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Old 09-08-2009, 01:33 PM   #11
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Re: Question, Airing Down for off road?????

Ford_6L_350 speaks the truth. At Bill Burke's Heavy Truck 4WD Class in Moab, we aired down to 30-35 psi for all of the Sportys for rough 4WD trails. He tried to get us stuck in heavy sand in one wash to demonstrate how to recover a vehicle, but the sand was too wet to really auger in, even at those tire pressures.


SMBs are real heavy vehicles and you have to closely watch your tire pressures.
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Old 09-08-2009, 02:41 PM   #12
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Re: Question, Airing Down for off road?????

Seems like starting your calculation with road pressure is arbitrary as people may run very different pressures on the road. I'd start with max pressure tire is rated for and reduce a percentage of that. This would make the advice given more reliable.

Also, I agree that a 10,000 lb SMB is a very different animal than a jeep.

Just my 2 cents....inter alia....

tom
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Old 09-08-2009, 05:20 PM   #13
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Re: Question, Airing Down for off road?????

So - what were the alia?
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Old 09-08-2009, 06:38 PM   #14
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Re: Question, Airing Down for off road?????

Quote:
Originally Posted by bluedog225
Seems like starting your calculation with road pressure is arbitrary as people may run very different pressures on the road. I'd start with max pressure tire is rated for and reduce a percentage of that. This would make the advice given more reliable.

tom
I'm not sure it is that arbitrary. Van run different tire pressures because they weigh differently and because they have different sized tires. But if the tires are running the proper pressure for the weight, then halving it is reasonable.

On the other hand, if two similar weight vans with the same tires are running very different pressures, at least one of them is running the wrong pressure.

Mike
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Old 09-08-2009, 07:02 PM   #15
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Re: Question, Airing Down for off road?????

That brings up a good question that has been bugging me. Say I have an 8000 lb van and 4 tires rated at 2500 lbs each at 80 psi. (I'm just making up numbers to make the math easy.)

What's the right pressure? I have always assumed that 80 psi was "best" and "safest' for road use.

Is there a formula using max psi, total weight, and capacity at max psi to calculate correct pressure? I have not seen a chart giving weight rating for less than max pressure. Is there one?

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Old 09-08-2009, 07:06 PM   #16
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Re: Question, Airing Down for off road?????

Quote:
Originally Posted by Psomaki
So - what were the alia?
Guy's doctor says "you have 2 months to live." Guy says "I want a second opinion." Doctor says "Ok, you smell bad."

and that alia I gotta say on dat subject....
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Old 09-08-2009, 07:31 PM   #17
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Re: Question, Airing Down for off road?????

Quote:
Originally Posted by bluedog225
That brings up a good question that has been bugging me. Say I have an 8000 lb van and 4 tires rated at 2500 lbs each at 80 psi. (I'm just making up numbers to make the math easy.)

What's the right pressure? I have always assumed that 80 psi was "best" and "safest' for road use.

Is there a formula using max psi, total weight, and capacity at max psi to calculate correct pressure? I have not seen a chart giving weight rating for less than max pressure. Is there one?

Tom
Old school answer:

The pressure rise from cold tire to hot tire should be 4-6psi. Less than 4psi rise means the cold pressure is too high, more pressure rise means the cold pressure is too low.

Best answer:

Get the actual weights on the tires and ask the manufacturer for the proper pressure for that weight.

My method:

Again, get the actual weight. Compare it to the max weight on the tire. Do some math.


Actual weight/Max weight X Max Pressure = Desired pressure

If you have 2500# on a tire rated at 3000#@80psi you get:

2500/3000 = .8333 .8333 x 80psi 66.66psi, which I round up to 70psi


Then I always round the pressure up. Much better to err on the side of too much pressure than on the side of too little pressure.

Mike
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Old 09-08-2009, 08:17 PM   #18
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Re: Question, Airing Down for off road?????

When I got new rubber from our local Les Schwab place (which I've praised in another thread), I got several opinions from different employees as to the proper tire pressure. One guy said just use what Ford put on the door plate. Others said no, that's way too little and the tires will start to break down. The most experienced guy (works with trucks and RVs a lot) said run 'em at 70 psi and watch the tread wear. General opinion seemed to be that lowering pressure to get greater comfort on rough roads is fine if you don't mind your tires failing sooner, that some tires can take this longer than others, and that if you do it, you gotta watch for signs of early failure before one of them blows. Or, in summary for Tom and any others who like Latin, caveat emptor.
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Old 09-08-2009, 08:24 PM   #19
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Re: Question, Airing Down for off road?????

Quote:
Originally Posted by Psomaki
When I got new rubber from our local Les Schwab place (which I've praised in another thread), I got several opinions from different employees as to the proper tire pressure. One guy said just use what Ford put on the door plate. Others said no, that's way too little and the tires will start to break down. The most experienced guy (works with trucks and RVs a lot) said run 'em at 70 psi and watch the tread wear. General opinion seemed to be that lowering pressure to get greater comfort on rough roads is fine if you don't mind your tires failing sooner, that some tires can take this longer than others, and that if you do it, you gotta watch for signs of early failure before one of them blows. Or, in summary for Tom and any others who like Latin, caveat emptor.
I doubt that airing down for off-road use will cause a tire to fail sooner. The real culprit is heat. And slow speed, off-road use doesn't generate nearly as much heat as high speed use does. If you air down for off-road and then drive 5-10 miles at highway speeds to a service station for air, all bets are off. But if you keep speeds down and don't overheat the tires you won't cause any problems. Rocks and other off-road hazards might cause problems, but they are more likely to cause problems at higher pressures.

And, for sure: caveat emptor. Or, in English: You take your chances and you pay the piper.

Mike
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Old 09-08-2009, 08:33 PM   #20
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Re: Question, Airing Down for off road?????

Mike, right on! I'm sorry that I did not do a complete job of recounting what all those tire guys said. They were talking about airing down and then driving at higher speeds over rough back roads, trails, etc. You are right - heat was the culprit for the most part, although one opinion expressed was that even at lower speeds, the sidewalls are forced to flex more when the tires are aired down and this can lead to early failure. All this was anecdotal, of course, based on these guys' experience, and they didn't all agree with each other.
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