Airing down tires

[agesilaus]

It's finally penetrated my head that maybe I need to air the tires down on these washboarded roads. Just how much do you guys air down, I have 80 psi truck tires. I have a 12V Viarr air compressor. I have to admit having the tires pop off the rim concerns me, whether it is reasonable I don't know.

[mptjelgin]

I drop the fronts to 50 (diesel engine is heavy) and the rears to 40.  It definitely softens the ride and there is zero danger of tires coming off of the rims.  

[agesilaus]

Thanks I'll give it a try.

[Ray,IN]

Just don't reduce air pressure so much the tires are overloaded, it can damage the sidewalls.

[Twotoes]

When I go off roading in my Jeep Rubicon I air down to 18 psi but that is only for off road where we usually only drive at 15 MPH. Never worried about the wheel coming off the rim. I would not air down below 30 PSI while driving on the highway at 55/65 MPH  I usually am at 38 PSI on highway. 

[agesilaus]

Yeah but don't a lot of those Jeep wheels have rim lockers?

[noteven]

Tires are 1st suspension component. On a truck (according to my Bridgestone training) they should squat so there is about 10% less distance from rim to the ground on the loaded spot of the tire vs where the tire is not touching the ground.

If you remove the valve stem and deflate to where "the whistle" just starts, that's good psi for warshboard / rock strewn / sand / driving at reasonable speed. 

On the truck camper SRW 4x4 truck I got myself into some what I knew better.. well anyway had to deflate - I dropped the front to whistle psi then reduced rear till the tires squatted the same as front but it wasn't whistle pressure because more load on rear. 

Probably 15-18 front and 30-35 rear. 

Lifted right out of the sand and drove off. 

On my little Kenworth dually 11R22.5 I run 70 in the drive tires so they have some give. If stuck in sand I would go as low as 35psi at low speed. Forget what's in steer but it is not heavy on the steer like a 12 or 15 litre engine truck. 

Edited February 15, 2021 by noteven

[packnrat]

less air will soften the ride. but you better have a onboard way to re air the tires back to correct psi.

on my cj-7 i drop down to 5 psi.  35X8X15 and simple steel rims, no bead lock required. in the decades of use never poped a bead. but that is ONLY for off road usage. heck dirt roads get full psi.

[noteven]

On my Diesel Dodge 3500 the door sticker numbers for stock sumpin sumpin 17's was Front 65psi always, rears 40psi empty 80psi at max load. 

When I changed to 285 sumpin 17's, the load and inflation chart for those tires wanted 55psi front for what the truck weighed, and 70psi rear for max axle weight.

Back in the truck business days tire reps would share the secret SPEED included rating numbers for running low pressures with inflation systems for off highway work. 

packnrat is correct don't be lazy when you need to return to high speed roads - inflate the tires correctly. 

As per a lot of RV advisors - to be "safe" you can run MAX sidewall air psi at all times. The well built RV trailer doesn't rattle to pieces all that fast. 

[Vladimir]

Airing down on washboard roads??  Is the cure worst than the disease???

In my entire professional career nobody has ever discussed OR suggested airing down to drive on a washboard road.

I know lots of 4wd and other "club" users did it for situations other than washboard roads.  But I view those folks as "extreme" sports folks.  The Federal government was not paying me to test the limits.  I just drove slow.

It was only actually after I retired and was towing a light trailer on a washboard road that I slipped the truck into 4wd.  THAT REALLY helped.

Your retired, why do you care if it takes you a few more minutes to arrive at your destination??

Oh, you do know that washboard roads are caused by excessive speed for the road surface???

[noteven]

Vladimir - 

I live in a relatively dry gravel road country.  We have 1000's of miles/kms of gravel roads. We know washboard. It's everywhere. Every intersection, every uphill....

Using 4wd absolutely helps on washboard and it helps prevent it getting worse because your two wheel drive wheels aren't skipping and scratching looking for traction. 

Overinflated tires have less traction on loose surfaces. Powerful modern vehicles are happy to be "wiping their feet" looking for traction on loose surfaces. Washboard begins to form. Which causes more slippage and more washboard. Softening tires helps with traction and absorbing some of the washboard vibration. 

Now groups of many potholes (called "cluster bombs" on forestry roads) are not washboard. The secret here is speed up and get the whole vehicle airborne as much as possible. Reduced pressure helps but safe reduced pressure is always LOAD and SPEED related. 

[Vladimir]

Your in a "different league" than normal drivers, and even those of us that drove those roads every day working.

It is a great explanation.   Thanks for that.

Late in my Forest Service career, someone told me that "WE, Forest Service" had people that could out hike, out hunt, out horse ride, out snowmobile, out x-country ski, out motorcycle, etc, etc. anybody in the public. 

That is no longer true.

Just before I retired I got a phone call from a jeeper that wanted the Forest Service to layout "trails that low-pressure tiire jeeps could use in the winter time".

I told him that if the area was open to motorized use in winter go out and enjoy your National Forests.  BUT you really don't want the Forest Service to lay out trails for you....those will come with regulations.  Just be careful and don't draw attention to yourself by getting stuck and requiring help.

Thanks again, for your explanation of washboards.  All the current Forest Service employees wish the public would read your post.

[noteven]

Thanks Vladimr - 

I forgot to mention warshboards look like a warshboard - many ripples. They are not holes but ripples. 

They also come in various frequencies - the close little high frequency ones where vehicles are trying for traction going uphill or leaving intersections - are caused by drive tires scratching for grip. This type is recognizable by the gawd awful vibration in your vehicle, and if in rear 2wd the way the back end skips sideways. When towing see: "stuff everywhere on the floor of the trailer." and "cubbards falling off the wall"

The long frequency ones can be caused by graders travelling too fast for moisture conditions with too little angle on the mouldboard. The profile starts and the drive wheels begin to walk on them and well ... it all goes downhill after that. Again I'm not certified as a certified motor grader trained operator but you will know this type when you drive on them. Maps unload off the dash, every horizontal cubby empties itself on the floor, coffee slops everywhere out of the cup in console...  On certain surfaces fast traffic "under power" can make these too. I can't recall seeing grader caused long washboards on any BLM or NFS roads - usually the grader people are doing a super nice job.  We got a dump of snow early this winter on roads that weren't frozen yet. Our grader had to blade snow off miles of road in a hurry. We had some classic long form warshboards till he had time to go over the roads again and shape them up. They are beautiful this spring here right now. 

Super big warshboards in the desert are called "whoops" short for whoop de dos... These are not normally found on roads that receive at lease some maintenances...