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Ask Dr. Tire – Temperature Effects?

Have questions? Dr. Tire has answers! How much air is in a trailer tire? How does temperature change my tire pressure? What pressure should I set my trailer tires to for winter storage?

Dear Dr. Tire – I have a 215/75R17.5 tire on 17.5×6 wheel. How much air is stored in my tire? And we are getting ready to store our rig for the winter. If I last checked my tire pressure in September at 90F and store it now at 50F, what should I see in my pressures and what should I put them to for storage? Does tire pressure go down in cold weather?

Thanks,

Midwestern 5th Wheel Fanatic

Hi Midwestern!

Great questions on tires! Answers are in the gold font!

Volume:

Let’s tackle the air volume question. To simplify the math, we can think of two cylindrical objects. A tire and a wheel, where the wheel is acting as a void (think donut…mmm, donut). For these calculations, we need to know the radius and height (or width) of our air tube. Uppercase R and H will represent the tire inputs and our wheel will be lowercase r and h.

The radius of our tire, R, is calculated as section width times aspect ratio, as percentage of 100, plus one-half of our wheel diameter, converted into metric – 383.50 mm. And the height, H, of our tube will be the section width of the tire – 215 mm.

The radius of our wheel, r, is straightforward: one-half of our wheel diameter, converted to metric – 222.25 mm. And our height, h, is the wheel width, in metric – 152.4 mm.

To figure out approximately how much volume the tire can hold, we’ll calculate the two cylindrical shape volumes, subtracting the wheel’s volume out. Leaving us with an approximated volume for tire only.

VT1 – VT2, where V = π*r2*h

In this case, the 215/75R17.5 tire holds approximately 20.00 gallons of volume! This is a great number to know if you are looking to match an air compressor. Be sure to select an air compressor that has at least a 20-gallon tank when filling up tires from 0.

Pressure Change:

Winter is approaching. Time for that warm apple cider, more donuts…mmm, donuts…and planning out next seasons trips! Great question about what happens when the temperatures drop from summers’ warmth to crisp fall levels. To answer: “Does tire pressure go down in cold weather?”. Simple answer is YES!

Gas behavior is super neat! Gases all have different chemical properties; however, they all, minus extreme temperatures and pressures, obey the gas laws! Gas laws help us understand how gases react in respect to pressure, temperature, amount, and volume. To answer this question, we’ll need the Ideal Gas Law to help us calculate the temperature changes. This law works well for compressed air (78% nitrogen. 20.95% oxygen, pinch of other gases, and very small sprinkle of water vapor) and “clean” compressed nitrogen (>90% pure) at the pressure ranges we see in trailer tires: 50-125 psi.

Ideal Gas Law: Pressure*Volume = n(moles)*R(Boltzmann constant)*Temperature

where R = 0.08206 (L/atm)/(mol*K)

From your questions, you’ve given us much of the needed information. We know your starting temperature and volume. But we will need to figure out the amount, in moles, to help us get to more answers.

A 215/75R17.5 calls for 125 psi for the best load carrying operation. Quick conversions for pressure in psi to atm. And temperature from Fahrenheit (F) to Kelvin (K).

Start Temperature = 90F = (90 – 32) * 5/9 + 273.15 = 305.37K

Initial Pressure = 125 psi = 125/14.6959 = 8.51 atm

Now we will need one more constant, number of moles, to get our end answer calculated. We can adjust the equation to solve for n, moles.

n = P*V / R*T

Number of moles of our gas is 25.69. Now we can see how going from 90F to 50F changes our pressure!

We will hold the volume and moles constant for these calculations and only will change the temperature from the initial 90F to the 50F you are experiencing now.

PFinal = n*R*T / V

Final Temperature = 50F = (50 – 32) * 5/9 + 273.15 = 283.15K

Final Pressure = 7.89 atm = 7.89*14.6959 = 115.9 psi

Starting at 90F and going to 50F will lower the pressure by 9.1 psi!

Storage:

Recap the findings, if you initially set your tire pressures when it was 90F outside, then you should find that your tires are around 116 psi if the weather is in the 50Fs now. You may see a few psi lower as its nature for some air to permeate out of the assembly though the bead area and valve seals. Generally, 1-2psi a month.

For winterizing your rig, go ahead and inflate the tires back up to their COLD inflation rating, in your case – 125 psi. And if you live in a region that will be experiencing colder temperatures, you may want to recheck your tire pressures monthly throughout the winter to maintain COLD inflation rating – 125 psi.

Thanks for writing in!

Dr. Tire

(And if you have a question for Dr. Tire, write us at askdoc@tredittire.com and maybe he’ll answer your question next!)