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Old 09-22-2007, 12:45 AM   #15
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Quote:
Originally Posted by tetstream
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 !!
I can't answer for Goodyear but I can direct you to a Goodyear memorandum regarding inflation pressures for Marathon tires when speeds exceed 65 MPH. Goodyear advises owners to inflate pressures to 75 psi, i.e. 10 psi above the rating on the sidewall. It may be that Airstream wants the higher inflation pressure to reduce tire sidewall and tread flexing to minimize heat buildup. Airstream's recommendation for our 2005 25' CCD is 65 psi.

You can get to the memorandum from the Goodyear webpage on the Marathon ST tires and selecting the link at the bottom:

"Helpful Information On Tires Used In Towing Applications" or click the link I have attached below:

http://www.goodyear.com/rv/pdf/marat...nfo_032806.pdf
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Old 09-22-2007, 06:51 AM   #16
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I'm going to run mine at 75 psi based on the memo. They look a little squishy at 65 psi.
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Old 09-22-2007, 10:42 AM   #17
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Quote:
Originally Posted by ptrvr
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.
Most of what I know on the topic has already been mentioned by 2airishuman. That said...

1. Keep in mind that inflation/load/speed ratings for ST tires are different than for 'normal' tires and that for ST tires any recommended inflation is for a load at speed.

2. In hot weather test your tires. Infrared thermometers are fairly cheap and easy to use. Others swear by the 'hand test' - if your tire is too hot to keep your hand on it without hurting then you are going too fast for the temperature outside.
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Old 09-22-2007, 10:53 AM   #18
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oh..and I forgot...

The full goodyear brochure is here

http://www.goodyear.com/rv/pdf/rvbrochure.pdf
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Old 11-03-2007, 10:52 PM   #19
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Cold and colder tire pressure

Yesterday I was checking the tire pressure on the Marathons to see if the dealer put 65 lbs. in them. On the east side, 64 or 65 lbs., on the west, 61. It was late morning and the Colorado sun had been heating up the tires on the east for hours. This morning, just before the sun came up (temp. about 35?) 3 tires were 61, and one on the east side, 62. I made them all 65 lbs. so they would be the same at least.

With temps changing 20 to 30 degrees in a matter of hours here, you can have 65 lbs. at dawn and 70 by mid afternoon. I'm not sure this proves anything except that pressure is approximate, especially depending on the quality of the pressure gauge.

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Old 11-03-2007, 11:27 PM   #20
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hi gene

no question that an accurate gauge is important.

it's not easy to find a gauge better than 1-2% +/-...

my understanding is that for every 10 degrees in ambient temp change tire pressures move 1 psi.

air temp is the usual marker, but road surface temp is important too...

and road surface temps may moderate the wider daytime variations in air temp in spring/fall...

while adding directly to the issue in summer and winter.

max sidewall psi is about 1/2 to 5/8s of the rim blow off pressures...

and running 'slightly high' is much safer than 'slightly low' for most tires.

my understanding is that more tire side wall heat is generated by underinflation.

you have another potential issue as well, pressure changes with altitude....

IF my trailer sat idle in the rockies for months, i'd inflate to 70 psi to offset the gradual decline and cold weather...

then drop them to 65 for travel.

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Old 11-04-2007, 06:42 AM   #21
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Originally Posted by CrawfordGene
. I'm not sure this proves anything except that pressure is approximate, especially depending on the quality of the pressure gauge.

Gene
Gene,

It proves that your pressure gauge has excellent precision. Tire pressure should change 4 psi for a 27 deg. F change in temperature.
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Old 11-04-2007, 09:06 AM   #22
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Quote:
Originally Posted by markdoane
Gene,

It proves that your pressure gauge has excellent precision. Tire pressure should change 4 psi for a 27 deg. F change in temperature.
A very interesting point.

No manufacture includes a temperature/pressure compinsation factor on their inflation chart but rather just suggests inflating tires while at abient. That's OK when I leave NJ in January headed for Fl. but not so good for anyone leaving Fl in Jan. and headed for NJ, not that anyone would do that.

I have long and often suggested to that individuals refer to the manufactures inflation charts when selecting a tire pressure. Now it is becoming increasingly clear that that advice may not be an all inclusive idea.

For the last 100,000 miles plus I have always set my tire pressure by an inflation chart. When I bought my 2004 Ford Excursion, a month ago, I used the same logic. This appears to have been a major mistake. Having almost lost the trailer due to sway I posted on this and other Ford Excursion sites for comments. 85% came back that one should not use the charts when dealing with an Excursion but rather increase the pressure almost to max.

If this is in fact the case with the Excursion how many other situations fall outside the manufactures recomendations? To date I have not tested this idea of higher inflation pressure because it flys in the face of all historical engineering recomendations. But it is causing me to think and look for strong supporting evidence on either side.
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Old 11-04-2007, 09:53 AM   #23
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Simple rule of thumb - Weigh and Inflate according to the tire manufacturer's recommendation. That said, the Airstream recommended 50 psi for my trailer's original "C" rated Marathons appears to provide more than a reasonable safety margin for potential asymetrical loading. My scale weight is 7,400 lbs, loaded, with 990 lbs on the ball. At 50 psi the tires have a capacity of 2,150 lbs/each, or a net of 8,600 lbs. I have since gone to "D" rated Marathons (---replaced due to age) - but I still use 50 psi. I tossed around the idea of increasing the pressure to the max rating for the "D" tires (65 psi) but decided that I didn't have any reason to do so. I tow at no greater than 65 mph, but usually around 60 mph on the highway and much less on my preferred secondary roads. I've never had a tire failure in over 40 years of trailering all sorts of rigs - including one 12,000 mile summer trip with the notorious small tires on a heavily-loaded large pop-up camper. I also run my dually rear tires at a much lower pressure than that shown on the door sticker - but that discussion has already been well hashed over.
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Old 11-04-2007, 10:41 PM   #24
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Toyota has no information in the driver's manual or on the tire pressure sticker on the truck about tire pressure while towing or if loading the bed with a half ton of cinderblocks. Maybe other manufacturers do include that information. Toyota's only recommendations are for 30 front, 33 rear. I assume the higher pressure for the rear is to account for the load in the bed that probably isn't there on most pickups since they are used as a family car. It surely doesn't account for the weight of all the options on the truck—about 500 lbs.—or what part of the truck they are on.

If I hadn't been reading these threads, I wouldn't have even thought of increasing the pressure as obvious as it is once I read about it. Thinking about all the many variables—temperature, sun, altitude, weight, accuracy of gauge, leaks, type of tire—makes me crazy. I decided to try the max on the truck tires and see how it worked. It did. The truck front end came down and it stopped feeling too light. We could easily see the hood was lower. I have no idea why more air pressure in all the tires should do that, but why argue? I thought of maintaining Toyota's 3 lb. difference between front and rear, and maybe I'll try that later.

By the way when I check tire pressure in the spring (in Colorado, about 5 minutes long) when temps one day may be 80˚ and the next 20˚, I learned long ago there's a variation of 2 or 3 lbs.—less than Mark suggests, but I wasn't being scientific. I just set them according to temp averages for the season.

My gauge appreciates Mark's compliment. It's a cheap, old round guy that reads up to 60 lbs—I have to guess above that. It gets dropped a lot too. One step above the kind with the stick that pops out. I was thinking of getting a new fancy one, but I will now trust it.

Gene
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