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Old 08-23-2018, 01:43 PM   #1
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Airing down question

So for years I have always aired down my jeeps and buggys' before going offroad. This past weekend I took my SMB up a nasty rocky fire road but was not sure if I should air down and if so how much. I have never driven anything this heavy off road so I just crawled it. Looking for some advice.
It is a 2003 E350 SMB EB 4x4. V10, chateau roof, Agile offroad kit, and running 285 E rated BFG AT2's. gotta weigh in @ 5 tons.

Thanks!
Patrick
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Old 08-23-2018, 03:41 PM   #2
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I have my Staun tire deflators set to drop my pressure down to 35 when I get ready to set off on the dirt. Sometimes I will a little more air in the tires if its just a short dirt section or mild fireroad. I go as low as 20-25 for deep sand. My van does not have a heavy build out like an SMB so probably close to 8000 lbs currently.
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Old 08-23-2018, 03:54 PM   #3
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I've gone down to as little as 20psi with 285-16 BFG A/T's if I recall correctly.

I look for 'foot print size and sidewall bulge' more than tire pressure, as those two are a function of vehicle weight, pressure, and tire construction. I shoot for about 1/3rd reduction in sidewall height, which in turn makes the tread footprint larger.

More than that, and I get concerned I'll pinch a sidewall of break a (cast aluminum) wheel if I smack a ledge or sharp rock.

In the sand, you can go 'flatter and fatter' than that if needed, you just have to keep the tire from popping the bead.
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Old 08-23-2018, 04:40 PM   #4
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Touch the sidewall...if it is "hot" (like close to burning your hand), you're going too fast or running too little air for the speed/weight. Get in the habit of touching your tires after running on the freeway or off road - you'll get a sense for when it it too hot. Tire will be internally damaged due to thermal or mechanical stress for under pressure issues.

You can also blow a bead and be more! ( Yes more) suspectible to sidewall punctures if your down too much. Wish I had some better guidelines (like actual temps or psi)...

Also, it is not sidewall bulge that matters when airing down - in fact, you really don't want to have much sidewall contact as it is not designed for it (sand is a different story) the tire footprint lengths and this is what adds traction.

All that being said, I'd start with 30-35psi...and go down from there. (I run 10-12 on my jeep and 18-28 psi on my sprinter van that is sitting on 35s).
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Old 08-23-2018, 06:40 PM   #5
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I usually drop to 30 front and 35 rear on gravel roads. Seems to ride and drive just fine at those pressures. Running 265/75 16 KO2s.
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Old 08-23-2018, 08:54 PM   #6
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'..nasty rocky road/s..'- I crawl along smooth and easy, picking the best line, and, I haven't had to air down yet.
Sand and powdered dirt, I got nothing.
Backcountry rocky roads have been our main haunts and if anything it's when to fire up the lockers; maybe we'll learn a lesson the hard way, but so far our 285/75 - 16 bfg at ko's appear to not know the difference.
Hat's off to Agile RIP by the way...
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Old 08-23-2018, 09:13 PM   #7
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Depending on the sharpness of the gravel on gravel roads...you will also see less punctures. Northern Baja as an example—airing down will save you punctures. I speak from experience...
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Old 08-23-2018, 09:45 PM   #8
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We have EB350 with cruiser top and V10 which looks to be similar in size and weight to yours. We typically drop our pressure to 50 in the rear and 40-45 in the front. This seems to work well for us.
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Old 08-23-2018, 11:25 PM   #9
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Been discussed here a few times, this is a good thread: http://www.sportsmobileforum.com/for...ate-16832.html

I’ll do as low as 25 offroad, depending on speed and conditions. Dropped them down to 10 to drive out of a dune in Baja. But I wouldn’t go very far with less than 25.

This is 15psi in my old 285s:
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Old 08-24-2018, 07:34 AM   #10
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‘...this is a good thread:...’
Thanks for the Thread Link RPA, solid info in there.
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