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Old 08-13-2019, 10:13 AM   #29
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As an added comment, the "Max Pressure" is there as a "not to exceed" since most folks don't know their TT weights...is what I was told when I asked the GY tech folks. Notice also they have a Max Speed...I surely don't expect to drive 87mph either with my AS! Still feel following the mfg. specs are the way to go! YMMV!
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Old 08-13-2019, 10:16 AM   #30
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This discussion has sort of stuck a stick in Tireman9. I don't doubt for a minute he's an expert, that he understands tire failures, or that he's learned a lot during his professional career. But I suggest he doesn't have as much real world experience as Andy Thomson at CanAm RV in selecting tires and running them at pressures that won't damage our trailers.


Thatís an interesting way to package your thoughts. Couldnít you just as easily say that Andy doesnít have real world expertise in designing, engineering, and testifying as an expert in court on tires?

I think the 2 are experts in different areas. They have different disciplines and perspectives. Neither is a complete source of expertise on everything and both have opinions worth considering for whatever final decisions owners make with their trailers.

I donít see TM9 having a stick in him because his discipline is different than someone elseís.

But what do I know?
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Old 08-13-2019, 11:12 AM   #31
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We have two threads on this tire pressure question that are running in parallel, so I'll just post here.

I get it -- TM9 and Capri advocate running at sidewall maximum pressure. That's 80 psi for the GYE. Their focus is interply shear. Andy Thomson recommends 45 psi for GYE tires on the 25 foot models, for a decent ride. So why this difference?

Using Andy's view that 45 psi is correct for the 25' trailer, let's look at the Goodyear tire inflation table for the Endurance tire. That table says, at 45 psi, the GYE will support a maximum load of 2,020 lbs. per tire.
Using Airstream's data, the FC25 has an UVW of 5,503 lbs. and a CCC of 1797 lbs., for a GVW of 7,300 lbs. It's a little hard for me to imagine putting a 1,000 lbs. of "stuff" in this trailer, but lets assume we do, so -- a ready to camp weight of 6,500 lbs, or 1,625 lbs per tire if weight per tire is even (and it probably isn't). Even so, that's a 24% safety margin (2,020 per Goodyear's inflation table divided by 1,625 lbs. load per tire in my example)

So why isn't 45 psi a "safe" pressure with plenty of margin, both from a load-carrying perspective and adequate protection from interply shear?
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Old 08-13-2019, 02:22 PM   #32
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So why isn't 45 psi a "safe" pressure with plenty of margin, both from a load-carrying perspective and adequate protection from interply shear?

I think thatís a great question and would welcome some more input from tire engineers and Andy on the interply shear question - Iím not sure whether they look at this from the same lens.

There seem to be three opinions

1) match the load to the table with some margin (10-25%) for a comfortable ride with enough capacity to not overwhelm the tire

2) go with the max cold pressure on the sidewalk to reduce instance of interply shear (stiffer sidewalls I believe)

3) go with the placard the trailer manufacturer affixed to the trailer. In most cases that was likely the same as #2 above - but in my case, original tires were the GYMs D load and the placard says 65 PSI - I now have the GYEs E loaf where the max is 80psi but my placard still says 65 PSI (obviously).

I think if youíre a tire engineer - your lens might be avoiding shear at all costs and perhaps you de-emphasize smooth ride.

If youíre a dealer known for custom setups - your lens might be most comfortable ride that meets minimum load table requirements and perhaps you de-emphasize interply shear.

You might bring a different lens to the table - I donít know that thereís a single right or wrong, just different reasons to chose one approach vs another.
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Old 08-13-2019, 02:42 PM   #33
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What about a Bambi?

Interply shear is not an issue for those of us towing a Bambi (unless I lack some understanding of the problem).

Does that factor into the pressure question in some fashion?
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Old 08-13-2019, 02:51 PM   #34
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Tire pressure for Goodyear endurance

Its only an issue with two or more trailer axles. Itís because one tire (or two) is dragged hard sideways in a tight turn mostly on the inside of the turn. It leaves a lot of rubber, which indicates a lot of shear force on my rig.

I need to keep pressure fairly high so the tires can resist the forces better.
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Old 08-13-2019, 03:39 PM   #35
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Originally Posted by SteveSueMac View Post
I think thatís a great question and would welcome some more input from tire engineers and Andy on the interply shear question - Iím not sure whether they look at this from the same lens.

There seem to be three opinions

1) match the load to the table with some margin (10-25%) for a comfortable ride with enough capacity to not overwhelm the tire

2) go with the max cold pressure on the sidewalk to reduce instance of interply shear (stiffer sidewalls I believe)

3) go with the placard the trailer manufacturer affixed to the trailer. In most cases that was likely the same as #2 above - but in my case, original tires were the GYMs D load and the placard says 65 PSI - I now have the GYEs E loaf where the max is 80psi but my placard still says 65 PSI (obviously).

I think if youíre a tire engineer - your lens might be avoiding shear at all costs and perhaps you de-emphasize smooth ride.

If youíre a dealer known for custom setups - your lens might be most comfortable ride that meets minimum load table requirements and perhaps you de-emphasize interply shear.

You might bring a different lens to the table - I donít know that thereís a single right or wrong, just different reasons to chose one approach vs another.
I think thereís a lot of merit in #1 and #2. As for #3 I think AS just copied the max sidewall pressure on the tires the installed on the trailer. I donít think they applied any science to it. My trailer (2017 23D) has 65lbs listed on the placard .... it came with GYMís. The 2018 23CB (new designation for the 23D) I saw at a show had a placard listing 80 lbs. Same GVWR. why? It had GYEís mounted.

I think a practical approach is start high with pressure (sidewall max if you want) and reduce until you have no issues with rivets, etc...while not going below that recommended by the load tables.

I run my GYEís at ~75 psi, which is way over what I need for a 6000 lb GVWR trailer. Donít have any vibration issues. I do have Centramatics installed, however.
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Old 08-13-2019, 06:10 PM   #36
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If the manufacturer puts out a trailer that beats itself to pieces with tire pressure that they recommend, they’ve failed. The tires and suspension should work together to absorb road bumps etc. and still give a reasonable ride quality. I have GYM’s on my TT but do have GYE’s E rated on a horse trailer with a GVWR of 7k and a empty GVW of just under 3k. At 55psi the empty horse trailer will bounce all over the place, at 80psi it doesn’t. Even at such a low weight I still get nearly a full width contact patch with the GYE’s. The advantages of the higher pressure is less rolling resistance, less heat, and higher cornering stiffness with full contact patch which = a higher resistance to sway.
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Old 08-15-2019, 12:06 PM   #37
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I run the endurance tires at 62 psi for my 25í forward bed.The right front tire was running about 6to7 degrees hotter so I added 3to 4 psi and now the temperatures are similar.Attachment 348051

If you have a tire position running higher pressure and you have confirmed this is not relaated to external heat (Sun) than that is an indication of one end of the axle supporting more than 50% of the axle weight.


Pressure changes about 2% for each 10F so you can do the math.


Sorry but I can't provide a Temperature or Pressure per pound load conversion.
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Old 08-15-2019, 12:42 PM   #38
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Some general observations and comments.
Capri and I are both Tire engineers with significant Forensic Tire Inspection knowledge. Our focus is on getting a better tire life and seeing fewer structural tire failures.
Out there, I would guess there is a pop rivet expert who would ask why the size, number or type of rivet being used is failing at a high rate. Airplanes are riveted together but I don't hear about "poped" rivets in that application.

Very few RV owners know the actual load on their tires. Simply dividing the scale weight by two or 4 doesn't provide the correct answer as it is easy to have one tire position to be hundreds of pounds heavier than another and they only respond to the actual load on that tire and not some mathematical average.

Also, I don't know the loaded weight of each year/size/model TT so can't provide an informed estimate on the inflation that TT should run.
I do know that having a MINIMUM of 10% Reserve Load (tire capacity - actual load) is a new requirement (2017) from RVIA. DOT has no margin requirement ( Zero%). Capri and I are on record of suggesting at least 15% margin. The Interply Shear numbers would suggest that TT need to have a reserve closer to 25% if you want tire life more like a Motorhome gets (5 - 7 years).

Since it is the air pressure that supports the load and not the tire simply going up in Load Range but not changing the inflation will in all probability gain you anything in actual load capacity.

The pressure on the tire sidewall isn't really the "Maximum" you can safely use but is the MINIMUM inflation needed to support the MAXIMUM load that is marked on your tire sidewall. Personally, I do wish the wording on tires was more technically accurate but I can only guess that the lawyers and bureaucrats decided to use "max" in relation to the pressure for some reason.

Here is a question I wish someone could answer. Why are ST tires rated to support 10% to 20% more load than the same size LT tire? In the past, the reason was that ST tires were only rated to 65 mph to compensate for the increased load. Now ST and LT tires have almost the same speed rating but ST still carry the extra load capacity? If tire companies could all simply improve their ST tires almost overnight, why don't they use the same "magic" tire construction to increase the load rating of their LT type tires?

Anyone?
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Old 08-15-2019, 03:18 PM   #39
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Could it be that the GYE's are constructed better than the GYM's, as in heavier with a much more substantial sidewall>I noticed the heavier weight as soon as they were delivered.🤔

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Old 08-15-2019, 04:46 PM   #40
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Answer to TM9 2 posts back.

I already concluded that the ST endurance is calculated in maximum load for the same 65mph as the old ST tires.
In the official european formula the speed for wich the maxload is given AT reference- pressure, is called the reference-speed.

For P- tires above Q speedrated, the referencespeed is also different from maximum speed of tire. S,T,H,V speedcode all have referencespeed of 160kmph/ 99mph.
W max 270kmph, refspeed 190kmph
Y max 300kmph, ref 220kmph.
Converse yourselves to mph.

That Endurance does not give 10psi higher needed for 66 to75mph and 10% reduction of maxload for 75 to 87mph., as old ST gave, is to my conclusions for reason of replacement for the old ST , so more reserve when using the prescribed maximum pressure ( in real referencepressure, wich you may not go over cold nowadays, in earlyer days it was allowed.
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Old Yesterday, 05:05 AM   #41
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A little correction to my former post.
Read back the copy of the official european formula. It gives " reference load-capacity acording to load index" , and " reference inflation pressure" , but the reference-speed , I filled in myself.

But wait 10 years, and TT makers will provide the new made TT's with Endurance tires, with yust enaugh maxload to cover the GAWR +10%, and then the tirefailure- affaire will happen again, though a bit less percentage , because of the prescribed 10% extra .
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Old Yesterday, 06:10 AM   #42
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Airplanes are riveted together but I don't hear about "poped" rivets in that application.

Capri and I are on record of suggesting at least 15% margin. The Interply Shear numbers would suggest that TT need to have a reserve closer to 25%...

If tire companies could all simply improve their ST tires almost overnight, why don't they use the same "magic" tire construction to increase the load rating of their LT type tires?

Anyone?
There are many experimental aircraft that use "pop" rivers and at least one certificated airplane manufacturer uses them -- the Maule series.

The inflation pressures suggested by Andy Thomson do seem to offer those reserves, albeit at pressures well below the maximums molded into sidewalls.

I suspect (1) the stronger sidewalls of the ST tires produce a more harsh ride that the LT tire users wouldn't accept and (2) the LT tires are strong enough.
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