Steering Wheel NOT Straight - Page 3 - Audi Forum - Audi Forums for the A4, S4, TT, A3, A6 and more!

08-19-2010, 05:59 PM

#21

mikeh

Audi Forum - Posts like an HORCH

Join Date: Dec 2009

Location: Burlington, Ontario

Posts: 2,719

Re: Steering Wheel NOT Straight

Quote:

Originally Posted by hoeser

Respectfully gentlemen, you are both incorrect. The pressure inside the tire will increase the higher you go above sea level. The volume of air inside the tire will not change as you increase in altitude (aside from leakage, the seal isn't 100% perfect). However with that being said, most gauges read in pressure above atmospheric at sea level (around 14.7psi). For example, an "empty" or "flat" tire at sea level still has air in it, but it's the same as the surrounding air pressure, the gauge will then read 0, but the tire actually has 14.7psi of pressure inside. If you were to add 1psi to the tire, it would have 15.7psi in it but the gauge would read 1.

As you increase in altitude, the air thins, and the ambient pressure is therefore less. At 10,000ft its around 10psi... so a tire that reads 30psi at sea level will then read about 34.7 at 10,000ft. (4.7psi less ambient pressure = 4.7psi higher tire pressure). In a complete vacuum the tire would read 44.7psi as there would be no pressure on the outside of the tire at all.

The end result is, the higher you go, the harder the tire becomes as the air inside wants to escape - the less pressure pushing on the outside means more pressure pushing on the inside. If you put 55 or 60psi in a tire at sea level, and threw that same tire in a vacuum, it would likely explode (assuming its max rating was around 50psi).

Also, I have too much time on my hands.

Hey there "Too much time on your hands" you are correct and very accurate. This is one of those questions I like to ask to get people thinking.

here is a synopsis of what you said above.

Significant changes in altitude affects tire pressures when traveling from one elevation to another. Fortunately this influence is relatively small and can be easily accommodated.

Atmospheric pressure is the force exerted on objects by the weight of the air molecules above them. While air molecules are invisible, they have mass and occupy space.

However as altitude increases, atmospheric pressure decreases. For example, atmospheric pressure pushes against the earth at 14.7 pounds per square inch (1 kilogram per square centimeter) at sea level, yet drops to only 10.1 pounds per square inch at 10,000 feet as indicated in the following chart.

Altitude (ft.) Air Pressure (psi)
Sea Level 14.7
1,000 14.2
2,000 13.7
3,000 13.2
4,000 12.7
5,000 12.2
6,000 11.7
7,000 11.3
8,000 10.9
9,000 10.5
10,000 10.1
When it comes to measuring tire inflation pressure, it is important to realize there is a difference between atmospheric pressure and gauge pressure. Most pressure gauges (including all tire pressure gauges) are designed to measure the amount of pressure above the ambient atmospheric pressure.

Imagine removing the core from a tire valve and allowing the air to escape. Even after the air has completely stopped rushing out of the valve, the tire is still experiencing 14.7 pounds per square inch of atmospheric pressure. However, a tire pressure gauge would read zero pounds per square inch of tire inflation pressure because the pressure outside the tire is equal to the pressure inside.

Since a tire mounted on a wheel essentially establishes a flexible airtight (at least in the short term) pressure chamber in which the tire is shaped and reinforced by internal cords, it retains the same volume of air molecules regardless of its elevation above sea level. However, if tire inflation were set with a tire pressure gauge at sea level (where the atmospheric pressure of 14.7 pounds per square inch is used as ambient atmospheric pressure by the gauge), the same tire pressure gauge would indicate the pressure has increased at higher elevations where the ambient atmospheric pressure is lower. Those measured at the 5,000-foot level (where an atmospheric pressure of only 12.2 pounds per square inch is the ambient pressure) would indicate about 2-3 psi higher than at sea level. On the other hand, traveling from a high altitude location to sea level would result in an apparent loss of pressure of about 2-3 psi.

However, the differences indicated above assume that the tire pressures are measured at the same ambient temperatures. Since tire pressures change about 1 psi for every 10-degree Fahrenheit change in ambient temperature, the tire pressure measured in the relatively moderate climate typically experienced at sea level will change when exposed to the colder temperatures associated with higher elevations.

This means that in many cases differences in ambient temperature may come close to offsetting the differences due to the change in altitude. Depending on the length of their stay at different altitudes, drivers may want to simply set their cold tire pressures the morning after arriving at their destination, as well as reset them the morning after they return home.

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