RV Tires - Page 3 - Fiberglass RV


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Old 01-01-2013, 05:24 PM   #29
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Wow- I had no idea that Burros are that heavy!

Per trailer tire discussions AKA The War That Never Ends:

Yew ain't seen nuthin until you've seen the ongoing 93 pages on the subject at the Open Roads Forum.
And I've read 'em all! Even posted there some...with predictably mixed results:

Anyway...here's a pastable version of the inflation chart for Marathons...and I'd add that I did a 2,000 mile road test this fall to test some of its recommendations. I'd intended to post my results but haven't been in the mood for a ('nother) fight!





Francesca
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Old 01-01-2013, 07:20 PM   #30
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Quote:
Originally Posted by rabbit View Post
Yes, I actually am, in size 205/75R14. wgt. on axle 3120lb. or 1560lb per tire. But, hey, I'm just having a bit of fun here about general advice and specific prescriptive advice. You don't even have to look at the Goodyear chart. I could run my tires at 43psi according to Goodyear and no suggestion anywhere that any inflation figure on there is a "minimum" figure--the numba is the numba.

So far today is the worst day of the year (surprise) and more to come so once it warms up enuf that we see the sun for two days running, maybe I'll hook up, dump the air down to 43 lb. and take a five mile run with a few items unsecured in the trailer. Couple of expansion joints should tell me all I want to know about shock absorption and damping. I should say that I am not singling out Brian and Floyd as the occasion of my little homily. Every camping and boat trailer forum has this very discussion where the blind men touch the elephant and tell you what they "saw" at least twice a month.

jack
This is a frustrating topic, since it always results in everything from "Wikigenius" advice, to advice from people with a lifetime of training and experience.

This venue doesn't allow for divining the difference.

Many of the posts above contain nuanced errors which are important enough to effect property and safety.
Each of us will just have to choose which is which and take personal responsibility for the consequences.
It looks as if you have come to that same conclusion.
That's as it should be!

After you touch the elephant, let us know what you "See".
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Old 01-01-2013, 08:20 PM   #31
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Deal, Floyd. I'll go so far as to make a couple of short runs with mine. It's documented in this thread as to curb wgt., tire rating, max pressure so we won't have to argue anecdotal apples to oranges in a pickup game of this is his story, this is her song. Also no direct reading sensors, monitors. Just the circumstantial bounce-o-meter that registers the observable occurence of displaced objects in the trailer. You'll have to trust me to not load the dice (not the best feature of this experiment). You'll have to trust my judgment that I'm doing the same route at the same average speed. I guess we'll have to assume that my torsion arms aren't dead at 15 yrs of age. I think they're not as I managed to stuff a rim/tire combo in there that's an inch larger in diameter than originally equipped. Frame to ground was originally 14"; it's now 13". Subtract the .5" gained in the wheel swap comes down to 1.5" loss of travel in the arms (I think?). So a couple of pulls over the same route same direction same average speed same maximum speed. All same same ANAP.

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Old 01-02-2013, 12:09 AM   #32
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It seems counterintuitive to me that an object with a relatively high resistance to deformation is the best at cushioning impact but I do see a hint on the horizon that cushioning and damping may not be synonymous. As you say, compression rate and return rate (recovery from deformation) may be different for tires, springs, and shocks.

jack
It sounds like you've got it, Jack.
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Old 01-02-2013, 12:19 AM   #33
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I guess we'll have to assume that my torsion arms aren't dead at 15 yrs of age. I think they're not as I managed to stuff a rim/tire combo in there that's an inch larger in diameter than originally equipped. Frame to ground was originally 14"; it's now 13". Subtract the .5" gained in the wheel swap comes down to 1.5" loss of travel in the arms (I think?).
It's the rubber that dies, not the arms, but I get the idea.
1.5" is most of the travel of a typical rubber torsion axle - if the suspension really sits that much lower than than it did originally, it seems like significant deterioration.
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Old 01-02-2013, 12:30 AM   #34
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I could run my tires at 43psi according to Goodyear and no suggestion anywhere that any inflation figure on there is a "minimum" figure--the numba is the numba.
Each weight number in the chart is the capacity of that row's tire size at that column's pressure. Since capacity increases with pressure, those are minimum pressures - there's no need to label them as such.
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Old 01-02-2013, 01:01 AM   #35
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Sad commentary on empirical science that no one is able to establish inflation pressures which are optimal (or at least balanced to give best wear, traction, ride control).
This optimum cannot be determined with only tire data and an axle load, but a value (which may not be the compromise you want) is determined for every seriously developed vehicle... which unfortunately does not include trailers, which are low-production vehicles with zero model-specific chassis development. Look on the tire inflation placard of your tow vehicle for an example.

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... maybe I'll hook up, dump the air down to 43 lb. and take a five mile run with a few items unsecured in the trailer. Couple of expansion joints should tell me all I want to know about shock absorption and damping.
That's perfectly valid. Some things are much more practical to determine empirically than theoretically. This does not mean that an understanding of the underlying principles is unimportant. An infinite number of monkeys hitting random keys on typewriters for an infinite amount of time may produce the complete works of William Shakespeare, but there are not enough moulded fiberglass trailers to take that approach to chassis tuning with any success.
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Old 01-02-2013, 01:03 AM   #36
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Ok, minimum pressure as a synonym for pressure threshold or level which supports a certain load I understand. I believe what evolves from this logic is that the minimum pressure which supports the next listed load must be the maximum pressure "necessary" to support the previously-listed. One might cavil with this use of the term "maximum necessary" as nonsense, which it is except that it makes clear the existence of a range of pressure values adequate to carry a certain load. The increment is such that the upper limit is not significantly more pressure than needed to support the load. I do not see how the terms minimum pressure and adequate pressure are distinct in this context.

You may be right about the rubber rods, Brian.

jack
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Old 01-02-2013, 01:22 AM   #37
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Is the upper inflation limit on the Marathon STs also a "minimum"?
It is the minimum to reach the tire's maximum capacity. This is not true of all tires - at least some "P" tires reach their maximum capacity at (as I recall) 35 psi, but allow inflation up to 44 psi in case higher (than 35 psi) pressures are desirable for some vehicles.

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Seems it might be as it's acceptable to the manuf. to add another 10psi to tires on rims rated for that pressure.
I've never seen that statement from a tire manufacturer... only the statement that if you inflate to 10 psi higher than required for sufficient capacity at up to 65 mph, you can have that same capacity at a higher speed. Example: if Jack's ST205/75R14 LR-C tires only needed to support 1530 lb each, they would need only 40 psi (and be limited to 65 mph); if run at 50 psi, they would still be within the maximum pressure shown on their sidewalls and would be allowed a higher road speed at that 1530 lb load. That doesn't mean the tire manufacturer endorses inflating those tires to 60 psi.
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Old 01-02-2013, 09:16 AM   #38
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A very good qualification concerning maximum psi. I personally have been confused by statements that maximum is not always maximum for trailer service tires. I also understand why Floyd discounts the role of tires in the suspension system. Thanks for your replies and certainly your patience.

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Old 01-02-2013, 12:53 PM   #39
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Shall we further muddle the issue by discussing this Goodyear Marathon ST bulletin that advises users of ST's how to exceed the supposed 65 mph speed "limit" of the tire?

Quote:
• Industry standards dictate that tires with the ST designation are speed rated at 65 MPH (104
km/h) under normal inflation and load conditions.
• Based on these industry standards, if tires with the ST designation are used at speeds between 66 and 75 mph (106 km/h and 121 km/h), it is necessary to increase the cold inflation pressure by 10 psi (69 kPa) above the recommended pressure for the rated maximum load.
Does anyone read that to mean that one should also exceed the maximum P.S.I for the tire by ten pounds?

Or are we to refer to the numbers on the chart previously posted and adjust from there, thus staying under/at the tire's stated maximum pressure???

Francesca
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Old 01-02-2013, 01:37 PM   #40
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Originally Posted by Francesca Knowles View Post
Does anyone read that to mean that one should also exceed the maximum P.S.I for the tire by ten pounds?
Not me.

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Originally Posted by Francesca Knowles View Post
Or are we to refer to the numbers on the chart previously posted and adjust from there, thus staying under/at the tire's stated maximum pressure???
That is my interpretation; however, the explicit direction to not go over by more than 10 psi suggests that Goodyear may actually be advising exceeding the sidewall maximum. Personally, I'll stick with the more conservative approach.
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Old 01-02-2013, 01:41 PM   #41
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Your second statement (Francesca) is true. You would add 10psi so long as you don't exceed the maximum on the tire. And this allowance is only allowed on "improved", i.e. paved roads. Actually the operative word is "highway" use.
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Old 01-02-2013, 03:14 PM   #42
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Thanks for the catch on my "Michelin" blooper, Brian- fixed on edit!

Per exceeding stamped maximum sidewall pressure:

It should be noted that the bulletin does specifically caution against exceeding the PSI limit of the wheel that the tire is mounted on.

I wonder how many folks even know that there is such a number?

Since it's usually stamped on the tire-bearing surface, demounting the tire is required to see it...

Francesca
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