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Old 06-02-2018, 10:38 PM   #21
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The foregoing sounds like the final word on this subject to me.
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Old 06-03-2018, 07:25 AM   #22
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Quote:
Originally Posted by BobC View Post
Here's a site maintained by Roger Marble, a retired tire engineer who posts on IVR2 as Tireman9. The link I've given is to an interesting trailer-specific article but if you go to his home page, there are links to a lot of pertinent info:

RV Tire Safety: Search results for trailer

Anecdotal information from those of us who aren't experts in the field isn't as good as that from an expert who has a much broader, better informed and balanced perspective.

As you stated earlier the article says pressure is a dynamic thing based on load. He also does not straight out advocate filling them to their max pressure but adjusting them for the load. He more or less says do what n1as is trying to do by adjusting the pressure to the load.



There looks to be some good subjects so I've bookmarked to read later.



Quote:
Originally Posted by LarryW View Post
Please note what America’s Tire Store says at https://www.americastire.com/learn/trailer-tire-faqs

First “check” mark. “A trailer tire should be inflated to the maximum inflation designated on the sidewall of the tire to provide the full load carrying capacity


Again someone on the internet who is not an authority and is talking worst case. Let's go to a couple of authorities:



https://www.goodyearrvtires.com/pdfs/rv_inflation.pdf



https://www.maxxis.com/trailer/trail...nflation-chart



Notice they do not say fill to the max pressure on the side wall but provide pressures based on weight. There is a reason for this as an over inflated tire is not good either. An over inflated tire can affect how the tread contacts the road which can lead to things like hydroplaning or other forms of traction loss. It can also result in faster tire wear although tire tread life isn't an issue for most TT owners.



I'm done arguing with you. As I said previously you can do what you want. As for set it and forget it I'll go with the federally mandated sticker as it is based on the mathematics of the engineers and is based on tire loads.
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Old 06-06-2018, 05:52 PM   #23
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Using load & inflation tables to determine tire inflation

Tire mfrs specify an optimum pressure for trailers tires the same way they do for RV and truck tires -- by load.

This table is from Goodyear, other mfrs are similar:

https://www.goodyearrvtires.com/pdfs/rv_inflation.pdf

The various tire sizes have a range of recommended pressures depending upon load -- everything from 25 to 80 psi. Some go to 110 psi.

It really is best to use a scale to get the actual load and then use the tire mfr's inflation tables to determine the proper psi.

Under inflated tires can be dangerous, but so can over-inflated tires.

This is not a case where more = better.
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Old 06-06-2018, 10:18 PM   #24
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weigh it yes, refer to tire pressure rating, on the tire, you can load per tire by the label. the manual doesnt always reflect tire manufacturers ratings. 60 is TOO low, I have the rating of 85 cold for a load of 2405 each tire. 60 you will over heat the tire. remember to inflate cold
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Old 06-06-2018, 10:51 PM   #25
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Originally Posted by n1as View Post
Micro Minnie. 3900 lbs dry, I estimate 4500 lbs or less for a weekend trip. Four tires, each rated at 2040 lbs at 65 psi.

2040 lbs divided by 65 psi = 31.38 lbs-load / psi

4500 lbs trailer divided by 4 = 1125 per tire. Add 30% for dynamic margin = 1462 lbs.

1462 lbs per tire / 31.38 lbs per psi = 46 psi. Hmm, seems a bit low.

Add 10 psi for "over 65 mph travel" (though I'll be running 65 or less) and then round up to 60 for extra margin.

I don't want a blow-out due to under inflation. Nor do I want a rock hard tire that has no give so that any road hazard takes it out.

60 psi. Good or bad? :-)
You are smart to be concerned with both under and over inflation. Both can be dangerous, as you point out.

What size tires does your trailer have?

According to this table from Goodyear, even the smallest of their tires (ST205/75R14) only needs 35-40 psi for the 1,462 lb load you calculated.

Note: these "ST" tires are rated for speeds up to 87 mph. There is no spec'd pressure increase for speeds over 65 mph. It sounds like your tires may be different. Needless to say, it would be best to use the inflation table provided by the tire mfr. I used this Goodyear chart just as an example from a major mfr.

I've never heard of "dynamic margin". What is that?
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Old 06-07-2018, 11:42 AM   #26
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I would suggest you google "tire load inflation pressure chart". Then look at the chart and your tire size and type, ie. 205/75R14 type C. for this tire it is 1760 pounds at 50psi
multiply for 4 tires which is a total of 7040 total weight carrying capacity. This is the maximum for this tire and should far exceed you weight.
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Old 06-07-2018, 06:43 PM   #27
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Using load & inflation tables to determine tire inflation

This Goodyear inflation table says the same thing -- 1,760 lbs @ 50 psi:

https://www.goodyearrvtires.com/pdfs/rv_inflation.pdf

Using 1,462 lbs per tire (which includes 30% for dynamic margin), 40 psi is more than enough.

If the load is equally distributed and each tire carried 1,125 lbs, just 25 psi would be more than enough, according to Goodyear.
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Old 06-07-2018, 07:25 PM   #28
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My tires are 205/75/14 load range D. With 4 of them I have more load capacity than I do axle capacity so clearly running at max PSI is over-kill.

I believe anyone who just says to run the tires at sidewall pressure without considering load capacity is either ill-informed or over-simplifying. I suspect tire dealers fall into the latter category because they are battling the common problem of under inflation and it is safer to tell the ill-informed crowd to just inflate them to max and be done with it. That way if they forget to check pressure for a month or two years, there is a good chance they'll still have enough air to be safe.

Back on topic, I've searched for load charts and they are few and far between. Curious. Perhaps the tire manufactures don't trust us either.

Oh, and "dynamic margin" is a term I used to refer to the fact that the load from a moving trailer will change as the trailer hits bumps, pot holes, turns corners, etc. I guessed that the extra load would be 30% or less. Pure guess.

All of this still leaves me unclear regarding which PSI to actually use. It would be easier for sure to pretend none of this exists and just inflate to the sidewall pressure.
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Old 06-07-2018, 09:44 PM   #29
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Using load & inflation tables to determine proper tire inflation

Quote:
Originally Posted by n1as View Post
My tires are 205/75/14 load range D. With 4 of them I have more load capacity than I do axle capacity so clearly running at max PSI is over-kill.

I believe anyone who just says to run the tires at sidewall pressure without considering load capacity is either ill-informed or over-simplifying. I suspect tire dealers fall into the latter category because they are battling the common problem of under inflation and it is safer to tell the ill-informed crowd to just inflate them to max and be done with it. That way if they forget to check pressure for a month or two years, there is a good chance they'll still have enough air to be safe.

Back on topic, I've searched for load charts and they are few and far between. Curious. Perhaps the tire manufactures don't trust us either.

Oh, and "dynamic margin" is a term I used to refer to the fact that the load from a moving trailer will change as the trailer hits bumps, pot holes, turns corners, etc. I guessed that the extra load would be 30% or less. Pure guess.

All of this still leaves me unclear regarding which PSI to actually use. It would be easier for sure to pretend none of this exists and just inflate to the sidewall pressure.
Excellent post.

I think you are spot-on about the all too frequent advice to over-inflate car/RV/trailer tires. The easiest thing for tire dealers to tell customers is to inflate to the absolute maximum pressure (molded on the sidewall).

Apparently the thinking is that gross over-inflation is safer than under-inflation. That may be true, but of course over-inflation has its own problems: tire is more prone to blowout from impact; more likely to get punctures; excessive center tread wear; less rubber on the road; extremely rough ride, etc.

Tire dealers may be concerned about liability, but one would think they would be liable if they recommend that a customer put double or triple the recommended pressure in their tires and they blow out from hitting a modest sized pothole (one that other vehicles have no problem with).

When we bought our View, the tires were all inflated to at least 80 psi! The previous owner said the local shop inflated her tires. The WGO recommendation is 61 psi. When we weighed the rig and looked at the load & inflation tables we found that 61 psi good for about 12,200 lbs -- approx 1,200 lbs over the GVWR!

Many well meaning but inexperienced employees at tire shops simply don't know any better.

I can't explain the lack of load charts. I know our View manual specifically says to weight the rig -- all 4 corners, and inflate the tires according to the tire mfr's inflation table.

The Michelin charts are easy to find, as was the Goodyear chart I linked to. IDK about the other mfrs though, I haven't looked for them.

I'm curious about "dynamic margin". We don't tow, so I don't know much about trailers. I would have thought that the forces acting on a trailer tire are similar to those affecting a car or RV tire.

The load and inflation tables are based on static weight. To be most accurate, it is best to measure the weight on each tire (rather than each axle and divide by 2). If one end of an axle has more weight, then both tires (or both sets of duals) should be inflated to the psi appropriate for the heavier end. All tires on an axle should be equally inflated.

Unless there is something different about trailers, would get the static weight and go by the tables.
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Old 06-08-2018, 12:30 PM   #30
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Trailer tires have higher lateral stress than a vehicle tire which is why the ST tire came out. The firmer sidewall is supposed to handle the additional lateral stress. My previous post had the Maxxis load range chart in addition to the Goodyear chart.
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Old 06-08-2018, 04:02 PM   #31
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Quote:
Originally Posted by old_engineer View Post
Trailer tires have higher lateral stress than a vehicle tire which is why the ST tire came out. The firmer sidewall is supposed to handle the additional lateral stress. My previous post had the Maxxis load range chart in addition to the Goodyear chart.
Thanks Old Engineer!

I just now looked at both charts.

I had only looked at the Goodyear chart previously to get the correct info for my posts. I figured that between the two, Goodyear would be the best to use for load and inflation numbers.

Using 205/75R14 as an example, I found that both Goodyear and Maxxis specify the same amount of load for each specific pressure, up to 50 psi (where the Maxxis chart ends).

However, the same size Goodyear ST tires can be inflated to 65 psi (max) with a load rating of 2,040 lbs -- vs 1,760 lbs @ 50 psi for the Maxxis.

One thing I'm curious about is the "D" in the Goodyear chart, in the 65 psi column, for the top 3 tires. What does that designate?

Links:

https://www.goodyearrvtires.com/pdfs/rv_inflation.pdf

https://www.maxxis.com/trailer/trail...nflation-chart
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Old 06-08-2018, 09:51 PM   #32
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IMO is to run the sidewall pressure if you are running ST tires. They have a stiffer sidewall than LT tires so running them at the min pressure for max weight will not harm them.

I suggest you get the tires balanced. Had the tires balanced on our 5er and one tire took 5 oz of weight.
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Old 06-09-2018, 12:07 AM   #33
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Just the messenger...

If trailer tires were meant to always be inflated to the maximum pressure on the sidewall, then there would be no reason for tire mfrs to publish load & inflation tables.

Here is some info I found with a Google search:

Basics of Trailer Tires | www.trailerlife.com

"Understanding your tire load rating is crucial to long tire life and safe travel. The
pressure figure on the sidewall of an ST tire is the minimum pressure necessary to carry
the maximum load. In other words, if a tire reads “Max load single: 3000 lbs at 65 psi
cold,” 65 psi is the minimum cold-inflation pressure required to carry the maximum-load
figure. Some RVers may be inclined then, to inflate the tire in our example to 65 psi – but
unless that tire is carrying its maximum load, that inflation pressure may not be
necessary. Tire manufacturers like Goodyear and Michelin publish load/inflation tables that
indicate how much weight a tire can carry based on inflation pressure. Therefore, it is
recommended that you weigh your trailer when it is loaded and ready for travel (including
fuel, freshwater, supplies, etc.) to determine the amount of air necessary to carry your
load. Weigh the trailer from side to side if possible (individual wheel weights are
optimum) and inflate all tires to the pressure of the tire(s) carrying the heaviest load.
Also, regardless of which side is heavier, make sure that tires on the same axle have the
same inflation pressure."

~

A few more links:

https://info.kaltire.com/the-right-t...isnt-the-best/

https://www.kenjones.com/blog/traile...ion-guide-faq/

https://www.tirerack.com/tires/tiret...jsp?techid=196

https://www.thefitrv.com/rv-tips/how...re-in-your-rv/

https://www.discounttiredirect.com/learn/tire-pressure

https://www.tirerack.com/tires/tiret...e.jsp?techid=8


Over-inflating tires makes them more likely to blow out from hard impacts; more likely to get punctured; and the tread will wear in the center prematurely. Also, there is less rubber on the road, so maximum braking force (before skidding) is reduced.
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Old 07-19-2018, 07:57 AM   #34
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You just hit it on target. Mtg of trailer knows what carry and they do this to cover their backsides.
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Old 07-19-2018, 04:28 PM   #35
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You just hit it on target. Mtg of trailer knows what carry and they do this to cover their backsides.
If there is a difference in recommended pressures, I don't know that I would give priority to the trailer mfr specs over the tire mfr specs.

I can say that with our View, I go with the Michelin RV tire load & inflation tables.

My reasoning is that WGO cannot know how the rig will be loaded, so they err on the high side. That's the way to go if you have to guess -- but we do not have to guess. We can use a scale and weigh all 4 corners of the rig and set the pressure accordingly.

IDK about trailers, but the View owner's manual specifically says that the best way to set tire pressure is to weight the motorhome and use the tire mfr's tables. WGO even has clear instructions in the manual for how to weigh the rig.
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Old 07-19-2018, 04:31 PM   #36
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Thanks for info
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