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Old 06-04-2017, 03:01 AM   #11
glasseye
 
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Default Re: Optimal tire deflation for corrugated and sand roads

[QUOTE=Midwestdrifter;553851]
Quote:
Originally Posted by FourWheelers1987 View Post

I agree about washboard speeds, obviously going crazy fast is a dumb idea. However, depending on the shocks you have, there is often a "sweet spot" speed wise. There are also natural resonance frequencies of the suspension, that if you hit them, the entire vehicle starts shaking to pieces.
Isn't it suspension resonance that causes washboard in the first place?
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Old 06-04-2017, 03:26 AM   #12
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Default Re: Optimal tire deflation for corrugated and sand roads

[QUOTE=glasseye;561093]
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Originally Posted by Midwestdrifter View Post

Isn't it suspension resonance that causes washboard in the first place?
Suspension plays a role. However, and wheel rolling over a loose particulate surface will naturally produce washboarding. This is true for blades being dragged repeatedly over the same type of surface. Any small disturbance in the surface starts the process, with each additional pass moving more material from the valleys to the peaks. And extending the washboard further forward and to the sides. High tire pressures, and air suspensions tend to exacerbate this effect, as they produce higher peak contact pressures. At least that is how I understand it.
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Old 06-05-2017, 02:36 AM   #13
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Default Re: Optimal tire deflation for corrugated and sand roads

Here are some photos of various inflation levels for my rear axle.

Here is fully inflated at ~60psi

IMG_20170601_082553 by J Luth, on Flickr

Here we are at 40psi

IMG_20170531_134621 by J Luth, on Flickr

And this is ~30psi

IMG_20170601_082604 by J Luth, on Flickr

40 rear and 30 front is becoming standard for well maintained dirt roads recently. I limit my speed to 80KPH at this pressure. This is just shy off 20% sidewall reduction.

30 rear and 25 front is what I use for corrugated roads.

25 rear and 20 front is what I use for badly corrugated roads. I limit my speed to 50kph at this pressure. This is right at 25% sidewall height reduction, and is the "ideal" pressure for my van in off road situations.


For really soft surfaces, I will do 20 rear and 15 front. My speed at this pressure is usually 40kph or less, and aggressive maneuvers are not recommended.

Keep in mind my vans weight and tire size. For a lightly loaded van, the pressures will be lower
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Old 11-08-2019, 09:25 PM   #14
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Default Re: Optimal tire deflation for corrugated and sand roads

On a recent trip to the beach, I thought the sand was firm enough to drive the van on. At road pressures stock rims & Goodrich At ko2 225 75 r16 , I managed to get about 50ft onto the sand before I felt it going in. Knowing that the worst thing to do was to keep going digging holes & grounding out I stopped. Spent half an hour scraping the sand away, & another 20 minutes on my belly with a stick letting the pressure down. Deflated tyres & tried it out. My 4x4 sailed out first time. I knew the tyres were on the low side, so I went very easy on the clutch, trying not to grab it, & even easier on the way home (6miles away). When I got home I was horrified to find the tyres had only 8 psi in them. Two lessons learnt was that modern tyres can be very forgiving, & that taking chances with soft terrain is never worth it, Deflating tyres is a hassle, but so is walking home from a deserted beach with an incoming tide. Don't take any chances. Life is much better if you get a tyre deflator, I bought an ARB EZ-deflator after buying some Staun type screw on jobs, that were unreliable, especially in the sand, at my age I don't won't to spend too much time on my hands & knees in the soft stuff. On sand & rocks, drive gently, but on greasy mud you'd better floor it, & don't let quite so much air out. Thanks Midwestern, for the good advice as always.
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Old 11-09-2019, 11:58 PM   #15
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Default Re: Exploring Australia From The Left

This is the exact information that I have been wondering about during and since my trip this past summer where I drove lots of washboard roads out in Colorado. Thanks for posting.

I just gutted my Sprinter Westfalia and as part of the refitting I am planning on mounting a higher quality 12 volts compressor, like an ARB, on board to more easily facilitate deflating/reinflating my tires.

Quote:
Originally Posted by Midwestdrifter View Post
I do regularly deflate when driving on rough roads. I vary my pressure with the speed at which I want to drive, as lower pressures means lower speeds.

For speeds of 65kph, I need around 30 rear and 25 front minimum based on my last time on a scale.

I will go down to 20 rear and 15 front on very soft surfaces. However I need to keep my sustained speed 35kph or under, and unseating a bead is possible with aggressive driving.

I base my deflation pressures on the sidewall height rule.

My tires are 245/75 profile. The sidewall measured from the rim to the ground is about 175mm at normal pressure (60 rear 55 front). I reduce the sidewall height by ~15% for crap roads, and 25% for sand etc.

For those unaware, at 25% sidewall height reduction the tire reaches its optimal footprint. This is as long as it will get without the sidewalls rubbing on the ground. This means optimal flotation and maximum bump absorbing.

Anyways, the van wasn't rattling itself to pieces. Other than my aftermarket radio coming out, it was pretty uneventful (other than the slow onset of insanity).

For those interested, the van weighs about 8600lbs. 3800lb on the front axle, and 4800lbs on the rear axle.
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