ST225/75R15: Load range "D" or "E" ?

[rickst29]

I'm ready for new tires - because of their age (not due to blowout or visible "issues"). I have TPMS, and pay attention to it's readings.

My 2619 (2006) was built like a 3023 with not just the 15" wheels, but with the bigger 5000 lb Torflex axle too. My old Marathons did a goo job,

Buy ST225/75R15-D tires like my current set; and "tune down" the inflation from the 65 PSI sidewall max, in "moderate" amounts.

For load values less than 2600 lbs/wheel, Goodyear provides a table which recomends reduced PSI; Then, after applying that factor, they also suggest adding 10 PSI if you're going to drive on highways at high speed. For my TM, the "lighter load" reduction factor takes me about 45 PSI, but I don't go below 50. And then, in non-California States with fast highways, the second rule puts me back up to 60 PSI (cold, of course).

Or;
Buy ST225/75R15-E tires, at slightly higher cost, and "tune down" the inflation MUCH FURTHER from the 80 PSI sidewall label values which most of them specify.

Would it actually be harmful to the tires to buy "stronger" sidewalls, but then take out a much higher proportion of the max inflation value to create a softer ride for the sake of TM contents, frame, and Torflex? Or is "stronger" pretty much always "better", even if you soften them up A LOT, with inflation pressures as low as 50 PSI?.

This is a controversial topic.

You go onto RV.net, and you'll be told to ignore the inflation table and use the max psi of the tire. The argument is that ST tires are nowhere near as good as LT rated truck tires (where the inflation table should be used) and and trailer tires take more abuse; especially double axle tires which need a stiff sidewall to counteract scrubbing around corners.

Carlisle says to use the max pressure on its ST tires.

Goodyear says to use the max pressure on its ST tires unless that leads to other problems (like shaking your trailer to bits?).

Maxxis doesn't seem to care (maybe because their tires are of such high quality that they can be run at lower pressures without problems?).

I've also seen a recommendation to use the inflation table but add 10psi for a double axle trailer. And then there's Goodyear's recommendation to add 10psi if you intend to travel at 65-75 mph.

Personally, I wouldn't go below the 65psi in a D rated Maxxis. But would also use 65psi for an E rated Maxxis. And I would prefer the E rated tire, 'cause I figure the 2 xtra (rated) plies would help with potholes and road debris.

[rickst29]

The table which they provide (for reducing pressure) is linked from the Marathon page in the "tire selector", and it provides separate tables for "ST" and "LT" tires.

The "LT" tire Table doesn't list anything below 35 PSI, and the max load for a load range "D" in a "single" configuration is only 2095 lbs. The "ST" table, in contrast, starts at only 15 PSI (with max load of 1060 lbs) and goes up to 2540 (at 65 PSI).

I'm in agreement with you about being "not too aggressive with Tire Pressure reductions" - Owners of TM's equipped with 14" Marathon tires have a long, sad record of tire trouble when running at or near the ultimate limits of those tires. Goodyear's "ST" table values for size P225/75R15-D are:

Inflation     Tire Load
Pressure      Rating
(PSI)         (Lbs)

 15           1060          
 20           1260
 25           1430
 30           1600
 35           1760
 40           1880
 45           2020
 50           2150  (@ 50 nominal PSI limit for Class "C")
 55           2270
 60           2380
 65           2540  (@ 65 nominal PSI limit for Class "D")

If I wanted the maximum amount of sidewall tire "flex", then I would run close to the limit - and on return trips (with fresh water empty, and with black/gray tanks dumped at an RV park, and no wine bottles left) this would be accomplished by using only 35-40 PSI. Instead, I run at 50 PSI ("max load" 2150 lbs), and add on my 10 PSI for high-speed Interstates above that (60 PSI, 2380 "max load").lbs lbs. My actual loading is 70-80% of the Max value.

Your suggestion, use roughly the same PSI on the "E" rated as the "D" rated, makes really good sense - I'll start at 65 PSI on pair of "80 PSI" E-rated tires, and if damping isn't enough, take out another 5 lbs or so. Probably red-line at 55 PSI minimum.

[Harry Womack]

I run the load range E at 70 PSI on my Maxxis tires.

[ShrimpBurrito]

I run load range E tires at 65 psi (and they are rated for 80 psi, which was a disaster). I'd rather have more plies at a lower air pressure than less plies at a higher pressure.

Dave

[wbmiller3]

I have Goodyear Marathon ST225/75R15 Load Range D and run them at 65 psi. So far so good. They are rated for 2540 lb each (5080 total) which should be a good amount of margin for our lightly loaded 2619.

[davlin]

Originally Posted by ShrimpBurrito

I run load range E tires at 65 psi (and they are rated for 80 psi, which was a disaster). I'd rather have more plies at a lower air pressure than less plies at a higher pressure.

Dave

Dave, what sort of "disaster" occurred when towing with 80 psi? I have E load range tires on my 3124, and I run them at 80 psi with no issues. Loaded (we never tow with water on board) my TM is probably around 4250 lbs., or about 2125 per tire.

Dave

[ShrimpBurrito]

Originally Posted by davlin

Dave, what sort of "disaster" occurred when towing with 80 psi?

Loaded, our TM is about the same -- about 4,000 lbs. But when we towed with 80 psi, everything inside vibrated. The toilet hinges broke (just try to find replacements...), the water heater and oven started falling out after about 100 miles, stuff in your cabinets flew out. Our tires are balanced, too. Not sure what is different between your setup and mine.

Dave

[davlin]

Originally Posted by ShrimpBurrito

Loaded, our TM is about the same -- about 4,000 lbs. But when we towed with 80 psi, everything inside vibrated. The toilet hinges broke (just try to find replacements...), the water heater and oven started falling out after about 100 miles, stuff in your cabinets flew out. Our tires are balanced, too. Not sure what is different between your setup and mine.

Dave

Wow. We just took a 700 mile trip, and everything in the trailer stayed right where it was. It was all "hard surface" road, however. Did you have some rough roads for some of that 100 miles? And what have you lowered your tire pressure to now? Has that made a difference?

Dave

[ShrimpBurrito]

Our roads are far from perfect out here -- there are lots of bumps, dips, ridges, potholes, etc. even on the interstates. I lowered the pressure to 65 psi, and that resolved all the problems.

Dave