Tire Pressure

Gkconfer · 04-12-2011, 04:38 PM

We just returned from picking up our new Rockwood 2306 Mini-Lite from RV Wholesalers in Ohio. We were told during the PDI that the tire pressure should be set at 50 PSI and that RVW had already checked that the tires were properly inflated during the prep. After parking today, I thought the tires looked a little low and checked the pressure with a digital gauge. The tires, which are ST175/80R13, were all between 34 and 36 PSI which, I assume, means they are badly under-inflated. Shame on me for not checking this myself before we pulled away from RVW.

I'm now wondering if we were in jeopardy of having a problem due to the under-inflated tires. We had no issues on the drive home, but I assume that, if nothing else, we would've gotten better mileage if the tires had been properly inflated. My tv averaged 22 mpg on the way out and 11 mpg pulling the trailer home.

Additionally, the laminated chart attached to the rear of the tt says my tt has a dry weight of 3678 and the maximum cargo is 954 for a total weight of 4632. Does this pertain to the tires and the maximum weight they can safely carry? If so, it sounds as though it may be wise to put fourteen inch tires on my tt. Thanks for your input.

Greg

DAISY BOYKIN · 04-12-2011, 04:43 PM

Greg, Check the DOT and VIN stickers on the left front side of your TT. The DOT tire info sticker is yellow. This will tell you the correct pressure.

skip1955 · 04-21-2011, 09:26 PM

always check your tires when they are cool

Quote:

Originally Posted by Gkconfer

We just returned from picking up our new Rockwood 2306 Mini-Lite from RV Wholesalers in Ohio. We were told during the PDI that the tire pressure should be set at 50 PSI and that RVW had already checked that the tires were properly inflated during the prep. After parking today, I thought the tires looked a little low and checked the pressure with a digital gauge. The tires, which are ST175/80R13, were all between 34 and 36 PSI which, I assume, means they are badly under-inflated. Shame on me for not checking this myself before we pulled away from RVW.

I'm now wondering if we were in jeopardy of having a problem due to the under-inflated tires. We had no issues on the drive home, but I assume that, if nothing else, we would've gotten better mileage if the tires had been properly inflated. My tv averaged 22 mpg on the way out and 11 mpg pulling the trailer home.

Additionally, the laminated chart attached to the rear of the tt says my tt has a dry weight of 3678 and the maximum cargo is 954 for a total weight of 4632. Does this pertain to the tires and the maximum weight they can safely carry? If so, it sounds as though it may be wise to put fourteen inch tires on my tt. Thanks for your input.

Greg

LadyWindrider · 04-21-2011, 11:51 PM

ALWAYS READ THE SIDEWALL ON THE TIRE!!!!!! Never believe a sticker on a trailer. LOOK AT THE TIRES!!!!! The dealer may have swapped tires, factory may have installed wrong tires. LOOK AT THE TIRES!!!!. No your gas mileage would not have increased. Did I mention, READ THE SIDEWALL INFORMATION ON THE TIRES!!!!! Your weight and tire size suggest load range "B" tires, in which case 35 psi is about max. CHECK THE SIDEWALL on the tires and look for "load range" and also max "inflation pressure" Sorry if I'm over stressing this, but if you have a tire problem, they will ask what pressure you were running and where did you get the info. If you say from a sticker on the trailer, the tire manufacturer is gone because you did not follow inflation instructions printed on the tire.

**Herk7769** · 04-22-2011, 06:51 AM

While Skip is correct that tires are ALWAYS checked COLD, the pressure would have gone UP not down when checked hot. Boyle's Law of Gases.

If the tire was checked hot at 36 PSI, they were severely under inflated. I hope the trip was a short one. I would complain bitterly, and yes I would be a PITA until they documented it in your jacket.

IMO, (NOT a tire guy; but have had a tread separation at speed), those tires could be damaged.;ESPECIALLY, if they were driven at high speed for a long distance.

Anecdote:
I had to get involved with my tire dealer when I bought tires for my truck. The mounting machine they use did not go to 80 PSI, so the mechanic was going to mount them at 50 PSI; the highest the machine could go. I insisted I wanted 65 in the front and 80 in the rear. The mechanic whined, "but I will have to do another step to do that!" Sheesh...

Glenn5995 · 04-22-2011, 08:19 AM

You will be fine.

50 PSI is the maximum pressure the tire can hold. The sidewall will be marked with the maximum PSI the tire can hold and the corresponding weight the tire can bear at that pressure. As the pressure decreases, so does the tire's load bearing capability.

Here is a tire inflation chart from Goodyear which gives the the load bearing capability of their tires at given pressures.

http://www.goodyear.com/rv/pdf/rv_inflation.pdf

You had just picked it up, so it should be empty. The dry weight is 3678, but lets add 400 for optional equipment, so that brings you to 4078. Subtract about 400 for tongue weight and that leaves the axles supporting about 3678. I believe that trailer is a tandem axle, so you have four tires. Although weight is never equally distributed in an RV or most vehicles for that matter, that makes each tire roughly supporting 919.5 pounds. According to Goodyear's chart (most manufacturer's will be similar), 30 PSI is sufficient to bear that weight.

Of course, if you have the cheap Carlisle tires which come with many new RVs or any Chinese manufactured tire........you are on borrowed time anyway, but that is another story!

**Herk7769** · 04-22-2011, 08:28 AM

Thanks Glenn, a masterful addition.
You get a thumbs up from me.

tentcamper1 · 04-22-2011, 09:46 AM

Quote:

Originally Posted by herk7769

Thanks Glenn, a masterful addition.
You get a thumbs up from me.

hit the "thanks" button!!

youroo · 04-22-2011, 10:11 AM

You picked it up in Ohio,and went to PA. Wonder what the tire press was from Elkhart,IN to RVW???? Youroo!!

Glenn5995 · 04-22-2011, 12:24 PM

Tire pressures vary quite a bit with temperature. That may be part of the problem depending on the ambient temperature at which they were originally checked and the ambient temperature at which the OP checked.

When I check the "cold pressure" on the trailer tires on a 115 degree Phoenix day and then drive to a camping area at 9500' in the White Mountains of Arizona and check them the next morning at 45 degrees, they will be 10 to 12 PSI less. If I leave them alone and return to Phoenix they will be back to the original pressure. If I were to fill them back up at the colder temperature, when I return to Phoenix I will find they are now over inflated.

That is one reason some people prefer Nitrogen filled tires, they do not vary pressure as much with changes in temperature. In hot climates, I usually keep the tires a few PSI above where I think they should be and at colder climates allow them to be under a little. In moderate temperatures they will be right on. Otherwise I would constantly be changing tire pressure.

Gkconfer · 04-22-2011, 02:30 PM

Thanks everyone for weighing in. When I checked the tires, they were cold. According to the rating chart that Glenn attached, I was well within a safe range with the weight we were pulling. The tire sidewall does indicate a max 50 psi with a max weight of 1360 lbs. That info, btw, is very difficult to read but here is what is clearly legible: Made in China. I don't know about you, but I have zero confidence in the quality of Chinese products based on the horror stories that have come out of China in recent years regarding their lack of quality control.

08flagvlite · 04-22-2011, 03:26 PM

I don't know if it is relevant or not, BUT I purchased a bag of Doritos in Jacksonville, Fl. where I live, and went to Tennessee on the Blue Ridge pk.way, and that un opened bag of chips was making all sorts of funny noises inside a Wal mart bag in the back seat; we kept wondering what the heck was making that noise, and discovered that bag was about ready to burst! Apparently, the air pressure when filled and sealed in sea level Florida, was so much greater at 3500 feet(guess) above sea level, it made the bag do that. Also, I recall a kid's TV show many moons ago, where they took normally inflated balloons down in a sub, and they lost all of the air in them. Upon returning to the surface, they magically went back to normal. This same thing effect tires???? HMMMMM
Randy

**Herk7769** · 04-22-2011, 03:43 PM

I am researching this thread regarding two "head scratchers."

1) What IS the coefficient delta between nitrogen and a 78/20 mix of oxygen and nitrogen. IE if air changes 5 PSI due to a temp change what would 100% nitrogen do under the same conditions.

2) Would anybody care if I did.

Glenn5995 · 04-22-2011, 06:02 PM

I hope I didn't give the impression I was advocating Nitrogen for inflating tires. I was just noting that less variance with temperature is one of the claims made by its advocates. I have not figured out how much difference there would be between air which is already 78% Nitrogen and pure Nitrogen (as you noted), but there are those which swear it is so. I personally think (that's scary isn't it!) that the result is not because of it being pure nitrogen, but the fact that it is "dry", i.e no humidity/moisture. If you removed the moisture from plain old regular air, you would probably have about the same result with more consistent pressure.

Just my thinking, feel free to rally to my cause

or revolt

as you deem appropriate.

08flagvlite · 04-22-2011, 06:10 PM

1.)My POINT IS does low air in the tires change at a higher altitude, or not? And, if so, then if i topped off my 50 lb. tires at 50 lb. cold in Fl, and drove to say the smokies, or someplace like that, would my air pressure now be dangerously over pressurized, risking a blow out or not?
2.)I for one would like to know the answer to Herk's puzzling strange phenomenon.

lbrjet · 04-22-2011, 07:40 PM

From everything I have read there is a slight increase in tire pressure in higher elevations, but not nearly as much as with temperature changes. I saved this formula from someone over on RV.NET which seems accurate. I have no idea if nitrogen filled tires react differently.

"
The complete formula is (P1 * V1)/T1 = (P2 * V2)/T2. It's fairly safe to assume the volume doesn't change in a relatively minor change in temperature. Normally, the "T" number is Fahrenheit Absolute, which is regular F +460.

As an example, if you set the pressure at 120 psi when it's 40 F and the recheck it at 80F, and ignore the tire volume, P2 woud be 120*540/500, or 129.6
"

Glenn5995 · 04-22-2011, 08:37 PM

My understanding, and this is not an area in which I claim a lot knowledge, so correct me if I am wrong, is that there is no change in pressure in a SEALED container with changes in elevation assuming all other variables remain constant.

In the example of a sealed chip bag, when the chips were placed in the bag, no air was pressurized into the bag. The air inside was at atmospheric pressure, which matched the exterior pressure and so they canceled each other and the bag was pliable. Once you went up in elevation the atmospheric pressure decreased. The bag still contained the original higher atmospheric pressure, but now the pressure on the exterior of the bag has decreased and so the pressure inside the bag is greater than that outside and the bag expands under the force of the greater pressure, but the pressure in the bag stayed the same. This is assuming all other variables remain constant.

The change in pressure in tires is primarily from changes in temperature.

**Herk7769** · 04-22-2011, 10:55 PM

Quote:

Originally Posted by lbrjet

The complete formula is (P1 * V1)/T1 = (P2 * V2)/T2. It's fairly safe to assume the volume doesn't change in a relatively minor change in temperature. Normally, the "T" number is Fahrenheit Absolute, which is regular F +460.

As an example, if you set the pressure at 120 psi when it's 40 F and the recheck it at 80F, and ignore the tire volume, P2 would be 120*540/500, or 129.6
"

Gas laws - Wikipedia, the free encyclopedia

We could go crazy here with formulas, but the question posed has more to do with the "pure Nitrogen" vs "Air" debate of which there are over a hundred separate threads on this forum alone.

It is true, that if you keep the volume of a gas constant (ie in a fixed non-inflateable tire) and increase temperature, the pressure will rise. Decrease temperature and the pressure will fall.

Similarly, in a fixed volume, raising or lowering the pressure OUTSIDE the container; NOTHING inside the container changes. Pressure stays the same since the volume of the tire is fixed.

If the tire was flexible enough to inflate like a balloon, it would expand until the pressure inside was the same as the pressure outside (or it popped). If you put it underwater it would deflate as the outside pressure increased. However tires are not that flexible so temperature is the only thing that changes internal pressure.

MtnGuy · 04-23-2011, 06:59 AM

FWIW, it is almost impossible to get 100% (pure) N2 in tires. Unless there is some way to completely purge the existing air out of a tire, a 95% N2 fill is about all that can be expected.

Glenn5995 · 04-23-2011, 09:20 AM

Lou,

Sorry, I have not been on the forum long and did not realize there was a "nitrogen vs air" debate. I wasn't trying to start anything.

Also, thank you for explaining what I was attempting to say about the laws of gas far more clearly than I did.

I generally try to confine my comments to things about which I have knowledge, but after awhile the silence gets to me and I have to say something.......regret it.......start the process over.

Glenn

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