Tire pressure

[Roadtoaster]

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?

Thanks

Greg

[richinny]

i'm confused. pressures for the truck are different than the trailer. each is filled to handle the load and rating of the tire.

[Beginner]

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).
Beginner

[overlander63]

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.

[68 Overlander]

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.

[Roadtoaster]

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

[68 Overlander]

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.

[Airperson]

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?

[thecatsandi]

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.

[Roadtoaster]

Thanks Michelle.

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

Greg

[thecatsandi]

see the thread on valve stems.... Metal.

[overlander63]

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.

[2airishuman]

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?

cheers
2air'

[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 !!

[Motoman]

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

[markdoane]

I'm going to run mine at 75 psi based on the memo. They look a little squishy at 65 psi.

[ljmiii]

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.

[ljmiii]

oh..and I forgot...

The full goodyear brochure is here

http://www.goodyear.com/rv/pdf/rvbrochure.pdf

[Gene]

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.

Gene

[2airishuman]

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.

cheers
2air'


so many issues, so little time!