16 replies to this topic

[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

Edited by klahanie, 08 May 2018 - 02:50 AM.

[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.c... Tire Deflators