Tire Pressure Management for Off-Road

[Bunga]

Hi,
I've been running the 37lbs in a LX570 and that does seem like the right pressure for the 285/17 for city and highway. But when I get to my wheeling spot, I'm at 41lbs with the driving, so I air down and set it to 37lbs, where it says pretty stably until shutdown. I have a C02 tank for refill, so I air back up for paved driving. I definitely like the traction at 35-37, but that is still well above any normal recommendation for airing down. So what is the right approach to airing down pressures given the loss of load support at lower pressures?

 

[Mogwai]

I don't think there's any magic formula. If my cold psi is 38, normal wheeling I would go down to 18psi or so (50%). If I'm in the desert and know I'll be going a little faster than normal I'd stay a little more inflated, call it 25psi. If it's slow going and technical you could go down to 12 to 15 and you shouldn't have to worry about rolling a bead but it could happen.

If I have to hit the road to get to another trail, I'll only do that without airing up if I'm around 25, and only for a few miles, otherwise I air back up then air back down.

 

[grinchy]

I run 34 or 35 daily psi on my 129 load e/lt tires. They have a very stiff bead and I think I could run really low pressure, like 10 or 12 if needed. Haven’t had to. I run 20 on forest road type stuff, and 15 if sloppy snowy or rocky.

Misc tidbit. This weekend was on a four mile long wide flat and very potholed forest road. Tried it without airing down but after 1/2 miles gave up and put on the stauns, mostly for comfort. Several rigs passed me, a 4Runner trd, lx470, a couple pickups and a hatchback while I waited. None aired down. I figure it takes 10 min for my stauns to deflate.
I passed all of those rigs in the remaining 3.5 miles. So not only is it better for puncture and flatting, but aired down pressures are much faster to drive on (and way more comfortable). Even over a short 4 mile road, even including air down time.

 

[Bunga]

I'm using dual filler hoses so that I can air up faster. Drag to one side and fill two, with a helper, at the same time. If the loss of load capacity is OK then I'll drop mine to 30 and see.

 

[rlynch356]

Normally the first question is what are you doing off road? (rocks, dirt road, etc)
Second is what tire/wheel combo.. 18" have less sidewall, and the A/T tires on them are typically not E rated.
17's have more sidewall, and greater tire selection (E rated, Mud tires etc..)
Third, is truck load which affects the F/R balance for pressure.

The 200 is a heavy truck, which will deform the sidewall at lower pressures fairly significantly. Lower pressures(15-20Psi) gives you maximum tire section and "grip" but also comes with caveats, namely rolling the bead off, or slicing the sidewall on sharp rocks.
10-12Psi is probably too low for non-E rated 17" tires,

So it depends.. on 1 & 2 above.

For most stuff with a heavy truck I usually recommend 20psi give or take load and weight distribution (normally a bit higher in the rear) and adjust from there up or down.

 

[TeCKis300]

Lots of variables in this and it's not an exact science either. I didn't catch exactly what size tire you're running? 285/17?

As mentioned by others, depends on tire size, sidewall, terrain, speeds, and weight of your rig. One of the primary governing principles is to make sure the pressure is not so low, for the highest speed you expect to travel at, for the given weight of your vehicle, to not overheat the tires. Overheating tires will fail from the inside out, delaminating, and cause structural failure of your tire in ways that may not be visible from the outside until too late.

Other thresholds making sure to not go too low are rolling a bead, but also not pinching a sidewall.

Not trying to make it sound undesirable to air down too far. I'm actually a proponent of airing down more. Just have to be conscious of those things to not go too far.

For my 305/55r20s with a RCTIP of 35psi, on a relatively stock weight rig, I usually go straight to 20psi for most everything off-road. For faster smooth track stuff, I may keep it higher like 26psi. If attacking soft stuff like sand, 12-15 PSI depending on how dry or sugary the sand is. Lower, will not want to steer as hard to avoid rolling the bead.

 

[grinchy]

One 'rule of thumb' I have seen a couple places for RCTIP on an 'unknown' tire is to put air in it until the cold/hot temp difference is within 10%. I don't know if it applies to aired down, but I keep an eye on my aired down pressure, and if it went up by more than 10% than would slow down. Haven't needed to yet, but I watch it. Particularly on a fast road where I'm running 40+.

 

[eatSleepWoof]

If I'm going down fairly smooth FSRs, I don't bother airing down.

If I'm going down a real trail, I drop down to 18PSI (hot) regardless of the vehicle/tire (assuming AT/MT)/etc. It's a generic ballpark that works well for me.

 

[Bunga]

This is all great advice. Thanks. The truck is so capable it certainly can do very well even with A/T 21s at stock pressures.

I run LT285 /70 R17 116Q C1 BS. C-load: I general lower to stock for fast dirt/gravel, as it raises from 37 to 41 after driving on pavement an hour or so to get to the trailhead (unloaded), and I've been playing with lower pressures. Pretty clearly 37 is way too high for rolling up bedrock stairsteps and and real sand. But I'm leery about getting really low on the pressure for the pinch/rollover issues and the massive weight of this rig...

So I'll drop to 30 on my tracks and watch the temps and see how it goes. I don't do serious crawling as my passengers get pretty twitchy on sidehills and exposure, but they don't mind sand blasting or mud.

 

[eatSleepWoof]

This is all great advice. Thanks. The truck is so capable it certainly can do very well even with A/T 21s at stock pressures.

I run LT285 /70 R17 116Q C1 BS. C-load: I general lower to stock for fast dirt/gravel, as it raises from 37 to 41 after driving on pavement an hour or so to get to the trailhead (unloaded), and I've been playing with lower pressures. Pretty clearly 37 is way too high for rolling up bedrock stairsteps and and real sand. But I'm leery about getting really low on the pressure for the pinch/rollover issues and the massive weight of this rig...

So I'll drop to 30 on my tracks and watch the temps and see how it goes. I don't do serious crawling as my passengers get pretty twitchy on sidehills and exposure, but they don't mind sand blasting or mud.

Sand and 30PSI won’t mix well. Go down to at least 20PSI. You’ll be fine (and better off).

 

[CruiserFan88]

Normally the first question is what are you doing off road? (rocks, dirt road, etc)
Second is what tire/wheel combo.. 18" have less sidewall, and the A/T tires on them are typically not E rated.

What would you do for a P-metric A/T tire? Does the 20PSI average still apply? I've never had a problem running FSR type trails at the RCTIP of 33PSI, but i'm curious to know if airing down is advisable/necessary for these conditions.

For my situation specifically, I encounter a mix of uphill and downhill stretches of mild rock/stone while the majority of the trail ranges from dirt/grass/mud. I'm not sure if going over the rock sections at 33PSI (probably more like 35-36 due to drive time to the entrance of the property) is acceptable.

Another thing is elevation. Trailhead sits at around 2500ft while camp is 4200ft.

 

[rlynch356]

I’d probably start at 25psi and adjust up/down from there... I would optimize for the most common terrain. Weight of the rig is going to be the biggest factor in how low you can go before you run the risk of rolling the bead off (and tire/wheel Contruction)

For instance in my d90 we have some trails I use 25psi (255/85-16 km2 E rated) because of the sharp shale and rocks, giving up grip for sidewall strength. in uwharrie mud and rock climbs mostly)I’m at 15psi... to maximize the contact patch.


just make sure you have a way to inflate before you deflate.

 

[CruiserFan88]

I’d probably start at 25psi and adjust up/down from there... I would optimize for the most common terrain. Weight of the rig is going to be the biggest factor in how low you can go before you run the risk of rolling the bead off (and

Thanks for the feedback. I wasn't sure if airing down a P metric tire would be any safer than tackling the terrain at normal operating PSI as the P metric doesn't have the sidewall strength that an LT tire does. Grip usually isnt a concern unless the area gets a lot of rain so I'm more concerned about rock punctures. At this point I don't need/haven't really needed an LT tire and not interested in the compromises that come with it. When the LC isnt my daily driver I will probably switch over.

So i've been looking into installing an onboard air system and that's one of the main reasons I posed the question. If airing down makes sense for the P Metric than I can justify doing it. If its safer to not air down a P metric at all, then I would hold off. Having an air compressor on board though is still useful regardless so may still consider doing it.

 

[afgman786]

Thanks for the feedback. I wasn't sure if airing down a P metric tire would be any safer than tackling the terrain at normal operating PSI as the P metric doesn't have the sidewall strength that an LT tire does. Grip usually isnt a concern unless the area gets a lot of rain so I'm more concerned about rock punctures. At this point I don't need/haven't really needed an LT tire and not interested in the compromises that come with it. When the LC isnt my daily driver I will probably switch over.

So i've been looking into installing an onboard air system and that's one of the main reasons I posed the question. If airing down makes sense for the P Metric than I can justify doing it. If its safer to not air down a P metric at all, then I would hold off. Having an air compressor on board though is still useful regardless so may still consider doing it.

When I was on my P metric street tires and hitting trails, I would air down to 25/26 but no lower. Same reasons you mention, the sidewall wasn't as tough and I didn't want to destroy the tire but still make the ride a little smoother.

 

[Cruiserdrew]

I guess it depends on if you need more traction or not. If you need more traction, then below 20 PSI psi is the answer. I've run my heavy 60 and 80 on the Rubicon at 15. In Moab 18-20. Death Valley 20 psi. In deep sand, you'll need to go much lower. Like 10psi.

If it's just a comfort thing, then only you can decide that, and a few PSI either way makes no difference.

I'm going to venture a guess that there is very little change in contact patch size between 37 and 30. An old rule of thumb is to look for the amount of tire in contact with the earth to increase by 50%. For the average off road tire, that's usually between 15 and 20 psi.

Do some real experiments-like a real trail, at 15 psi vs 30. You will be impressed even though you will at first be nervous.

 

[CruiserFan88]

Does airing down also help reduce risk of sidewall punctures? Im new to the concept and while I understand it I never used it. The tacoma 33's always stayed at 35 and 11 years i've never had problems, Uwharrie is the worst its ever seen but it been through the entire park no issues. Sure it was a stiff ride. None of us ever aired down for anything though throught out the years im thinking its because we didnt know we should be.

Its talked about a lot on the forum so I wanted to expreiment with it as I have a few rocky spots that will need some work eventually. Perhaps the better question would have been 'can a P metric tire be aired down safely and to what psi?'. Real life expirements are the best way, youre right about that. Now im going to try 25-26 with some confidence based on afgman's suggestion.

Thanks for the help

 

[TeCKis300]

Interesting comments from Joe Bacal who is a factory test driver, that helped develop some of Lexus's race campaign trucks for Baja, also a test driver for many tire manufacturers. You can find his comments on his youtube channel.

re: P-metric factory highway tires
He's said that he's done many thousands of miles on factory cars, with factory tires, at factory spec tires. That's how they develop them, and that's how they run 'em. He's rarely ever had punctures or issues even in that configuration.

The one place where you almost must air down is sand. Particularly fine hot dry sand. Perhaps not as much as AT/MT tires as highway tires actually have an advantage in this type of surface, but lowering to 20-25 psi (AT/MT need to go lower) can be the difference between confidently traversing sand, or bogging down to the frame rail.

With the right tires, airing down can be a huge advantage everywhere else.