Airing down tires with camper on

[JaSAn]

My experience driving on ice, snow, wet clay, grass, dirt, loose gravel:
- RWD vehicles can transition from oversteer to understeer quickly. An inexperienced driver does not handle that well.
- A 4WD vehicle is closer to neutral steering in all those conditions.

From my experience ice racing FWD, RWD, 4WD; the vehicle dynamics information is there, it just feels different.

 

[JHanson]

Let's remain on topic and debate one safety issue at a time - we can come back to seat belts after we conclude this discussion.

My argument is that driving in 2WD provides more available information to the driver regarding available traction when compared to driving in 4WD. This additional information is helpful in the short term since it immediately provides the driver with an accurate measure of traction where this vehicle is at the present moment. This additional information is also helpful in the long term since it directly enhances driver skill.

Your counter argument is that reducing the available information regarding traction to the driver is safer in the short and long term. And that's exactly what driving in 4WD when it is not necessary does - it reduces the level of detail regarding available traction to the driver. Is this your argument?

No. My argument is that increasing traction on a loose surface increases safety, period. And I disagree that being in four-wheel drive reduces the information available to the driver. Why would it? In either case there is going to be a point at which traction is lost or is about to be lost, and any competent driver can discern when the limit is approaching. On a loose surface, at any speed, you will have more traction in four-wheel drive than you will in two-wheel drive. More traction equals more safety.

 

[ski3pin]

We are going to allow Mr. Hanson to have the last word here on 4x4 vs. 4x2 and return to the topic discussion on airing down tires.

Enough said.

 

[Wallowa]

We are going to allow Mr. Hanson to have the last word here on 4x4 vs. 4x2 and return to the topic discussion on airing down tires.

Enough said.

Good call Ski....

 

[steve whiteside]

To increase comfort and reduce shaking the camper to bits, we reduce from 45/62 psi down as low as 15/20. Washboard isn’t as bad as big rocks, so we reduce to about 25/30. Were not rock crawlers but enjoy getting to places on pretty rough roads.

Me too. It was surprising to me that others did go as low of psi. I don't really see a downside to going to ~25-30psi unless you fear the bead loosing integrity or the sidewalls getting cut with rocks. My heavy F350 quad cab and BF Goodrich All terrains has no issues in less than 30psi.

I do get stuck at least every year in the beach sand of Baja. Not every time but maybe 1 in 10 or 1 in 15 times. Usually needing to turn on a side slope or going up the beach grade. Then need to drop the trailer; drive the truck to firmer sand/less slope and recover the trailer in some other manner.

I never drive onto a beach without a recovery plan. I get stuck way too often.

Steve

 

[Mighty Dodge Ram]

265/75-16 LR-E Toyo AT-2s. I try to keep the math simple: FS roads etc pressure reduced by 25%, washboard reduced by 50%, sand (meaning beach driving, which I don’t do much of) 18 psi. I adjust from these initial set points as needed. I run a 3/4-ton longbed but am seldom at max GVW.

 

[rubberlegs]

The FWC Fleet weight is 100% on the rear axle on our access-cab Tacoma. So my philosophy is the front tire pressure would be the same with or without the camper. For the rear we use the same ratio as the front. I've not done the math to see if this is "correct" per the tables, but for slow driving off pavement it's more of a comfort issue. Frankly we've rarely had a traction issue.

I kinda like the ratios discussed here, and use a similar ratios. I hadn't realized I was using ratios, which really simplifies thinking about it. But on Forest Service roads, which aren't that bad here in Washington State, we use pavement pressure. Most of our trips off pavement here are pretty short. It's in the desert where there are long and rocky roads that we'll drop pressure.

I've read that low pressure on sharp rocks may have fewer punctures. This kinda makes sense, and kinda doesn't. But the comfort factor is huge with all that stuff rattling around in the camper. Everything not on the floor tends to end up there, and the dinette table, no matter how hard we tighten the table leg, tends to rotate. So we pad the table with blankets and pillows, and put everything heavy on the floor.

I think the ratios work for E-rated tires, but not for P-rated tires, which have much lower pressure on pavement.

 

[JHanson]

You do have to pay attention to conditions and react accordingly. In Egypt some years ago we needed to get three Land Cruisers up a 300-meter-tall dune escarpment. Time to air way down? Except the escarpment was also littered with razor-sharp limestone outcroppings called kharafish. We had to leave the tires almost fully inflated and rely on momentum and multiple tries. We made it up and only sliced open one tire.

 

[ri-f]

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Bad, long, washboard roads get my attention. To take the edge off, I'll air down from my pavement pressure of 65 PSI, all around, to 35 PSI, on my E-rated Nitto Ridge Grappler 285/75R16 10-ply tires, on my Cummins/Hawk combo (8,700 lbs split equally on both axles - the Cummins diesel is heavy enough to keep the front axle the same weight as the rear with a slide-in Hawk shell on my flatbed). When airing down I use Coyote automatic deflators preset to 40 PSI (my tires inflate/deflate +/- 5 PSI, the differential between a sunny, hot afternoon and cool night and morning temps). The Coyote deflators are adjustable from 1-65 PSI if you want a lower or higher preset.This takes me just two minutes for all four tires and is easy enough and the Coyotes are excellent with accurate repeatable results. I know I can go lower, but don't want to risk sidewall damage or losing a bead unnecessarily. At 35 PSI my tire rim is only 1/2 inch lower to the ground and the sidewall is not compromised. I suppose I'm playing it rather conservatively. I also go slow on washboards and rougher roads, regardless of whether I'm aired down. or not. The oscillations of a washboard might allow me to go faster if it helps to dampen the ride. But generally, slow is usually easiest on my truck and on me. Of course it does take a longer time to get from A to B, but the tires don't heat up as much and the sidewalls have held together well over time. I've only had one puncture in the last few years on service roads, and that was clean through the middle of the tire tread on a front drivers side tire, and caused by a sharp, long lag bolt hiding in a pothole. Also, I don't air down on forest service roads mainly because my tires have plenty of traction and don't spin in most circumstances, and because I drive slow enough on the rougher sections as to not put a tremendous amount of shock loading on my suspension. When I'm ready to air up again, after a few or after a lot of slow miles on a miserable corrugated road I pull out my Viair 400p air compressor, which I love, and air up back to 65 PSI. This takes me about 16 minutes to add 30 PSI to all four tires (~ 4 minutes per tire).

Ocassionally, if the road is slick with slimy mud or slick with icy snow or I need some extra traction while beginning to climb a slippery grade of loose rock and dirt or snow or move through an obvious wet, muddy patch I'll drop into 4WD to get through the dicey parts and not air down. I don't want to rehash this whole 4WD vs 2WD thing here, but in my case my truck is equipped with part-time 4WD, so staying in 4WD is not necessarily preferable for me on mostly dry, switchback dirt roads where I have decent traction. The type of forest service roads that I tend to drive along don't really resemble extreme, crude, narrow, wonky Jeep trails. With part-time 4WD as many older trucks have, the truck will tend to understeer, unless on a slick surface, and also, the drivetrain components will wear out prematurely if we leave 4WD engaged all the time on dry roads with traction, unlike full-time 4WD as I understand it. I can certainly see the benefits of full-time 4WD and might use it more frequently if I had it, but since I only have part-time 4WD with both a high and a low gear range available, in addition to a manual 5 spd. tranny, I'll only use my 4WD on dicey surfaces and will stay in 2WD, which has never been a problem for me while driving the forest service roads of the Northern Cascades to the Southern Rockies, unless I see a real need to drop into 4WD.

Rich

 

[wicked1]

I just got back from a 2 week trip in Colorado and Utah yesterday. Drove through the 'point of no return" road in sand dunes national park. I didn't air down at first.. Well, I was slightly aired down from driving on a really rough road near the park (the waterfall). I'm usually at 50 front, 60-65 rear on the paved roads.. I was at about 40/50.
Got to the soft sand spot, made it most of the way across, then got stuck. I aired down to about 25/35 and drove right out, w/out any problems.
Then we were out on some trails on BLM land with really rough rocky trails.. (used to be roads, but are no longer maintained). I had to do a lot of road building there.. stacking rocks, etc. Was super fun! The wife didn't handle it well, though, heh.
I have read on a bunch of 4x4 offroad forums that you should air down on rough rocks to let the tires flex rather than puncture, so I did.. And it was smooth riding w/out any problems. Was at about 30/40 for the rocky roads.

 

[rando]

If I am going to be on dirt roads for more than a few hours, I typically reduce tire pressure from street pressure of 50/55psi front/rear to 30/35psi. It makes the ride more comfortable and reduces the stress on the vehicle and camper. I drop the pressure with a set of Coyote deflaters (2 for the front at 30 and 2 for the back at 35psi) which take around 5 minutes. Re-inflating is about 15 minutes. You can get a good idea if your tires are getting too hot by watching the tire pressure through the TPMS readout. If the pressure goes up by more than 15% then your tires are getting hot and you may need to slow down or increase tire pressure.

I also shift into 4wd as soon as there is any sign that I am loosing traction, or there is a specific obstacle coming up that looks like it will require 4wd. Four wheel also seems to provide better handling on fast washboard roads (which we seem to spend too much time on) even though it may not require it for traction. It seems to help maintain steering control even if you are driving fast enough to 'skip' across the corrugations.

 

[ramblinChet]

Airing down is not supported by any tire or vehicle manufacturer that I am aware of and should be considered on a case-by-case basis.

While working at NASA LaRC and completing my MS in Aeronautics with a focus on safety, one of my studies was on aircraft and spacecraft tires. I immersed myself in technical cases with some involving vehicles such as a Ford/Firestone debacle. I have a particular interest in tires since for several years I raced superbikes on the national circuit and was always amazed by the fact that just a few square inches of rubber contacting the surface of the race track provided enough centripetal force to counter the extreme cornering forces of me carving the bike through turns at speeds well in excess of 100 MPH. Or just that tiny patch of front tire was enough to decelerate me from 170 MPH down to nothing in seconds. Without going into much additional detail - my professional knowledge of tires goes well beyond average.

When I talk with engineers, technologists or technicians we quickly establish a mutual respect and can dive into very interesting areas. If you have ever wondered why manufacturers will not publish the results of testing at low pressure it is because the tires were never designed to operate under those conditions and it is dangerous. The most information I have ever received from a manufacturer was a few years ago at a national event - I was speaking with their engineering staff following a wonderful presentation and brought up the common practice of airing down tires for additional comfort and limited improvement in traction is some conditions. The very most they would even state was that in an emergency condition it was considered somewhat acceptable to air the tires down up to a maximum of 1/2 the vehicle manufacturers recommended pressure to extract yourself from a stuck condition. They were adamant with regarding to only being off pavement, staying below 15 MPH and only doing this for emergency purposes.

And just because you have a buddy that knew a guy that read somewhere on the interwebs that this guy in a CJ5 ran his E-rated boggers at 0 PSI on the trail and never had problems - that really does not matter.

What I can tell you with absolute scientific certainty is that heat is the primary enemy of tires and the greatest source of heat results from tires operating below their recommended pressure. Am I am absolutist, not really. But just because you run washboard roads though the desert at 45 MPG with your loaded down camper and your tires at 15-20 PSI and heck, they are just fine, never had any problems...that does not equate to all being well. The simple fact is that components rarely fail catastrophically during one incident - it's most often a progressive failure that goes largely unnoticed over time and at some undetermined time in the future the culmination of past minor failures results in a final and catastrophic event. Hopefully this does not involve blowing a rear tire at 75 MPG coming down a mountain pass with your tires at normal operating pressure.

Just in case you are interested the absolute worst condition you can put any tire in a under a heavy load, for an extended period of time, on a hot day, at a high speed, at an unsafe pressure. This creates a dangerous amount of heat on a molecular level and internal components of the tire begin to break down. This are micro-fractures that can not be seen with the human eye and progressively grow. What is the limit? I don't know but I know that until I find specific information from a manufacturer detailing the pros and cons of tire safety and how it relates to running tires underpressure for extended periods of time I will proceed with much caution.

And we must never forget that there exists such a wide variety of vehicles, tire sizes and construction, driving conditions and styles that we can only speak in the broadest of generalities regarding the effectiveness of airing down a tire. Most of it is about comfort anyway but in life nothing is free and in some cases when you ask for more comfort you forfeit the safety of not only yourself but others.

 

[JWL]

Airing down is not supported by any tire or vehicle manufacturer that I am aware of and should be considered on a case-by-case basis.

While working at NASA LaRC and completing my MS in Aeronautics with a focus on safety, one of my studies was on aircraft and spacecraft tires. I immersed myself in technical cases with some involving vehicles such as a Ford/Firestone debacle. I have a particular interest in tires since for several years I raced superbikes on the national circuit and was always amazed by the fact that just a few square inches of rubber contacting the surface of the race track provided enough centripetal force to counter the extreme cornering forces of me carving the bike through turns at speeds well in excess of 100 MPH. Or just that tiny patch of front tire was enough to decelerate me from 170 MPH down to nothing in seconds. Without going into much additional detail - my professional knowledge of tires goes well beyond average.

When I talk with engineers, technologists or technicians we quickly establish a mutual respect and can dive into very interesting areas. If you have ever wondered why manufacturers will not publish the results of testing at low pressure it is because the tires were never designed to operate under those conditions and it is dangerous. The most information I have ever received from a manufacturer was a few years ago at a national event - I was speaking with their engineering staff following a wonderful presentation and brought up the common practice of airing down tires for additional comfort and limited improvement in traction is some conditions. The very most they would even state was that in an emergency condition it was considered somewhat acceptable to air the tires down up to a maximum of 1/2 the vehicle manufacturers recommended pressure to extract yourself from a stuck condition. They were adamant with regarding to only being off pavement, staying below 15 MPH and only doing this for emergency purposes.

And just because you have a buddy that knew a guy that read somewhere on the interwebs that this guy in a CJ5 ran his E-rated boggers at 0 PSI on the trail and never had problems - that really does not matter.

What I can tell you with absolute scientific certainty is that heat is the primary enemy of tires and the greatest source of heat results from tires operating below their recommended pressure. Am I am absolutist, not really. But just because you run washboard roads though the desert at 45 MPG with your loaded down camper and your tires at 15-20 PSI and heck, they are just fine, never had any problems...that does not equate to all being well. The simple fact is that components rarely fail catastrophically during one incident - it's most often a progressive failure that goes largely unnoticed over time and at some undetermined time in the future the culmination of past minor failures results in a final and catastrophic event. Hopefully this does not involve blowing a rear tire at 75 MPG coming down a mountain pass with your tires at normal operating pressure.

Just in case you are interested the absolute worst condition you can put any tire in a under a heavy load, for an extended period of time, on a hot day, at a high speed, at an unsafe pressure. This creates a dangerous amount of heat on a molecular level and internal components of the tire begin to break down. This are micro-fractures that can not be seen with the human eye and progressively grow. What is the limit? I don't know but I know that until I find specific information from a manufacturer detailing the pros and cons of tire safety and how it relates to running tires underpressure for extended periods of time I will proceed with much caution.

And we must never forget that there exists such a wide variety of vehicles, tire sizes and construction, driving conditions and styles that we can only speak in the broadest of generalities regarding the effectiveness of airing down a tire. Most of it is about comfort anyway but in life nothing is free and in some cases when you ask for more comfort you forfeit the safety of not only yourself but others.

WOW! Really makes me stop and think about it.
So what about the opposite side? Most of us are running larger tires and PSI above what is stated on the vehicle. I’ve only gone one size up in my tires but the E rated Coopers have a max PSI of 80, I’ve found (through chalk tests etc) that 65 and 60 is the sweet spot for my rig. Definitely not the 30/33 stated on the sticker! I know that was for the P rated tires but…? Again it’s what the manufacturer suggests.
So many decisions, so much to think about, what’s right and what is wrong? I guess maybe ya just gotta go with your gut.

 

[wicked1]

Manufacturers aren't known for taking risks. Quite the opposite.
So yeah. No surprise they aren't going to 'suggest' lowering pressure in any circumstance. Then some idiot who has no idea why they are doing it, will decide to do it when there's a bit of snow on the ground, and run at highway speeds at 1/4 pressure of something. They have to think of the worst case, possible litigation, etc.
We aren't in regular circumstances. We are making educated decisions. We are driving (at least I am) at about 10 mph when aired down to about 2/3 pressure. We are aware of issues and check our tires regularly. I don't see any issue.

 

[rubberlegs]

I’ve never understood why E rated tires must have higher pressure than P rated tires. Is the thicker sidewalk less efficient for cooling?

 

[ramblinChet]

In regards to running higher pressures the same technical staff members I mentioned above unanimously agreed that running too much pressure is safer compared to running too little. As most of you know when you run too high of a pressure the tire loses some traction for a number of reasons. As the pressure increases the contact patch does become smaller but the majority of the traction lost has to do with the fact that your tire is a key component in your suspension system and now that it is harder it is transmitting more forces into the remaining components in your suspension system. The system is out of balance and not designed to operate that way.

The good thing is most manufacturers provide load and inflation tables to use as a reference - search for "tire load and inflation table" and read up.

But don't be too quick to weigh your truck on nearby scales and say since your 8k pound truck has 4.5k on the front axle and 3.5k on the rear you can just use the table and inflate to that exact pressure. When a vehicle manufacturer determines optimum tire pressure this is a very deep and time consuming issue - their goal is to provide the safety that you expect balanced with performance, comfort and longevity. When you weight your truck you are only taking into account static loads. When an engineering team works to provide a solution they must also take into account dynamic loads in normal and emergency scenarios.

Will I air down my tires on a case-by-case basis, sure. Will my tires be at an optimum pressure while driving on paved roads, definitely. Will I continue to study this topic and ask questions to learn more, most assuredly.

 

[rubberlegs]

ramlinChet, you bring up some good points. I've read everything I can find online, but it's still pretty confusing (and I used to design landing gears for a large airplane manufacturer, but not the tires).

The placards on our truck state 30 psi. It came with P-rated kevlar tires, which per the numbers should be capable of handling the load of our camper (despite our rig being 6-8% overweight -- we won't talk about other components). After getting a nasty sidewall puncture on a pretty rough road, we upgraded to E-rated tires. The only guide with real numbers I can find is this Toyo publication.

It definitely states that pressure in the E-rated tires are higher than the stock P tires, but doesn't that change the braking and suspension characteristics?

As for front and rear pressure, to me the contact area does make sense. if there's 4.5k on front, 3.5k on rear in your example, it seems for both tire safety (overheating), and braking/suspension that the heavier loaded tires should have more pressure, thus contact area is consistent with load.

 

[ri-f]

It's true that airing down is thought to be risky business and a potential for liability as far as tire manufacturers are concerned, regardless of proven real-world experience and practice by offroad enthusists. As an example, here is a tire manufacturer that has a dual message. One is its marketing stategy, the other is its liability/warranyt policy, which says it all:




Marketing

Roadian MTX Mud Terrain Xtreme tire. Although most off-road tires have dual sidewalls, Nexen carried the load-bearing tread pattern from the top of the tire over the side wall providing even strong gripping power. With an F-load rating on all flotation sizes and exceptional low air pressure performance, the Roadian MTX can be aired down to extremely low PSIs with reduced risk of rollover and the best part -- no bead lockers required.

Press Release describing the merits and safety of airing down their offroad tires/


And here is the reality of their warranty and warnings (despite their press release):

Warranty
2. WHAT IS NOT COVERED BY THE WARRANTY NON-ADJUSTABLE CONDITIONS A. Irregular wear or tire damage due to: • Road hazards such as punctures, cuts, snags, scufs, carcass bruises or impact breaks • Fire, wreck or collision • Improper infation, overloading, high-speed spinning, improper mounting or demounting, running fat, of-road use, racing, vandalism, willful damage or abuse
AIR PRESSURE Check the pressure in your tires, including your spare, at least monthly, and always before and during extended driving — when tires are cold (at least 3 hours after the vehicle has been stopped and before it is driven more than 1 mile/1.6 kilometer). Do not reduce pressure when tires are hot: Use an accurate air pressure gauge to check pressure and maintain it at the level recommended on the vehicle tire placard or in the owner’s manual. Under-infation produces extreme fexing of sidewalls and builds up heat to the point that premature tire failure may occur. Over-infation can cause the tires to be more susceptible to impact damage. Cold tire pressures, however, should never be higher than the limit molded onto the sidewall.

The point being: the manufacturer has no indemnity clause built into their legalese. They take no responsibility for their tires if they fail offroad or aired down and give you no guarantees. Ironic, since they market these tires as uniquely offroad capable and tout their ability to be aired down to as low as 11-15 PSI for rock crawling - see their press release above. Basically, offroad or air down at your own risk is what they are saying.

 

[Wallowa]

Ok...probably none us with time off road are going to change the driving practices that have safely and effectively got the job done over the years and many miles of varying conditions....hopefully, we learn from our experiences and do not just make decisions on theoretical.

I see the "manufacturers" mentioned....the tires we use, pressures we use and loads we carry, surfaces we drive on are not in the matrix that can be related to the vehicle manufacturer's recommended tire pressures for OEM tires and recommended max vehicle GVWs...

However the tire manufacturer's load ratings for specific size and class tires can and should be considered for on or off road, but you need to know what the load is that you are putting on the axles and total vehicle "wet" weight with all gear and persons aboard.

To each their own about tire pressures and of course tire selection..just remember to error on the conservative side of the equation since a failed tire can mean getting stuck in BFE or worse and most of us, if we admit it, are over GVW. Not rocket science but really pretty straight forward..

Oh, in passing, lowering pressure does increase tire contact patch by lengthening not widening the patch, and yes, lower pressure does increase tire pressure as has been stated.

 

[JHanson]

I'm going to try not to get into another argument here. However, running a tire at, say 50% pressure at 30mph on a dirt road will not have the same effect on that tire as running it at 50% pressure at 75mph on pavement, period. The heat-inducing hysteresis is vastly reduced at low speeds and on surfaces with less than perfect traction.

And as it happens, at least one tire manufacturer, Michelin (in Europe at least), does in fact advise on off road pressures for various scenarios, with a minimum of 1.5 bar (about 21 psi) for sand at a maximum of 12 mph. Clearly they recognize that pressure and speed are interdependent, and that it does no harm to a tire to run it at 12 mph at 21 psi in soft sand.

Please: Do yourself and your passengers, your tires, your suspension and cargo, and the trails we share a favor and AIR DOWN to appropriate levels to suit conditions. Trust the collected wisdom on this—and Michelin.

I'll let that be my last word.

Reference Tom Sheppard's Four-by-four Driving, among many others, for solid advice on this.