# Thread: Altitude effects on Tire Pressure

1. Originally Posted by pffog
Not sure what the contact patch area actually is, and it varies with the tire, wear etc, but if we say it equates to an approximate 2.5" circle on each tire, that is about 10 square inches of surface area, put bike and rider weighing 800 lbs on it and you have 80 psi deforming the tire.
The math for this is fairly simple. It's pounds per square inch.

Let's take your 800 lb. bike with tires inflated to 40 psi. The contact patch is 800lb./40 lb./sq. in. The 'lb.' terms cancel, leaving 800/40 sq. in., or 20 sq. in. This is exactly the 10 sq. in. you hypothesized. Lower pressure means more square inches, as does higher weight but, as you can see, it's very easy to calculate the area. It is not, however, 80 psi in any sense. It's 40 - in each tire.

2. Originally Posted by 535is
The math for this is fairly simple. It's pounds per square inch.

Let's take your 800 lb. bike with tires inflated to 40 psi. The contact patch is 800lb./40 lb./sq. in. The 'lb.' terms cancel, leaving 800/40 sq. in., or 20 sq. in. This is exactly the 10 sq. in. you hypothesized. Lower pressure means more square inches, as does higher weight but, as you can see, it's very easy to calculate the area. It is not, however, 80 psi in any sense. It's 40 - in each tire.

Think my calcs are closer even though I was only estimating the contact patch. Yours would be correct should a tire have NO structural rigidity, and was pliable like a balloon, but it is not, and thus the structure supports some of the weight, plus the rigidity allows the internal pressure to have a larger surface area acting to support the mass. It could all be calculated using the Modulus of rigidity, thickness and angular forces.

The 80 PSI is probably pretty close to the actual force on the tread surface where it contacts the road.

3. If inflation delta p carries half of an 800 # load with tire rigidity carrying the rest, you could put the same tires on a 300# bike with a 100# riderand not inflate them at all.

It ain't that way.

4. Originally Posted by SPOKESMAN
If inflation delta p carries half of an 800 # load with tire rigidity carrying the rest, you could put the same tires on a 300# bike with a 100# riderand not inflate them at all.

It ain't that way.
You are assuming that the tire rigidity is NOT dependent on tire pressure which is wrong. The wrapping and cord structure require inflation to become properly tensioned and thus have rigidity.

Look at air pressure and tire structure like spokes of a wheel, it is not just the spokes perpendicular to the load that support the mass, it is the "assembly" of all the support structures that does.

If it was just pressure above the contact patch maintaining the tire shape then if you took the handle of a screwdriver that had 1 square inch of surface area and pushed it hard into an inflated tire with 50 or 60 lbs force (exerting 50 -60 psi) the tire would deform severely. Guess what it doesn't.

5. I think tire pressure should be monitored with altitude change. My experience was with my car. I started in Toronto and drove to Colorado. In Colorado the 32 psi tire pressure was now reading 50psi. Lesson: Be sure to adjust tire pressure when altitude changes for an extended period of time.

6. Anybody up for Boyles Law or even better the modern ideal gas law?

The state of an amount of gas is determined by its pressure, volume, and temperature. The modern form of the equation is:

pV = nRT

where p is the absolute pressure of the gas; V is the volume; n is the amount of substance; R is the gas constant; and T is the absolute temperature.

In SI units, p is measured in pascals; V in cubic metres; n in moles; and T in kelvin. R has the value 8.314472 J?ĀK−1?Āmol−1 in SI units).

The temperature given in the equation of state must be an absolute temperature that begins at absolute zero. In the metric system of units, we must specify the temperature in Kelvin (not degrees Celsius). In the Imperial system, absolute temperature is in Rankine (not degrees Fahrenheit).

7. Originally Posted by walterK75
I started in Toronto and drove to Colorado. In Colorado the 32 psi tire pressure was now reading 50psi.
Sorry, the only way you could have gained 18psig going to Colorado was if you were in the Space Shuttle orbiting over Colorado !!

9. I use a mix containing about 78% nitrogen. Seems to work well.

10. Originally Posted by marchyman
I use a mix containing about 78% nitrogen. Seems to work well.

Actually, 32% nitrox works pretty well too, but watch MOD..... 20.9% nitrox works pretty well too. Kinda like the 78% nitrogen mix. .... just sayin'

11. Lived in Reno / Sparks for years and used to ride down to the California valley and then back up over I-80 and Donner summit at night in VERY cold conditions. Never gave a thought to tire pressure. Too busy trying to stay warm.

12. Originally Posted by marchyman
I use a mix containing about 78% nitrogen. Seems to work well.
I use 78.09% in mine.

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