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Recommended tire pressure?

11K views 3 replies 3 participants last post by  TJTJ 
#1 ·
I realize this question has likely been asked a billion times, so I'll preface with stating that I did try to search and do some research.

This post proved extremely useful. Tire master, wow.

So I'm running BFG All-Terrain KO2s (LT 265/75/R16). So 32's on stock 7" wide rims. Looking on the BFG website it states: 3,415 lbs at 80 psi.

Doing TJ's math: 35% of GVWR is 1820, then we take 1820/3415=53%, and 53% of 80psi is 43psi

43 seems crazy high, no? Do I have the wrong tires for the truck?

My main question is around airing down for off-road. I've seen numerous posts stating that running at 15 psi is good for muddy off road conditions. Is the minimum (safe) off-road psi a function of your highway PSI? I'm assuming that someone who has to run 43psi on the road would have a different off-road pressure compared to someone running 30psi on the road?

I have a Viair 400P portable compressor for airing up, but no on-board air. I also have no clue how to reset a tire if the beat came off - so I'm very interested to ensure that doesn't happen when I'm in the middle of nowhere! Hoping to get a sense of a 'safe' minimum for airing down my BFGs for getting maximum traction off-road.

To add some context to my question, I was exploring deep in the Caribou range this past weekend and after many kilometres of driving remote roads came across a set of switchbacks heading up to an alpine ridge - it was uber steep, a full on V-groove eroded in the road, exposed cliff on the side, and some rocks/gravel in the groove and sides of the road to make things extra exciting. I turned around, not wanting to risk it, but was curious how low I can air down to try and crawl up it...
 
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#2 · (Edited)
Yeah, you have to let go of what "sounds right" on tire pressures, especially if not used to higher load range/truck tires...some of which are rated for 80 psi for example.

43 psi is not "crazy high" - its "crazy LOW".....for an 80 psi tire, on the street.

:D




If you are only running at ~ half the tires load PSI, it means that the tire will not be performing optimally....that happens closer to ~ 75% PSI.

When choosing a tire for a rig, you want to consider the load range/PSI that it would require.

If you have a rig that weighs 4k lb for example, that's ~ 1,000 lb per tire to hold the rig up...plus that ~ 35% MINIMUM to account for weight transfers during stopping and cornering.

(For example, ~ 70% of a rig's weight is transferred to the front tires during hard braking, or ~ 35% per tire, etc).



So, you want an 80 psi tire inflated to AT LEAST 65% of its max to run well, or, ~ 52 psi.

Yes, 52 psi supports more weight than the MINIMUM required, but, the handling will be less squirrelly, the tire won't over heat as badly at high speeds, etc.

The ride WILL be a bit rougher than at a lower psi of course, as the tires are meant to support a full sized pick-up truck loaded with cargo...and you are mounting them on a mid-sized SUV...with no bed of cargo in back.


On the plus side, the heavier duty tire will last longer with your lighter loads, and generally be more damage resistant.





15 psi is a number that works for some rigs, in some terrain, with some tires, and which is too much, or, too little, for everyone/everywhere else.


What you really want to do, for YOUR rig/tire combo, is to use ~ 50 - 60 psi for daily driving on roads, etc...(Street psi)

...and, to know what to air down to, as a guideline:


1) Park the rig with a fully inflated (street psi) tire on a stump or rock. (Using stump for example) Pick a stump that has a crest, so that the tire is resting on a small area smaller than the tire's foot print. (A log, etc, anything that would force the tire to wrap it as it deflates, is fine for this purpose)

2) Deflate the tire until the tire wraps around the stump, so you see the tread on the opposite sides "grab" the stump, like fingers.

3) Measure the psi at that amount of deflation....as that's about the PSI you want that day.

You can experiment to see what works in specific terrain...but, the CONCEPT is you want the sidewall to bulge out a bit, enough to maximize the tread's contact patch, but NOT MORE THAN THAT.

If you keep deflating, eventually the contact patch is no longer in good contact, its just a flat tire, etc.


SOME tires end up at ~ 3 psi, where others are too deflated at 18 psi, so you do want to do the experiment.





This is an example of a tire deflated enough to wrap the terrain.

Notice the sidewall is still supporting the rig, but the tread is "grabbing" the stump.

Its able to pinch the stump between the opposite sides of the tread.





Notice the tread blocks can get a finger-like grip on the stump, to claw over it?






Here's on on a rock. Notice that even on that small area, the tread blocks are able to grab the subtle irregularities at the top of the rock, and pinch them between the blocks, to grab them for traction?


On the STREET, traction is almost entirely about friction, rubber on the road, hence racing slicks, etc. OFF road, except on, ironically, slick rock, etc, the traction is MOSTLY about the tread conforming to the terrain...less about friction, and more about grabbing edges, etc.





So, deflate ENOUGH to get YOUR tread to conform to the terrain, and, that will work for YOU.

:D


If you blow a bead....all that means is that you need to pop the lip of the tire back into the lip of the rim....and put air back in.

So, a tire with a blown bead is deflated all the way...so, its easy to push the tire bead back where it belongs. The hard part, when you have a weak air source, is to get the air to stay IN the tire while you are trying to get the bead to be seated enough to not let the air out again.


A cinch strap for example, wrapped around the circumference of the tire, and tightened so as to force the tire to bulge out against the rim lips, will typically give you ENOUGH of a seal to re-inflate it.

OBA with a tank, at 150 psi, or a CO2 set-up with high flow regulator, etc....can release such a large blast of air in one shot, that the strap, etc, is not typically required to get the bead to reseat/seal....but, with a smaller compressor, and a strap, typically, you can get'r done.

There are also tricks with lighter fluid, where you light it, and the heat makes the air expand quickly and pop the bead back on, etc...but, off road, if you are not "already good at it" this is risky due to forest fires, burns, self immolation, etc.


:D
 
#3 ·
Good info TJTJ. I am going to have to learn more about this and re-evaluate my street pressure.

On a recent off-road trip to Colorado I aired my BFG AT TA/KO's (265 75R 16's) down to about 20 psi for the more difficult trails. I felt like the traction and ride quality were noticeably improved. Surprisingly, I also felt like the slight reduction in ground clearance was noticeable as well. I was generally struggling with ground clearance more than I was with traction, so I ended up airing back up to about 32 PSI for most trails. This is all based on my stock vehicle carrying 2 people, 14 gallons of water, and maybe 300 lbs. of vehicle gear, camping gear, and food.

I don't know anything, just sharing my recent personal experience…
 
#4 · (Edited)
Good info TJTJ. I am going to have to learn more about this and re-evaluate my street pressure.

On a recent off-road trip to Colorado I aired my BFG AT TA/KO's (265 75R 16's) down to about 20 psi for the more difficult trails. I felt like the traction and ride quality were noticeably improved. Surprisingly, I also felt like the slight reduction in ground clearance was noticeable as well. I was generally struggling with ground clearance more than I was with traction, so I ended up airing back up to about 32 PSI for most trails. This is all based on my stock vehicle carrying 2 people, 14 gallons of water, and maybe 300 lbs. of vehicle gear, camping gear, and food.

I don't know anything, just sharing my recent personal experience…

Your observations are correct....airing down shortens the sidewall height, and that makes your diffs, leaf hangers, etc, closer to the ground, along with everything on top of that.

Off roading requires a unique blend of compromises and considerations to balance various capabilities.


IE:

High enough to not hit stuff below can = too tippy when off camber.

Airing down to increase foot print reduces ground clearance.

Adding armor reduces risk of damage, but increases weight, which can stress components/mpg, etc.


Ideally, we'd have a bag of rigs, like a golf bag full of clubs, and we could pick the club/rig for each shot, etc....but few of us can afford that, so we decide on a 9 iron or whatever seems to make sense, and putt and drive and chip, etc...with the one club for everything.





>:D
 
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