ECO-Fit DiscussionThreads discussing the pursuit of ultimate economy, hypermiling techniques & maximizing your MPG
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Myths about pressure
Let's put to rest some common misconceptions. The tires will not balloon out creating a peak in the center portion of the tread when tire pressure is above 35 psi. There is a steel belt that prevents this from happening.
Sorry, officer but tire engineers will assure you that the tread of a tire will bow out in the middle when overpressurised. The steel belt is no more rigid across the tread than it is around the tread. stiffer than nylon yes but not that rigid. It is that bowing that reduces rolling resistance and yields better gas mileage.
And I can show you many tires that were worn to the cords on the center and half tread depth left on the edges from showroom stock racing at 45 psi. But not on Crown Vics.
On a Crown Vic 45 psi is not overpressured but on a Fit they are. Except for autocross or track or economy runs.
Everything else is excellent. Good advice.
Just an FYI. I was at tire store 2 weeks ago and in speaking with owner and actually seeing, they had put 125 lbs of air, in a tire to seat it on the rim. They where leaving the tire sit as it was a problem to get mounted with employees walking around. Tire and employees were all fine. 100 psi and tire exploding sounds like myth to me.
Just an FYI. I was at tire store 2 weeks ago and in speaking with owner and actually seeing, they had put 125 lbs of air, in a tire to seat it on the rim. They where leaving the tire sit as it was a problem to get mounted with employees walking around. Tire and employees were all fine. 100 psi and tire exploding sounds like myth to me.
doesnt mean its a good idea to drive on it like that
All I can say is.... Different size tires have different volume resulting in different characteristics with different air pressure...
eh.... PV=nRT for those who know what it means.
Also, the article mentions nothing about directional vs non directional tires.
PV=nRT has nothing to do with structural strengths of anything. And it has nothing to do with differing characteristics with differing pressures any more than PV/t =PV/t.
Just an FYI. I was at tire store 2 weeks ago and in speaking with owner and actually seeing, they had put 125 lbs of air, in a tire to seat it on the rim. They where leaving the tire sit as it was a problem to get mounted with employees walking around. Tire and employees were all fine. 100 psi and tire exploding sounds like myth to me.
Yeah, I'm not sure how far the tire was filled. My coworkers seemed quite saddened by the event as if they knew or knew of the kid killed. Otherwise I wouldn't have taken it seriously either.
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Using nitrogen to completely fill the tire will help (not entirely fix) the problem of increased pressure with increased temperature. when using atmospheric air, it can compress and expand greatly with temperature. If using pure nitrogen, it will expand and contract less. This would allow you a great safety margin when carefully measuring the pressure of your tires to keep them within the safety limits.
I also thought of argon, but I am not completely sure about its properties relative to nitrogen, but if anything, it would also be better than atmospheric air pumped into the tire.
CO2 would not be a good choice to fill tires with though, as it expands and contracts with temperature to a greater extent than nitrogen or argon. Think of the compressed gases, and liquid compressed gases you can purchase at welding supply shops (airgas, linde gas, etc.)
It appears as though several people are missing a major point regarding tire pressures. The recommended pressure from the automobile manufacturer -- the one found in the door jamb and in the owners manual -- and the recommended maximum pressure from the tire manufacturer -- the one stamped onto the sidewall of the tire -- are both COLD tire pressures. They both assume the measurement is being taken before the tire has been in use. In addition, the maximum pressure on the sidewall is the maximum pressure at maximum load, so it takes into account extremes of heat-related pressure increases.
In short: there is no need to get all freaky about how much the pressure might go up as the tire heats up. The engineers behind the numbers you see already took that into consideration.
I heard a story about twenty years ago when I lived in Hawaii about a kid who was killed pumping up an automobile tire. The story was he believed since auto tires are beefier than bike tires, they should be inflated higher than bike tires. Bike tires easily exceed 80-100 psi. The auto tire exploded killing the kid. Might be urban legend, who knows.
The story your talking about was a "kid" not sure how old maybe 18 to 20 something he was working at a tire store and he over inflated a car tire when he was mounting it. The tire exploded killing him. I was working back then at Sears auto and when this happened we were all told about it. When mounting a tire you need to add sometimes a fair amount of pressure before the tire pops on to the bead. If you didnt put enough lube on the bead of the tire (soapy water solution) the pressure required would sometimes go up quite high to get the tire to pop onto the edges of the rim. So the story your most likley referring to is that one.
Is it worth replacing the air in my '09 Fit with liquid nitrogen from the very start? Would this really increase MPG? How often would the tires need to be checked/refilled? Two different tire dealers had two completely different interpretations on the advantages/disadvantages of LN over air....
Is it worth replacing the air in my '09 Fit with liquid nitrogen from the very start?
Only if you are racing at a very high level, where +/- 0.5psi makes a real difference. Also please do not use liquid nitrogen; frozen-solid tires are not conducive to either ride quality or rolling resistance.
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Would this really increase MPG?
No. The only thing that will increase is the net profits of the nitrogen seller.
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How often would the tires need to be checked/refilled?
Because nitrogen is a smaller molecule than air, you would want to check them more often. But since you are already wise enough to realize that nitrogen is of no benefit to you, you can go ahead and check them every two weeks or so.
Because nitrogen is a smaller molecule than air, you would want to check them more often. But since you are already wise enough to realize that nitrogen is of no benefit to you, you can go ahead and check them every two weeks or so.
I am pretty sure Nitrogen is a larger element (not a molecule) than oxygen so technically you would have to check less often;
With that said there are supposed to be other advantages by loading up on pure dry nitrogen gas; your tires will not suffer from oxidization from having oxygen diffuse through the tire membrane which should extent the life of the tire; there is also an advantage by reducing the humidity compared to compressed air which would mean your tires would be more resistant to overheating and have a more consistent operating range.
Playing devils advocate to my own post; I don't think it is worth it to pay extra for nitrogen as you will probably only get 90% anyway (if you purge the tire a couple times you can increase that to as much as 95%). The atmosphere already has 78% nitrogen so really you are paying for a tiny increase. Studies have shown that air retention is negligible (something like 1.3psi over a long period of time different) between a tire filled with air and a tire filled with nitrogen. You would be better off to spend you 5$ on a tire gauge and check it regularly.
There might be something to the oxidization theory, fleet studies have been done and show an increase in tire life.
Also using a dry air or nitrogen is better for demanding conditions than standard compressed air for keeping a more consistent operating range. Most race cars will use nitrogen for this reason alone.
My own conclusion: Nitrogen has some advantages, but seems like a giant rip off to me. I am always making adjustments to my tire pressure and there is no what I am paying 5$ a hit, just isn't worth it.
Hmmmm, anybody ever consider filling their tires with Argon lol
EDIT lol I just caught your Argon statement:
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Originally Posted by gnef
Using nitrogen to completely fill the tire will help (not entirely fix) the problem of increased pressure with increased temperature. when using atmospheric air, it can compress and expand greatly with temperature. If using pure nitrogen, it will expand and contract less. This would allow you a great safety margin when carefully measuring the pressure of your tires to keep them within the safety limits.
I also thought of argon, but I am not completely sure about its properties relative to nitrogen, but if anything, it would also be better than atmospheric air pumped into the tire.
CO2 would not be a good choice to fill tires with though, as it expands and contracts with temperature to a greater extent than nitrogen or argon. Think of the compressed gases, and liquid compressed gases you can purchase at welding supply shops (airgas, linde gas, etc.)
The expanding and compressing is negligible between them, like I was saying before the main issue is humidity.
The only thing I am wondering about with other inert gases is weight, a heavier gas would mean a heavier tire would it not? C02 would be an interesting one to try as well.
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Last edited by Sugarphreak; 08-24-2008 at 02:06 PM.
All very nice if you are comparing pure oxygen to pure nitrogen, which I don't believe we are. (Pure oxygen is a tremendous fire hazard so let's not entertain that idea, hmmm?)
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