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Old 12-21-2022, 11:47 AM   #61
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Originally Posted by hairyclive View Post
! way to check this is to run the tires over some paper and look at the pattern. If most of the wight seems to be supported at the centre of the the tire the pressure is too high. If most of the weight is supported at the edges the pressure is too low. Since you are running the tires at about 70% load 70% of full allowable pressure would be a good range to start at. If you find the pressure that gives the best contact patch a little wonky to tow, with too much side wobble, increase the pressure to stiffen the sidewalls.

https://kktse.github.io/jekyll/updat...footprint.html

https://www.enginebasics.com/Chassis...t%20Patch.html

Too much pressure will wear the centre of the tread out sooner and will give the trailer a harsher ride causing some extra frame and body flex.

There is lots of info on selecting correct tire pressure to improve handling and tire contact at the online racing sites etc.\

CapriRacer discusses that here:

http://www.barrystiretech.com/inflationpressure.html

And here, relative to your 70% suggestion:

http://www.barrystiretech.com/sttires.html
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Old 12-21-2022, 02:14 PM   #62
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Michelin is not the only tiremaker that gives (mostly 10psi), higher maximum allowed cold pressure, then the reference-pressure, as it is officially named.

On the 2 pictures a Viking tire, in the Continental groop. Enlarge them and see on one 65 psi, and the other behind the maxload-story "maximum inflation pressure" of 519kPa( 75 psi).

Here in Europe, the Continental groop gives often on C-tires ( EUR eqivalent of LT) a 10 psi higher " maximum inflation pressure" then the reference pressure directly given behind the service-descriptions ( loadindex and speedcode)
Other brands in the Continental groop.
Uniroyal ( the rain tire, original Belgium)
Continental
Vanco ( tireseries of Continental)
Barum
Viking
and all give often the 2 pressures in Europe.
But there stil is a lot of confusion created by the tiremakers, using AT for maxcold, maxpress for the referencepressure, etc.
P-tires only give maximum allowed cold pressure of between 44 and 51 psi, and an ocacional XL 60 psi, while the reference-pressure is Standardload 35 or 36psi, and XL 41 or 42psi.

Higher then given maxload, the tiremaker dont support, but to laws of nature, at higher pressure more load still gives no overheating of tirematerial when driving the reference-speed constantly, wich is the goal of pressure determination.

So if you overload the tire, wich you must not do, you better use that higher pressure then behind AT, officially called the reference'pressure, to prevent overheating.
Other solution is drive slower then the reference-speed of mostly 99mph, above Q speedrated.
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Old 12-21-2022, 02:31 PM   #63
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https://www.irv2.com/forums/f87/tires-600628.html
In this topic in post #6 and #10, same size and Michelin agillis Crossclimate, with max 90psi.and the pressure/loadcapacity list maxload given at 80 psi.

Wanted to make pressure loadcapacity list for that topic, but what specifications to use?
LI 115 AT 80 psi ( LT)
LI 118 AT 77psi ( C-tyre)
LI 121 AT 83 psi ( C-tyre)
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Old 12-22-2022, 05:04 AM   #64
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Originally Posted by tjdonahoe View Post
Iím happy with my 235/75/16 E defenders on our 13 31í classic fully loaded 7800#ís on the axlesÖ..I run 68#ís air
I don't think they produce that tire anymore - but WARNING - They do produce a P metric tire in that "size". Do NOT confuse the 2!!
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Old 01-23-2023, 09:21 AM   #65
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Sorry for being late to this party.
1. As a fellow Tire Engineer at a different company, I generally agree and support the posts by Capri
2. A comment on "Max" pressure. The correct way to understand this is to remember that basically if you want more load capacity you need to increase the inflation BUT the pressure on tire sidewalls really should say "XX Psi is the highest cold tire inflation for increased load capacity. Further increase in cold inflation above this number will not result in an increase in load capacity".
3. I do have a basic question. Why do so many people seem to want to run the lowest level of Reserve Load capacity in their tires? The primary reason IMO for the shorter tire life in RV trailer use is High levels of Interply Shear coupled with low levels of Reserve Load capacity. If you investigate you will learn that most passenger cars are running 20% to 30%+ Reserve load. his is a major reason for so few passenger car tire failures. If you want to try and offset the significant increase in Interply Shear in trailer applications you should be running a minimum Reserve Load capacity of 24%.
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Old 01-23-2023, 01:41 PM   #66
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Sorry for being late to this party.

3. I do have a basic question. Why do so many people seem to want to run the lowest level of Reserve Load capacity in their tires? The primary reason IMO for the shorter tire life in RV trailer use is High levels of Interply Shear coupled with low levels of Reserve Load capacity. If you investigate you will learn that most passenger cars are running 20% to 30%+ Reserve load. his is a major reason for so few passenger car tire failures. If you want to try and offset the significant increase in Interply Shear in trailer applications you should be running a minimum Reserve Load capacity of 24%
That question as it pertains to the Airstream community is trailer ride quality.
The harsher the ride, the more potential rivets will pop out. Either externally or internally.

Which means dancing in the narrow window of what is required for the load and not too much to perserve the trailer requires a lot more attention. And some are just not up for that kind of inspection and topping off pressure to get back into the window.

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Old 01-23-2023, 02:09 PM   #67
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I would second Actionís response. I donít want to beat up the trailer and the stuff in it.

I looked at the numbers for our combination to test your statements.

The tow car tires are 95Vs, rated for 1521 lbs. Front GAWR is 2684 lbs. When towing, I probably have 2600 lbs on the front axle, using WDH. Thatís 1300 per tire, or 85% of the rating. I have had no issues with car tires using them this way over many years. I typically buy Michelin or Yokohama.

Perhaps trucks have more reserve capacity with their specified tires.

The trailer is single axle, so I assume interply shear is not a serious issue. Loaded weight on the axle on a two week trip with full water was 3900 - letís call it 1950 per side (although the left probably carries a bit more due to the fridge and water heater, depending on how much heavier stuff is carried toward the right side of the trailer).

Tire capacity at 55 psi is 2270. Load is 86% of rated capacity at 55psi. So far, no problems.

I still think STs are generally lower quality, although the Goodyear Endurance has so far performed very well.
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Old 01-23-2023, 04:31 PM   #68
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Originally Posted by Tireman9 View Post
Why do so many people seem to want to run the lowest level of Reserve Load capacity in their tires?
Maybe they downloaded the "Good Year Inflation Chart" and took it seriously.
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Old 01-23-2023, 06:36 PM   #69
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But, folks, if you are not adding 15% to your actual tire loads, and another 10% if you are using anything other than STs (ie. P-meterc, metric, lt metric or european c tires)....then taking that result to the tire load charts, you are under inflating your trailer tires.
At least that is my understanding.
You should perform the pressure rise test regardless of what pressure you decide on.
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Old 01-23-2023, 07:12 PM   #70
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Maybe they downloaded the "Good Year Inflation Chart" and took it seriously.
Wait a minute Rich.
Are you saying there more to it than just reading the inflation chart?
There is more to it than finding my tire and inflating my tire to what is listed?
I should have a margin too?
And the inflation chart is just a minimum recommendation?

OMG that is so much work!

(OK sarcasm is off now.)

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Old 01-24-2023, 01:17 PM   #71
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Wait a minute Rich.
Are you saying there more to it than just reading the inflation chart?
There is more to it than finding my tire and inflating my tire to what is listed?
I should have a margin too?
And the inflation chart is just a minimum recommendation?

OMG that is so much work!

(OK sarcasm is off now.)

Action



Yes the chart just states the MINIMUM inflation for the stated load. The chart and DOT and AS all assume you load the RV such that each end of an axle carried exactly 50.0% of the total load on an axle.


You don't have to do all this work, measurement and calculations but the "Pay me now (work) or Pay Me later (changing a failed tire)" laws of reality can bite you when you are 50 miles from service location.


Sorry no sarcasm. Just Physics and reality.
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Old 01-24-2023, 08:11 PM   #72
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Maybe they downloaded the "Good Year Inflation Chart" and took it seriously.

WHAT...are you telling me that GY has its OWN "inflation chart".
Cheese and crackers got all muddy I never realized, I thought they were all the same.

BTW...they are.

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Old 01-25-2023, 07:09 AM   #73
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But, folks, if you are not adding 15% to your actual tire loads, and another 10% if you are using anything other than STs (ie. P-meterc, metric, lt metric or european c tires)....then taking that result to the tire load charts, you are under inflating your trailer tires.
At least that is my understanding.
You should perform the pressure rise test regardless of what pressure you decide on.
Its sooner the other way around, ST needs more reserve then an LT-, C- or P-tire ( etzetera)
This because ST is given maxload for 65mph ( yes also the Endurance with N speedrating for 87mph), and all the others for 99mph.
Same size and referencepressure in ST and LT, need same pressure to get the same temperature of tire- material.

The ST tirematerial becomes yust at hot, when driving 65mph, as the other tires driving 99mph, if pressure from the list for that tire is determined for the load on tire. St then gives lower pressure then LT for the same load. Giving ST then the same pressure as the same LT, yust as hot.

Trucktires are a different story, mostly for 75mph the maxload is given.
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Old 01-25-2023, 07:36 AM   #74
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Its sooner the other way around, ST needs more reserve then an LT-, C- or P-tire ( etzetera)

This because ST is given maxload for 65mph ( yes also the Endurance with N speedrating for 87mph), and all the others for 99mph.

Same size and referencepressure in ST and LT, need same pressure to get the same temperature of tire- material.



The ST tirematerial becomes yust at hot, when driving 65mph, as the other tires driving 99mph, if pressure from the list for that tire is determined for the load on tire. St then gives lower pressure then LT for the same load. Giving ST then the same pressure as the same LT, yust as hot.



Trucktires are a different story, mostly for 75mph the maxload is given.
Hmmm, I'd like to have CapriRacer's take on that. I don't recall that info from his website.
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Old 01-25-2023, 08:18 AM   #75
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I believe the extra 10% for non-ST is to help ensure against interplay shear, which the st tire is constructed for. All get the 15% margin, as I said above.
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Old 01-25-2023, 09:03 AM   #76
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Now to the other question in this thread: the 10% factor.

That factor only applies to P type tires used in light truck and trailer usage. It does not apply to LT and ST tires because they both have that factored in per the load tables.
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I believe the extra 10% for non-ST is to help ensure against interplay shear, which the st tire is constructed for. All get the 15% margin, as I said above.
CapriRacer stated in this thread 10% is not for non-ST tires it is only for P-type tires.(added bold)
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Old 01-25-2023, 09:14 AM   #77
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Ok, perhaps my confusion is the statement,
"If you only know the total weight on the tires (but minus the hitch weight):

Divide by the number of tires and add 10% to the value. Then add that 15%."

From "Barry's tire tech" (CapriRacer's site) which covers what probably most of us know from our CAT scale tickets.

BTW, I just drove from Chicago to Phoenix this last week and the pressure rise test dictates a 25% total margin is needed with my LT tires.
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Old 01-25-2023, 09:33 AM   #78
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I believe the extra 10% for non-ST is to help ensure against interplay shear, which the st tire is constructed for. All get the 15% margin, as I said above.
This would be for the P type tire.

I do not believe it is for a LT tire. And the LT tire is constructed that it will give a harsher ride than a P tire. Which is counter to using a lower inflation pressure because of ride quality.

Inflating beyond the minimum of the tire inflation chart or manufacturer recommendation is not a bad thing. At that point, the user is in the area of comfortable ride or not. I keep the tires in my daily driver which is a full sized SUV inflated 4 to 5 PSI greater than the manufacturer recommendations. I mostly travel by myself and really don't notice a different in ride quality.

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Old 01-25-2023, 09:48 AM   #79
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This would be for the P type tire.

I do not believe it is for a LT tire. And the LT tire is constructed that it will give a harsher ride than a P tire. Which is counter to using a lower inflation pressure because of ride quality.

Inflating beyond the minimum of the tire inflation chart or manufacturer recommendation is not a bad thing. At that point, the user is in the area of comfortable ride or not. I keep the tires in my daily driver which is a full sized SUV inflated 4 to 5 PSI greater than the manufacturer recommendations. I mostly travel by myself and really don't notice a different in ride quality.

Action
Well, did you read CapriRacer's site? It is down the page on the "ST tire" section menu. Perhaps Barry can clarify.
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Old 01-25-2023, 10:01 AM   #80
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Originally Posted by Action View Post
This would be for the P type tire.

I do not believe it is for a LT tire. And the LT tire is constructed that it will give a harsher ride than a P tire. Which is counter to using a lower inflation pressure because of ride quality.

Inflating beyond the minimum of the tire inflation chart or manufacturer recommendation is not a bad thing. At that point, the user is in the area of comfortable ride or not. I keep the tires in my daily driver which is a full sized SUV inflated 4 to 5 PSI greater than the manufacturer recommendations. I mostly travel by myself and really don't notice a different in ride quality.

Action
Why would you add pressure to your daily driver? In some email traffic (also mentioned in his website), CapriRacer indicates that as a result of the ford/Firestone issue, most, if not all vehicle manufacturers added a margin to their door placard recommended pressures. Most are north of 15% . Verify this by taking your placard gawr, divide by 2. Then consult the appropriate load chart. You will probably find a pressure lower than placard. And you will find that gawr/2 + the amount of load difference between the chart pressure and placard pressure comes to somewhere between 15 and 20% .
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