Don't forget about partial pressures. That's why I said oxygen and argon would leak into a nitrogen-filled tire.
how? the water doesn't compress, so you might gain a little on making the suspension work more, but the added weight will kill handling and ride quality. [tinfoil] i.e. the lighter the unsprung weight the faster the wheels can move to get back into contact with the road, better ride, better traction, better stopping, better takeoff. ;)
and geronimotwo, that's the point. the sticker is just for the supplied tires. as soon as you change tires the numbers could be anything. unless you buy tires the exact same size. [hard to do, each maker measures them differently] and with exactly the same load handling numbers [hard to do, each maker measures it differently] and exactly the same max pressure. then you add the weight of passengers, and any gear. [popcorn]
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Not really. There are several factors at work here. The goal is to reduce pressure changes (constant volume assumed) with temperature changes AND to reduce leakage.
Nitrogen leaks just as easily as air, but it effectively eliminates any water vapor content--which is the largest contributor to pressure changes with a conventional air fill. (leakage IN of air constituents is effectively zero due to the pressure difference). While dry air could be used, it's difficult to dry the air sufficiently, and just more cost effective to use nitrogen.
Leakage is driven mainly by molecular size, and nitrogen (or oxygen) are larger than neon or argon, so noble gas fills are bad from that perspective (though SF6 would be great!)
In terms of pressure changes with temperature, since it's non-adiabatic, all the gases are similar, once you have eliminated any condensible parts (water vapor), so that's a wash.
So in terms of ideal gas fillers, we have nitrogen (advantage low cost and dry) and sf6 (higher cost, but very little leakage), so now it comes down to weight (ignoring cost for now). A 235/50-17 tire has an internal volume of about 47 liters. If pressurized to 30psi (gauge), the gas fill (relative to the air around it, so correcting for buoyancy) would weigh ~110g for nitrogen and close to 820g for sf6-- an unsprung weight increase of 1.5 pounds per tire! For larger tires (285/35-19) the difference is even greater, ~120g vs 950gr...and for high sidewall formula tires, even more.
So there is your result, when cost is a factor, nitrogen gives the best performance as it's totally dry and leaks relatively slowly, and if cost is no option, still nitrogen, as it weighs less than the more leak-proof gasses.
The ideal fill would likely be methane, as its molecular size isn't much smaller than nitrogen, it's unreactive with rubber and metals, doesn't condense, and is lighter than nitrogen (saving 50+g of unsprung weight). Of course, it's flammable, and the methane in a tire if it blew out and ignited would be the same as a pound or more of TNT...so that's probable not good.
Dude, lighten up a little. Look up the phrase "tongue in cheek"... ;)
Brad
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how? the water doesn't compress, so you might gain a little on making the suspension work more, but the added weight will kill handling and ride quality. [tinfoil] i.e. the lighter the unsprung weight the faster the wheels can move to get back into contact with the road, better ride, better traction, better stopping, better takeoff. ;)
and geronimotwo, that's the point. the sticker is just for the supplied tires. as soon as you change tires the numbers could be anything. unless you buy tires the exact same size. [hard to do, each maker measures them differently] and with exactly the same load handling numbers [hard to do, each maker measures it differently] and exactly the same max pressure. then you add the weight of passengers, and any gear. [popcorn]
this is why it is good to monitor tread wear. if you see the tread wearing on the inside the tires likely are overinflated. the opposite can be said if the tread is wearing on the outside, assuming they're not mounted on the front of a ford pickup. ;)
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my chalk method(ok i just can't remember where i picked it up) will tell you before you've ruined a set of tires. ;)
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I read something authoritative a while back that said to go by the medallion. But I forget the reason behind it. I'm thinking it's because the rating on the tire represents the max that the tire should be inflated to under any condition, but the figure on the medallion is the inflation for the condition of that particular vehicle's weight and handling characteristics.
DD
The tires on my truck are listed as 44 max psi on the sidewalls IIRC. The door lists 35psi as the pressure they should be inflated to, and thus I keep them at 35psi.
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Dude, lighten up a little. Look up the phrase "tongue in cheek"... ;)
Brad
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)
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Kinda.
The sidewalls and bead PULL the wheel upward. Basically, the weight of the car pushes the rim downward, which pulls down on the top of the tire (think of the bead and sidewall as a rope pulling upward on the bottom of the rim). The ground pushing up on the bottom of the tire results in the top of the tire trying to expand outward (due to the internal pressure), which is restrained by the sidewalls and bead pulling downward. The tire is acting in tension which is distributed by the internal pressure. sidewalls have little compressive strength.
You might want to double check your FBD on that one.
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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.
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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.
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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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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 :)
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But it would glow...with the color and intensity of 30,000 HID headlights :)
So....that would be a "yes"....
(https://armedpolitesociety.com/proxy.php?request=http%3A%2F%2Fwww.rectorcom.com%2Fwp-content%2Fthemes%2Ftylium%2Finc%2Ftimthumb.php%3Fsrc%3Dhttp%3A%2F%2Fwww.rectorcom.com%2Fwp-content%2Fuploads%2F2010%2F07%2F1T-shirt-Chumlee-Awesome-copy.png%26amp%3Bh%3D150%26amp%3Bw%3D240%26amp%3Bzc%3D1&hash=a3c4c9a6267732a6bd1e70493e92ac0f5d2e44b5)
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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.
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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.
(https://armedpolitesociety.com/proxy.php?request=http%3A%2F%2Fimage.motortrend.com%2Ff%2F31572925%2Bw786%2Bar1%2F2011-chevrolet-silverado-HD-wheel-cutaway.jpg&hash=0750c267d1565a47df6f373890c85059420bf4ab)
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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.
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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.