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Old 09-21-2007, 02:57 PM   #1
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Tire pressure

I posted this question in response to another post. Since it did not get a response and was somewhat off topic to the original post, thought I would give it another shot as I'm interested in knowing more.

AS says inflate the Marathons to their maximum 65 lbs. The tire placard on my truck says 53 lbs front and 80 lbs rear. When hitched, my rear weighs about the same as the front when not hitched, yet the fronts are ok at 53 lbs. The Goodyear site says inflate based on known loading.

So, why are we maxing out pressures? Is the truck/trailer more stable because the sidewall is stiffer; is there less heat produced for the same reason? One thing is for sure, more pressure means a much harsher ride in the truck and for the trailer. Anyone out there running pressures based on load? Are lower pressures more/less prone to failure?

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Old 09-21-2007, 03:07 PM   #2
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i'm confused. pressures for the truck are different than the trailer. each is filled to handle the load and rating of the tire.
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Old 09-21-2007, 03:12 PM   #3
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Tire pressure

ptrvr
It is my contension that the the ST tire is Speed Rated at 65MPH. This appears on Goodyears tire web site. The reason so many people have trouble with the ST tire is because of heat. The rubber compounds used cannot stand the heat of 65 MPH on a very hot summer day. This is evidenced by the number of people losing these tires, and the fact that appearntly Goodyear is having a "silent recall".
The majority run the Marathon at max pressure to minimise the flexing of the sidewalls and tread area (high pressure/hard tire) hense reducing the generation of heat which makes the tire fail.
All tires have a heat limit above which the rubber compounds loose their integrity and the tire comes apart. The general indicator of the compounds that will stand the higher heat is the Speed Rating (according to the NHTSA engineer I talked to on the phone).
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Old 09-21-2007, 03:39 PM   #4
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Tire pressure recommendations are based on many factors. Vehicle manufacturers will specify a lower pressure than max for the tire to compensate for defficiencies in the vehicle's suspension, or unusual wear patterns that would result from using maximum inflation.
The higher the tire pressure, the less rolling resistance, the less sway from sidewall flexing, and less heat generated from sidewall flexing. On the other side of the coin, running maximum pressure on a lightly loaded vehicle can cause a harsh ride, bruising of the center of the tread, and excessive wear in the center of the tread.
Underinflation can cause excessive sway, excessive heat generation due to sidewall flexing, sidewall failure due to the heat, and tire wear on the outer edges of the tread and, in extreme underinflation cases, wear on the sidewall.
My truck manufacturer recommends 51psi front, 80 rear, I run 55 all the way around based on loading. My trailer is supposed to have 50 psi with load range C tires, I have D rated tires, and run 60, instead of the maximum of 65.
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Old 09-21-2007, 04:06 PM   #5
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I agree with Beginner and Terry because heat is the cause of more than 85% of all tire failures whether a leak started it or not. I had total tread separation on an ST tire two weeks ago and I was traveling 70 on a 100 degree day. Wasn't pretty but luckily I only ruined about four feet of the under belly wrap behind the tire. Now rethinking tires AGAIN.
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Old 09-21-2007, 04:23 PM   #6
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And I agree also. But when GY publishes a load/inflation chart for ST tires, it cetainly makes one believe they are comfortable with something other than max pressure, assuming less than max load.

I used the truck analogy because of the similarities in load between front and rear when hitched/not hitched. I probably made the analysis about as clear as mud.

I run the AS tires at max pressure just as I do the rear truck tires. But surely GY knows the stresses an ST tire is subject to and published the chart after analysis. I'm not willing to chance it but was curious to see if others might have.

Greg
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Old 09-21-2007, 04:42 PM   #7
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Quote:
Originally Posted by ptrvr
And I agree also. But when GY publishes a load/inflation chart for ST tires, it cetainly makes one believe they are comfortable with something other than max pressure, assuming less than max load.

I used the truck analogy because of the similarities in load between front and rear when hitched/not hitched. I probably made the analysis about as clear as mud.

I run the AS tires at max pressure just as I do the rear truck tires. But surely GY knows the stresses an ST tire is subject to and published the chart after analysis. I'm not willing to chance it but was curious to see if others might have.

Greg
You are indeed correct. There are many references to inflating trailer tires to max PSI but that doesn't mean they are right.
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Old 09-21-2007, 06:22 PM   #8
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Am I a newbie or what? Could someone please explain to me the difference in a rubber valve stem and a metal one. Which one is best?
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Old 09-21-2007, 07:10 PM   #9
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I am runing pressures based on load. I am currently runing 60 PSI. Works well, tires are wearing evenily. My 06 truck is 75/75. My 04 truck was 55 front/70 rear.
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Old 09-21-2007, 08:07 PM   #10
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Thanks Michelle.

Well, that is one person inflating based on load and I know she travels a lot.

Greg
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Old 09-21-2007, 08:35 PM   #11
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see the thread on valve stems.... Metal.
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Old 09-21-2007, 09:01 PM   #12
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Quote:
Originally Posted by ptrvr
Thanks Michelle.

Well, that is one person inflating based on load and I know she travels a lot.

Greg
That is two. See my post above.
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Old 09-21-2007, 10:42 PM   #13
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hi ptrvr...

g/y marathons are NOT speed rated at 65 mph...

they are NOT speed rated at all.

the marathons (and ALL ST tires) are LOAD rated at 65 mph....

this is significantly different than passenger tire speed rating.

a/s recommend the max inflation 50 or 65 as a safety/liability issue.

running the marathons at max inflation generates the least heat and sidewall deflection.

so it's easy and safe for a/s to go with 'max cold inflation'....

besides they'd have to recommend endless combinations otherwise and weight every trailer axle....

which they don't do.

clearly the marathons can be used at pressures from 30 to 65 psi...

but how many folks actually weigh the trailer or each axle to determine an honest load/pressure level?

not many.

also IF you are planning to tow with the marathons at speeds OVER 65 mph...

one must add 10psi to that load/pressure calculation up to the max cold inflation pressure....

IF the marathons were speed rated to 65 mph (which they aren't) why would g/y include official company advice....

on what to do between 65-75 mph?

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Old 09-22-2007, 12:27 AM   #14
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The tire inflation sticker on the side of my new AS says that my pressure should be at 75 psi. My Bambi came with D rated GY Marathon 225/75 15's. The spec on the tire sidewall says max psi of 65.

What the heck ?? I've got them at 65 because that's straight off the tire.

Single axle trailer ...... I don't want any stinkin' blowouts !!
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