Double checking load table info

[DaveMc]

Hi all! Setting off on a three-week trip tomorrow and I'm checking tire pressures this morning on the TV and trailer. I've got a year-old set of Goodyear Marathons on our '05 Safari 30-ft bunkhouse. (Small aside is that these will be going on my cattle trailer later this summer, to be replaced by the new Endurance tire for the Airstream).

Total trailer weight is 5800 lbs with a pretty good "loaded for camping" scenario. GVWR is 8400 lbs. I'm adjusting my hitch to spread the load better between axles, but here's the info I have now:

Front left = 1775
Front right = 1850

Rear left = 1150
Rear right = 1025

Goal is planning for 1500-1600 lbs/tire. I believe these readings are (potentially) dangerously out of whack.

In looking at this load tables for Goodyear trailer tires: http://www.goodyearrvtires.com/pdfs/rv_inflation.pdf

If I'm reading this right (and there's no guarantee there!), I should be running between 30-40 psi per tire for loads between 1600-1880/tire.

Am I reading table correctly? Thanks in advance for any insight!

Dave

[Al and Missy]

I think what the table says is not that you should be running a particular pressure, but you should be running no less than the pressure listed. I would take the highest load of any tire after you get it all tuned up, add about 10% margin and then pick the pressure appropriate for that. What does the placard in your trailer say about pressure?

Al

[A W Warn]

I agree those numbers on the chart are minimum pressures for a specific load.

I normally air up to no less than 5 lb below the max so that when the tire us in use expansion does not create much higher than the max. The higher the pressure the less the sidewalls will flex (less heat created from friction). The higher the pressure the better heat will dissipate. The higher the pressure the more likely the tire will not fail when turning sharp (inter-ply shear). The higher the pressure the less likely the wheel will be damaged if there is an impact with road debris.

What is of more concern to me is the balance of your load. Those numbers you posted tell me that you have to much weight forward of the axels. Move load that is forward to somewhere between the axles.

Was your trailer tongue slightly low when you weighed? Low tongue adds weight to the front axle. High tongue adds weight to the rear axle.

[HowieE]

You did not list what tire you are using. If it is an LT 225 75 R16 than a 1600 pound load would inflate at 40 lbs.

While the chart is Goodyear the pressure hold for a given tire size no mater how made it. They are standardized by Federal Regs.

Post what tire you are using

[DaveMc]

Sorry I forgot tire size!!!
225/75/15.

AW-- agreed. I'm adjusting the receiver hitch offset correct that. Despite seeming level with the nose slightly down, clearly I erred in my estimation.

Thank you all! I was a little incredulous at those inflation pressures, but looking at them as minimums make sense.

[HowieE]

If the trailer is tongue low the front trailer tire will be carrying more load. Airstrean doesn't use self equalizing axles and thus the need to ride parallel to the ground.

If you do get the load equalized out the and the highest tire is under 1600lbs. I assume you are running ST 225 75 R15 I would inflate to 45 lbs.

Running a higher pressure will reduce the tire contact area and result in poor braking and center tire tread ware.

[Action]

Originally Posted by A W Warn

I normally air up to no less than 5 lb below the max so that when the tire us in use expansion does not create much higher than the max.

It is my understanding that the above is partially flawed.

All tires will experience a rise in temperature and pressure during use. If the tire was properly inflated prior to any use the rise in temperature and/or pressure is not relevant.

If the tire was inflated to max pressure listed on side wall AND it is not overloaded AND is not too old to need replacement, the pressure noted after use WILL be above max and that is acceptable. In addition it is not relevant to max cold fill pressure listed on the sidewall as the tire is not cold. The sidewall only lists max cold fill pressures. There are no pressures given for any tire that have been in use or that are hot. Checking tire pressure that has been in use will produce a reading. However that reading means nothing as there isn't a published spec for that type of reading.

>>>>>>>>>>>>Action

[HowieE]

Those that are using a TPS system have experienced the change in tire pressure and temperature during usage. In colder weather tires will commonly rise 5 degrees and several pounds while on the highway. Additionally they will rise an additional 5+ degrees if exposed to
the sunning side for a while.

[A W Warn]

Originally Posted by Action

It is my understanding that the above is partially flawed.

All tires will experience a rise in temperature and pressure during use. If the tire was properly inflated prior to any use the rise in temperature and/or pressure is not relevant.

If the tire was inflated to max pressure listed on side wall AND it is not overloaded AND is not too old to need replacement, the pressure noted after use WILL be above max and that is acceptable. In addition it is not relevant to max cold fill pressure listed on the sidewall as the tire is not cold. The sidewall only lists max cold fill pressures. There are no pressures given for any tire that have been in use or that are hot. Checking tire pressure that has been in use will produce a reading. However that reading means nothing as there isn't a published spec for that type of reading.

>>>>>>>>>>>>Action

You are correct and I agree that checking tire pressure when tires are hot is not the thing to do. I'm not saying you are wrong at all, but maybe we have different opinions. I think that may be because we have different goals. My goal is to have high pressure as possible for obvious/multiple reason, but also soft as possible too to keep from beating the trailer too badly.
My opinion - the lower pressure I stated is best for my trailer, but not necessarily the best for my tires. It's worked for me, not one tire failure, even towing on those GYM's for more than 25 years (yes, I've been lucky).

[HowieE]

Originally Posted by Action

I
Checking tire pressure that has been in use will produce a reading. However that reading means nothing as there isn't a published spec for that type of reading.
>>>>>>>>>>>>Action

Not sure what you point is about checking tire pressure while hot. The second rule of tire management is never check the pressure at other than ambient temperature. The first rule is inflate the tire to the proper pressure and that pressure is not what is on the sidewall. What is printed on the sidewall is the MAX Cold Pressure.

[GM Airstream]

We did a little engineering check... seeings how we had the accel-romters... we put it in the AS 31... in the center of the isle.. in front of the kitchen sink and refrig on the other side...

we started out with the TOW -Max... blow'n up to max pressure... wow... you don't need the instruments to tell you that the trailer really takes a hit inside as the thing bounces down the road... (Andy... once said that AS's need to ride soft)... and I believe it... letting the air out of the tires... seemed to only add heat to the sidewalls... yet the load was not their... (we changed to alu rims to help get rid of the heat in the tire as well as the brakes when on.. and found that it really does make a difference between steel and alu rims... about 5 deg cooler running)

We changed to alu rims and 225-75-16 10 plu LTX tires... (from 1700 to 3040 lbs rated EACH tires) and did the same run and experiment with max air pressure... not much difference... but due to the loading of the new tire... we then decreased the air pressure... and the results were spectacular

When we ran 80 lbs of air... things moved in the inside of the trailer... (showed a 2.- G loading on some bumps...we then lowered the pressure the next day to 60... about a 15% reduction in acceleration (vibration inside the trailer according to the instrumentation was less than 1.7..)... the following day we dropped 'em to the suggested pressure for the weight (THAT IS THE IMPORTANT WORD HERE... PRESSURE FOR THE WEIGHT)...

Changing the air pressure around we found that for OUR trailer the ideal pressure for the weight is about 48 lbs... in each tire... and the reduction in vibration/acceleration inside the trailer at the floor was a whopping 56% LESS...than what we started with... (not one spike over 1.2- g loading)

indeed a glass of water on the counter would have stay'ed their while you were going down a bumpy road... the axles were working along with the tires...

As to side load and squirming... didn't see much of that with 8/10 ply tires... and so turning allowed some give in the tread... yes your going to wear it off if you do less than 18 ft radi circles... but then again... something has to give...and you give a little rubber... then again.. I have yet to see someone WEAR OUT A SET OF Mitch LT 16 in tires on theirs'... most of the time... age takes its toll before wear sets in...

So what is a good indicator of tire loading.... well drive down the road for about 15 min... stop and go back and shoot the tire wall with a IR temp gun...(harbor freight special or better)... you ARE going to get a rise in temp over the air... but it should be stick around 5-15 degrees over the current air temp... ( we have been told that the tire should get up to about 100-125 deg F. on the sidewall...(70 deg day) ... which allows the oil in the rubber to do its thing also...

Yellow line... critical run'n temps should not be above 130... (again with the air temp being standard)

If your above that temp... 140- up... your in the red line area... and need to re-evaluate your LOAD , SPEED... or check for something wrong...

All of this is what we found about tires and AS... but one thing for sure... Andy was right... AS do want to ride soft... or you will start seeing strange thing happening... such as cracks in the interior.. and lots of things swap'n places at the end of the trail...

I was surprised to find that most owners never have their trailer weigh'ed wheel to wheel... and only go with someone elses general rule of thumb... (put max air pressure in and go with the flow... all while bending the heck out of the trailer and working over the axle rubbers...

The best advise I can give is to load the trailer.. weigh the trailer...make sure its level towing... and then work on achieving the ideal air pressure for the load and conditions your towing through... don't go with the flow about max pressure... The charts help but... you need to know your WEIGHT PER WHEEL... to set the air pressure correctly... (also speed of towing figures in too)

Good luck...

[DaveMc]

I love every single one of you guys and girls! Thanks so much and thanks to the admins and the techs that make this collective knowledge possible! I mess things up twice before getting it right, so having y'all to ask is a godsend.

Make plans for the kids/bunkhouse rally if you can. Looks like a great park and I know I'm better informed on my way there because of you guys!

Dave

[Action]

Setting aside tir loading as that is another piece, my point is that stopping short of max pressure assuming the pressure will rise after the tire is in use to achieve max pressure is an issue.

Good for you that the ride you desire is achieved at less than max pressure. Kudos as well you have not had a tire failure. (Road hazards abound which speaks to your driving and luck in addition to tire prssure)

If one is looking to get max load capacity from a tire, max cold fill pressure is required. Anything less than that means the tire will not handle the max load correctly. Because 5 pound less than max assuming the pressure will rise to max is not max pressure or loading. And I have seen posts in this forum of users that have that belief. And bleed off air pressure at a stop on a trip on a hot tire because it was filled at max cold and when hot is greater than max sidewall pressure.

>>>>>>>>>>>>>Action

[ShinyPete]

DaveMc-

Don't run the Marathons less than the 65 psi the tire calls for as well as Im betting the tire data sticker on your coach calls for. Just my opinion, but we did that for 3 yrs on our 7600 gvwr 27fb, loaded to about 6800 lbs for travel, and didn't have any problems. Not with tires, nor with things moving inside, nuthin.

We're 1000 miles or so on a set of the new Endurance Goodyears, and I'm very happy with them. Smoother ride, virtually no air loss, and I worry a lot less. They are 30x the tire the Marathon is, my opinion; my tire dealer agrees after seeing them that Goodyear really stepped up finally with a decent ST made on the USA.

Oh, and I put the Marathon takeoffs on my cargo trailer, as they are 10x the tire as the kenda junkers that came on it.

Hope this helps.

[A W Warn]

Originally Posted by Action

Setting aside tir loading as that is another piece, my point is that stopping short of max pressure assuming the pressure will rise after the tire is in use to achieve max pressure is an issue.

Good for you that the ride you desire is achieved at less than max pressure. Kudos as well you have not had a tire failure. (Road hazards abound which speaks to your driving and luck in addition to tire prssure)

If one is looking to get max load capacity from a tire, max cold fill pressure is required. Anything less than that means the tire will not handle the max load correctly. Because 5 pound less than max assuming the pressure will rise to max is not max pressure or loading. And I have seen posts in this forum of users that have that belief. And bleed off air pressure at a stop on a trip on a hot tire because it was filled at max cold and when hot is greater than max sidewall pressure.

>>>>>>>>>>>>>Action

Thanks again! I agree with everything you said.

You said "If one is looking to get max load capacity from a tire" one must use maximum air pressure. I totally agree.

I was trying to point out; everyone is not looking to get max load capacity from their tires.