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Old 07-22-2017, 09:55 AM   #1
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What Pressure to Air Down To?

Our 4wd 2016 144" wheelbase RB Sprinter weighs in at 8,600 pounds, just a bit over GVWR. Weight isn't the issue here, the Sprinter handles it just fine.

My question here is about the correct pressure to air the tires down to when off road. The Sprinter sits on BFG KO2 265-70R17's and after 15,000 miles I can report that I am very happy with them. They are quiet on the highway, they handle well off road, and they are wearing well.

On our recent trip to Coyote Flat (a mixture of dirt, sand, gravel and rock, with the emphasis on rock on the steep elevation parts of the trail) I took the front down to 25 and the rear down to 30 and the van performed very well on all of the various surfaces, plus it gave a smooth ride when we could get up a little speed.

My son-in-law, a dyed in the wool long travel Tacoma owner, is adamant that I didn't go low enough, that I should have started at 20 up front and 25 in the rear and most likely have gone down from there.

But what does a Tacoma owner know about SMB's? I am here to put the question to the experts! What say you? What pressure would you recommend and for what conditions? Sand (I still want to take the van out to Pismo Beach) is one condition, sharp rocks on a mountain trail with a steep climb is another, with lots in between.

Let the education begin! (And thanks in advance for contributing.)
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Old 07-22-2017, 12:02 PM   #2
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My empty van is just over 7k lbs, 8400 loaded. Empty, 8psi is absolutely as low as I can go before the sidewall folds over on itself, I found that out accidentaly when I aired down to 7psi once. I pretty much only go that low when we're going into the dunes (Oceano OHVR) in my 2wd, not just driving on the beach.

Loaded, I generally try not to put it anywhere that requires airing down, but I have gone down to 12psi with travel trailer in tow going a long ways off the beaten path in the desert, with other vehicles of course.

I would say that with your slightly heavier van, with slightly smaller tires (lower volume) I'd keep it in the 15-20psi range to be safe.

If you're going lower than 30psi, just make sure you have an air source nearby as you don't want to be driving on the highway at low pressure.
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Old 07-22-2017, 03:28 PM   #3
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Supplemental question: do the load and inflation tables apply when we are off-road and not driving at a full range of speeds?

And are there different tables for each tire/brand or is it standardized? This PDF on the Toyo site was interesting but I don't know if it'd apply to any other tire.

https://toyo-arhxo0vh6d1oh9i0c.stack...s_20170203.pdf
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Old 07-22-2017, 07:45 PM   #4
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If you're going lower than 30psi, just make sure you have an air source nearby as you don't want to be driving on the highway at low pressure.
Onboard air is provided by an ARB CKMTA12
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Old 07-22-2017, 11:27 PM   #5
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Do you have beadlock wheels? (If no, would you consider them?) It would be good to know how much lower you can go with them since they prevent sidewall/rim separation. I would think, though, that going so low that having the rim pressing into doubled over rubber is not a good thing.

Anyone with pros/cons of beadlocks, I'm interested in feedback.
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Old 07-23-2017, 04:19 AM   #6
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Pritikin: airing down has much the same affect as being grossly overweight for a given tire, you are reducing the load carrying capability in exchange for a larger footprint, and more compliant tread face. They will get noticeably warm from normal offroad use, and continue getting hotter as speeds pick up. Not only do different brands have different ratings, but each individual size has its own load inflation table. Higher load rated tires generally have a stiffer sidewall and a higher margin for error when running lower air pressures.

TomH: Having owned many sets of beadlock wheels, and mounted many more, I'll say that I just don't see them being worthwhile for the kind of offroading that is likely to be done in an RV type van. Most are not DOT approved, and most do not meet the load rating for a heavy vehicle. The more common type of beadlocks (outer beadlock ring only, attached with lots of 5/16" bolts near the tires bead) will regularly break bolts, even just going down the highway, so they'll require very regular inspections/maintenance to be safe. I'd imagine that an even heavier vehicle will increase the frequency of necessary wheel maintenance.

I've had a few different sets of Hutchinson internal beadlocks (both 8 bolt and 12 bolt style 16.5" humvee, and 17" aluminum), and they are a much safer assembly, but much heavier, much more costly (except the 16.5", they're still dirt cheap) and most tire shops will either refuse to mount them, or just flat out have no idea what they are looking at. The humvee versions even have magnesium and/or rubber runflats inside.

I've got more experience with beadlock wheels than I would care to share, but I would strongly recommend against them for the average weekend warrior/daily driver type vehicle.

Standard wheels have an inner lip that helps retain a tires bead even at low/no pressure (that "pop" you hear at the tire shop when they're mounting your new rubber is the tires bead jumping over that little lip) until it sees a significant side load. The larger the rim diameter, the better it can hold a bead simply because there is more tire in contact with it. You might be surprised how well a 17" will hold a bead even at "sidewall folding" pressures, just try to avoid spinning donuts when you're aired way down.

The trade off there is that more rim equals less tire sidewall for a given tire height, which means you'll have to air down even lower to get the same compliance. Generally speaking, wheel sizes that end in ".5" (16.5", 17.5", 19.5", etc.) do not have this inner safety bead, so they are not suitable for low air pressures.

There also comes a point of diminishing returns from reduced air pressure. Again, the goal is to get the tire soft enough to conform to irregular ground surfaces to increase it's contact patch and reduce ground pressure, which also improves ride quality since the tire can conform over obstacles instead of transferring the impact through the suspension. If you air down too low, the rim can absolutely hit and at best, jar the heck out of you, worse it could cut the tire or break the wheel. You also risk spinning the rim inside of the tire (think: drag car tire) which usually results in an instant loss of your last bit of air pressure.

It's easy enough to stop and take out a little more air, but it can be time consuming/ PITA to add air or reseat a bead. I'd start with a conservative 20psi and if all is well there, take a couple more psi out. Continue that process until you're down to your comfort zone. You can test in your driveway to find the point of "too far" (when the sidewall begins to fold) so that you know to never go that low in the field. That reference "minimum pressure" is actually more useful than you might think since often times you won't be able to see your sidewall at the time of airing down if you already started spinning/digging in sand/mud/snow so you'll haveto trust your gauge.

Sorry for being so long winded, I hope it at least added some value.
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Old 07-23-2017, 06:00 PM   #7
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Sorry for being so long winded, I hope it at least added some value.
Please don't apologize; that was amazingly helpful. So it sounds like airing down to the point of the bead breaking has no advantage to begin with. That being the case, I presume there also is no point in going with an inner tube to address the bead break issue????

(And thanks for the tip on the radii that end with point five. That is really good to know.)
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Old 07-23-2017, 11:43 PM   #8
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My 2011 E-350 weighs 10,400 or so and I run 60/80 front/rear. My go to off road tire pressure is 30/40 for most dirt roads - as much for my comfort as anything else. I go down from there depending on how sandy and how long the trail. At Pismo beach I ran 18 lbs all around all day and had no issues. A friend in a similar van running similar pressure got stuck running a bowl and slid sideways downhill for 20' or so. He broke beads on both downhill tires and it was a bit of a pain to get them re-seated in the field. I got stuck at Pismo once (in 2wd running street pressure). I took it down to 20 lbs and drove right out. Do watch for sidewall bubbles. I've lost two tires to them in 15,000 pretty hard miles likely due to running lower pressure on highways between trails...a bit lazy, I know.
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Old 07-24-2017, 12:26 AM   #9
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Do watch for sidewall bubbles. I've lost two tires to them in 15,000 pretty hard miles likely due to running lower pressure on highways between trails...a bit lazy, I know.
Does the constant flex of the sidewall cause heat buildup and the bubbles came from the heat, or does the constant flexing just wear the sidewall too thin and weak?

When young, I converted a VW diesel Rabbit pickup into a mini-camper. My Michelin tires were 7 y.o. but seemed in fine shape. I did not then know that polymers break down when they get older and loose strength. In the late 80s we drove across the Mojave desert on blacktop at 80 mph on a really hot day. All 4 tires bubbled all over the sidewalls. We were in the middle of nowhere in a small town. In Sacramento, the tires were $50/ea (small tires-30 yr. ago). I went to a tire shop and was quoted $200/ea. I asked if there were other shops in town. The guy belly laughed, told me the name of the one other shop, and said their price was identical. He was right. Collusion/price fixing is illegal, but in a small town, what are ya gonna do? The local prosecutor likely was their friend and didn't care. At least I was in a place that did have tire stores.

Anyway, the moral of the story is you don't want sidewalls bubbling out in the middle of nowhere.
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