Victor

Hi. I'll be driving to Seattle next month. We usually take the wife's TDi due to its amazing gas mileage but I talked her into taking the 4Runner so I can do some wheeling while I'm up there.

I live at 3958 feet altitude and wonder how much air I should expect to put in the tires as I drop in altitude. I probably won't air up until So Cal or Central Cal.

supascout

For what it is worth...

On a "standard day" you can expect to loose 1"Hg per 1000 feet! I dont know what that equaits to in PSI! I use to live in Gunnison CO at +/- 8000 and would frequently drive home to Dallas without ever adding any air! As I recall I lost about 5 pss though. the drop from 40 psi wasent eneouf to bother with for a week long trip though!

be safe

_travis

Victor

An Hg is a mm of mercury - we use that standard of measure in blood pressures. I have no clue what kind of conversion that is though.

Last summer we lost about 5 psi in my wife's tires so I was wondering if I should add about 2 psi on the beginning of my trip to make sure my mileage stays good. I've averaged 22 MPG before on trips so I'm hoping I can do about the same on this trip. We had noticed that her mileage was a little lower than normal especially in central Cal and again in close to Portland.

midiwall

You can multiply Inches of mercury (in Hg) by 0.4912 to get PSI. So, if you figure you're dropping about 3500', then that's 3.5 in Hg or 1.7psi.

Reference: http://xtronics.com/reference/convert.htm

Victor

You rock! I'm beginning to doubt that 2 psi is worth worrying about but that is one less thing to check when I get to Seattle.

midiwall

Yeah... it's not a lot to worry about. Just set yourself up right as you leave El Paso and you'll be fine.

Have a safe trip! *TAKE PICTURES*!


Mark

gwhayduke

According to USA today, the barometric pressure for Seattle is 30.46 inches of Mercury (Hg) versus El Paso's 29.85.

If you convert those pressures to PSI, Seattle has 14.96 PSI versus EP's 14.66 PSI. An affect of 2% on your V&P

The temperature will also affect the V & P of your tires:

Seattle: 60F or 289K
El Paso: 96F or 309K

Which will also affect your V&P by another 6%.

Nice little problem...if only you knew the volume of your tires in El Paso or the moles of gas you put into them.

Victor

You bet I will. I think the big change in tire pressure was also due to the temp difference. We have 100' weather right now so couple that with 30-40 degrees in temp change and the altitude change and you get a big tire pressure change.

Victor

Joel you really know how to floor me!

gwhayduke

This just an Ideal gas problem: PV=nRT where P=pressure, V=volume, n=number of moles, R=Ideal gas constant and T=temperature in Kelvin.

These are fun little problems.

FattyCBR

This thread cracks me up, I never would have even considered that my tire pressure would change with a loss of elevation. What I'm really curious about is why is the pressure able to change since the tire is theoretically a sealed system? Isn't a good analogy if you take a sealed coke bottle under say 20psi from ground level to 30k feet in a plane isn't it still at 20 psi?

midiwall

Nope, because the air pressure in the plane's cabin is artificially made constant by on-board systems. If it wasn't, all the passengers would need to be breathing oxygen above appx 12,000'.

The air pressure in the tire changes because the atmospheric pressure outside of the tire changes.

Victor

Information I can use from my flight physiology courses. As altitude increases the distance between air molecules increase. The distance between these molecules causes a baloon to increase in size. Therefore as you decrease altitude the baloon will appear to deflate due to the molecules coming closer together. Same thing happens inside your tires.

PutzRunner

I have learned so much since joining this forum.

Formulas and math are not my forte. So instead of figuring out how much I will lose, I just take the tire gauge out of the runner and measure the tire pressure. If it is low I add air, if it is too high I let air out.

I know this sounds simple, but that is the way it works.....



j/k

P.S. The technical stuff is interesting.

Victor

Having flown at 42000 feet let me tell you that no aircraft's pressurized cabing is 100% efficient. You can actually see soda cans swell a small amount if the pilot isn't dilligent in watching the cabin pressure. Oh yeah - and then you fly over Mexico where there is no radar and the pilot dives to give you zero gravity. It was all a great physics lesson.

FattyCBR

So you guys are saying it's not at 20 psi anymore even though the container is sealed?

gwhayduke

The Coke analogy also doesn't work because the CO2 is more soluble in cold liquids and high pressures. Which is why nobody wants to open a hot coke.

Example...
Two cokes at identical temperatures from identical batches at the plant...

#1 at sea level and #2 at 10,000 feet.

Which one spews the most when opened?

The one at 10,000 feet because there is less atmospheric pressure forcing the CO2 into solution.

Now back to the tire....
The tire, like the coke can has multiple forces acting on it. The pressure from outside the tire (atmospheric pressure and the weight of the vehicle on the contact patch) and the counterbalancing air pressure inside the tire.

Change any one of the forces and the others must change as well.

Example...
Take a balloon and put it in a bell jar, lower the pressure in the bell jar and the volume of the balloon will increase until the forces exerted on the inside walls of the balloon match the forces being exerted by the elasticity of the balloon rubber and atmospheric pressure.

A real neat demo of this is to take a large garbage bag and have a kid step into it, then gather the bag around their neck, keeping their head out of the bag of course. Now stick a vacuum cleaner hose into the bag and turn it on. You can really feel the 14.6 PSI of atmospheric pressure squeezing you.

Victor

The pressure inside the object doesn't increase. Joel is absolutely correct. I in my idiocy am equateing the pressure inside the bottle when it is actually the pressure outside that changes. Damn science teachers....

As far as the tire goes as you go to a lower altitude there is more pressure pushing on the outside of the tire and then your tire pressure drops inside. So we topped off our tires last summer and drove back to El Paso. When I checked my tire pressures the next morning after we arrived they were all 6 PSI over - less pressure pushing outside the tire and of course the heat here were both factors.

The more dense area (air) is why engines perform better at sea level.