Topic: Why Air Down Your Tires Off Road

[blackdiamond]

This discussion keeps getting repeated on the chat so I am starting a thread where we can all keep up.

There was an article that I read more than a decade ago that was showing that having uber flex wasn't always better. It explained the physics of the contact pressure on the tires in a Jeep YJ with no flex and three points of contact vs. a Jeep buggy with quarter elliptical rear springs with four tires on the ground. The 4th tire on the ground had so little weight on it the end result was just less overall traction because the rear tire was not being pushed to make contact as much.  Flexible suspension can give you more stability but doesn't always mean better traction overall. Higher contact pressure can be helpful on ice which is why ice racers often run super skinny tires. When you're talking about going off road you're comparing one or two lugs of the tire having high contact pressure when aired up vs. having a large section of the tire in contact when aired down. Yes the contact pressure per sq-in may be less, but the mechanical traction gained from the tread being able to mold into the terrain is a much more significant factor.  Even on smoother off road terrain the benefits of airing down (maybe not to extreme low pressures) is there because it keeps the tires in better contact when the sidewalls become part of the suspension and the tires will simply absorb small things rather than bouncing along. Being aired down puts a lot less stress on everything.

[Rockcrawlintoy]

I thought they used stuff like this for ice racing

[blackdiamond [OP]]

This is what I was thinking.

[YooperYota]

This discussion keeps getting repeated on the chat so I am starting a thread where we can all keep up.

There was an article that I read more than a decade ago that was showing that having uber flex wasn't always better. It explained the physics of the contact pressure on the tires in a Jeep YJ with no flex and three points of contact vs. a Jeep buggy with quarter elliptical rear springs with four tires on the ground. The 4th tire on the ground had so little weight on it the end result was just less overall traction because the rear tire was not being pushed to make contact as much.  Flexible suspension can give you more stability but doesn't always mean better traction overall. Higher contact pressure can be helpful on ice which is why ice racers often run super skinny tires. When you're talking about going off road you're comparing one or two lugs of the tire having high contact pressure when aired up vs. having a large section of the tire in contact when aired down. Yes the contact pressure per sq-in may be less, but the mechanical traction gained from the tread being able to mold into the terrain is a much more significant factor.  Even on smoother off road terrain the benefits of airing down (maybe not to extreme low pressures) is there because it keeps the tires in better contact when the sidewalls become part of the suspension and the tires will simply absorb small things rather than bouncing along. Being aired down puts a lot less stress on everything.

What is your argument...?

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[blackdiamond [OP]]

What is your argument...?

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This thread was posted based on several discussion on this topic in the chat window. 300k advocates not airing down in a lot more scenarios than most.

My argument is that airing down almost always has a benefit off road.

[YooperYota]

This thread was posted based on several discussion on this topic in the chat window. 300k advocates not airing down in a lot more scenarios than most.

My argument is that airing down almost always has a benefit off road.

I didn't realize that was arguable. Airing down (to an extent) I'd always a benefit. In both rude quality and traction.


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[Willard]

Lol....agree with this not being an arguable discussion. Airing down has a benefit in sand, boulders, granite, mud, dirt wait it works for everything in an off-road environment. I was amazed at the difference in traction I achieved with my bead locks and the peace of mind to be able to go to a lower psi. This is not an arguable discussion, airing down always wins.....I would like to hear anyone who disagrees and has a damn good reason why. Also when do people cross the line of being aired down vs not being aired down? What is considered aired down? My last trip on the Rubicon I started out at 8 in front and 12 in rear. Ended up at 10 in rear and dropped to 7 in front. Worked great! It also may depend on tire size and tread type I'm sure. This may turn out to be a long discussion!!


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[blackdiamond [OP]]

I find that the 10-12 psi range for my 35s is sufficient for ride quality and traction in 90% of wheeling. When I need a little extra I will go down to 8 psi without any real concern for having any bead issues. The difficulty of the wheeling and how far I plan to drive before airing up are factors in how low I go. It appears that my Coopers may handle low air pressure a little better than the BFG KM2s I had before meaning that they squat less at the same psi.

I have seen super low psi causing rigs to hop more than they need to be. On the last trip Travis was running really low pressure and when he brought the psi up a bit his Jeep stopped bouncing as much on the ledges.  All things in moderation.

[blackdiamond [OP]]

Oh. I have run 20 psi on the street before and for 35s would consider 15 psi aired down as that's where I can start to notice a difference.

[Gnarly4X]

I agree with YooperYota and Willard.

I don’t see how that airing down vs not airing down can be much of a logical debatable topic.

With that said, in today’s PC world of total absurdity, I suppose you can create any argument.

First, I can only go on my off-roading experiences… very active from 1977 to 2001… about 24 years. Like many here on this forum, I have wheeled in elevations from Death Valley to Dusy Irshim.  In ambient temperatures from about 15d F to 120d F in practically every terrain – rocks, sand, mud, snow, ice, water, dirt and dust - crawling to driving like crazy - like racing other vehicles up a canyon out in Florence, AZ in the middle July with running water 2 feet deep over rocks that we could not see at idiotic speeds.... dumb.  Or racing up the beach below El Golfo Sonora Mexico with 15 vehicles, wide open, as fast as my truck would go (tires air down to 4 psi).  Several of us hit a cut in the sand from water run-off.  The rut was 12" deep and about 4 feet across.  4 vehicles hit it.  I  hit it at about 50 MPH.  I went air borne, and flew over 30 feet before hitting the sand.  The guys in front of me had stopped and were looking back at me, they said they could see my rear diff, I was so vertical!!  It smashed my front skid plate and bent my tie rod.  I expected to see more damage.  One Jeep busted the motor mounts.  That little ditch could have caused some serious carnage! We were about 20 miles from any medical attention.  Fun at the time.  We agreed it was careless and stupid.  Both me and my passenger were seat belted.

Secondly, 300k doesn’t have as much experience as many of the guys here, so I would question some of his comments and motive for his comments.  Many of his comments appear to clearly indicate a lack of experience and often have a tone of arrogant condescension - I wouldn't hold that against him - he's not alone here on this forum.  I think he likes the attention he gets with his jabbing comments, but he is smart enough to know the facts.  He is in a learning curve.

It would be difficult for me to be convinced that airing down has ANY disadvantages in the typical 4-wheeling we do.  In other applications, like off-road racing, tire air pressure may play a significant roll since the higher PSI the more heat the tire can absorb.

You can easily apply the laws of physics to tire wrap, sidewall flex, contact patch, and contact PSI.

Some guys don’t like to air down just because they have to drive on the highway after they get off the trail. I’ve driven many times on the highway for short distances with tires at 10 or 15 PSI, and never had an issue at 50 to 60 MPH, however my 1985 Standard Cab only weighed 3100 lbs.

Can there be a 4-wheel drive vehicle or a 4-wheeling situation that we typically get into that clearly demonstrates that a higher than lower tire air pressure is “better"… perhaps… I just haven’t seen it myself.

Gnarls.

[SqWADoosh]

There has been no discussion on this topic. There has been a 16 year old who thinks he knows everything talking out his ass and EVERYONE else telling him he doesn't know what he is talking about.  Pointless thread.

[Gnarly4X]

There has been no discussion on this topic. There has been a 16 year old who thinks he knows everything talking out his ass and EVERYONE else telling him he doesn't know what he is talking about.  Pointless thread.

SqWADoosh.... is your failed fabrication/suspension build a "pointless thread"?

Gnarls.

[SqWADoosh]

SqWADoosh.... is your failed fabrication/suspension build a "pointless thread"?

Gnarls.

Seeing as its a bunch of armchair experts going off of half a dozen pictures and trying to act like they know everything about the link job. No I don't think its as cut and dry as the benefits of airing down off road that have been well established for what, 70 years?

[H8PVMNT]

I will go ahead and bite because I love to see the moment when a new wheeler gets the light bulb above their head when I force them to air down to 4-5 psi on their first good snow run.  Truly an "aha, I get it now" moment.

I will share the experience I had this morning.  Not off road, but a good practical lab experiment for contact pressure. We got our first snow yesterday and the commute to work was an inch of wet ice with wet heavy slush ruts and 12" blowing snow drifts up on the bench.  I had about 32 psi, nice road pressure for my 35s.  In these conditions I was all over the place at 32 psi.  My top speed in 2WD was about 35 mph while quartering slightly sideways. About half way to work I pulled over and yanked valve cores, aired down to about 17 psi.  This is not really "aired down" per-se, but my tread patch was longer than it is wide (all important) and softer with more rubber and edges touching the road surface.  At 17 psi I had total directional control and could scoot along comfortably on the sheet of wet ice at 55 in 2WD until I got behind a farmer in a new F150 doing 25 mph with nice skinny AT tires at max psi.  He was practically sledding down the hill into town and had no control in these conditions, all over the road even with what must have been 8-10 times the contact pressure I had and probably ABS and traction control up the wazzu.

I understand the contact pressure theory, and in some cases it may work OK to run a lot of pressure, especially in a super-capable rig,  but in my experiences a longer, softer tread patch is almost always better than a shorter harder one.  In snowy conditions the longer tread contact makes you track better and not get pushed around with wide tires.  Dirt is much the same as snow.  In rocks with less air the tire can actually wrap around things like a hand grabbing for a hold on something.  We even tried the skinny tire thing and even the old 33x9.50 bfg and the 34x9.50 bias TSLs worked better at low psi than street pressure.

A lot of times I don't think we need to air down quite as much as we do, probably 10-12 is nice for most trails and still OK for driving home, but in all cases I have experienced in any kind of wheeling a gushy tire works better than a hard tire.

My advice to anyone struggling with "tire pleasure" is to try it yourself, high pressure, low pressure. Experiment, experience all pressures in all conditions and draw your conclusions from that.

This is all based on what I have found to be true, not what I think, but everybody has to try it for themselves.

[Gnarly4X]

Profound enlightenment!!   

Thank you, H8PVMNT! 

Gnarls. 

[YooperYota]

That's hilarious, where is this chat? I've gotta see his arguments, haha. The Tapatalk chat doesn't seem to have it.

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[blackdiamond [OP]]

That's hilarious, where is this chat? I've gotta see his arguments, haha. The Tapatalk chat doesn't seem to have it.

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The chat window doesn't keep history beyond a certain number of posts which made it impossible to have this discussion without a useless thread.

[YooperYota]

The chat window doesn't keep history beyond a certain number of posts which made it impossible to have this discussion without a useless thread.

Ah. Well, it would have been dumb to have a useful thread for a useless discussion, so I guess it all works out in the end.

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[Gnarly4X]

Ah. Well, it would have been dumb to have a useful thread for a useless discussion, so I guess it all works out in the end.

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Whether a thread is useless, useful, or pointless is a matter of opinion and perspective.

Gnarls.

[blackdiamond [OP]]

SqWADoosh.... is your failed fabrication/suspension build a "pointless thread"?

Gnarls.

 

Do we need a new forum rule that "jabs" at SqWADoosh about the BBT in discussions that are unrelated is prohibited?  This doesn't need to be brought up every time he expresses a different opinion.

[Mudder]

Shhhh blackdiamond, you're thinking logically here. I remember join this board a couple years ago and it wasn't nearly as toxic as it now. Airing down has been around for a long time because it works. There is no debate.

[SqWADoosh]

Shhhh blackdiamond, you're thinking logically here. I remember join this board a couple years ago and it wasn't nearly as toxic as it now. Airing down has been around for a long time because it works. There is no debate.

That was before the Arizona invasion. 

[Gnarly4X]

Do we need a new forum rule that "jabs" at SqWADoosh about the BBT in discussions that are unrelated is prohibited?  This doesn't need to be brought up every time he expresses a different opinion.

Bravo Sierra!  What's good for the goose is good for the gander!

Gnarls.

[Gnarly4X]

That was before the Arizona invasion. 

That is discriminatory.... and hurts my feelings.

Gnarls.

[300k]

ok, I guess I should start my novel here. This is not a debate of which performs better offroad, but more of a debate of if you should even air down or not. I know that's not what's up on the table, but that's what it was skewed into by others. I'm disregarding snow and beach/dune sand as I haven't driven on those.

Airing down seems to be one of those "well Pa done it this way so its gotta be right!" things...if the tires are touching dirt, someone is going to be aired down. I think there is more to it than offroad = air down. There are a few major benefits to airing down : tire flex, contact patch, and ride comfort (There may be others, but I am grouping most of the mechanics of airing down within "tire flex"). That all sounds great, but I think it is purely terrain dependent, and a few other factors. I'll list some of the factors.

How long will it take to air down
How easy is it for you to air up
How long will it take you to air up
Will airing down even benefit you?

So, if you're sitting at the trail head jamming a stick in your valve stem, and you're trying to pull 21 PSI out of a 37 at 30 PSI, you're going to be there for 20-30 minutes. So you get done with your 2 miles of dirt, and then you're sitting at the trail head with a bike tire pump trying to put that 21 PSI per tire back in those 37s, well you're going to be there all night. That's a little unrealistic, so let's say you did the most logical thing and drove to the gas station...trying to put 21 PSI into each 37 is going to cost you like 25 dollars in quarters when you're all done. Well, what if they have free air (we drove an extra 6 miles for this) and you can air up for free? It took me about 5 minutes per tire to air back up, and for the first two tires I had to keep running back and punching the button. Those 37s took about 4-5 button presses per tire. So what is the final product? you spent 30 minutes airing down, 40 airing up (20 minutes + 20 minutes there and back), an hour and 10 minutes for...what? I would have liked to run that trail at street pressure but the first 4 miles were soccer ball sized boulders...not fun! at 9 PSI there was an incredible improvement in ride quality, but I noticed that my rear tire would slip off almost every rock I put it on. I degraded my offroad performance, I wasted an hour and 10 minutes, but in the end I had to because the ride was unbearable at 30 PSI. HERE IS THE TRAIL I AM SPEAKING OF WITH ME DRIVING AT 9 PSI

None of this matters if you can pull valve cores to air down and stick a 6 CFM compressor on your tires, you may as well air down for pot holes

So let's move onto terrain. The terrain I wheeled is mostly consistent of a mini Moab type deal. the dirt is as hard as cement, and the rocks are massive, smooth, and slightly steep. HERE IS A SIMILAR LINE I HIT AT 30 PSI (same trail) : I'm willing to say had his tire not buckled under the car, he wouldn't have flopped. there are other aspects to the flop, wheelbase, a horrible line, no clutch to press in, jeep guys maybe just can't drive, but perhaps that if he was aired to 30 PSI he wouldn't have been on his side. This is why I like 30 PSI. You have tons of tire structure which is almost a requirement for our type of wheeling.

OOPS posted THIS NEAT LITTLE TABLE, but I think it is slightly misleading. The saving grace from not airing down is that you're distributing force through a smaller contact patch, which equals more outright grip. But if you're on the rocks, only part of your 30 PSI tire will actually be on the rock, which increases the grip by half or even two thirds! at 30 PSI, I NEVER slipped a tire, and 9 PSI, I was losing the line nearly every time.

Also, when I was in the parking lot of the gas station, I took a corner hard and truck was about to drift...they started doing that low tone chirp...what the heck? I thought I was supposed to have drag tire grip

I may be forgetting a lot of my points, but I'll finish with this. I'm not advocating that you ride at 30 PSI, I'm just saying (and this is all I've always said) airing down depends on what you're riding on. You guys have made it into hitting every trail at 40 PSI, nope. Do I have a huge troll face on the entire time as you guys get mad at me for saying this? totally...

[YooperYota]

That's not how friction works on ideal surfaces. Friction is u*N, surface area doesn't come in to play technically. I'm willing to bet that if the jeep had taken a less sucky line or had better tires he would have walked up regardless :grinpimp: . Where the increased contact patch comes in to play (disregarding snow/sand) is that it allows the tire to grip the terrain more effectively. Because you're not utilizing the full contact of the tire when aired up, there's more force per unit of tread and the tread then deforms and/or slips, and you lose traction. When you're aired down you have that larger contact patch so the force normal to the tire tread in the opposite of the direction of movement is distributed over more tire tread and the lug is less likely to slip.

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[H8PVMNT]

Yes, sometimes airing down is hardly worth it, but most of these guys go wheeling for day or two at once, when it is worth it. Also nearly all of them have the tools to air down and air up quick.  I understand now you were not trying to debunk the whole theory of airing down, so I will let you live another day.

So now, get some tire deflators:

 https://www.amazon.com/TRAILHEAD-Tire-Deflators-Billet4x4-OFF-ROAD/dp/B01MRLDHEU/ref=pd_sbs_263_14?_encoding=UTF8&psc=1&refRID=KS1QZBQG53V8T40CYN9T 

Get some decent onboard air, and do some testing and recording.

Prove the contact pressure theory on certain terrain, tell us what you experience. Wheel the same place at 30 psi, 20 psi, 10 psi, 5 psi...  Break a bead, reseat a bead. Tell us where you were right, where you were wrong, with even more pictures. Pictures of the tire gripping, pictures of the tire slipping, videos even.  Get a funny hat.  Eat sardines.  Build a fire without matches.  Kill a chicken with your bare hands.

Street cred Grasshopper.

[H8PVMNT]

https://www.youtube.com/watch?v=qlFMZ03YfxY

[Snowtoy]

Airing down seems to be one of those "well Pa done it this way so its gotta be right!" things...if the tires are touching dirt, someone is going to be aired down.

No, "Pa" didn't have anything to do with it, it was learned from trial and error when open diffs and 27's were what most people wheeled with, the lower psi allowed for better traction and reduced the chances of blowing a bead when dropping onto a rock or trying to bump/hop up a ledge.

How long will it take to air down
How easy is it for you to air up
How long will it take you to air up

Never understood the issue with time when wheeling, taking 20-30 minutes to air down, and 30-60 to air up was just part of wheeling.

So, if you're sitting at the trail head jamming a stick in your valve stem, and you're trying to pull 21 PSI out of a 37 at 30 PSI, you're going to be there for 20-30 minutes.

Pulling the valve core or using an air chuck resolves the issue of time and sticks.

So you get done with your 2 miles of dirt, and then you're sitting at the trail head with a bike tire pump trying to put that 21 PSI per tire back in those 37s, well you're going to be there all night. That's a little unrealistic, so let's say you did the most logical thing and drove to the gas station...trying to put 21 PSI into each 37 is going to cost you like 25 dollars in quarters when you're all done. Well, what if they have free air (we drove an extra 6 miles for this) and you can air up for free? It took me about 5 minutes per tire to air back up, and for the first two tires I had to keep running back and punching the button. Those 37s took about 4-5 button presses per tire.

Ever heard of 5gallon air tanks(something we used in the '80's early '90's), CO2(something we used from the early '90's to '00's), or 12 vlt compressors?  All of these remove the need to use a bicycle pump, or drive 20-30 miles to air up.

So let's move onto terrain. The terrain I wheeled is mostly consistent of a mini Moab type deal. the dirt is as hard as cement, and the rocks are massive, smooth, and slightly steep. HERE IS A SIMILAR LINE I HIT AT 30 PSI (same trail) : I'm willing to say had his tire not buckled under the car, he wouldn't have flopped.

Line and wheel base lead to the rollover, you can see he was already heading for a roll as soon as he started to ascend.


When the rear axle was at the same point, all the weight was to the rear, street psi wouldn't have prevented the roll.

[blackdiamond [OP]]

"Also, when I was in the parking lot of the gas station, I took a corner hard and truck was about to drift...they started doing that low tone chirp...what the heck? I thought I was supposed to have drag tire grip." - 300k

I think you are misunderstanding what is actually happening. When you're feeling like you're about to drift that is simply the sidewalls rolling which is a downside to having tires aired down at speed. This doesn't mean that the tread is slipping.

If you have ever driven on the pavement with your tires aired down you can feel how much more the tires are hugging the road surface compared to street air pressures. The "barking" is the sound the tire makes when it is really gripping and then slips. You don't hear at street pressure because the tires slip easier.

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.