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Old 12-05-2007, 12:24 PM   #1
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Air down to what pressure?

Hi,

I've got an 05 PSD RB-50 with the SMB 4x4 conversion on BFG 285/75/R16-D tires. I'm wondering what air down pressures I should be using.

All of suggested pressure guides I can find don't even approach the weights of our vans.

In a recent thread on Tire Selection (for a slightly larger tire), canyonclan wrote:
"I run them at 50 psi on road for long hauls, 30-35 psi on rocks & 20-25 on sand."

Do these seem like reasonable values? What do you use?

David
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Old 12-05-2007, 02:36 PM   #2
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Yes they do. It becomes a visual decision, seeing how soft your tires are getting as you air down. You will figure it all out with experience. It's not that critical, so start with those numbers.
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Old 12-05-2007, 04:57 PM   #3
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A big factor in your airing down pressure is the type of tire and the normal traction. Some tires will provide more soft surface traction than others and thus airing down will be less important.

As for the criticality, I wonder. When you air down there is more sidewall flex. Sidewall flex produces heat, and heat is an aging process. More heat means more quickly aging tires.

This is increased when you take into account that todays tires have less steel and more polyester.

I wonder if more care should not be given to airing down or airing down less to help preserve tire integrity. We have heard a number of stories about tire problems. These are heavy vehicles, and when non-aired down driving is done at or near max pressure and max load, I think we need to be very careful about how we treat our tires.
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Old 12-05-2007, 05:59 PM   #4
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Re: Air down to what pressure?

Quote:
"I run them at 50 psi on road for long hauls, 30-35 psi on rocks & 20-25 on sand."
This is also pretty much the same as what i do, with the addition of 40-45 psi for decent not-washboarded dirt roads. I run 55 psi on the highway.

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Old 12-05-2007, 11:27 PM   #5
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I'm at 65lbs in the front and 70 in the back, per salem kroger's recommendation. The air ride suspension does a lot to offset the nvh I would normally pickup at those pressures. haven't needed to air down yet, but probably won't go much lower than 40, even with internal bead locks.
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Old 12-05-2007, 11:41 PM   #6
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The issue with a beefy 4x4 system & solid axles is not so much the vehicle's suspension, but all the unsprung weight. Every time you hit a bump, those heavy wheels, tires, and axles get violently bashed out of the way. By airing down, you're adding a "suspension" between those lower parts and the road, so they don't have to move so much in response to bumps.

Buji, if you've never aired down, I'd recommend giving it a try sometime! See what 40 PSI feels like when you leave pavement on your next backroad adventure.

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Old 12-06-2007, 12:00 AM   #7
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Oh when I'm on washboards I go down to about 45 or so all the way around, but I really don't like the feel of it on pavement.
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Old 12-06-2007, 12:37 PM   #8
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I run 45psi up in front and 48 in back most of the time..
50psi and more is just to bouncy
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Old 12-06-2007, 10:27 PM   #9
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Anything below 25 psi is risky IMHO. Especially in off camber terrain and rocks. The beads will come off the rims. That's been my experiance with other 4x4s. I've not lost a bead yet on my SMB.
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Old 12-07-2007, 07:39 AM   #10
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I run around 45 psi (somethimes a little more depending on my load) on the pavement and down to as low as 20 psi in the dirt/sand.
When I was working (yuck) we did a LOT of research with all the major tire companies about reduced tire pressure (RTP). This was for log trucks and belly dumps hauling on dirt roads. We wanted to keep 'em off the roads till late spring, or until all the moisture was gone, as the higher tire pressure was really rutting the roads and causing greatly increased road maintenance. All the tire manufactures said the only problem with RTP was driving too fast and having heat buildup due to increased sidewall flexing. We found that with a serious buldge in the sidewalls the roads were actually in better shape, the truck drivers were super happy 'cause of the better ride and they weren't getting insignificant heat buildup at speeds up to 40 mph.
I have found that the best way to go is to reduce pressure till you're happy with the ride and then get out every few miles and check the sidewall temps with your hand. Very warm is OK. If it's hot, ya need to slow down or add air.
Here are some shots of my tires.

This is my Toyota truck tire at 10 psi that I use here in Baja for all my running around.



This is my SMB tire from the rear at 21 psi.



The SMB tire from the side, again at 21 psi.


I really don't use a tire gage when I'm reducing my tire pressure, I go by the bulge in the tire which is what gives me a bigger foot print and a softer ride. The tire pressure will be different for every tire and rim combination and for the load the tire is carrying. With this amount of bulge, I keep speeds below 40 mph and never get any significant heat buildup. I know it's possible to roll a tire off a rim or break a bead, but in over 35 years of doing this I've never had that happen.
This is just what I do and what works for me (and lots of other guys that I run the dirt with). Everyone needs to do what works best for them and what they are comfortable with. Experiment when you're out running around and see what works for you.
The biggest problem with RTP and a big bulge in your tire is that the sidewalls are much more exposed to sharp rocks and cactus. Sidewalls are the weakest part of the tire so....
Anyway my two cents worth.
Just do something, even if it's wrong.
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