Proper tire pressure for the weight?

Our TM is a 2431KS. Bought it new in 2005, and of course it came with Goodyear Marathons. We had our first blowout when the original tires were 2 years old with about 18,000 miles on them, and we replaced all of them with new Marathons. Over Memorial day weekend this year we had another blowout, but again these “new” tires are 3 years old with at least 15,000 miles on them.

We’ve read the posts, stories and comments from others with tire issues, and understand that one school of thought is that under inflation is the principle cause of sudden failures. By the way, I’ve always made sure the pressure was 65 PSI cold before the start of every trip, and we never pull our TM with fresh, grey, or black water in any of the tanks. The “stuff” we carry in it doesn’t seem all that heavy, but until yesterday I had never weighed the loaded trailer.

Thinking that the load range D Marathons just weren’t man enough for the job, I bought and installed 3 brand new Maxxis M8008 ST load range E tires hoping that a heavier-duty tire might not blow out so readily. The Maxxis chart shows that these tires can handle almost 300 pounds of weight more than the Marathons, but at 80 PSI rather than 65. Fine, except that even at 65 PSI with the Marathons we noticed a lot of items inside the trailer were bounced and vibrated loose during each towing. Surely at 80 PSI the pounding would be even harder as the suspension on these TMs is not exactly cushy.

But then I weighed my fully-loaded TM.

Total axle weight the way we tow it: 3680 pounds
Driver’s side wheel: 1900 pounds
Passenger (door) side wheel: 1780 pounds

Why the difference? Well, we have removed the awning and its housing from the door side (it’s heavier than you think!), and the fridge (packed with food) and water heater (which we don’t usually drain) are on the opposite side.

But wait a minute – those Marathons were supposed to handle 2540 pounds at 65 PSI! We weren’t even close! Was I OVER-inflating them??

The Maxxis chart for the tires we NOW have shows that for 1900 pounds (the heavier side) we only need 40-45 PSI of pressure. That seems awfully low to me. Sure, the trailer won’t be as “bouncy” (my wife says “destructive”) at lower pressures, but I’d like some reassurance that 45 PSI is enough, and I’m not missing something obvious.

The recommended pressure for the 15" Load range E, Maxxis 8008's I just installed is 65psi. Must be different sizes?

[rickst29]

Search is often your friend. You might have found this post from long ago, in which I quoted some figures from Goodyear's recommended "match your Marathon tire PSI to your tire loading in pounds" table: http://www.trailmanorowners.com/foru...09&postcount=5

Per my discussion there (and confirmed by your experience of trailer contents getting rattled and shaken hard), the tires at 65 PSI were running too "stiff", with no ability to "flex" in response to the road surface and loading changes during turns. When the tires don't flex properly, a number of bad things can happen. The TM manuals from my year, and at least a fews years afterwards, always said to use the maximum PSI on the sidewall. For many of us, that's utterly terrible advice!
Here's the Trailmanor-Relevant lines of data in that post:

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)

As you see, you should have been reducing the pressure to about the same figures as Maxxis recommends for your M8008 tires: 45 lbs, maybe even a bit less. Goodyear, however, adds another "rule" in the other direction: If you're going to drive down the freeway at speeds in excess of 65 MPH, you should add 10 PSI to reduce excess tire squirm at the front of the tire, where the rubber hits the road.

I have a 2619 with the class 'D' 15" Marathons. Although it's the smallest TM model, and we don't have an awning, we always load it up really heavy -- about 1900lbs per tire, after the WDH has loaded some of the weight "back to the TM". Goodyear's tuning table thus recommends pressure of about 42 PSI, but I like to have a bit of a safety factor on the high side: I use 48 PSI, and that works really well. 65 PSI is WAY out of spec. (And probably dangerous, as I described in one of my previous posts. If you're curious, follow the links form the one above.)

REDHAWK and RICKST29:

Thanks for the super-fast response. Here’s a link I should have included for the Maxxis M8008 ST tires. The size I bought is ST225/75R15:

http://www.maxxis.com/Repository/Files/m8008load.pdf

So yes, I’m familiar with the tire loading and pressure relationship, and with the reasons for adding a bit of pressure if speeds may exceed 65 mph.

My errors seem to have been these:

1. Yes, I believed the TM recommendation to keep the Marathons at 65 PSI cold and I did so religiously regardless of actual weight.
2. Given number 1 above, I never really cared about the actual towing weight of the trailer because I knew we never “loaded it up” and we never pull it on the highway with anything in the liquid tanks.
3. I DID search through just enough posts about tires to see repeatedly that UNDER inflation was the ultimate sin, but not enough posts to find yours RickST29, or others concerning OVER inflation.
4. Then, when I had my blowouts, I figured I needed “better” tires, since many posts complained about the Marathons.

Okay. That said, it seems the BEST thing I did was to load up my TM to the way we actually tow it (almost always the same, give or take maybe 100 pounds) and take it to a local public scale. Check your local phone book or Google to find one. I used the one at our local Allied Van Lines location. For a total cost of a whopping SIX DOLLARS(!!), no tax, I was able to weigh just the TM axle but still attached to my tow vehicle and WDH, and then each wheel of the TM axle independently! What a bargain. If I’d only known, I’d have done it years ago.

So, thanks to your help and better information I will pressure up my new Maxxis M8008s to 50 PSI cold (safety factor included), avoid some of the wear and tear on stuff we carry in the TM due to less shaking and pounding, and hopefully enjoy decent tire life for a while.

(I hope so, ‘cause those 3 Maxxis tires set me back a total of $485!)

I’m also covering up those pricey tires with RV wheel covers when not in use, and using my stabilizers to take much of the weight off the tires while it’s parked in my driveway (other recommendations I garnered from this TM Owners Forum site).

We’ll see. But thanks again for the help.

John

Thanks John, now you got me wondering! I inflated my Maxxis 225/75/15 to 60 psi for their first trip. I had no problems, but now wonder if I should let them down a bit?
What are others inflating the Maxxis tires too?

Redhawk,

Given that the "chart" Rickst29 included in his reply (above) is identical to the pressures for wheel-weights in my Maxxis chart (linked to above), those ratios probably apply to other tires as well. Our expensive Maxxis tires are likely little different from Marathons or other ST tires in terms of strength or durability.

I too will be interested in hearing if others have actually weighed their TMs (or accurately estimated their weights) and adjusted tire pressures down to as low as the charts seem to call for.

John
TM: 2005 2431KS
Tow Veh.: Ford Expedition
WDH

Wow! I've been blindly following the TM owner's manual specification of 65 p.s.i. for the 15" Marathons on my 3023. Fortunately, I have not had a blow-out, but now I am seriously considering reducing p.s.i. following the guidelines in this thread. I too will be very interested in knowing what other folks are doing...

I didn't see a reference to excess pressure causing blowouts - only causing things to ride rougher. Did I miss something?

Grakin (Joel),

I didn't mean to imply that my blowouts were caused by over inflation. Nor do I think I've ever read that. I think over inflation just caused excessive bounce and pounding and rattle, etc., to the stuff inside the trailer. Probably contributes to shaking things loose and increases the "wear and tear" on the TM. Not a good thing, but not a cause of blowouts.

If you've missed something, I've missed it as well!

John