RV Tires

[Brian B-P]

Originally Posted by rabbit

If I have determined rather the minimum inflation pressure, do you mean minimally adequate or the minimum that is wholly adequate or something else altogether?

Yes, it's the minimum for safe and reliable operation.

To put this in perspective, my Toyota van came with tires that are at their maximum load (after derating the P tires for van use, just as for a trailer) when inflated to their allowed maximum pressure and loaded to the gross rear axle weight rating. While owners complain about tire wear, there is no reliability problem with them.

[rabbit]

Great explanation, Brian. After a few thought experiments with baseballs and red rubber balls and the transfer of energy, it occured to me that soft tires might be expected to decrease the amplitude (absorb) of vertical motion from pavement discontinuities transfered thru the tire to a greater degree than would hard tires. Take the example of tennis balls which will hardly bounce (evidence of receipt of energy expressed as change of direction) when the pressure of the injected gas is lost. 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

[rabbit]

One further thought on "minimum" inflation recs in the chart. 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). Is the upper inflation limit on the Marathon STs also a "minimum"? Seems it might be as it's acceptable to the manuf. to add another 10psi to tires on rims rated for that pressure.

jack

[Francesca Knowles]

Originally Posted by rabbit

One further thought on "minimum" inflation recs in the chart. 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). Is the upper inflation limit on the Marathon STs also a "minimum"? Seems it might be as it's acceptable to the manuf. to add another 10psi to tires on rims rated for that pressure.

jack

The upper limit is just that. Since it's impractical to make vehicle-specific tires, they have to be suitable for a wide range of vehicles, so tires can be run at a variety of pressures under the stated maximum.

That's why the first thing a tire manufacturer says per pressure is "follow your vehicle maker's recommendation".

Barring access to a manufacturer's recommendation, appropriate pressure for trailer tires can be determined by the weight of the trailer itself combined with the tire manufacturer's chart of weights/pressures.

Side note:
There's a body of thought that says that exceeding a tire's stated maximum pressure (under certain conditions) by perhaps 5% is acceptable. I for one am too cautious to promote this practice...

Francesca

[rabbit]

Here's the dog whistle, Francesca. It's obvious to me from the previous discussion that no matter how sophisticated one's view of physics may be, most of us are concerned enuf about blowout at speed that we don't loose a lot of sleep over trailer tire tread wear (the side walls will crack before it's down to the markers) or the ride quality as experienced by our bedding, pots and pans (we're not iin there!). Everywhere I go, I find highly-nuanced discussions the upshot of which is always dynamite em for stiff, cool sidewalls. Add to this the caveats about constant monitoring of pressure, physical inspection of sidewall, etc. and it would seem that the worry beads are what keeps our rigs on the road.

I'd be mighty beholdin to anyone who will take my axle wgt. and tire specs and the Marathon chart at Brian's link and tell me at what pressure I should inflate. Nome sane?

jack

[Carol H]

Originally Posted by Francesca Knowles

The upper limit is just that. Since it's impractical to make vehicle-specific tires, they have to be suitable for a wide range of vehicles, so tires can be run at a variety of pressures under the stated maximum.

That's why the first thing a tire manufacturer says per pressure is "follow your vehicle maker's recommendation".

The above is very true but I would be very cautious about following a trailer manufactures recommended tire pressure if the trailer is 25 or more years old & the pressure recommendation stated by the trailer manufacture was made an equally number of years ago. The odds are the trailer tires you are buying today were not even around when the trailer manufacture made their pressure recommendations 30 years ago. IMHO in that situation you would be far better off going with what the actual tire manufacture recommends and use your actual trailer weigh as a guide.

[Francesca Knowles]

Originally Posted by rabbit

H
I'd be mighty beholdin to anyone who will take my axle wgt. and tire specs and the Marathon chart at Brian's link and tell me at what pressure I should inflate.

Nome sane?

jack

Well...part of the time I do...

Two questions per your request:

1) Are you actually using Goodyear Marathons, and if so what size?
and
2) (Axle weight being only another "capacity" number re. maximum capacity of that suspension component, it's necessary to know):
What does your trailer actually weigh?

Francesca

[rabbit]

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

[Francesca Knowles]

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

[floyd]

Originally Posted by rabbit

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".

[rabbit]

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.

jack

[Brian B-P]

Originally Posted by rabbit

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.

[Brian B-P]

Originally Posted by rabbit

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.

[Brian B-P]

Originally Posted by rabbit

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.

[Brian B-P]

Originally Posted by rabbit

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.

Originally Posted by rabbit

... 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.

[rabbit]

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

[Brian B-P]

Originally Posted by rabbit

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.

Originally Posted by rabbit

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.

[rabbit]

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.

jack

[Francesca Knowles]

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?

• 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

[Brian B-P]

Originally Posted by Francesca Knowles

Does anyone read that to mean that one should also exceed the maximum P.S.I for the tire by ten pounds?

Not me.

Originally Posted by Francesca Knowles

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.