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Old 02-06-2016, 01:04 PM   #211
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Old 02-07-2016, 06:36 AM   #212
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Originally Posted by Flyingsilver View Post
Greetings,,

I have been reading this thread from the beginning, over and over and I have to admit that the longer this thread goes the more confusing it gets. I thought it has been mentioned before in a previous posting that this thread was exclusively about the performance of these tires on a trailer, air pressure, loads, derating the max load , and so on. But here, quote, " the max load is 2183# (or 1985# depending on what vehicle is on)
So this tire gets derated on a SUV use? or a trailer use?, please explain
P type tires are load derated by a factor of 1.1 when used in certain types of service - trucks, vans, (although the popular interpretation doesn't include minivans), multipurpose vehicles (which includes SUV's, but doesn't seem to include crossovers!), and trailers.

If a P type tire is used in passenger car service, it is NOT derated.

LT type tires include the factor in the calculations - as does ST tires. It only applies to P type tires.

Why? I've tried to get to the source and have traced it back as far as 1968 - about the time the US federal regulations on tires began. I do not think this has anything to do with the federal regulations, only that whatever thinking was taking place occurred in the same timeframe and was adopted as a regulation. But I can only find the rule, not the justification behind the rule.

My best guess is that the types of vehicles mentioned tend to have stiffer suspensions, and to be driven on ...... ah, let's call it less smooth roads, than passenger cars. Both of those are factors when determining the load carrying capacity of a tire.

Quote:
Originally Posted by SteveSueMac View Post
Thanks. Very helpful. I see you didn't mention anything about "interply shear" we hear a lot about with double (or triple) axle trailers. In your view - does max pressure (50 psi in this case) offer an advantage there?


I may try mine at 45 PSI this season and see what differences that makes.

Thanks again!
Interply shear is covered by my comment on failures.

A word of caution: When talking about fatigue type tire failures - and tread separations are the most common type - there are very few ways to determine that any particular thing makes things better or worse. Ya' see, we can't actually measure how much fatigue life is left in a tire. We can draw the inference that operating temperature is an indication - and I'm sure that the change in rubber properties is a factor here! If we measure the load on a tire, we can draw an inference there as well.

However, there are some early warning signs that we all should be on the alert for:

a) Bulges: Bulges are a sign of things within a tire failing.

b) Cracks: Cracks are a sign of either too much stress or deterioration of the rubber - both of which are bad things.
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Old 02-07-2016, 10:42 AM   #213
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Derating of the "P" tire falls under this regulation:

49 CFR 571.110

Tire selection and rims and motor home/recreation vehicle trailer load carrying capacity information for motor vehicles with a GVWR of 4,536 kilograms (10,000 pounds) or less.

S4.2.2.1

Except as provided in S4.2.2.2, the sum of the maximum load ratings of the tires fitted to an axle shall not be less than the GAWR of the axle system as specified on the vehicle's certification label required by 49 CFR part 567. If the certification label shows more than one GAWR for the axle system, the sum shall be not less than the GAWR corresponding to the size designation of the tires fitted to the axle.

S4.2.2.2

When passenger car tires are installed on an MPV, truck, bus, or trailer, each tire's load rating is reduced by dividing it by 1.10 before determining, under S4.2.2.1, the sum of the maximum load ratings of the tires fitted to an axle.

S4.2.2.3

(a) For vehicles, except trailers with no designated seating positions, equipped with passenger car tires, the vehicle normal load on the tire shall be no greater than 94 percent of the derated load rating at the vehicle manufacturer's recommended cold inflation pressure for that tire.

(b) For vehicles, except trailers with no designated seating positions, equipped with LT tires, the vehicle normal load on the tire shall be no greater than 94 percent of the load rating at the vehicle manufacturer's recommended cold inflation pressure for that tire.
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Old 02-07-2016, 11:00 AM   #214
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switz,
I'm not a lawyer, but I don't think this CFR applies to me when I select tires for my travel trailer. I believe this is for vehicle manufacturers. I'm not saying this is something I wouldn't/shouldn't do as a good practice, I just don't believe this code applies to me when I'm making tire decisions.
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Old 02-08-2016, 07:18 AM   #215
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switz,
I'm not a lawyer, but I don't think this CFR applies to me when I select tires for my travel trailer. I believe this is for vehicle manufacturers. I'm not saying this is something I wouldn't/shouldn't do as a good practice, I just don't believe this code applies to me when I'm making tire decisions.
Top,

This derating appears in the tire standards as well - meaning it is part of the Physics of Tires, and not just a regulator admonition.
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Old 02-08-2016, 07:43 AM   #216
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I have no issue with following the tire standards from the tire manufacturers. Please post that information from the tire standards book. The CFR that is often copied and pasted here is a regulation that pertains to tire selection and sizing for vehicle manufacturers.
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Old 02-09-2016, 07:05 AM   #217
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Originally Posted by Top View Post
I have no issue with following the tire standards from the tire manufacturers. Please post that information from the tire standards book. The CFR that is often copied and pasted here is a regulation that pertains to tire selection and sizing for vehicle manufacturers.
Unfortunately, I would have to scan a page from the Tire and Rim Association Yearbook, and my printer/scanner isn't seem to want to communicate with my computer.

Sorry.
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Old 02-09-2016, 10:31 AM   #218
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Michelin Defender LTX M/S 235/75-15 109T XL

Why not just let all those who care on this thread buy their own copy?

http://us-tra.org/publications.html
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Old 02-09-2016, 01:11 PM   #219
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Originally Posted by SteveSueMac View Post
Is that a single or double axle? If single - the Michelin you're talking about has to be de-rated 10% for use on a trailer so it basically carries 1985# per tire. If a single axle, you have 3970# of load carrying capacity though most would suggest you want to leave about 10% "headroom" meaning your trailer (if single axle) shouldn't weigh more than about 3600# loaded for camping. If a double axle, you'd have 7840# of carrying capacity or with some headroom, 7200# loaded for camping.

Given that and the weight of your trailer, you should be able to decide whether those Michelins will work for you or not. As an FYI - my double axle 27FB weighs in at about 6000# on the scales and I use the Michelin 15" tires. I'll be starting my 4th year of camping this spring - only trouble was when I scuffed them on a curb and replaced them out of an abundance of caution (with very few miles on them and an assurance they'd be ok...).

Good luck!
Single or double axle is not the determining factor. It is the type vehicle i.e. not being a standard passenger car.

Station Wagons, SUV, P/U, Trailers all require "P" type tires to be de-rated i.e. Load/1.10 which is considered -10% by many.
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Old 02-09-2016, 01:19 PM   #220
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Originally Posted by Bill M. View Post
I think the statement "max load at 50 psi " Is true and accurate. What is also true is that they will carry the same max load at 44 psi. So on a trailer what are the pros and cons to running at 50 versus 44 psi?

You can find the answer if you Google


"Interply Shear RV tire)


or

Pro 50 will decrease likelyhood of tread separation and improve fuel economy
Con 50 may increase likelyhood of impact damage but this is not a condition I see very often of tires that are 70, 75, 80 or 85 series aspect ratio
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Old 02-09-2016, 05:18 PM   #221
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Originally Posted by Tireman9 View Post
Single or double axle is not the determining factor. It is the type vehicle i.e. not being a standard passenger car.

Station Wagons, SUV, P/U, Trailers all require "P" type tires to be de-rated i.e. Load/1.10 which is considered -10% by many.

Yes my language was awkward there. I meant if single axle - the derated load probably doesn't provide enough capacity and certainly not with another 10% headroom on top of that (unless his trailer is sub 3500# loaded for camping) - where a dual axle with those tires would provide 7900# of capacity. What I wrote sounds like there's a difference in derating P tires if single vs dual axle and that wasn't my intent. Thanks for catching that!
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Old 02-10-2016, 02:16 AM   #222
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Quote:
Originally Posted by Tireman9 View Post
Single or double axle is not the determining factor. It is the type vehicle i.e. not being a standard passenger car.

Station Wagons, SUV, P/U, Trailers all require "P" type tires to be de-rated i.e. Load/1.10 which is considered -10% by many.
I have a serious question, what is the difference from a p car or a station wagon or minivan or a trailer, does the tire knows where is mounted? Lets say the p car is a heavier car than a mini van, an old car you use it as a taxi with lots of people and luggage, then what? do you derated or the tire has a beeeppp beeep to let you know... doesn't make any sense to me at all, the van carries flowers for example..of course it is designed to carry a heavier load... however, the car is carrying a heavier load..then what ..
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Old 02-10-2016, 07:38 AM   #223
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Originally Posted by Flyingsilver View Post
I have a serious question, what is the difference from a p car or a station wagon or minivan or a trailer, does the tire knows where is mounted? Lets say the p car is a heavier car than a mini van, an old car you use it as a taxi with lots of people and luggage, then what? do you derated or the tire has a beeeppp beeep to let you know... doesn't make any sense to me at all, the van carries flowers for example..of course it is designed to carry a heavier load... however, the car is carrying a heavier load..then what ..
First, in a manner of speaking a tire does know it is on a different vehicle because of the way it is treated.

If you compare a trailer with a car, the first thing you should notice is that the trailer doesn't power itself. No torque is supplied to the tires to make the vehicle move forward - and the tire can sense that.

And other vehicles have different characteristics that distinguish them: Road surface being driven on, suspension stiffness, etc. - all of which affects the load carrying capacity.

And it isn't the vehicle as much as it is HOW those types of vehicles are used that determines what type of tire is used.

So taking your example of a van carrying flowers - I knew a florist once and they used a van because of its interior size, not load carrying capacity. The tires were not strenuously used.

But that same van could be used to haul heavy objects - and the tires would still be expected to perform.

Tires are fairly robust (boy, I hate that word, but nothing else fits as well!) with regards to operating conditions. You don't have to have the inflation pressure exactly right - you don't have to have the same tire in all 4 positions - you don't have to operate only on smooth roads (and the list goes on). In fact, I can use a generic tire in a lot of different types of service - it's just that the tire won't be the best performer.

Tire manufacturers shouldn't design for the best conditions. They should design for the worst! And those adjustments that are made for differing operating conditions take that into account.

So when the standards say to derate a passenger car tire, they are saying that because the operating conditions are typically different and while the tire is the same, the tire is still supposed to be survive those different conditions.
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Old 02-10-2016, 08:52 AM   #224
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Originally Posted by CapriRacer View Post
First, in a manner of speaking a tire does know it is on a different vehicle because of the way it is treated.
This is starting to get deep...I never knew tires had feelings

I guess that's why they blow...when they feel they are abused not being treated right!
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