Tire Air Pressure - Fiberglass RV


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Old 07-02-2016, 08:11 AM   #1
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Tire Air Pressure

Getting ready to take off for the Holiday. Doing a Boler walk around and checking tires. Brand new 185-80-13 tires. Tire shop aired them to 38 lbs. Tires say max 50 lbs cold.

Will be towing to campsite 45 miles from home in 85 degree weather.

What pressure should I air them up to.
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Old 07-02-2016, 09:21 AM   #2
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What type of tires are they? If they are STs(Special Trailer) you should inflate them to the maximum inflation listed on the sidewall.
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Old 07-02-2016, 09:42 AM   #3
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Yes ST Thank You
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Old 07-02-2016, 10:07 AM   #4
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I've always read to go with the max pressure listed on the sidewalls. That's where they carry their rated load. Running too low air pressure will cause the tires to over heat.

As an example, I remember having a flat in my Corolla. I was only about .1 mile to a turn off. I slowed down and limped to the turnoff. The tire had gotten SO hot in that short distance, I couldnt touch it! I know that's extreme, but it made a believer in me about running low tire pressures!
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Old 07-02-2016, 10:23 AM   #5
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Another vote for 50 PSI.
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Old 07-02-2016, 11:01 AM   #6
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Running tires at max pressure will make for a bumpier ride.
Get a Load/Inflation chart for the tires
Weigh the trailer to know the actual load on each tire.
Air up accordingly.
Rule of Thumb: If you don't see any bulge in the sidewalls; if the tread is not contacting full width on the ground, you have too much air in them.
After running 50 or so miles at highway speeds, feel the tires. If just warm, you're OK. If hot, add air.

For example: from the LOAD/INFLATION chart:

ST185/80R13 = Max speed rating is 65 mph.
At 50 psi is good for 1480 lb per tire or 2960 axle load
at 45 psi ... 1400 psi, or 2800 axle load
at 40 psi ... 1300 lb
at 35 psi .... 1200 lb.
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Old 07-02-2016, 11:28 AM   #7
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Wayne. I follow your general approach, but how did you locate the specific LOAD/INFLATION chart?
I've never understood the idea that ST tires should be inflated to the rating max regardless of the load, unlike other style tires.

Walt
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Old 07-02-2016, 11:39 AM   #8
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Don't forget... Marathons are restricted to 26.2 miles per trip, I read that somewhere on the internet!
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Old 07-02-2016, 11:43 AM   #9
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Here we go again.

Walt
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Old 07-02-2016, 12:01 PM   #10
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Quote:
Originally Posted by WaltP View Post
Wayne. I follow your general approach, but how did you locate the specific LOAD/INFLATION chart?
I've never understood the idea that ST tires should be inflated to the rating max regardless of the load, unlike other style tires.

Walt
Go to <tire and rim association load inflation table> or the manuafacturer's site

Load/Inflation charts apply to all makes and types of tires.
I got into it a lot with earthmover tires on scrapers, graders 4WD loaders, etc.
The MAX load at XX psi is used to select the minimum tire for the application. i.e. if you want to carry a heavier load in your trailer, you would have to get tires with a higher load capacity .... or take a chance on an early failure.
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Old 07-02-2016, 12:59 PM   #11
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I understand where you're coming from. The destructive effect of sidewall flexing is dependent upon how much the tire flexes. How much flex depends upon
(1) the load being carried by the tire and
(2) the tire's internal pressure, plus I suppose we must include
(3) the inherent stiffness of the sidewall design for a particular tire.
One can visualize a proportional relationship between PSI and load; as the load carried becomes lighter, the tire pressure needed to avoid damaging levels of flex also declines.

However, that third factor can be a bit of a bugaboo. Sidewall stiffness and susceptibility of a given tire brand/type design to sidewall flex can vary. I'm not sure that a 'one size fits all' chart like the one cited can account entirely for this variable.

On the other hand, maintaining max rated PSI cannot somehow cause excessive sidewall flex in a tire. Having more air than needed is greatly preferable to having less than needed, since the latter can have unpleasant and expensive effects. The downsides of having more inflation than necessary are a somewhat stiffer ride and a little faster wear on the center of the tread than on the outer parts. Considering that most trailer tires age out before they wear out, the uneven wear issue may not be worth worrying about. As for the stiffer ride, my personal preference is to accept this in lieu of potential blowouts from sidewall flex/degradation. But everyone must balance this equation for himself.
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Old 07-02-2016, 01:25 PM   #12
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Quote:
Originally Posted by Mike Magee View Post
I understand where you're coming from. The destructive effect of sidewall flexing is dependent upon how much the tire flexes. How much flex depends upon
(1) the load being carried by the tire and
(2) the tire's internal pressure, plus I suppose we must include
(3) the inherent stiffness of the sidewall design for a particular tire.
One can visualize a proportional relationship between PSI and load; as the load carried becomes lighter, the tire pressure needed to avoid damaging levels of flex also declines.

However, that third factor can be a bit of a bugaboo. Sidewall stiffness and susceptibility of a given tire brand/type design to sidewall flex can vary. I'm not sure that a 'one size fits all' chart like the one cited can account entirely for this variable.

On the other hand, maintaining max rated PSI cannot somehow cause excessive sidewall flex in a tire. Having more air than needed is greatly preferable to having less than needed, since the latter can have unpleasant and expensive effects. The downsides of having more inflation than necessary are a somewhat stiffer ride and a little faster wear on the center of the tread than on the outer parts. Considering that most trailer tires age out before they wear out, the uneven wear issue may not be worth worrying about. As for the stiffer ride, my personal preference is to accept this in lieu of potential blowouts from sidewall flex/degradation. But everyone must balance this equation for himself.
Another negative effect of overinflation is a greater chance of breaking the cords under the tread when you hit a sharp object like a rock or curb.
With the "correct" pressure the tire is able to flex and absorb the impact without damage.
You are right on about different sidewall constructions.
I want to get tires for our Prius which will give a softer and quieter ride.
the trade off is shorter tread wear.
DUROMETER is the hardness of the rubber. The best ice and snow tires have a low durometer and wider gaps between the lugs, but they wear out faster.
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Old 07-02-2016, 01:34 PM   #13
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Exclamation inflating the trailer tires

The post that suggests inflating the tire when it is hot is a recipe for disaster. The tire is hot because being under inflated at the start of the trip it flexed and heat build up was the result causing an increase in psi. If a hot tire has additional air added it will be over inflated when cold. Always correct your tire pressures when the tire is cold and never run on an under inflated tire.
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Old 07-02-2016, 01:58 PM   #14
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This issue is one that never gets resolved on these forums, but let me use my own personal example. My Bigfoot came with 13 inch tires max rated at 50psi. The manufacturer's sticker inside the closet says inflate to 32PSI. I have upgraded to 15" tires, max rated at 80PSI.
Sorry etrailer, but no way will I run them at 80PSI. As a compromise I choose to run them at 50PSI.
I would love to discuss this with a master tire designer, which etrailer clearly is not. But etrailer's story just does not make sense to me.

Walt
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