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Old 09-02-2013, 08:21 PM   #11
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A question for you Tireman, that I've never had a clear answer to. Lets make this specific about Goodyear Marathons, and let us know if your answer is specific to those.

On mine, the Max Cold pressure is 80, with speed limited to 65. The Goodyear webpage says that you can increase pressure by 10 pounds to increase the Max Speed to 75. Does that mean that you can increase the pressure to 90, or is it that you can increase pressure above that needed when you are at lower loads, but NEVER over "Max Cold" of 80?

Thanks.
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Old 09-03-2013, 12:41 PM   #12
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Originally Posted by BarryD0706 View Post
A question for you Tireman, that I've never had a clear answer to. Lets make this specific about Goodyear Marathons, and let us know if your answer is specific to those.

On mine, the Max Cold pressure is 80, with speed limited to 65. The Goodyear webpage says that you can increase pressure by 10 pounds to increase the Max Speed to 75. Does that mean that you can increase the pressure to 90, or is it that you can increase pressure above that needed when you are at lower loads, but NEVER over "Max Cold" of 80?

Thanks.
OK this gets a little technical.
First, if you have followed my posts here and in my blog I strongly suggest that tires on multi-axle trailers be inflated to the pressure molded on the tires. I suggest this so that you have a chance at improving tire durability.
I know the GY Tech Bulletin is not clear but having discussed this with GY they say you should not exceed the inflation molded on the tire sidewall.

I do understand the concept of changing speed rating with an inflation change. What is also true is that the +10 mph 65>75 is the maximum speed increase covered by Goodyear in their document.

My concern is that Goodyear fails to consider the special side loading seen on multi-axle trailers.
In my blog post on "What should Trailer owners do" I discuss some of the unique problem they face. The unique side loading causes what is known as "Inter-Ply Shear" between the belts and edge of the tread. This force is trying to tear the tire apart from the inside out. You can lower these high forces but not eliminate them by decreasing the tire load or lowering the "Sip Angle". You can lower the slip angle by increasing the inflation.
My concern is that ST type tires like the GY Marathon have higher load rating based on the assumption that you will go slower (less heat) and the tread depth is less (lower heat). BUT if you increase the speed you may be generating heat faster than it can be dissipated so you end up doing permanent irreparable structural damage to the tire.
This technical paper explains how increasing the tire pressure lowers the tire distortion when cornering. Lower distortion means lower inter-ply shear. Lower inter-ply shear means longer tire life. "Changing inflation pressure affects tire force and moment characteristics. This is because changes in inflation pressure alter the size, shape and contact pressure distribution in the footprint of the tire. In general, increasing inflation pressure will cause the size of the footprint to shrink, raise the contact pressure near the center of the footprint and allow less tire distortion"

So you have a choice.
You can increase the inflation above what is needed for the load (which I suggest)and get longer tire life at the same max speed

or

you might increase the inflation and run up to 75mph but have less tire durability. It is your choice but you can not expect the increase in inflation to give both longer life and higher speed capability and you cannot increase the inflation above that molded on the tire sidewall.
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Old 09-03-2013, 12:49 PM   #13
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Originally Posted by 5verdriver View Post
Go by the tire stamp, not the sticker on the trailer. Same goes for the tow rig. I know people that go by the sticker on the door jamb on their truck, but what if the tires they're running are different from the factory ones?
Sorry not quite correct.
For TT you give the correct info but for the wrong reason. For motorized vehicles your suggestion is most likely wrong.

Vehicles sold for highway use by law have a Certification label or "tire Placard" applied by the vehicle mfg. This provides the tire type, size, Load Range and inflation recommended by the vehicle mfg.
The tire company does not know the weight of the vehicle a tire may go on so can only provide information based on what inflation is needed to carry the max load.

Cars, pickups and motorhomes can adjust the tire inflation so that the tire can carry the actual load on the tires.
TT with multiple axles could do the same but if the TT has multiple axles there are special considerations which usually result in the inflation recommendation being the inflation molded on the tire (see my prior post)

If an owner changes tire size or load range it the vehicle owner's responsibility to know the actual tire loads and to establish the proper inflation based on actual loads.
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Old 09-03-2013, 01:36 PM   #14
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Tireman9,

EXCELLENT information, and the first logical engineering based explanation that I have seen.

Anyone that has turned their rig into a tight spot on asphault and has watched the tires scrub around and try to tear themselves off the rim has a small sense of the "special" side loads that you are talking about. I have LR E tires on a trailer that could handle with LR C and I keep them at close to the Max pressure (75psi) figuring the increased pressure keeps the tire cooler, the side loads lower AND they will be replaced LONG before the center tread wears out anyway. Trailer sure does bounce alot tho, oh well ther are tradeoffs everywhere.
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Old 09-03-2013, 02:28 PM   #15
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Thanks. Sometimes I worry bout going too technical. I do plan on an expanded post or two on this topic on my blog. If you subscribe you will only get a notice when I make a new post. I usually do about 3 a month.
Roger
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Old 09-03-2013, 02:40 PM   #16
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Excellent information Tireman9!!!
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Old 09-03-2013, 04:57 PM   #17
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Originally Posted by Smokin View Post
I think my head just exploded. I'm a little confused and I'm not sure I know all of the answers but they are ST tires. The GVW is approximately 11,500 pounds.
Then also give the number of tires that ride on the road and the number of axles.
And from the tires the maximum load and that AT-pressure .

We now know they are ST tires wich may bare more because of the lower speed, wich heat up the tire less so at more deflection same heat-production as an LT tire for instance at that higher speed. Its not even to say that the heat does the damage . Shal play an important role in it all, but the repeated strain of 10 to 20 times deflecting and flexing back of every segment of the tire, could do the damage.

But if you have an LT tire the speedcode gets important, to know for wich speed they are calculated . But I did not knew that you had ST tires in my last post, so also wrote about the speedcode by a letter.

And ST tires sometimes even get damaged at the AT-pressure ,
Better is to give them the same deflection as an LT tire of same sises and AT-pressure, if that can be done by 10 psi higher pressure then AT-pressure.
For truck tires even 20 psi higher is allowed ( when is it called a truck tire?).

So what I am trying to make clear is , calculate it as it where an LT tire of same sises, and if then pressure is 10 psi above AT , you are save to laws of nature, and the TRA cant say you have used a to high pressure, because they allow it.
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Old 09-03-2013, 05:57 PM   #18
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Smokin

I have a 2012 290RLT with 235/80/16 tires and have always run them at 80 PSI with no problems in 6K miles. I have weighed my trailer at each wheel position while hooked to my TV. The actual weights are as follows RF 2,050 RR 1,850 LF 1,900 LR 2,800
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Old 09-04-2013, 11:53 AM   #19
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Thanks for the info. One question. Are your tires ST or LT?
Hint: When writing a tire size it helps if the letters in front of the numbers are included. It also helps if you include the "Load Range" letter. C, D, E, F etc.
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Old 09-04-2013, 12:19 PM   #20
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Originally Posted by Tireman9 View Post
OK this gets a little technical.
First, if you have followed my posts here and in my blog I strongly suggest that tires on multi-axle trailers be inflated to the pressure molded on the tires. I suggest this so that you have a chance at improving tire durability.
I know the GY Tech Bulletin is not clear but having discussed this with GY they say you should not exceed the inflation molded on the tire sidewall.

I do understand the concept of changing speed rating with an inflation change. What is also true is that the +10 mph 65>75 is the maximum speed increase covered by Goodyear in their document.

My concern is that Goodyear fails to consider the special side loading seen on multi-axle trailers.
In my blog post on "What should Trailer owners do" I discuss some of the unique problem they face. The unique side loading causes what is known as "Inter-Ply Shear" between the belts and edge of the tread. This force is trying to tear the tire apart from the inside out. You can lower these high forces but not eliminate them by decreasing the tire load or lowering the "Sip Angle". You can lower the slip angle by increasing the inflation.
My concern is that ST type tires like the GY Marathon have higher load rating based on the assumption that you will go slower (less heat) and the tread depth is less (lower heat). BUT if you increase the speed you may be generating heat faster than it can be dissipated so you end up doing permanent irreparable structural damage to the tire.
This technical paper explains how increasing the tire pressure lowers the tire distortion when cornering. Lower distortion means lower inter-ply shear. Lower inter-ply shear means longer tire life. "Changing inflation pressure affects tire force and moment characteristics. This is because changes in inflation pressure alter the size, shape and contact pressure distribution in the footprint of the tire. In general, increasing inflation pressure will cause the size of the footprint to shrink, raise the contact pressure near the center of the footprint and allow less tire distortion"

So you have a choice.
You can increase the inflation above what is needed for the load (which I suggest)and get longer tire life at the same max speed

or

you might increase the inflation and run up to 75mph but have less tire durability. It is your choice but you can not expect the increase in inflation to give both longer life and higher speed capability and you cannot increase the inflation above that molded on the tire sidewall.
Great write-ups. Very informative. I run my trailer at tire max pressure which was 80psi on my old trailer and higher than the sticker pressure on the trailer. I will eventually be replacing my D classes on the new rig with E class radials for the slip angle reason you mentioned above. My trailer has a wider axel spacing than is typical and you can feel them flex and drag during sharp turns.

My trucks tire pressure I run at tire max (50 psi) when towing. The rest of the time I back it down to sticker pressure (35 psi) to protect suspension and steering components.
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