Airing Down with a Camper

[rando]

We are headed out to the Maze in Canyonlands next weekend, and expect to do 100 - 200 miles of rough dirt roads over 5 days of exploring. I am not particularly worried about traction for this trip, but I have heard the roads in the Maze have been getting rougher, so it seems prudent to reduce air pressure at least for comfort.

My question is how low do folks go, given the heavy loads that our trucks carry?

For reference, I have 235/85R16 load range E BFG AT KO2's and normally run 55 psi in the rear and 48 psi in the front based on load inflation tables. For previous trips (eg Death Valley) I have lowered the pressure to 45/40 front/rear, but this didn't seem to make much of a difference, so I am wondering if it would be safe/prudent to go lower. On my pop-top Landcruiser, I would go down to about 25 - 30 psi for trips like the Needles/Elephant Hill and that seemed to improve slick rock traction and soften the ride.

Thanks!

 

[kmacafee]

I had the same size tires on my Tacoma and carried a FWC Eagle on the back. I typically went to 25 in rear and 20 in front and never had issues.

 

[CougarCouple]

Hay Rando
One thing I would mention is hard cornering slow or fast increases chances of rolling off the bead. Carry a strong ratchet strap to help reseat if this should happen.
Also starting fluid for reseating should one come off. YouTube that one, to learn how not to be taken lightly.

Russ

 

[JCatt]

I would look at the tire load tables and get the truck on some cat scales. With your load range E you could probably drop them quite a bit depending on weight

 

[rando]

After doing the research I should have done BEFORE starting a new thread, I found this:
http://www.wanderthewest.com/forum/topic/11408-off-road-tire-pressure-how-low-do-you-go/

Unfortunately it seems that opinions on how low to go are all over the place - from about 17 - 45 psi. I may start at 40/45 for the fast washboard stuff (en route to Hans Flat ranger station) and then go lower for the slower stuff beyond there.

The thing that confuses me is that on road the wisdom for keeping a certain tire pressure is to stop the tire flexing, heat building up, and the tire failing. For much of the dirt road driving that many of us do, driving on generally decent but washboard roads, at 40 - 50 mph, it seems that you could have a lot of heat build up from airing down, as the whole point is to absorb some of the shock in the softer tires. It seems like a catch 22.

 

[rando]

I would look at the tire load tables and get the truck on some cat scales. With your load range E you could probably drop them quite a bit depending on weight

That is the thing - the toyo tire load table is how I came up with the my 55/48 on road pressure. I don't see how you can use these tables to calculate a minimum pressure, but maybe I am doing it wrong.

 

[rando]

I had the same size tires on my Tacoma and carried a FWC Eagle on the back. I typically went to 25 in rear and 20 in front and never had issues.

Good info. Did you limit your top speed when aired down?

 

[PackRat]

The question I think you want answered is this:
"What tire pressures for my rig should I use if I go off-road where I plan to travel under these circumstances"
1) estimated 100-200 mile trip
2) washboard roads
3) no sand/mud expected

I think the key to your last question is paramount: You plan to drive at 40-50MPH and only want to soften the ride somewhat, right? As you note, when tires are run for any length of time, the sidewall suffers. That is greatly increased by the speed at which you drive on them. In addition, since you are on washboard roads, not muddy tracks or sandy tracks, under inflation heat build up on the sidewalls is increased I believe when you go that fast.

Where I am going with this is just that I don't think you can have it both ways on what may be a washboard surface as opposed to a more forgiving on at those speeds.

The reality is you are going to get bounced around but you are trading a smooth ride for improper tire inflation based on the SPEED and the SURFACE of the road. Unless you plan on stopping many times on the journey where the tires could cool down, heat will build up and that isn't good for the tires.

Others have mentioned the risk of under inflation causing the tire to come off the bead/rim as well. Maybe it's just me, but I would approach what you have described at a much lower speed anyway. Furthermore, if some reduction in tire pressure is your goal to soften the ride why not try about five miles and see how it goes and if you wanted to, THAT is the time to reduce pressure by 10-15% to see if that helps. However, before I did that I would reduce my speed to 35-40 and see how the ride is then.

I know, its not an interstate highway so the ride should be expected to be not as smooth. You are trading SPEED for a comfortable RIDE at a possible cost of tire damage/potential dismounting.

Have you asked the mfger about running at that pressure on those tires and given them the conditions you outlined here?

Just sayin'......

 

[rando]

Definitely a good idea about an iterative approach - lower the tire pressure a little, see how it handles and check the tire temperature again. Things also change once you get into the Park, and then we are talking about 5-20Mph speeds, so lower tire pressure is also safer and we could lower further.

My experience has definitely been that there is an optimum (and also a worst) speed for washboard when your suspension system can cancel the oscillations, and you are well away from the resonant frequency that caused the corrugations. So either faster or slower than the 'typical speed' that caused the washboard. This often seems to be around 40mph. I guess you could try and drive much slower than the resonant frequency, but given the distances, I am not sure driving slower would be a reasonable option. I am not talking about 4wd roads here, but wide gravel roads with washboard, of which there a many of in Robbers Roost/The Maze area.

I guess you have more faith in CS reps for BFG than I do. Their advice appears to be to fit the same size tires as OEM and follow the guidance on the data plate on your door jamb. I guess I understand this given that they gain nothing but opening themselves up to liability by suggesting anything else.

 

[ntsqd]

FWIW with our 9100 lbs. CTD/Camper on 315/70R17 Yoko's I normally run 50 psi on pavement (max for this tire). I drop that to 40 psi on rough or washboard. If I had better shocks than the 5160's I wouldn't drop it at all. I've found that most washboard roads live up to the old cliche' of them being "4-40 roads", and I usually opt for the 40.
Desert racers run their tires at pavement pressures, not rock crawler pressures.

 

[klahanie]

I use the same L&I table. ie Table figure for min pressure for my weight when starting out plus 2-4 psi for highway (measured cold). Off highway I might deflate to the same table figure (but as hot) then "maybe" deflate another 5psi as a possible second step down. After that I reduce speed (even more) to improve comfort.

It's not uncommon for me to see a 10psi rise from cold to hot for both front and rear tires. So for eg 45/60 cold + highway use bump + any elevation gain + any ambient air temp rise + a couple kms of bumpy gavel and I might be at 58/73. I take that down to 45/60 (measured hot). EDIT: only for comfort, as desired.

Our rig is heavy compared to a lot on here and has relatively high spring rates so airing down helps immensely for comfort off highway. And we drive thousands of kms of gravel per year.

I have two reasons for not usually going lower. First, invariably the road surface changes and I may be able to drive faster a few kms down the road. Here I don't want to be too underinflated for the speed that might be being driven (under steer can sometimes occur and possible loss of control). Second I've gone lower and the tires can get hot, which I don't like. I remember one time we were days from home and a long way in from the highway and the fronts (tire, rim and hub) were warm enough to concern me. I was worried about the wheel bearings.

So I'd also use temp as a guide. If the air coming out of the valve is hot enough to burn my wrist then, yeah, that's low enough.

EDIT: YMMV, weight, speed, road surface, tire rating are all factors

 

[rando]

All good points. I hadn't heard the term "4-40" roads before, but it makes sense. I circumnavigated Australia a number of years ago and that involved thousands of km's of washboard. We got pretty good at finding the "sweet spot" for wash board, as much as there was a "sweet spot" in an '84 HJ47.

The tire temperature will be something to keep an eye on. My Tacoma gives real time readout of the pressure in each tire, so one trick I have used is to convert the pressure rise into a temperature rise. By the ideal gas law, temperature (in K) is proportional to pressure. So if my pressure rises from 55 psi cold to 57 psi (typical for the highway) I know my temperature has risen by about 4% of 300K or 12C. This is not super accurate but will let you know if you have an issue as you drive.

 

[nikonron]

I just did the Maze in the Fall in my Tacoma w/ Eagle, great trip. I air down to 18 lbs., no problem. You can walk faster than you can drive much of the roads. I never slipped a tire and Teapot Canyon is a very rugged section but no problem. Ron

 

[Advmoto18]

Jonathan Hansen (an expert in this and many other overlanding subjects) usually chimes in on a subject such as this...when he has the time.

Jonathon published this article and worth reading for establishing a good baseline to air down.

Note the section in the article "How Low Can you Go?"

I can comfortably drop down from 65 psi to 28 for Baja/beach deep sandy conditions in my F-350 with a Hawk. Never had a tire issue. For wash board I use 35 psi.

Most folks don't want to go low because it can take a long time to air up with an inferior compressor. I suggest investing in a heavy duty air compressor with a continuous duty cycle suitable for off road and overland driving. I have been using an Extreme Outback ExtremeAire Magnum 12V for 3 years now and very pleased with its performance. It is expensive, but, considered an investment with a very good ROI (namely inflates my tires 275/75-20 Load E tires fairly quickly and gets my moving again).

 

[kmacafee]

Good info. Did you limit your top speed when aired down?

Absolutely. I rarely went over 40-45 when aired down, usually less. The last time I was in the Maze, I never went near 45 mph.

 

[rando]

To close the loop on this - I took the advice here and lowered the tire pressure in stages. Started at highway pressure 50/55 F/R then lowered to 40/45 on the way to Hans Flat Ranger Station, this made a small difference in ride comfort, but not huge, so I dropped it to 30/35 and this made a significant difference on washboard and rocky roads. We had no issues with traction even on steeper slick rock sections, so I left it at 30/35. Watching the pressure rise during the day as the tires warm up, it didn't make much difference what the pressure was, the pressure rise was ~10%, suggesting about a 30C rise in tire temperature, which is about what I see at highway pressure/speed.

Handling was fine even on the high speed run out to Hite.

Now my issue is that my automatic tire deflators only work at pressures below 30psi, so I will need to find some new ones. I also managed to break a bolt in the OME BP-51 shock mount (again), so that will require a little rework. These shocks give a great ride and firm handling, but I am not sold on their durability.

Overall a great trip!

 

[pvstoy]

Rando thanks for "closing the loop" as this is good information to know for future trips.

I have different sets of Staun deflectors and just bought a set of adjustable 15 to 50 psi. For the heavier Dodge truck. Spendy at $80 but have always worked for me. Here is a link to Amazon for viewing.
https://www.amazon.com/Staun-Tire-Deflators-HD-15-50/dp/B009X23XY6/ref=sr_1_4?s=automotive&ie=UTF8&qid=1526497321&sr=1-4&keywords=Staun+Tire+Deflators