Off Road Tire Pressure - How Low Do You Go?

[Shadyapex]

Oh yes, C02s? That would take a 5 gallon bucket of cylinders to even start to inflate a truck tire...


Phil

http://outbackequipped.com/
I get it filled up once a year and it airs up the Tacoma tires in a couple of minutes. Super convenient for fine tuning the level at camp via the airbags.

 

[Wallowa]

Shady...ah, different size cylinders! Not the 38g size indeed!

Thanks for info..Phil

 

[Shadyapex]

Shady...ah, different size cylinders! Not the 38g size indeed!

Thanks for info..Phil

Indeed, it's a pretty handy system. The only annoyance is that the CO2 leaks from the tire much faster than air so you have to check your pressure again in a couple of days and, ideally, eventually air back up with the compressor in the garage. But it'll get you home, or to the next rough section. Side note, I used to live in Bend and love your corner of the state.

 

[Wallowa]

I should know but of course I don't...why C02? Less pressure change with temp?

Bend is beautiful but as you know we are 7 hours North East of there and of course it is much, much prettier here!

Phil

 

[Shadyapex]

I should know but of course I don't...why C02? Less pressure change with temp?

Bend is beautiful but as you know we are 7 hours North East of there and of course it is much, much prettier here!

Phil

Oh, I know, love the Steens, too. CO2 is used because it can be compressed far more than air and so packs more energy. The 10 lb. tank we carry will air up our tires many, many times. I have the tank filled at the local welding supply shop once a year, sometimes less. Smaller molecule, though, so tire pressure drops in just a few days.

 

[Wallowa]

Wow...fascinating. What is the psi when full and cf? Diving was my profession and the cylinder looks like a 50cf @ 3,000 psi dive tank. And CO2 is an escape artist!

I looked up the periodic table, damn it looked strange after all these years, and found C next to N and they both have associated O...but I did find out that the molecular structure of CO2 is linear and thus "sharp" [I know not very astute] while air of course is a combination of many gases beside N and O so does not have a molecular shape.

For me with restricted space, I will go with a small version of a good compressor...besides I have no idea what the volume of my 265/75 16 tires are at the normal 40 psi. Assuming the worst and a flat I might need a lot of volume. Besides I lugged around thousands of dive tanks over many decades and toting Hp cylinders anymore does not catch my interest..

Great food for thought and thanks.

Sorry to digress from the tread topic but Bishop and that side of the Sierra Nevada Mts. ["the right side"] is a favorite spot on the planet for me. I grew up in Reno in 50s when there was nothing but sagebrush and clean air!

Lastly, the Steen..Alvord..Trouts and Owyhee areas are truly off road heaven...pity those that have never been there..

Now if you drop down into the Rubys .....

Take Care,

Phil

 

[Shadyapex]

Wow...fascinating. What is the psi when full and cf? Diving was my profession and the cylinder looks like a 50cf @ 3,000 psi dive tank. And CO2 is an escape artist!

I looked up the periodic table, damn it looked strange after all these years, and found C next to N and they both have associated O...but I did find out that the molecular structure of CO2 is linear and thus "sharp" [I know not very astute] while air of course is a combination of many gases beside N and O so does not have a molecular shape.

For me with restricted space, I will go with a small version of a good compressor...besides I have no idea what the volume of my 265/75 16 tires are at the normal 40 psi. Assuming the worst and a flat I might need a lot of volume. Besides I lugged around thousands of dive tanks over many decades and toting Hp cylinders anymore does not catch my interest..

Great food for thought and thanks.

Sorry to digress from the tread topic but Bishop and that side of the Sierra Nevada Mts. ["the right side"] is a favorite spot on the planet for me. I grew up in Reno in 50s when there was nothing but sagebrush and clean air!

Lastly, the Steen..Alvord..Trouts and Owyhee areas are truly off road heaven...pity those that have never been there..

Now if you drop down into the Rubys .....

Take Care,

Phil

Sorry Phil, but I don't know the volume or pressure when full. It holds 10 lbs of gas and will fill my tires many times. I usually air down to 25 for the rough stuff and run 45 on pavement and this routine is repeated at least a couple of dozen times each year. I have the tank filled once a year and it's never been anywhere near empty.
Still planning a trip to the Rubys. I hear the skiing is great, well, as long as one has snow. Fingers crossed.

 

[Wallowa]

Geeee...I never dealt in Lbs of a gas only volume @ psi but looking up CO2 it appears that 10 Lbs of it = approx. 87 cf....a lot of gas. So that tank is bigger than I thought or the pressure is higher..but I am betting it is the standard 3,000 psi Al cylinder..no idea what pressure they are using.

BC skiing is almost here in the Wallowa Mts; 5" here at home....yes, skiing the Ruby Mts is on my list of must do..Hence the Hawk!

Take care..Phil


==============Disregard my estimates...website shows only 100psi fill; wonder what the burst disk pressure is...and yes 15 lbs empty and 25 lbs. for the rated 10 lbs of gas...chart still shows about 87 cf and that squares with you experience.....fooled me.

Read more at website...at 155 degrees the cylinder pressure that started at 100 cold will reach 3,000 psi and the burst disk will rupture releasing the pressure..not to worry a 3,000 psi rated/working pressure tank can do survive 4,000 psi [may permanently weaken tank] but disk will let loose long before that and drain tank...the trunk of a car can go over 250 degrees so keep out of sun!

 

[Shadyapex]

Geeee...I never dealt in Lbs of a gas only volume @ psi but looking up CO2 it appears that 10 Lbs of it = approx. 87 cf....a lot of gas. So that tank is bigger than I thought or the pressure is higher..but I am betting it is the standard 3,000 psi Al cylinder..no idea what pressure they are using.

BC skiing is almost here in the Wallowa Mts; 5" here at home....yes, skiing the Ruby Mts is on my list of must do..Hence the Hawk!

Take care..Phil


==============Disregard my estimates...website shows only 100psi fill; wonder what the burst disk pressure is...and yes 15 lbs empty and 25 lbs. for the rated 10 lbs of gas...chart still shows about 87 cf and that squares with you experience.....fooled me.

I'm afraid we've kind of hijacked this thread. The tank isn't too big, it fits under that back seat of the Taco Access Cab with room to spare. And yes, it holds a remarkable amount of gas. The tank is rated in pounds so I just weigh it to find how much is left as the tare is listed on the bottle. There are, however, warnings on the site about leaving the full bottle in the cab in the heat of summer. I fill it in the fall and hope for the best.
Hoping for some back country action this year as well, it's been 4 years of lousy snow pack so I've got my fingers crossed for El Nino in spite of the damage that it is likely to cause to those is so cal in dangerous canyons. Are you getting some precip from this recent storm?

 

[JHanson]

The only annoyance is that the CO2 leaks from the tire much faster than air so you have to check your pressure again in a couple of days and, ideally, eventually air back up with the compressor in the garage.

Interesting. I have a CO2 tank as well as several compressors. I use the tank for tire repair classes at the Expo since it's faster then even my Extreme Outback compressor, and quieter. I need to set up a test to compare loss rates.

 

[craig333]

I'm curious to see what you get as the guys that run CO2 that I know haven't brought this issue up. I just went out and checked the spare on the Jeep which was last aired up with CO2 after losing the bead. 15 psi. Unfortunately I don't recall what it was aired up to then but likely wouldn't have been much more.

 

[smlobx]

Not to rain on anyone's parade but I'm having a hard time following the logic of using CO2 over other gasses...

The best gas to run in tires is actually Nitrogen which is about 70% of what generic air is made up of.
People who race cars/trucks for a living from the Baja 1000 to Formula 1 all fill their tires with Nitrogen because it is inert and not affected (as much) by temperature fluctuations.

The reason the OP can fill more of his tires with CO2 is probably due to the cylinder more than anything else.

 

[Shadyapex]

Not to rain on anyone's parade but I'm having a hard time following the logic of using CO2 over other gasses...

The best gas to run in tires is actually Nitrogen which is about 70% of what generic air is made up of.
People who race cars/trucks for a living from the Baja 1000 to Formula 1 all fill their tires with Nitrogen because it is inert and not affected (as much) by temperature fluctuations.

The reason the OP can fill more of his tires with CO2 is probably due to the cylinder more than anything else.

I understand the superiority of using Nitrogen, it is a better gas for inflating tires. CO2 is handy just because you can store more of it in a bottle. It's my understanding that it's more compressible.

 

[Wallowa]

Well, why CO2 indeed. If I interpreted what my limited search produced here might be a reason. Volume of compressed gas vs size and pressure of cylinder. The aluminum 50 cf SCUBA tank has a rated and working pressure of 3,000 psi and contains 48+ cf of air. The CO2 cylinder in the article looks to be the same size cylinder as a "50cf" SCUBA cylinder, but this CO2 cylinder is filled to 100 psi and contains 87 cf of C02 gas. In short form, CO2 has more molecules per mole [amount of gas per volume compressed] so you could fill more tires with that cylinder of CO2 @ 100 psi than you could with it filled with air at 3,000psi... 55+% more.

But hey, I could very well be wrong! Phil


Ps..What Shady said..

 

[JHanson]

The reason the OP can fill more of his tires with CO2 is probably due to the cylinder more than anything else.

Exactly. As Wallowa said, it's strictly about available volume in a reasonably sized cylinder. No benefit to the gas itself, and possibly a disadvantage if it really does permeate through the tire that quickly. I know it will do so through a butyl bicycle inner tube.

Powertank, one of the pioneers in marketing CO2 tanks as a replacement for a compressor, claims that a tire filled with it will deflate more slowly than one filled with air, because "the CO2 molecule is larger than a nitrogen molecule, which comprises 78 percent of air." My understanding is the CO2 is actually soluble in butyl and thus does not have to permeate. But I'm going to experiment.

 

[Shadyapex]

This is interesting. I had always thought that the CO2 was a smaller particle than N but some quick online research seems to refute this. My experience has always been that I have to adjust the pressure in my tires much more frequently after airing up with the CO2. But I have never done a side by side comparison.

 

[Shadyapex]

This is from a cycling page but I thought it might be relevant. Many cyclist use a small CO2 cartridge to refill after a flat rather than a pump.
Dear Lennard,
Since a CO2 molecule is larger than either an oxygen or nitrogen molecule, why does it leak out of a bicycle tire faster?
Glenn

Dear Glenn,
Upon receiving your question, I put CO2 in a clincher tire with a Michelin butyl inner tube (latex tubes leak air quickly, as you’re probably aware). This particular tire and tube hold air pressure faithfully for weeks on one of my road bikes without needing pumping. And sure enough, within three days after inflating with CO2 to 90psi, the pressure had dropped to 45ps
...
Permeation by diffusion predicts gas leakage rates proportional to the inverse of the square root of their molecular weights. Using air as a reference the predicted leakage rates for common gases are: helium 2.7, air 1.0, nitrogen 1.02, oxygen 0.95, argon 0.85, carbon dioxide 0.81.

It turns out however that the leakage rate of CO2 is huge, and the reason is that it is actually soluble in butyl rubber and is thus not constrained to normal permeation loss, it can transfer straight through the bulk rubber resulting in severe tire pressure loss on the order of a single day.

Here's the link: http://www.baltimorespokes.org/article.php?story=20090207131052655

Thoughts? Does this sound legit? I think JHanson mentioned the possibility of solubility as well.

 

[JHanson]

Thoughts? Does this sound legit? I think JHanson mentioned the possibility of solubility as well.

It seems to be well accepted in the bicycling community, where small CO2 cartridges are popular for flat repair. I'll be back where I can compare two BFG AT tires side by side in a week or so. I'll report.

 

[ntsqd]

In skimming the recent posts I think a very important aspect about CO2 tanks was missed. The reason that you can get a LOT more tire fills out of a CO2 tank than a comparable sized N2 tank or air tank is rather simple. Phase change due to pressure. N2's phase change pressure isn't reached in a normal tank so even at 2500 psi it is still a gas, but CO2's is and it goes to a liquid state. Change that gas to a liquid and you can get a whole lot more volume of CO2 in the tank than you can get N2 in the same tank. Propane also phase changes to a liquid in the tank, but propane filled tires is too scary to think about. shudder......

 

[Wallowa]

ntsqd,

Very interesting. You are you saying that when his CO2 tank is filled to the 100 psi rated pressure it goes from a gas to a liquid inside the cylinder? Wow, I had no idea that such a low pressure could transform CO2 gas into a liquid. Very interesting.

Thanks for insight,

Ps...I just re-read the Outback specs on the cylinder charging...they fill to 100psi 'cold' [no mention of temp] and then state that at 70 F the pressure is 837 psi....not certain how all this folds into the possible phase change for gas to liquid.