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Old 07-30-2010, 09:29 AM   #1
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Airing Down

I had a question on airing down my SMB. I have BFG A/T 315/70/R17 and from what I have read, you are safe to air down to at least 75% of the recommended PSI, so in my case that would be 75% x 50=38 PSI, so I would let out about 12 PSI. I also note to keep it under 40 mph when aired down.

My question is that when it comes time to air down on back country roads, my tire pressure is of course quite higher, as the tire is heated up, and then I wonder if I let out the calculated 12 PSI or is the idea to get it down to 38 PSI, no matter what it takes?
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Old 07-30-2010, 09:48 AM   #2
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Re: Airing Down

This may start a bit on the technical side, but really, there is a point.

The ideal gas law is P * V = n * R * T

P = pressure, which is what you're focusing on.
V = volume
n = number of mols (think molecules) of air in the tire
R = a constant (doesn't change, so ignore it)
T = temperature

If you normally run at 50psi cold, and your tire temperature increases, then pressure must increase also to keep the two sides of the equation in balance (assume that the tire volume doesn't change significantly, which it doesn't). So, say you are 60psi warm. If you take out 25% of the air (n), then pressure will drop back down to 45 psi warm. Once the tire cools back down, the pressure should be at your target of 38 psi cold. If you had gone to 38 psi warm, you would be at about 32 psi cold.

Bottom line - if your cold "aired down target" is 75% of your cold "aired up target", then let out 25% of the current pressure when the tire is warm and you will be in good shape.

Somebody else will have pitch in with guidance about BFG A/T's and what that target % should be.
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Old 07-30-2010, 09:49 AM   #3
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Re: Airing Down

The object is to get to a ride that is good for the terrain and what you want for the ride. Some air down further than what you are aring down to such as in sandy situations. Tire bulge can be a concern when in sharp rocky conditions, and tire rub is always a concern. In wash board conditions speed like you posted about must be taken into account. I run E rated tires and am not worried about airing down to 35 in most situations but usually find 40 to 45 workd well for me plus I can air up faster.

There are several posts that discuss the correct pressures for specific circumstances
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Old 07-30-2010, 11:29 AM   #4
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Re: Airing Down

can you tell me what posts to search for for ideas on what tire pressure to run off road for e rated tires? I have been searching all morning and not finding exactly what I am looking for. We have an 07 with smb 4wd and run bfg all terrains, they are pretty small as they seem to work better in snow and it still fits in the garage, our our size 265. I am still trying to find a good number for technical off road use in the summer (not dirt roads but for difficult off road adventures). Sticker inside the van say max 80 rear 60 front. Is 35 lbs too low? I have the medium Staun deflators which go from 15-35 but I am now thinking I should of purchased the heavy duty ones which go up to 55. Actually I take that back, I think they only go up to 30 lbs. Anybody with heavy duty ones and big tires want to trade? They are brand new but can't be exchanged since I bought them last year.

Thanks much.
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Old 07-30-2010, 12:15 PM   #5
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Re: Airing Down

I don't think there are any hard and fast rules about airing down. My rule of thumb is 50% of normal is as low as I want to go - except for sand. Let some air out until the sidewalls have a nice bulge and try it.
If the ride is ok, then ok.
Keep the speed down to prevent the tires from overheating from the flex.
Air back up before you hit the pavement. And that is the hard part. You have to have a compressor and time.

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Old 07-30-2010, 08:02 PM   #6
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Re: Airing Down

We have an '08 with extra water and battery. The rig weighs somewhere north of 10,000# fully loaded. Off-road, we air down typically to 26#, hot. We don't go a million MPH when aired down, and they seem fine. I've got the same size as you. (315X70X17). We ran BFG's on our diesel F250 that we kept at our house in Baja, and ran them at 8-10# cold. Never had any problems. Go as low as you would like or need to go, and check them for heat. Hot is not good! But slow, 20-30 shouldn't hurt; just check them with your hand from time to time. A little common sense doesn't hurt either. In Baja, we've run them as low as they would go without falling off the rims (very often), and they were fine. Heat seems to be the enemy.
Bill
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