Deflating for comfortable ride

[offroadohio]

I would assume it is one of those things he has to say for liability reasons. If he tells you to air down to 20 pounds and then something happens to the tire you could potentially go back on BFG.

I Street Drive my 35 inch tires at 20 psi.

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[MA_Trooper]

I would assume it is one of those things he has to say for liability reasons. If he tells you to air down to 20 pounds and then something happens to the tire you could potentially go back on BFG.

I Street Drive my 35 inch tires at 20 psi.

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I think it has a lot to do with not knowing the person's experience/knowledge level. If he tells them to air down, they do, and then they drive a few miles down the highway (the added friction produces great amounts of heat) compromising the integrity of the sidewalls of the tires, which often results in a blowout later on down the road/trail. When you air down it is important to maintain lower speeds. If you are flying down a road, then you didn't need to airdown.

 

[Michael]

Ya, also depends on the tire. Is it E-Rated? The BFG KM2 is very durable. Under certain circumstances, the engineer is right. It's situational.

 

[clmautz]

@rmerron Sounds awesome! I'm building my Jeep OL rig down here in Auburn. If you get down this way, give me a shout, maybe we can meet up!

 

[Lassen]

That's an odd statement. While running at a lower PSI will mean you are running a little bit on the sidewalls exposing them to things, but, the lower internal pressure means less chance of actual puncture. Think of a balloon being inflated a little or a lot. the one inflated a lot is easier to pop. I think that engineer's answer was a little oversimplified.

I don't know, but balloon analogy could also be a bit oversimplified. Remember, a balloon is a very thin piece of rubber stretched to the max. Rubber on tires is substantially heavier, so to me it's like comparing apples and oranges.

The recommended internal pressure of a tire could be just enough to support the rubber so as to keep it from deforming too much and remain intact. Too little pressure and the rubber could collapse too much. Think of air as an internal superstructure of a tire.

Maybe we could ask Tom Brady about proper inflation! Sorry, couldn't resist!


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[Lifestyle Overland]

Just cleaned up our original post and created a blog post on this subject: http://www.lifestyleoverland.com/deflating-overland-travel/

 

[MVO]

I spoke to a BFG rep yesterday. The guidance in simple terms was: Air down to as much as 20 psi. In sand not less than 15 psi. Blowing the bead is the issue. The off road tires like the KO2's etc are very tough. Remember to pressurize before running at speed. Even short distances can build a lot of heat.

 

[offroadohio]

I Street Drive my 35 inch good years at less than 20 psi.

On the trail with beadlocks I go down to 5 PSI when I have no intentions to get back on the road. If I am out in the woods running a few miles between the trails I will keep them at 10 psi.

I have worked in the tire industry since 2005 I have been to Goodyear and Michelin training facility and I've always been told that I'm running my air pressure is too low and I have never in my 12 years of off-road driving had a tire issue caused by low PSI other than popping the beads when not running beadlocks.

I would not suggest running the pressure that I run to anybody else but it works for me, but you should take into consideration that the larger the tire the less PSI that is required to support the weight of the vehicle. There is a Formula somewhere to figure it out but I don't know what it is off hand

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[PNW EXPLR]

I had the opportunity to speak with an individual (who shall remain nameless) within the BFG corporation. This particular gentleman was an engineer who helped develop a lot of the tires we run today. He told me (officially) that he can not condone running their tires at a lower PSI than recommended. However, (unofficially) he confirmed that running at a lower PSI under the right circumstances would not degrade the integrity of the tire itself.
That being said; there are several factors to consider when selecting a proper PSI... this was some of his input on the subject:

1) Wheel size versus overall tire size.

So how much meat (rubber) do you have between the rim and the rock you're about to hit? If you're running 15" wheels with 35 or 37 inch tires you can go down in the 10 PSI range with no issues. If you're trying to run 18" wheels then 20 PSI is probably as low as you want to go if there's a chance you'll tag a rock. The reason for this is you can actually damage the interior of your tire if you were to pinch the tire between a rock and the rim. This type of damage will go unnoticed until you either remove the tire, or have a blowout...

2) Speed.

The rate at which the tire deforms and reforms can have an adverse affect on the sidewall as well. If you're running 55 MPH down a paved road to a trail head at 18 PSI you're generating a ton of heat due to the constant deforming and reforming of the tire footprint. It doesn't take long to do irreparable damage to a tire under these conditions (which is also hidden to the user). Consider airing up even if it's just a short run down a hard surface at speed.

3) Surface.

This factor goes hand-in-hand with speed. The type of surface you're running on can make all the difference on PSI selection. For example, if you're running on sand, the surface itself is helping support the integrity of the tire since it is also deforming to accept the footprint of the tire. You can typically run much lower PSI on sand due to this fact. Rocky trails at slow speeds are also good candidates for lower PSI since you want more deformation to avoid a puncture or cut. Gravel roads would be a bit less forgiving especially if they are hard packed. Consider a medium range PSI.

4) Aggressiveness of steer.

Basically this means; how hard will you been cutting the wheel at speed? You can run 2-5 PSI in sand... in a straight line. If you plan on doing donuts then you're probably going to blow a bead... and possibly roll your rig if you're carrying enough speed. Same for rock crawling, if you're at an extremely low PSI and try to cut hard while in a bind, you're likely to blow the bead.

The final decision on PSI is really up to you. It's your rig, so experiment with different settings until you find those sweet spots for your unit's wheels-to-tire size ratio and final weight. I highly recommend investing in some quality deflators (Stuan or ARB) and a 100% duty rated air compressor (Dual compressor ARB unit is incredible) to save a lot of headache and time. Also, get a quality air pressure gauge... that free one from NAPA ain't no good! (After tons of research I got a digital craftsman unit from amazon)

And if you do blow a bead... try a heavy duty ratchet strap (kept in the tool kit) for wrapping around the tire while re-seating.

Here are my typical PSI settings for our 2014 4Runner running BFG KO's in 275/70R17 Load Range E:

Every day travel: 40 PSI front 42 PSI rear (45 PSI in rear if loaded)

Highway between trails: 30-32 PSI (Used this in Colorado since our route had highways in between trail heads)

Gravel Roads: 20 PSI (24 PSI in rear if loaded)

Rocky Trails: 16 PSI (18 in rear if loaded)

Sand: 14 PSI (18 PSI in rear if loaded)

Great write up. Thanks for taking the time to post this.

 

[The other Sean]

I Street Drive my 35 inch good years at less than 20 psi.

On the trail with beadlocks I go down to 5 PSI when I have no intentions to get back on the road. If I am out in the woods running a few miles between the trails I will keep them at 10 psi.

I have worked in the tire industry since 2005 I have been to Goodyear and Michelin training facility and I've always been told that I'm running my air pressure is too low and I have never in my 12 years of off-road driving had a tire issue caused by low PSI other than popping the beads when not running beadlocks.

I would not suggest running the pressure that I run to anybody else but it works for me, but you should take into consideration that the larger the tire the less PSI that is required to support the weight of the vehicle. There is a Formula somewhere to figure it out but I don't know what it is off hand

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Agreed. What a lot of people don't understand is a light truck like your Bronco II on 35's will require a different PSI than a 1 ton diesel truck on 31's...

 

[Lifestyle Overland]

So I don't condone this... and probably shouldn't even share this... but I've been slowly pushing the limits of my KO2's lately and finding that I can run 55-60 MPH at 16 PSI on pavement for 15-20 miles without raising the sidewall temp more than I would with normal pressures. Now, ambient temp was in the 50's and it was cloudy so I highly doubt I would have the same result in the heat of the summer, but it impressed me that there was no substantial heat build-up in the sidewalls (checked with laser temp gauge). Again, I don't condone this (feel free to slap my hand), but I think these tires are a lot stronger than we think. Still, it's good to maintain a sufficient safety threshold.

 

[MA_Trooper]

So I don't condone this... and probably shouldn't even share this... but I've been slowly pushing the limits of my KO2's lately and finding that I can run 55-60 MPH at 16 PSI on pavement for 15-20 miles without raising the sidewall temp more than I would with normal pressures. Now, ambient temp was in the 50's and it was cloudy so I highly doubt I would have the same result in the heat of the summer, but it impressed me that there was no substantial heat build-up in the sidewalls (checked with laser temp gauge). Again, I don't condone this (feel free to slap my hand), but I think these tires are a lot stronger than we think. Still, it's good to maintain a sufficient safety threshold.

Consider your hand slapped.

 

[Lifestyle Overland]

Consider your hand slapped.

I know, I'm a bad boy. But at least I know that in an emergency situation I won't feel that airing up will be a stopping point in extreme extraction circumstances.
If nothing else, it's good debate material!

 

[MA_Trooper]

I know, I'm a bad boy. But at least I know that in an emergency situation I won't feel that airing up will be a stopping point in extreme extraction circumstances.
If nothing else, it's good debate material!

The devil's advocate, I see. Yeah, I don't know much about the temp limits of my KO2s. I do know that a lot of tires have sidewall failure later on and it is often attributed to the heat generated at high speed/low pressure. Limits are always going to be case by case because different manufacturers use different compounds and construction.

 

[offroadohio]

Agreed. What a lot of people don't understand is a light truck like your Bronco II on 35's will require a different PSI than a 1 ton diesel truck on 31's...

Exactly, in the weight difference of vehicles and the tire size to rim size Ratio or both something that I forgot to mention.

There is a big difference between a 1 ton diesel running 20 inch wheels with a six inch tall sidewall, as compared to my Bronco 2 that's running 35 inch tires with a 15 inch wheel.


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[The other Sean]

Exactly, in the weight difference of vehicles and the tire size to rim size Ratio or both something that I forgot to mention.

There is a big difference between a 1 ton diesel running 20 inch wheels with a six inch tall sidewall, as compared to my Bronco 2 that's running 35 inch tires with a 15 inch wheel.


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When I try to explain doing the chalk test and the fact that I run my E load range Duratracs at 35 and 37 PSI on my Frontier, I usually get the deer in the headlights look and some response like "But those are E load range tires...."

 

[Vyscera]

When I try to explain doing the chalk test and the fact that I run my E load range Duratracs at 35 and 37 PSI on my Frontier, I usually get the deer in the headlights look and some response like "But those are E load range tires...."

I recently adjusted my rear 37" interco M16s to 35 PSI when unloaded, 42 PSI front. Its a far stretch from the 80 PSI that the OEM rear tires ran. But when I get an alignment, without fail the shop puts all tires at OEM PSI.

 

[offroadohio]

When I try to explain doing the chalk test and the fact that I run my E load range Duratracs at 35 and 37 PSI on my Frontier, I usually get the deer in the headlights look and some response like "But those are E load range tires...."

Load rating doesn't have a single thing to do with the amount of air that should be ran in the tires on your vehicle.

A load range e is just to safe at 30 PSI as a passenger Rated Tire is.


I think the Ford Explorer blowouts of the late nineties put a lot more fear in people about air pressure than is needed


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[Big E]

So I don't condone this... and probably shouldn't even share this... but I've been slowly pushing the limits of my KO2's lately and finding that I can run 55-60 MPH at 16 PSI on pavement for 15-20 miles without raising the sidewall temp more than I would with normal pressures. Now, ambient temp was in the 50's and it was cloudy so I highly doubt I would have the same result in the heat of the summer, but it impressed me that there was no substantial heat build-up in the sidewalls (checked with laser temp gauge). Again, I don't condone this (feel free to slap my hand), but I think these tires are a lot stronger than we think. Still, it's good to maintain a sufficient safety threshold.

Damage to under inflated tires will not show up for a while. Its not just the heat, its the flexing of the side wall with low pressure. Kind of like folding a piece of paper over and over, it will take a while but is will tear at the fold. Your damage may be done and waiting to fail. I am slapping your hand.

 

[Lifestyle Overland]

Damage to under inflated tires will not show up for a while. Its not just the heat, its the flexing of the side wall with low pressure. Kind of like folding a piece of paper over and over, it will take a while but is will tear at the fold. Your damage may be done and waiting to fail. I am slapping your hand.

Not arguing the fact it shouldn't be done... but if simply flexing damages the tire then we shouldn't air down at all, right? It's the flexing plus the heat that causes degradation.
Driving 10 mph down a washboard road is going to cycle the tire flex just as much as a 45 mph pass will.
In the end, it's the speed at which the tire forms and reforms plus the rigidity of the surface you're traveling which provides resistance to the forming and reforming which ultimately causes heat to build up if the cycle rate gets high enough.

Again, slap is understood... and expected. Talking theory now.