Topic: Why Air Down Your Tires Off Road

[Gnarly4X]

...
Why do you air your tires up to get fuel economy?  Because it reduces the friction between the tires and road surface. Friction is very similar to traction. When the friction between the tire and road surface is higher the tire slips less.

I do understand the practical aspect of the time it takes to air down and back up. I often only air down to the 12-15 psi range for easier wheeling because I have no issues driving home which in some cases is several hours of driving. It does cause some uneven wear but so does a week in Moab on the rocks. I have found that Walmart is happy to air my tires up for free and they can do it really quickly.

On a recent wheeling adventure with RockcrawlinJK I aired down to about 9 psi and opted to accept the offer to use his compressor and brought the tires up to about 17 psi to drive home. I could have easily driven the 45 minutes home at 9 psi. It sucks a bit more gas, forces me to drive slower, but it's not a big deal.

Totally agree!

Very well stated.

Gnarls.

[300k]

try standing on a single nail...ouch. look up a video of someone laying on a bed of nails. same concept, weight is not really a variable in this situation, it's the distribution of force.

as far as the tires barking, that's not the point. some people don't seem to understand the concept behind airing down. you're not making the compound of the tire any more sticky, but you're allowing the tire to wrap around rocks.

[YooperYota]

  

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[blackdiamond [OP]]

try standing on a single nail...ouch. look up a video of someone laying on a bed of nails. same concept, weight is not really a variable in this situation, it's the distribution of force.

as far as the tires barking, that's not the point. some people don't seem to understand the concept behind airing down. you're not making the compound of the tire any more sticky, but you're allowing the tire to wrap around rocks.

The tires are barking because they are gripping the surface better.  I think you underestimate the value of mechanical grip from the treat compared to the traction from the rubber itself.

Now use your imagination and consider laying flat on a single nail (obviously impossible in reality) so that the point of that one nail is supporting you.  Now imagine that you're laying flat on a bed of 1000 nails.  It's clear that the single nail will have a lot higher contact pressure, but if I were to then grab your ankles and pull you horizontally, the bed of 1000 nails would provide far more resistance and the single point would just rip you open.  That's the difference between the contact pressure of a single tire lug and an aired down tires that allows the edges of the tread lugs to bite the terrain.

[blackdiamond [OP]]

some people don't seem to understand the concept behind airing down.

 

This might be the most ironic thing I've read today.

[300k]

This might be the most ironic thing I've read today.

is it really? I'm talking about the physics behind it and everyone else is talking about ride comfort. your analogy of pulling horizontally doesn't really work in that regard. if you had a spiked tire on rocks it would probably do pretty good due to the concept I'm trying to get across. If you want a more accurate example, this is why you cannot stand on one egg, but you can walk across a carton of eggs.

the tires barking was in a cement parking lot, so airing down really shouldn't have any mechanical advantage over a street pressure tire. This whole thing really is getting blown past my original "argument", which was, to ME, for MY TYPE of wheeling, tire structure and distribution of force are more important than ride comfort or the ability for tires to wrap around rocks (which is impossible with MY TYPE of wheeling) this was a simple statement that turned into "300k says you should never air down". Same thing for my "I don't engine brake on short, steep hills"...turned into "300k says you should never engine brake, even offroad"

[YooperYota]

You just have a poor understanding of physics man.

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[emsvitil]

The rear tire on my motorcycle airs down all by itself........


 

[Willard]

.....


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[Lewis Hein]

I really didn't want to get into the middle of this particular dogfight, but...

I'm issuing a plea for more evidence and fewer opinions. Everybody's got opinions, and some are based on evidence. The trouble is, when people with opinions argue but don't present any evidence, we have no way to choose between arguments.

I suggest: Do as H8PVMNT suggested and run the same trail at a whole series of PSI. Then come back here with evidence in hand to say: "Based on these data (see them for yourself) I advocate airing down under conditions A, B, and C but not under conditions D, E, and F". If you are right, this will be much more convincing than anything you have said before. Physics is good, and I applaud you for applying it. But empirical data is king.

OK -- convince us, I'm waiting!

Note: I am aware of the incredible irony of me trying to argue for fewer opinions in any discussion. Nobody needs to tell me that I have stunk up the MC forum in the past with my useless, pointless opinions, so save your breath. Let's just say that I'm trying to move towards more evidence and less opinion in what I say.

[Gnarly4X]

Unless blackdiamond convinces me that I’ve been wrong for the past 40 years, I will continue to believe airing down my tires to go 4-wheeling is a good thing.

Gnarls.

[Gnarly4X]

I really didn't want to get into the middle of this particular dogfight, but...

I'm issuing a plea for more evidence and fewer opinions. Everybody's got opinions, and some are based on evidence. The trouble is, when people with opinions argue but don't present any evidence, we have no way to choose between arguments.

I suggest: Do as H8PVMNT suggested and run the same trail at a whole series of PSI. Then come back here with evidence in hand to say: "Based on these data (see them for yourself) I advocate airing down under conditions A, B, and C but not under conditions D, E, and F". If you are right, this will be much more convincing than anything you have said before. Physics is good, and I applaud you for applying it. But empirical data is king.

I really didn't want to get into the middle of this particular dogfight, but...

I'm issuing a plea for more evidence and fewer opinions. Everybody's got opinions, and some are based on evidence. The trouble is, when people with opinions argue but don't present any evidence, we have no way to choose between arguments.

I really didn't want to get into the middle of this particular dogfight, but...

I'm issuing a plea for more evidence and fewer opinions. Everybody's got opinions, and some are based on evidence. The trouble is, when people with opinions argue but don't present any evidence, we have no way to choose between arguments.

I suggest: Do as H8PVMNT suggested and run the same trail at a whole series of PSI. Then come back here with evidence in hand to say: "Based on these data (see them for yourself) I advocate airing down under conditions A, B, and C but not under conditions D, E, and F". If you are right, this will be much more convincing than anything you have said before. Physics is good, and I applaud you for applying it. But empirical data is king.

OK -- convince us, I'm waiting!

Note: I am aware of the incredible irony of me trying to argue for fewer opinions in any discussion. Nobody needs to tell me that I have stunk up the MC forum in the past with my useless, pointless opinions, so save your breath. Let's just say that I'm trying to move towards more evidence and less opinion in what I say.

I suggest: Do as H8PVMNT suggested and run the same trail at a whole series of PSI. Then come back here with evidence in hand to say: "Based on these data (see them for yourself) I advocate airing down under conditions A, B, and C but not under conditions D, E, and F". If you are right, this will be much more convincing than anything you have said before. Physics is good, and I applaud you for applying it. But empirical data is king.

OK -- convince us, I'm waiting!

Note: I am aware of the incredible irony of me trying to argue for fewer opinions in any discussion. Nobody needs to tell me that I have stunk up the MC forum in the past with my useless, pointless opinions, so save your breath. Let's just say that I'm trying to move towards more evidence and less opinion in what I say.

OK -- convince us, I'm waiting!

Note: I am aware of the incredible irony of me trying to argue for fewer opinions in any discussion. Nobody needs to tell me that I have stunk up the MC forum in the past with my useless, pointless opinions, so save your breath. Let's just say that I'm trying to move towards more evidence and less opinion in what I say.

There is an over abundance of empirical data.  It has been posted here.

It seems we are trying to convince one person.  Although here in these forums it is all about helping one person at a time, I don't know that more discourse will be convincing.

Ignorance, obstinance, and arrogance are a combination in human personality characteristics that equal clueless.  Any one of those personality traits can cause a deficit in understanding or in the learning process... I'm speaking from personal experience.

Gnarls.

[Gnarly4X]

I really didn't want to get into the middle of this particular dogfight, but...

LOL.... you just did.

Gnarls.

[blackdiamond [OP]]

is it really? I'm talking about the physics behind it and everyone else is talking about ride comfort. your analogy of pulling horizontally doesn't really work in that regard. if you had a spiked tire on rocks it would probably do pretty good due to the concept I'm trying to get across. If you want a more accurate example, this is why you cannot stand on one egg, but you can walk across a carton of eggs.

the tires barking was in a cement parking lot, so airing down really shouldn't have any mechanical advantage over a street pressure tire. This whole thing really is getting blown past my original "argument", which was, to ME, for MY TYPE of wheeling, tire structure and distribution of force are more important than ride comfort or the ability for tires to wrap around rocks (which is impossible with MY TYPE of wheeling) this was a simple statement that turned into "300k says you should never air down". Same thing for my "I don't engine brake on short, steep hills"...turned into "300k says you should never engine brake, even offroad"

The only reason that I am still discussing this topic is that I am trying to get you so understand for fundamental flaw with your assertion that there's more traction available with the tires aired up.

Another real world example. I have a Detroit locker in the rear of my 4Runner. At street pressures it's very easy to get the inside rear tire to slip almost to the point of breaking loose  going around corners, but when driving around Moab with 10psi I can get the tires to bark a bit but simply don't have enough power to make the truly slip. The rear axles fights to go straight because the rear locker is turning the rear axles at the same speed. Try this with your locker engaged.

[blackdiamond [OP]]

I think a lot of this may be related to wheeling in a rig as capable as Red. It has the suspension, gearing, lockers, etc., to make the majority of wheeling easy. Anyone that has wheeled extensively in a stock type rig and pushed it to the limits will attest to the fact that airing down their tires was often the difference between making it or not.

My dad recently took is stock 4Runner down a road that dead ended and he had real concern about his ability to get back out. He's been wheeling since the early 70s and his first step was to air down. He assumed that he had one shot to make it up the snowy muddy hill without getting stuck. He hadn't aired down going in because he didn't need the extra traction and was a very long drive from home without a way to air back up.

[Gnarly4X]

.... Anyone that has wheeled extensively in a stock type rig and pushed it to the limits will attest to the fact that airing down their tires was often the difference between making it or not.

Absolutely true.

Gnarls.

[fasterspider]

When I started wheeling, the idea of airing down didn't thrill me at all and of course, traction suffered on my two drive wheels and I thought it was due to being one leggers front & rear so I got locked front & rear but, still had traction issues.
My buddy said to me "Air down to 15 psi and don't worry about how you're going to air up again, the Flying J has free air you can use to get home with but, at the end if a 5 or 8 hour run, I don't want to drive the other way to air up & go home. I drove the 50 miles home on low pressure tires and watched my gas gauge drop from 3/4 of a tank to empty.
I can't afford an ARB on board compressor, I can barely afford my rig but, I needed a way to put air in my tires after wheeling so I wouldn't waste most of the fuel in my tank to get home and bought me a $40.00 on sale Harbor Freight 3 gallon 110 volt compressor and plugged it into the 1500/3000 watt inverter I installed a year ago and it works great. I put 10psi in to each tire bringing them back up to 25psi which is good enough to get home and only use a 1/4 tank of fuel instead of 3/4 of a tank.
My BFGs are only 33" tall this year but, airing them down to 13 or 15 psi has increased my trail riding experience 100% and now I can air up again at the end of a trail no probs.

There is no passenger seat, I don't want anyone else in my rig with me since my wife is too good to go wheeling with me so a compressor is the perfect passenger and it is a slow ordeal filling those tires with 25 psi but, it is worth the effort.

[H8PVMNT]

I like bacon

[Gnarly4X]

I like bacon

Do you like it better air'd down? 

Gnarls.

[YooperYota]

Do you like it better air'd down? 

Gnarls.

Ya know, I've never aired down my bacon and I don't know why people even bother. Seems like it'd take a long time to air it down with a stick, it can't be worth it.

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[300k]

I think a lot of this may be related to wheeling in a rig as capable as Red. It has the suspension, gearing, lockers, etc., to make the majority of wheeling easy. Anyone that has wheeled extensively in a stock type rig and pushed it to the limits will attest to the fact that airing down their tires was often the difference between making it or not.

Nope! My 84 was on the trails since 2015 on bald 33 inch SUV tires at street pressure. then in late October 2016, I upgraded to 31x10.50 retread mud terrains. I wheeled until July of 2017 when I blew my truck up. My truck is open open with chevy springs in the rear, no front sway bar but still has the torque rod. I never got into airing down the 33s because they don't really have a sidewall to ride on and they were pretty thin.

When it comes to red, I ran some of the same trails but did not air down. it rode fine, never slipped a tire. Red at 9 PSI was kinda lame. take the line you had in your head and throw it out the window cause guess what, your rear tire just slipped and sent you 2 feet over...lol

[Mudder]

If that's the case that's is more driver or tire choice. Not because you aired down.

[SqWADoosh]

If that's the case that's is more driver or tire choice. Not because you aired down.

Its not the tire. Ran MTRs on that truck with zero issues. They hooked up great. Only reason I went away from them was they didn't balance worth a damn.

[Gnarly4X]

... My truck is open open with chevy springs in the rear, no front sway bar but still has the torque rod.

Hey 300k,

A stock Toy truck with open axles is not a 4-wheel drive, it’s a 2-wheel drive and some times a 1-wheel drive.

The use of the phrase “4-wheel drive” is a misnomer unless both your axles are locked or lockable.

I tried 2-wheeling in my first 1986 truck… It took me about 1 trip off-road to put a Detroit in the rear, and one more trail to put a limited slip in the front.  Then bigger and more aggressive tires.

So technically you have not really 4-wheeled your truck. You can drive your truck on as many trails as you can go on and never air down from 30 PSI, however not airing down is just a ludicrous mindset and nearly everyone who does real 4-wheeling knows that.  4-wheeling is about traction.  Without sufficient traction there will be no forward progress. Generally, with our type of typical 4-wheeling, you can produce more traction with tires that are air’d down.  Without getting deep into the equations of Newton’s 3 Laws, there are some basic explanations if you are truly interested in learning.

There has been extensive research, studies and scientific publications on tires and traction, for obvious reasons.  That data can be easily be overlayed onto any 4-wheeling scenario in virtually any tire to surface condition.

Here is a couple of interesting publications that shouldn’t require a degree in Physics and may help you to understand the physical mechanics of why a tire that can produce more contact area is more likely to increase friction, and thus more traction.

http://boson.physics.sc.edu/~rjones/phys101/tirefriction.html

http://ffden-2.phys.uaf.edu/211_fall2013.web.dir/connor_mattson/physics.html

I just thought this might help you to become more educated in your passion for 4-wheeling.

Gnarls. 

[300k]

Its not the tire. Ran MTRs on that truck with zero issues. They hooked up great. Only reason I went away from them was they didn't balance worth a damn.

Huh. We drove home 2100 miles at varying speeds of 1 to 91 miles per hour, not a single vibration. A great tire.

[SqWADoosh]

Huh. We drove home 2100 miles at varying speeds of 1 to 91 miles per hour, not a single vibration. A great tire.

Yeah because its been balanced. I said it was a pregnant dog to get balanced. Takes a metric ton of weight. Also has a very weak sidewall.

[24SHORTIE]

[blackdiamond [OP]]

"Further, the size of the contact area is very important in car tires because the traction is dynamic rather than static; that is, it changes as the tire rolls along. The maximum coefficient of friction can occur anywhere in the contact area, so that the greater the area, the greater the likelihood of maximum traction. Thus, under identical load and on the same dry surface, the wider tire has a greater contact area and develops higher traction, resulting in greater stopping ability." - useful info from link posted above

[blackdiamond [OP]]

Hey 300k,

A stock Toy truck with open axles is not a 4-wheel drive, it’s a 2-wheel drive and some times a 1-wheel drive.

The use of the phrase “4-wheel drive” is a misnomer unless both your axles are locked or lockable.

I tried 2-wheeling in my first 1986 truck… It took me about 1 trip off-road to put a Detroit in the rear, and one more trail to put a limited slip in the front.  Then bigger and more aggressive tires.

So technically you have not really 4-wheeled your truck. You can drive your truck on as many trails as you can go on and never air down from 30 PSI, however not airing down is just a ludicrous mindset and nearly everyone who does real 4-wheeling knows that.  4-wheeling is about traction.  Without sufficient traction there will be no forward progress. Generally, with our type of typical 4-wheeling, you can produce more traction with tires that are air’d down.  Without getting deep into the equations of Newton’s 3 Laws, there are some basic explanations if you are truly interested in learning.

There has been extensive research, studies and scientific publications on tires and traction, for obvious reasons.  That data can be easily be overlayed onto any 4-wheeling scenario in virtually any tire to surface condition.

Here is a couple of interesting publications that shouldn’t require a degree in Physics and may help you to understand the physical mechanics of why a tire that can produce more contact area is more likely to increase friction, and thus more traction.

http://boson.physics.sc.edu/~rjones/phys101/tirefriction.html

http://ffden-2.phys.uaf.edu/211_fall2013.web.dir/connor_mattson/physics.html

I just thought this might help you to become more educated in your passion for 4-wheeling.

Gnarls. 

Great links.

While I agree with you assessment of the true 4wd capability of a stock 4x4, I still think that wheeling a stock 4x4 is still four wheeling. You're getting too technical which isn't any surprise.

If 300k hasn't had any traction issues when wheeling his mostly stock 84 truck then he simply hasn't been anywhere that pushed the limits of the trucks capability. When my 85 was had open differentials and 32s I wheeled a lot of places that required everything it had. Lines were important, momentum was important.  Low air pressure was required.

One hill was called Chicken Scratch Hill and was a pea gravel covered hill with moguls everywhere from the Jeeps. The only reason I had a chance was my wheelbase didn't match the mogul spacing so I could thread a line that avoided completely flexing out. I did most of the difficult trails in Moab (obviously not all of the most difficult obstacles) including the Golden Spike where I only needed an assist at the crack. I have experienced the limit with open differentials and tire pressure was key.

[SqWADoosh]

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/thread