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On my lightly loaded? (6900 pounds on scale) 2019 MR Transit 250 with LT255/70/r16 KO tires everything is peachy running 60 psi but the second I hit any washboard the van rattles like a dirt compacting machine and the rear end wants to come around at any speed and super dangerous and teeth shaking. It literally cannot go on the washboard where I need to go.... even with old ladies in a Yugo speeding by.

The offroad guys said I need to air down to 15 0r 20 at the most and that should make it all better but i'm worried about the tires coming off the rims being a total newb. How low can you conservatively go?

I ordered a co2 tank and regulator for airing back up just need a baseline as to what pressure to run.
 

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On my lightly loaded? (6900 pounds on scale) 2019 MR Transit 250 with LT255/70/r16 KO tires everything is peachy running 60 psi but the second I hit any washboard the van rattles like a dirt compacting machine and the rear end wants to come around at any speed and super dangerous and teeth shaking. It literally cannot go on the washboard where I need to go.... even with old ladies in a Yugo speeding by.

The offroad guys said I need to air down to 15 0r 20 at the most and that should make it all better but i'm worried about the tires coming off the rims being a total newb. How low can you conservatively go?

I ordered a co2 tank and regulator for airing back up just need a baseline as to what pressure to run.
I take 10 or 15 OUT of my Yokohamas (245/75/16) and that seems to work well. On hiway they’re at 52 and 65. I would think at 15 or 20 you’d lose the bead.
 
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I just spent many days and lots of miles on washboard in southern Utah and aired mine down to 25/30. My van weighs 8700 fully loaded and I'm running 245/75/16 Yokohamas. It worked great - much smoother ride and the tires don't seem to have sustained any damage from the trip. I'd recommend getting a good portable compressor for airing back up as it takes quite a bit of air to bring them back up to highway pressure. With a compressor, count on it taking somewhere between 20 and 40 minutes to air back up depending on how powerful your compressor is.
 

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I concur with above posters. I've done at least several hundred miles of washboard on my 245/75-16 KO2s, and most recently I was airing down to about 45psi all around, and have yet to suffer any wheel or tire issues. Most of this driving has been in Death Valley and Utah, for reference, and I have a strong tendency to step on the accelerator until I can't feel the washboard anymore.

The offroad folks are probably used to running a highway pressure of 35psi, so going down to 15 or 20 for them is more like dropping to 30-40psi on a loaded transit. The convention I've heard before is to air down no more than half the sidewall height, and this should sufficiently increase the footprint without significant risk of the rim damage.
 
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