What he said. Seems like most people here don't air down that much. I on the other hand go as low as possible. I'm running 35" mtrk's on stock wheels. Ill run 10 psi in the rocks and have run as low 4 psi in the powdery snow. The difference is huge!

 

Yep, I used to run my KM2 35s at 11 psi in the rocks and dirt. Made a huge difference from even 14 psi. Now with Beadlocks I run around 8 with my 37s.

 

There was someone here who posted a little guideline about airing down that went a little something like this

On logging roads or dirt roads (15-18)
Offroad loose dirt/mud (12-15)
Rock Crawling (8-12)


Just think of a tank tread. Think about where a wider flatter track would be advantageous....

 

When I had 10" rims and 12.5" tires I'd lose a bead at 18#, now with 8" rims and 12.5" tires I got to 12# with load range C tires. I've been as low as 5 after burping a tire. I'm not running beadlocks. The higher the load range the more you need to air down for the same sidewall flex. To get a real advantage I think you need to go 15# or lower. Silver Lake sand dunes requires you go down 15# or lower to get out on the dunes.

 

Soft sand (5-8)

You will find also that vehicle weight and tire/wheel combos will change that. For rocks, I prefer 12-15 with my current set up on the JKU. This above is a good starting point. Start there and adjust as needed to suite your vehicle and tire/wheel combo.

 

Do you ever have trouble with loosing a bead. I run 35 Trail Grapplers on stock wheels but don't really go below 15 lbs, but im new to wheeling and really unsure what I can get away with. I run alot of trails with mud

 

I have 37" BFG km2's in stock moabs and when rockcrawling I air down to 12 lbs without any problems

 

I haven't yet. (Knock on wood) if your tire is a load range C then you can usually get away with a lower psi. The stiffer sidewalls of D and E range tires could burp air more easily so sometimes you can't run as low a psi. I had a lot of problems with a load range E tire last year. Lost the bead 3 or 4 times.

Here is a picture of my mtr/k's at 10 psi going over a tree root. Lots of sidewall flex.

 

Real practical approach. For rocks: drive up to a rock a 1' or so high, let air out of your front tire so that the tire deforms around the rock, yet has enough "cushion" so the rim isn't close to the rock. Depending on your tire, it will be between 8-12psi. You'll want to do the same on your rear, its frequently different than the front because of the weight differences front to back.

On snow, air down as low as possible without closing your tread, usually around 5-10lbs. Sand too. You can do this experiment too: The goal is to have as much tread on the ground, but keep your rim off the ground. Let air out of tire until your tread is about 2-3x more on the ground compared to full air (so for example A 4" footprint long when aired up, might be 8-12" when aired down.)

As others have said, even airing down to 15-20psi helps smooth out the ride. However, to really take advantage of the tire and performance on the rocks, an 8-12psi. It's a really common mistake (and I do it too sometimes) to run trails with too much air.

 

Ooops... posted on the wrong thread... sorry.