Tire Pressure Question - Page 2 - Fiberglass RV


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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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Old 11-18-2015, 05:00 PM   #21
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Originally Posted by Byron Kinnaman View Post
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.

It's your money, spent it as you wish. Just be aware too, that an overinflated tire is more susceptible to damage from rocks, sharp edges, curbs, etc. Hard impacts can break the ply cords inside, but that won't show up right away.
I will continue to opt for a softer ride. Yes, I do increase air in the rear tires in our TV to allow for the added load from the Scamp.
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Old 11-18-2015, 05:54 PM   #22
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Originally Posted by Wayne Collins View Post
It's your money, spent it as you wish. Just be aware too, that an overinflated tire is more susceptible to damage from rocks, sharp edges, curbs, etc. Hard impacts can break the ply cords inside, but that won't show up right away.
I will continue to opt for a softer ride. Yes, I do increase air in the rear tires in our TV to allow for the added load from the Scamp.
Wayne,

On our first long trip with a small trailer we drove across Labrador, a 1000 mile dirt road at the time. We did lower the tire pressure on that road. We drove much slower, actual average speed 25 mph.
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Old 11-19-2015, 05:34 AM   #23
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Originally Posted by honda03842 View Post
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.
There's a fifth reason that applies to some of us: If you have excess load capacity (>/= 10%) when inflated to 50 psi it provides a max speed cushion. At one time the Tire & Rim Assn. handbook allowed for an extra 10 mph on a non-speed rated tire. While few of us travel at that high of a speed, the cushion is welcome.

Referencing another post I would mention that lower pressure means a lower tire spring rate which is better for harshness. That being said and since few of us know exactly what our trailers weigh, running at the tire max pressure is directionally safer.
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Old 11-19-2015, 10:09 AM   #24
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There are many opinions on tire pressure , I will continue to follow the recomended pressure stated by the trailer manufacturer , and not fool around with tire size or type .
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Old 02-29-2016, 04:55 PM   #25
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Tire pressure and temperature | TireBuyer.com
I found the above article on ambient tempature and tire pressure informative. I am preparing for a trip to Texas which will expose my tires to a 50 degree swing in ambient tempature in about 19 hours. Just another thing to be aware of for us snowbirds.
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