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How to determine tire pressure for the trail?

3202 Views 22 Replies 19 Participants Last post by  SweetPee
We have these Pro Comp tires (285/75/16) on stock Rubi wheels. How do you determine what pressure to run on the trail (without risking breaking the bead?) Is there a rule-of thumb? Run 1/2 of the street pressure, 1/3 of street pressure, trial and error, or what??
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It all depends on the terrain and what you're running, what you want and what you want to accomplish. There are a TON of variables at play. The good news is the factory Moab rims hold a bead very well at low pressure. They are about the best rim I've run that holds a bead this well at low pressure in most conditions. Granted, you can still pop a bead, but you can get much lower on the Moabs than most other rims-at least in my experience. Lots of folks consider "airing down" dropping to 17-18psi. In my mind, that is nothing but a comfortable ride on a gravel road. You're not really gaining anything in terms of traction as the tire isn't ballooning out/expanding enough to do any good at that high a pressure. Keep in mind a tire elongates more so than it expands width wise when aired down. In the 10-12 range, you'll start seeing benefits. Drop to 6-8 range and you'll be amazed at what you can do that you can't even think about at 17-18. Depending on terrain, can mean the difference of struggling in 4 wheel drive, or crawling in 2wd. Or, being locked or not. If all you're running is gravel forest service roads, then by all means, have a luxury ride at higher pressures if you'll spend a lot of time on them. If you're really wheeling, drop it down. My Moabs hold 8 very well in about most all conditions. I like to run 6, and lower, but for me, at 6 it gets really dicey as to whether I'll pop the bead or not. Maybe a 50/50 chance if you will depending on terrain and if I'm really working them. I'd like beadlocks for this reason eventually.

As to popping a bead, a lot of folks fear this problem on the trail. I'm really not sure why. Popping a bead is by far the easiest thing to fix on the trail-at least in my opinion. If you have Co2 or a good compressor, just jack up that wheel and hit it with air. Clean the bead seat prior of course, but once that's done, work the tire around by hand till it's somewhat touching the rim and hit it with air. It'll pop back on in seconds. The ratchet strap method is a joke that I've never seen work so don't mess with that or anything you've heard. Just makes it harder to do right. Do it by hand, hit it with air and go. Be certain it's aired up and the bead is on, then drop pressure and keep wheeling. May have to massage it around a bit by hand, working different parts of the tire in or out with your palm, but with the air going in, it will seat. When you need to start plugging and patching, that's when it takes actual work and when it gets dicey as to whether or not your patch job will hold. My last took something like 13 plugs I think and held for a bit-till it tore up the tread block and wasn't worth messing with at that point as we'd wasted too much time already. Swapped the spare on and continuing down the trail without a second thought. Another problem I've had a lot is torn valve stems on my Moabs with long stems. Swapped them all out to shorter ones but still have the problem. Another reason I'd like beadlocks with a ring to protect the stems. This one really pisses me off on the trail to repair a torn stem as it means dismounting the tire, breaking the bead and pulling the stem, setting a new one then re-seating, re-mounting, dropping down again and going. Takes a fair amount of time. Would be great to have a tool that could insert a stem from the outside rather than inside the rim-would cut a LOT of work. Word of advice-carry spare stems in your tire repair kit-several of them actually. Reminds me, I used my last and need to restock my supply...

With your current tires, if you run rocks, my biggest concern would be having a good tire repair kit and a good air source-be it Co2 or compressor of some kind-or both. Pro Comp tires are notorious for weak sidewalls, and in the sharp or jagged rocks, be ready to slice and dice. I've seen many of them destroyed in rocks and sticks both, then running through muddy trails in the hills and catching a stick deep in the mud and coming back up with the stick through the sidewall of the procomps. My advice with them depending on tread life would be to run them till they're no longer serviceable the upgrade to a "good" tire, or if they are new with great tread, see if you can sell them off and put that money towards a "good" tire. Toyo MT's are excellent, Goodyear MT/R's are a great tire. The Nitto Grappler is another excellent tire though loud on the street. Depening on location, some folks like the new BFG MT KM2. Around here, they slice sidewalls too easy. Other folks have been happy with them in other areas. Just to name a few.

Something to think about. By different terrain, I mean things like snow or sand, I'll run nearly 0-basically pull the stem and wait for it to inhale, then plug and go. Anything under 4 for sure. I run most all terrain in 6-8 due to the Moabs. With beadlocks I'd likely be in the 4-6 range. General dirt trails, hill climbs and such, maybe open it up a bit to 6-10 depending on conditions. Gravel roads-15-18 is a nice smooth ride. Street pressure if you don't have far to go. Best thing to say for your setup is to run as low as you can go till you blow a bead, then reseat it and keep experimenting till you find the sweet spot you're happy with. Different tires like different pressures too-some more, some less. Start high and slowly work yourself down low till you find the setup that works best for you. Then, once it snows, do it all over again... :lildevil:

Best of luck,

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I have run the John Bull Trail with 9 lbs in my Goodyear MTRs. Sheesh! I couldn't believe they kept the bead and looked like jello.

Certain tires depending on their size and rim wide could be come a hazard if you take them too low and could break the bead. For your size, I would try going to 15psi and see how it handles in relation to side wall flex. For my 12.5" wide tires, I like using 13psi which is good for sand, mud and rocks which can be found all day. G
gregert12 said:
Use the chalk test.

Draw a line of chalk across the tire and without turning, drive forward 100 feet. If the chalk disappeares on the outside of the tire only, you do not have enough air; if the inside is gone, there is too much. Keep playing with this until all the chalk disappears.

For instance, I have 34 trxus x 12.5 for trail use and my daily driving pressure is suppose to be at 35psi. After using the chalk test, I found that my tire and rim combo really loves 24psi.
Thats for on-road, I would try 12 and then drop from there.
I wouldn't go nuts airing down too much. With my 33's and now 35's i'd run 30psi on the street and anywhere from 17 to 20psi on the trail, and can play in the rocks all day. With lockers i say you wont have to go to low. Of course you get more traction the lower you go but for the wheeling your rig will be seeing by its specs i don't think you need to go to crazy. just my 2 cents
I think '05TJLWBRUBY/Mike covered everything nicely. When I was on 33" TrXus MTs I had problems getting low enough to see any sidewall bulge due to their E ratings. Club members suggested driving home from the trail head a couple of times on the 10 PSI that I ran on the trail rides. It was only 15 miles for me but, after hitting the interstate at 70 mph a couple of times they really seemed to be more flexible. They held up nicely and now live on a tow rig because I traded them for doing the regearing on my rig.

Our Moab wheels are really good for holding the bead. As for the valve stem problems I remember seeing that DoctorD somehow blocked off the OEM location and installed different stems between the spokes. I wish I could relocate that thread and link it here for you.
Wow - thanks guys! Thanks Mike for your detailed analysis! Now I have a pretty good idea about where to start for tire pressure off-road.

On our Libby, we run All Terrain TAs and had extremely good luck with them - did quite a few trails around the San Juans and never had a failure (never aired them down, either.) I have them on my truck and had two of them lose air - tire shop found a couple of nails in each tire (same kind of nail), must have picked that up at the landfill :(

On our motorhome (32,000 Lbs), we air the tires according to Michelin's inflation chart for our XZA3s based on weight. Every so often we weigh the coach and adjust the inflation accordingly.

I had a feeling the Pro Comps weren't the best choice for rocky trails, but I suspect we'll make do with them until we trash them. When we tow the Jeep 4-down, we get quite a bit of tire scrubbing on the fronts since the Jeep front tires can't react to a turn instantly, so we'll probably get one season out of them towing and doing trails.

We will have OBA - I'm looking closely at the Viair Constant duty model. I want the air permanently installed so I won't have to worry (as much) about theft (we're a soft top.)

Carrying extra valve stems and a repair kit is a great idea.

Thanks again for educating me! :beerchug:
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My Goodyear MTRs needed to go down to about 5psi and still didn't get the flex and my KM2s got at 8-10psi. Load rating of tire means different stiffness.

I run beadlocks and don't worry about popping a bead.

When running with ZigZag (on this forum), he was taking a creek waterfall and popped a bead on 33s. We all pitched in and got the bead reseated. We preferred putting on the spare, but his position, lodged in the rocks, didn't allow for it. Once his tire was reseated and moving again, we found out two simple reasons we want to avoid: 1) the tire was now out of balance... a long drive to the tire shop to rebalance... and 2) since the bead was lost in water, the tire got water in it, making the tire extremely out of balance. As I recall, ZigZag drove the interstate at 30 miles an hour before he could get home and figure out the problem.

So, yeah, avoid popping that bead if you can. Great suggestions here. When I ran my Moab wheels, I'd go down to 10-12 without a problem.
Hi John, I think you'll like your Xterrains. I'm on my second year with my 315/75/16 xterrains. 95% of my use is off road, I've ran most of the bad stuff at Moab and a lot of rock trails in CO. My xterrains in Moad at the Easter Jeep Safari last year worked well where some people with different tires struggled. I run between 8-10 psi off road, love the ride at that psi. I use around 20 psi on the highway. Wife and I just got back two weeks ago from jeeping the St Elmo, White Pine area and then down to the San Jauns and most of the trails around Silverton, Ouray and Lake City area. I looked at a lot of different tires before deciding on the Pro Comps. I really hated the Good Years that came with our 05 Ruby we bought new in Oct. 2004. To much road noise and fast tread wear. They were about warn out at 22,000 miles when I installed my 4 1/2 RE LA last year. Have fun with the jeep and if you ever get up this way, Frisco, CO. we've got some good trails in this area and we would enjoy showing you around the area if you don't mind going with some old farts, been jeeping the rockies since 1955 when I got my first jeep, a 1946 CJ-2A.
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Hi Ron,

You're making me feel a little better about the Pro Comps on the Jeep! We're on our second set of All Terrain TAs on the Libby and after about 5 or 6K miles they get really noisy on the road. It will be good to put the Libby back to street tires since the LJ will be the wheeling bus.

The Xterrains surprised me - I thought they would be much noisier on the road (maybe they are, I can't hear them for all of the wind noise :cheesy: .) I'm anxious to get the LJ off-road and check everything out. Roger on the 8-10 psi off-road. I don't even know what air pressure the former owner was running on the street - I need to check the tires.

We love the San Juans, done several trails on two different summer trips. Did one trail in Moab as a day trip from Grand Junction this summer. The plans are to be back next summer - I want to spend a couple of weeks in Silverton again, and hit more of western and SW Colorado, Moab and New Mexico. Would love to hook up with you - I'm no spring chicken either, no worries there :laugh:
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I run 4-5psi with Hutchinson beadlocks on 35" KM2's. Problem now is i loose 2-3" of ground clearance! But they stick like glue!
Fremlin said:
I run 4-5psi with Hutchinson beadlocks on 35" KM2's. Problem now is i loose 2-3" of ground clearance! But they stick like glue!
Gotta love those rims! I ran the Rubicon with 4lbs in my tires on Hutchinsons, and look forward to going even lower when the snow gets here. I've seen a Jeep with Hutchinson's get winched completely sideways, aired down to about 4 - 6lbs, no problem.
With 37x12.50s on my JK with stock wheels I blew a bead at 12psi, D load range STTs. I was on the rocks, with the front right at least a foot off the ground and most of the weight on the left rear. I just got the LJ so I haven't really been anywhere to do any real wheeling, and I'm still waiting on the lift. I hope the TJ moabs hold a bead better.........
10 psi with 33's and steel wheels. Never an issue.
Prior to beadlocks, I ran around 12 lbs. Only lost a bead once.

With beadlocks, usually 10, sometimes a bit lower.
I may have missed it... if so my appologies. One thing I have not seen on here is taking into account what happens when you air down a "hot" tire. To explain... you go wheeling and you drive the jeep 50 miles or so to get there with an average speed on 50MPH or better. When you get to the trail your tires are going to be considered "hot", meaning the tread and the air inside is now more warm than that of the ambient air temperature.

Your OK to air down a hot tire. BUT after an hour or two on the trail, going through mud, streams, puddles etc those tires cool off some. As they cool, your air pressure in each tire will DECREASE. Just something to take into consideration if you are uber sensitive to these sort of things and/or trying to air down on a regular wheel as low as possible. This is USUALLY not a big issue. A hot tire aired down to 10 lbs (about my usual in a 35") will often settle and cool down to 9psi roughly.
I have 35" KM2's on 15 rims. I run 13-15psi and LOVE it. Like others have said, it all depends on the tire.
It depends on the trail for me ... places like Moab where traction is not a problem, 12-15psi ... places like the Rubicon where you are slow crawling for a couple days over big boulders, 8-10psi ... dirt roads at high speed, probably about 20ish psi hot.

This is 35x12.5" MTRs on 15x8 wheels.
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