Tire Pressure Question - Fiberglass RV


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Old 11-17-2015, 10:22 AM   #1
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Tire Pressure Question

We have a new SCAMP 13 footer, which we picked up from Backus on June 8th. Since we drove it home to Kentucky it has mostly been in the yard, except for a weekend camping trip nearby.
We are about to head out to Florida on Friday afternoon for 10 days of sun and fun. I checked the tire pressure and found every tire to be low! I had a 38 lbs and a 39 lbs and the spare was 42 lbs. The temps were in the mid 50's and was sitting in the driveway. The sticker on the camper said to inflate to 50 lbs.
I found it odd that all 3 tires were low. Making me think they were not 50 lbs from Backus.
What do all you folks inflate to? (in particular the 13 ft. scamp)
Should I inflate to 50 lbs as indicated?
Today the temperature is in the mid 50's again possibly approaching the low 60's.
Thanks...
Ralph
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Old 11-17-2015, 10:41 AM   #2
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Trailer tires should be set at "side wall pressure". In your case 50psi. That cold temperature measured in the morning before any driving.
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Old 11-17-2015, 10:42 AM   #3
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Inflate them to 50 pounds and check them periodically when cold(ambient temperature) to maintain them at 50 pounds.
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Old 11-17-2015, 10:58 AM   #4
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We inflate to a 50 PSI cold pressure on our Goodyear Marathons, suggested max. pressure. As we drive south from NH to FL the cold pressure increases so we check it regularly.
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Old 11-17-2015, 11:03 AM   #5
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...inflate to recommended sidewall pressure.
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Old 11-17-2015, 12:01 PM   #6
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Originally Posted by honda03842 View Post
We inflate to a 50 PSI cold pressure on our Goodyear Marathons, suggested max. pressure. As we drive south from NH to FL the cold pressure increases so we check it regularly.
And can I assume you do the same thing when driving north and add air as needed?
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Old 11-17-2015, 12:07 PM   #7
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I found that after a trip, my tires lose a few pounds of air. I inflate them to 50 psi before every trip.
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Old 11-17-2015, 12:23 PM   #8
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The temp and elevation both make a difference since gauge pressure is referenced to ambient pressure.
Also I believe that the Goodyear Marathon tires are rated to a higher speed inflated to 55 psi.


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Old 11-17-2015, 12:30 PM   #9
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Thanks guys... you are a big help! I will watch them as we go south and then coming back up north as well.
Ralph
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Old 11-17-2015, 12:51 PM   #10
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Originally Posted by redbarron55 View Post
The temp and elevation both make a difference since gauge pressure is referenced to ambient pressure.
Also I believe that the Goodyear Marathon tires are rated to a higher speed inflated to 55 psi.


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This is the first time I've heard of that. How does the tire gauge or any pressure gauge work referencing ambient pressure? Is there some sort of external sensor?
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Old 11-17-2015, 03:21 PM   #11
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After quick search I see there are temperature compensating pressure gauges. Prices are pretty high.
I've seen NASCAR use such thing, but no tire shop I've been in does.
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Old 11-17-2015, 03:46 PM   #12
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And can I assume you do the same thing when driving north and add air as needed?
Yes Bill. I do pay attention to our trailer tires. We have not had a problem.

I know that there is apparently information that if you inflate above 50 PSI you can drive at higher speeds. I do not do this and never drive over 65 mph, the max speed limit stamped on the tires.

As well we tend to drive early n the day to avoid the heat of the day for the tires.
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Old 11-18-2015, 11:00 AM   #13
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All good advice so I won't say more except regarding the changing pressure - perfectly normal as the outside ambient temp changes, especially with the seasons. You will want to check your pressure again once you're down south. You should also verify your pressure gauge, perhaps compare it with a mechanic's. I had one of the new, "digital" ones which I assumed was more accurate than my old mechanical one. Turns out it was reading five pounds light!
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Old 11-18-2015, 11:27 AM   #14
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Any tire filled with air will lose a little pressure over time especially as the temperature drops. Not sure if any of you are aware of it but tires filled with pure nitrogen will maintain their pressure better than tires filled with air. I believe the nitrogen molecules are larger than air molecules so their is less chance of them bleeding out of the tire. Of course this is all a moot point if you can't find nitrogen to fill your tires. If I'm not mistaken when you buy tires at Costco they do fill them with nitrogen but other than that I don't know where you would get it. I do know that you should check tire pressure when the tires are cold.
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Old 11-18-2015, 12:32 PM   #15
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Just to geek out a bit; Generally, the Ideal Gas Law (PV=nRT) governs inflation pressure and its relationship to temperature. Assume that tire volume is constant. Beyond that, all pneumatic tires lose pressure due to osmosis. A ballpark loss is about 1-1/2% per month. There is a wide range of performance based mostly on the makeup of the tire's inner liner.
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Old 11-18-2015, 01:30 PM   #16
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Just to geek out a little more.....
Even if the molecules did not leak out at all (size of Nitrogen molecules and Oxygen, Argon etc.) then if the temperature changes the pressure changes. The higer temp the higher the pressure.
Also since we use gauges that reference ambient air to measure the higher the barometric pressure (also the lower the altitude) the lower the indicated air pressure with the same number of molecules still inside the tire!
So if you go from a higher elevation to a lower then the pressure indicated on the gauge will be lower (still no leaks). There is an old aviation saying" high to low look out below!"
The pressure supporting the weight stays the same with the same indicated pressure.
The air holds up the trailer and the cords and tire structure contain the air.
(Of course the sidewalls do a little too, but not enough to make a real difference.)
If you air up your tires on the top of a mountain and travel down and then measure it at sea-level (assuming the same temperatures at both places for the tires) it will read lower by the change in the ambient air pressure.
If you blow up a balloon under water and let it rise it will inflate more as the surrounding water pressure decreases too.

As to the seepage of air I know that in ancient day of yore if you let your race car sit for a while the tires would definitely be flat very soon. The tires were optimized for traction and not holding air. The wheels also tended to be porous as well ( at least the magnesium wheels of the period).
I think if you just check the tires in the morning before you start for the day you will be OK.
Tires last longer if used since the rolling distributes the oils used in manufacturing the compounds gets moved around a little to help prevent deterioration.
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Old 11-18-2015, 01:48 PM   #17
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Originally Posted by Byron Kinnaman View Post
Trailer tires should be set at "side wall pressure". In your case 50psi. That cold temperature measured in the morning before any driving.
Once again (and you may find lots of previous discussion on this)
There is no need to keep tires at 50 psi, just because they are marked "50 Psi" If you look carefully you will see that it also gives the Maximum Load for that pressure, which, according to the LOAD/INFLATION chart is 1360 lb per tire or 2720 lbs per axle.

Your 13 ft Scamp, assuming the deluxe, per the Scamp website, is maybe 1600 lb total, allow maybe 400 lb for water and all your stuff, it might weigh 2000 lb. Of that 200 lb is on the hitch; which leaves 1800 lb on the axle, or 900 lb per tire. Per the L/I chart, you need 25 psi in the tires.
Bottom line .... Ya Gotta WEIGH the trailer to know for sure.
Simple rule of thumb is to note if there is any sidewall bulge at the bottom of the tire. No bulge? too much air! Big bulge? Not enough air!
Does the Scamp bounce a lot on the bumps in the road? Too much Air!

Thanks for asking, and, Oh!, get yourself a quality tire pressure gauge.
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Old 11-18-2015, 02:55 PM   #18
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Quote:
Originally Posted by Wayne Collins View Post
Your 13 ft Scamp, assuming the deluxe, per the Scamp website, is maybe 1600 lb total, allow maybe 400 lb for water and all your stuff, it might weigh 2000 lb. Of that 200 lb is on the hitch; which leaves 1800 lb on the axle, or 900 lb per tire. Per the L/I chart, you need 25 psi in the tires.
Bottom line .... Ya Gotta WEIGH the trailer to know for sure.
Simple rule of thumb is to note if there is any sidewall bulge at the bottom of the tire. No bulge? too much air! Big bulge? Not enough air!
Does the Scamp bounce a lot on the bumps in the road? Too much Air!
Wayne,

I load mine to 50 PSI for four reasons. First, I have a Scamp 16 which weighs more than a Scamp 13. Second the side to side weight rarely is split equally between the tires on a flat surface. Third, As you drive along the weight on a tire can vary depending on road surface. Fourth as the trailer attempts to move from side to side, stiffer side walls are helpful.

To me it's about margin.

By the way when we towed with our CRV we also increased the rear tire pressure on the CRV to 39 PSI from a normal 26 psi for stiffer sidewalls.

Maybe there's some weak reasoning but it's what we do.
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Old 11-18-2015, 03:30 PM   #19
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There seems to be some desire to avoid the simple answer of using maximum stated sidewall tire pressure. Can someone provide a valid reason for not doing so?


Norm, many others and I all set our tires at maximum cold pressure without problems.
Sure leaves out a lot of questions.



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Old 11-18-2015, 03:57 PM   #20
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There seems to be some desire to avoid the simple answer of using maximum stated sidewall tire pressure. Can someone provide a valid reason for not doing so?


Norm, many others and I all set our tires at maximum cold pressure without problems.
Sure leaves out a lot of questions.
In previous discussions on this subject there seems to be a misguided thought that you reduce the harness of the ride on the trailer by lowering the tire pressure. That's kind of the deal with Ford Explorer and Firestone a few years ago.

I have always and will continue to set the cold tire pressure to max side wall marked pressure.
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