TrailManor Owner's Forum - View Single Post

**rickst29** · 03-19-2007, 01:03 AM

slightly OT: Dixilou and Joe, our TM manual is wrong about tire pressure. (Maybe they didn't want to go into a full page of details about tire loading, so they just tossed out the biggest number the tire can handle.)

First, if you want to drive FAST on freeways/Interstates, Goodyear now approves running the Marathons at up to 10 PSI *above* the marked sidewall pressure. Read:
http://www.goodyear.com/rv/pdf/marat...nfo_032806.pdf

But, perhaps even more important, Goodyear recommends that you tune your tire pressure downwards when the tires are NOT fully loaded. Full document here:
http://www.goodyear.com/rv/pdf/rv_inflation.pdf

Following is copy/paste from an earlier Thread here:
http://www.trailmanorowners.com/foru...ead.php?t=3357
- - - - -
Here are the recommendations for the Marathon ST225/75R15, the tire I'm talking about:

max load <> recommended
per tire: <> cold PSI:

1060 lbs <> 15 PSI
1260 lbs <> 20 PSI
1430 lbs <> 25 PSI
1600 lbs <> 30 PSI
1760 lbs <> 35 PSI
1880 lbs <> 40 PSI
2020 lbs <> 45 PSI
2150 lbs <> 50 PSI (max load and PSI for class 'C' load rated)
2270 lbs <> 55 PSI
2380 lbs <> 60 PSI
2540 lbs <> 65 PSI (max load and PSI for class 'D' load rated)

Now... let's think about a 3023 (the most extreme example, because it has the biggest payload). Dry weight is only 2915 lbs, and TM indicates a load capacity of 1928 lbs. Under maximum load, the axle is at about 4770 lbs (with 13% of the payload moved forwards onto the tongue, but 4% of that pushed back to the TM axle by a WDH). That's less than 2400 lbs per tire, even when the TM is loaded to the absolute max.

Unloaded (less loaded to begin with, or coming back home, water tank empty, gray and black water dumped, etc.) you're probably around 3600-3800 lbs on the axle (remember, TM doesn't count options such as AC in dry weight, that's still there). Maybe 1900 lbs per tire, maybe even less. For CORRECT tire performance (the right amount of "give" is needed to make the contact patch large enough) Goodyear recommends only 40 PSI. Run it at 65 lbs, and your trailer is getting shaken to death on the rock-hard tires, and your "contact patch" with the roadway is way too small.

I don't run MY tires all the way down at 40 PSI, even though Goodyear says that I should. But 65 is way to much for a road surface which has typical bumps, it's definitely WRONG to run that way except on the very smoothest of roads.
- - - - -

BTW, my own 15" Marathons and "bigger" axle are supporting a nonstandard 2619: I have even MORE load capacity than the standard 3023 (if I center it), and even LESS weight when empty-- on the same axle, wheels, and tires. Except when adding 10 PSI to cruise FAST on freeways, 65 PSI would be completely nuts for me. I'm usually running 45-55 PSI, depending on the load and road conditions, and even this is 5-10 PSI more than Goodyear recommends for operating at normal speeds on "normal" roads.

03-19-2007, 01:03 AM	#8
rickst29 yes, they hunt lions. Join Date: Aug 2005 Location: Reno, NV Posts: 1,318	Our 15" Marathon tires SHOULD NOT always be run at 65 pounds! slightly OT: Dixilou and Joe, our TM manual is wrong about tire pressure. (Maybe they didn't want to go into a full page of details about tire loading, so they just tossed out the biggest number the tire can handle.) First, if you want to drive FAST on freeways/Interstates, Goodyear now approves running the Marathons at up to 10 PSI above the marked sidewall pressure. Read: http://www.goodyear.com/rv/pdf/marat...nfo_032806.pdf But, perhaps even more important, Goodyear recommends that you tune your tire pressure downwards when the tires are NOT fully loaded. Full document here: http://www.goodyear.com/rv/pdf/rv_inflation.pdf Following is copy/paste from an earlier Thread here: http://www.trailmanorowners.com/foru...ead.php?t=3357 - - - - - Here are the recommendations for the Marathon ST225/75R15, the tire I'm talking about: max load <> recommended per tire: <> cold PSI: 1060 lbs <> 15 PSI 1260 lbs <> 20 PSI 1430 lbs <> 25 PSI 1600 lbs <> 30 PSI 1760 lbs <> 35 PSI 1880 lbs <> 40 PSI 2020 lbs <> 45 PSI 2150 lbs <> 50 PSI (max load and PSI for class 'C' load rated) 2270 lbs <> 55 PSI 2380 lbs <> 60 PSI 2540 lbs <> 65 PSI (max load and PSI for class 'D' load rated) Now... let's think about a 3023 (the most extreme example, because it has the biggest payload). Dry weight is only 2915 lbs, and TM indicates a load capacity of 1928 lbs. Under maximum load, the axle is at about 4770 lbs (with 13% of the payload moved forwards onto the tongue, but 4% of that pushed back to the TM axle by a WDH). That's less than 2400 lbs per tire, even when the TM is loaded to the absolute max. Unloaded (less loaded to begin with, or coming back home, water tank empty, gray and black water dumped, etc.) you're probably around 3600-3800 lbs on the axle (remember, TM doesn't count options such as AC in dry weight, that's still there). Maybe 1900 lbs per tire, maybe even less. For CORRECT tire performance (the right amount of "give" is needed to make the contact patch large enough) Goodyear recommends only 40 PSI. Run it at 65 lbs, and your trailer is getting shaken to death on the rock-hard tires, and your "contact patch" with the roadway is way too small. I don't run MY tires all the way down at 40 PSI, even though Goodyear says that I should. But 65 is way to much for a road surface which has typical bumps, it's definitely WRONG to run that way except on the very smoothest of roads. - - - - - BTW, my own 15" Marathons and "bigger" axle are supporting a nonstandard 2619: I have even MORE load capacity than the standard 3023 (if I center it), and even LESS weight when empty-- on the same axle, wheels, and tires. Except when adding 10 PSI to cruise FAST on freeways, 65 PSI would be completely nuts for me. I'm usually running 45-55 PSI, depending on the load and road conditions, and even this is 5-10 PSI more than Goodyear recommends for operating at normal speeds on "normal" roads. __________________ TM='06 2619 w/5K axle, 15" Maxxis "E" tires. Plumbing protector. 630 watts solar. 450AH LiFePO4 batteries, 3500 watt inverter. CR-1110 E-F/S fridge (compressor). TV = 2007 4runner sport, with a 36 volt "power boost".