Optimal tire deflation for corrugated and sand roads

Midwestdrifter

Engineer In Residence
Re: Exploring Australia From The Left

MWD.
Glad you are enjoying some of OZ. Can I remind you that you will damage your vehicle if you continue to drive on such high tyre pressures. That corrugated road
is about 12-15 psi country. Eric.

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.
 

FourWheelers1987

A 2017 2500 144” Crew 4x4 High
Re: Exploring Australia From The Left

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

. . . 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."

Are you running the OEM size 6.5" rims? I'm still looking for the optimal (safety, durability, versatility, and coolness) size tire for my alloy OEMs. Like many "right coasters", I'll probably only be off-road 10%-20%. I'm leaning strongly to GY Duratracs which will fit the spare tire
carrier.
 

Ed463

Active member
I agree with Eric. I've run BFG AT's at 14psi for many many miles, at times as low as 10psi but that's pushing things imho. These tyres maybe regarded as a little old school now but they are seriously tough! Lower pressure also equals less possibility of a puncture.

Again, imho, there's an urban myth when it comes to driving over corrugations, the faster you go the better. No chance, it may feel smoother but it's destroying your suspension no matter what pressure you run.
 

Midwestdrifter

Engineer In Residence
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

. . . 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."

Are you running the OEM size 6.5" rims? I'm still looking for the optimal (safety, durability, versatility, and coolness) size tire for my alloy OEMs. Like many "right coasters", I'll probably only be off-road 10%-20%. I'm leaning strongly to GY Duratracs which will fit the spare tire
carrier.
NCV3 sprinters have 6.5" rims, my van is a T1N, so I have 6" rims. the 245/75R16 fits fine. It is technically too narrow of a rim, but I have had no uneven wear issues in the last 20k miles. I believe that 245/75R16 is the factory size for 4x4 NCV3s? Tire size depends a lot on what you drive on. Go as wide as possible for deep soft sand/mud/snow. The ideal aspect ratio is 85. The closer to this ratio you are, the better. This means your sidewalls are 85% the width of the tread. This provides a long (but comparatively narrow contact patch when aired down. This allows for good flotation, but it also means you have to "push" less mud/sand/snow out of the way to move forward. It also means the tire absorbs bumps across the road more readily.

I agree with Eric. I've run BFG AT's at 14psi for many many miles, at times as low as 10psi but that's pushing things imho. These tyres maybe regarded as a little old school now but they are seriously tough! Lower pressure also equals less possibility of a puncture.

Again, imho, there's an urban myth when it comes to driving over corrugations, the faster you go the better. No chance, it may feel smoother but it's destroying your suspension no matter what pressure you run.
I am currently running BFG KO2 AT tires. On past vehicles I have used 10-15psi to avoid getting stuck in mud.

The reason I mention the 25% sidewall height rule, is because tire pressure is only 1/3 of the equation. The other two parts are tire size, and the load on the tire. Many 4x4s have wide/big tires for their weight. Even on the highway they don't often need more than 35-40PSI. My van is heavy for its tire size (comparatively). So my highway pressures are higher. The same goes for my aired down pressures (to an extent). If I ran 10 PSI on the rear of my van, with 2400lbs on each tire, there is a real chance I could bend a rim, or pinch the tire sidewall. If I was running 300mm wide tires, or 85% aspect ratio, it would be a different matter. The larger contact patch (for the same reduction in rolling height) makes the difference.

On my toyota pickup truck, which has 1200lbs rear axle weight, and 185/75R14 tires, I would run 24PSI on the highway, and 10PSI in the wet fields or farm roads. I designed an open cockpit formula type racecar back in college. It had 10.5" wide tires, and weighed 650lbs with driver. We routinely ran ~8psi on the track, that doesn't mean street cars should do that! My point is that making pressure reccomendations is a tricky business.

Lower pressure does mean lower contact pressure, and less risk of tire puncture. But you do need some cushion between the rim and the ground. And the lower the pressure, the more heat builds up in the tire. SO obviously you can't drive too fast at low pressures. Once a tire hits a certain temperature, the rubber rapidly degrades, and a blowout happens.

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. This speed is one where your shocks provide a bit more dampening. Usually this is right at the transition from low to high speed dampening circuits. There are also natural resonance frequencies of the suspension, that if you hit them, the entire vehicle starts shaking to pieces. :idunno:

I have lots of dirt roads left in Aus, so If more experience in Aus changes my approach, I will report so here. :smilewink: The ultimate experiment as it were.

If I can find the time, I will measure my vans contact patches at various pressures. They should be the tire load divided by the pressure, and will increase roughly linearly until the sidewall is 25% lower.
 
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Ed463

Active member
As you say finding the right pressure is a little more scientific than my generic numbers:)

Your observations are great and definitely demonstrate how low you can / need to reduce pressure. I've noted some talk about reducing pressure down to 30 or 40psi. That's pretty pointless imho!

Unfortunately we're all a little vain when it comes to tyre size, bigger, fatter is better. Well it's not. I run 255/75/16. I should have 235/85/16 but of course they look rubbish:)
 

Tooth Fairy

Away with the fairies.
The corrugations are going jar your bones no matter the pressure/speed.
If you get out on a corrugated road and feel the corrugations they are just dust and stones plied up.

Im thinking a pair of counter rotating brushes out in front of the vehicle to level the corrugations as you go would be ideal.
 

Midwestdrifter

Engineer In Residence
The physics of how corrugations form is fascinating. Materials science, physics, vehicle engineering. Interestingly the biggest contributors of washboarb buildup, (really any road damage) are semi-trucks (due to the airbag suspension), trailers with no shock absorbers, and vehicles with too high of tire pressure (excessive speed makes it worse too).

I am not sure any brush system could keep up with a reasonable rate of driving. Plus the dust production would be insane.

Also, interestingly enough, wet/damp roads are much more resistance to corrugation formation.

A common tactic used on the unpaved roads in the midwest USA is to use a specific mix of gravel. usually 1" or smaller pieces, with 20% fines from the crushing process. It is important to not to mix in natural sand, as this exacerbates the problem. Of course this requires lots of gravel, which would be expensive in this country of few people, and many roads.
 

FFred

Member
OFF road motorcycles have methods of hanging on to the rim at low pressure , talk to a Mcy dealer?
 

Midwestdrifter

Engineer In Residence
OFF road motorcycles have methods of hanging on to the rim at low pressure , talk to a Mcy dealer?
Bead-locks are are a thing. Bead lock rims are expensive though, and have their own unique challenges (especially ones for tubeless tires).
 

sepudo

Member
Re: Exploring Australia From The Left

I recently went to Pismo Beach to play in the sand and get stuck on purpose and practice using my traction pads. Prior to entering the sand I aired down to 35lbs all corners and never got stuck... Great to read I can safely go to 15 in a pinch.
I did get stuck previously in Baja red clay mud with commuter tires and aired down to 25lbs and rocked it out... No traction pads at that time.

Lesson learned.

@sepudo on Instagram
 

Midwestdrifter

Engineer In Residence
Isn't it suspension resonance that causes washboard in the first place? :idunno:
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.
 

Midwestdrifter

Engineer In Residence
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
 

Roodster

Member
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.
 
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.

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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