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Discussion Starter #1
What are the rules tire pressure?

When should you lower the tire pressure?
How low can you go?
Does it help to reduce against punctures?

Chuck
 

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More Guidelines than Rules . . .

cfowler - 2/8/2005 9:36 PM
What are the rules tire pressure?
When should you lower the tire pressure?
How low can you go?
Does it help to reduce against punctures?
The answer is different depending on conditions.

Ultimately, the point of airing down is to increase traction, as airing down increases the tire's contact patch.

Here's what I do, along with all the caveats:

On the street, I (almost) always run at the mfgr's recommended pressure.

On long stretches of washboard dirt road, I'll take it down to 15-18 p.s.i., which helps prevent the washboard from rattling my teeth out. On more technical stuff, I'll go down to about 12 p.s.i.. I've seen some folks go as low as 9. Maybe for rugged terrain the goal is that you air your tires down to the point that the whole width of the tread is on the ground - but none of the sidewall. With my tires, you have to let A LOT of air out to make that happen. I stop at 12 p.s.i. with a little bit of the edge of the tread still off the ground, because the tires feel just a little too mushy for my comfort if I go further.

When you air down the tires, while you increase the contact patch, you decrease the effective sidewall. Both my G's have 15 inch rims and 33 inch BFGs. That gives me a lot of sidewall to play with. But a G500 with 18" rims can't air down as much or else it will run the risk of damaging the rims or tearing the sidewalls. Speaking of sidewalls, if you're going to air way down, you need tires which are designed to take that abuse. BFG MT's and GoodYear MT/R's have multi-ply sidewalls for that reason. I'm not sure about punctures. I would reason that with tires aired down and the sidewall ballooned out, there would be an increase in the risk of cutting the sidewall on rocks, etc., I've seen cactus spines puncture sidewalls before. They are the weakest part of the tire.

When the tires are aired down you have to watch your speed, because the sloppy sidewalls will adversely affect handling at speed, especially on pavement.

It's also prudent to consider that aired-down tires will also reduce the axle-to-ground clearance.

If my truck is heavily loaded, I'll adjust the pressure upward accordingly.

It's also good to have a plan for airing back up when you get back to the pavement.

I'm sure there's more to say on the subject. With the guys I've driven with, tire pressure seems to be a very individual decision. YMMV.

I hope that helps.

— Spalding
 

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Spalding pretty much covered it.

I find that I can run 25psi in both my trucks pretty much all summer. That gets me to and from the trails without having to mess with the pressure. It gives me a little more contact than the 40 I would run on the street. It softens up the ride a little and lets the tire flex.

On the sidewall issue, a catus spine will do it[B)] but I dont have any of those around here. I have seen even lesser tires wrap around sharp rocks you would swear were going to rip a hole. I have also lost a tire on a nearly flat section of rocky trail just by rubbing one wrong. The lower pressure generally will allow the tire to wrap around things without a puncture. At full pressure there is no give and I think you are more likely to cut through.

Again, guidelines as suggested. Depends on a lot of things but I think the above listed pressures are pretty good. If I wasn't so lazy, and the G so capable, I would go lower off-road and higher on road[:)]
 

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Chuck,

lower tire pressure

when? whenever a softer tire or a larger footprint is needed

how low? depends on the tire size and your vehicle weight - more volume of air in a tire (35x12.50R15) supports more weight and can be lowered substantially more than a tire with little volume (30x9.50R16)

and yes, a softer tire is much less likely to go flat due to puncture. In fact, since I started running 35x12.50 tires 16 years ago I have not had a single puncture (during the 2 years with 205R16 tires on my 300GD i sometimes had 2 flats a day on rough dirt roads)

Harald
 

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Discussion Starter #5
I guess this is another reason I need to exchange 18" wheels for 16".
I will be offroad in Ocala next weekend. I will play with the tire pressures and let you know.

Chuck
 
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