Tire pressure

[sanglant]

heh, anybody think these would be a good upgrade from a subaru. summer car, winter car. both from a 2001-2005, somehow it seems like they might be more fun to drive. 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. and is still running good.

[drewtam]

Then what holds the car up?

The sidewalls hold the car up.

The tire pressure forces the carcass to maintain correct shape.

[birdman]

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.

[Brad Johnson]

And don't forget the nitrogen fills because it's like, you know, better than regular old air.

Brad

[zxcvbob]

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.  

[sanglant]

or it could just be the n2 not expanding as much and not spreading the rubber thin enough to fit through.

[seeker_two]

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?.... 

[sanglant]

hydrogen for the win!!!

[Regolith]

hydrogen for the win!!!

Gives a whole new meaning to the term "blowout"...

[MillCreek]

I am thinking uranium hexafluoride gas, in case you need to stop and do some roadside fission.

[zxcvbob]

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?

[roo_ster]

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?

[birdman]

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.

[Tallpine]

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 

[wmenorr67]

Don't forget to change from summer air to winter air before long.

[zxcvbob]

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.

[Hawkmoon]

Don't forget to change from summer air to winter air before long.

But where do I look for winter-grade nitrogen?

[geronimotwo]

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.

[sanglant]

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. 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.

[Brad Johnson]

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

[geronimotwo]

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. 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.

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.

[sanglant]

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.

[Boomhauer]

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.

[birdman]

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)

[drewtam]

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.