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CWtheMan 12-19-2017 07:29 PM

RV Trailer Tires
 
A synopsis: My version of RV trailer weights, tire & axle fitments simplified.

Although automotive vehicles and RV trailers are governed by the same building regulations and standards the weights may differ. The one undisputed weight is the vehicle’s GVWR. It is the maximum acceptable operating weight specified by it’s manufacturer.

Because axle manufacturers do not manufacturer axles in weight increments that will coincide with a vehicle manufacturer’s needs, the vehicle manufacturer has the authority to set GAWR weights that differ from the axle manufacturer’s certified GAWR. Example; Your trailer’s axles may have a tag on them from their manufacturer certifying their maximum load as 4000#. The trailer manufacturer can derate them to 3700#. It’s commonly done to allow the fitment of tires. In this case it could be they want to use ST215/75R14C tires because of their load capacity and diameter. Bottom line; The vehicle manufacturer’s GAWR values supersedes the axle manufacturers values and are the official certified values for that fitment.

Tires fitted to RV trailer axles must provide a load capacity equal to or greater than the certified GAWR of the axle (s). RV trailer manufacturer’s are required to set a recommended inflation pressure for those tires that is appropriate for their fitment.

Tire and axle fitments for RV trailers differ from automotive vehicle fitments because of the way the regulations and standards are written. Load capacity reserves are not a requirement for RV trailer tires or the axles they are fitted to. Their only requirement is to have load capacities equal to the tasks they are required to perform.

In the automotive industry axles and tires must provide a percentage of load capacity reserves. Their builders can do that with the axles/tire fitments or both. Look on your tow vehicle’s certification label and you will find that when the GAWRs are added together, they exceed GVWR. GVWR is the limiting factor so the axel overage is load capacity reserves. The tires may also have some load capacity reserves but not always. Rear tires may be maxed-out (inflation wise) but the axle’s load capacity reserves prevents the tires from being overloaded.

Tire industry standards for RV trailer tire replacements pretty much supports 100% of the building regulations that are certified. Because the building regulations are providing a minimum safety factor the tire industry standards will not do less. So, replacement tires must have a load capacity equal to or greater than the Original Equipment tires. That’s the way it’s written, but, there are other factors. Tire design. There are antiquated DOT regulations that are still active and are or can be applied at any time. They set a precedent that disallows changing tire designs from the trailer manufacturer’s OE tire design. It’s why it’s called a misapplication and also why almost any reputable tire retailer will refuse to make the swap. There are exceptions, LT tires designated RST (Regional Service Trailer). Any tire designed for low platform trailer service. Those are mostly 17.5” tires. Some European/Asian commercial tires designed for trailer service.

Because a trailer manufacturer uses something other than ST tires on a specific model or series of models does not imply that design can be used on some other model (s). The vehicle manufacturer has the sole responsibility in tire fitments and the tire design for such fitments. If a trailer manufacturer’s normal fitments are ST tires, with an optional design offering, that offering ends when the trailer is certified. Final certification ends before the vehicle is signed over to a consumer/owner. (Under some circumstances an authorized dealer is allowed to change the certification label prior to first sale, but only with the consent of the trailer manufacturer).

Today’s RV trailers, from the smallest to the largest, have more storage space than ever before. They can easily be overloaded long before all the storage spaces are used-up. It takes a wise and dedicated owner to be diligent enough to manage their trailer’s loads and how they are balanced.

Going back to those ST215/75R14C tires. They provide 3740# of load capacity for a 3700# axle. RV trailer axles hardly ever have a balanced load, so it’s very possible that one of those tires is going to be overloaded all the time. Only a savvy owner that often goes to a set of scales for load balancing can come close to being successful at keeping those tires from failing, early. (Early being a couple of years).

Remember, brochures are not official documents. They may have accurate information but not 100% of the time. Information in the vehicle owner’s manual is official. It was mandated by NHTSA. It should always be your guide over and above what is said in writings such as this.

I know all about what so and so said. “What is good for the goose is not always good for the gander.”

Reference material:
FMVSS
CFR 49 part 567
US Tire Manufacturing Association

Hodgy 12-19-2017 07:48 PM

.

Wow !

I did not know there were so many words about tires . . . . .

.

Sherwolfe 12-19-2017 08:41 PM

Well, this should become interesting :popcorn:

rhagfo 12-19-2017 09:20 PM

Quote:

Originally Posted by Sherwolfe (Post 267030)
Well, this should become interesting :popcorn:

I agree, my 5er came from the factory with LT tires 235/85-16Es, 3,042# ea capacity 12,168# total on a 32 5er with a GVWR of 12,360# that is plenty in my book. The originals were 12 years old when I replaced them last summer.

Tinner12002 12-20-2017 03:09 AM

Mine came with axles rated at 2K more than the GVWR and with tires rated at 5480# more than GVWR, so figure that one out...not complaining though!

notanlines 12-20-2017 03:11 AM

Okay, I'll start it.
"The originals were 12 years old when I replaced them last summer."
I'm not sure I would have admitted this.

rhagfo 12-20-2017 05:09 AM

Quote:

Originally Posted by notanlines (Post 267035)
Okay, I'll start it.
"The originals were 12 years old when I replaced them last summer."
I'm not sure I would have admitted this.

Why not? They were LT's still in very good shape, always covered and the last couple of years most of our trips were about 150 miles.
I see so many post on here of those running ST tire get a blowout thinking back to maybe hitting a curb, or a bad set of tracks. Heck these tires did this many times and still preformed great.
So many on this site are so happy to get a set of ST tires for their 5er and pay less then $400 for a set of four, then bitch like crazy when one blows after a year of service. I paid over twice that for a quality set of LT same size and weight capacity as the OEM LT tires.
I see all this talk of the ST being "Special" to deal with the scuffing when turning. Well I don't buy that, had a neighbor that ran log truck for years, ran the same casings, be it be steer, drive, or trailer position. When drivers got worn a bit and still had ok tread depth, but traction was down, those were moved to trailer position, then when tread was worn out, they were capped. Could be capped for either drivers or rib tread for trailer use.

Just my personal experience.

CWtheMan 12-20-2017 09:57 AM

Quote:

Originally Posted by Tinner12002 (Post 267034)
Mine came with axles rated at 2K more than the GVWR and with tires rated at 5480# more than GVWR, so figure that one out...not complaining though!

Keystone has always been better with fitments for triple axle trailers. There is probably money involved with your trailer's axle, wheel & tire fitments. Those ST tires are much less expensive than the 16" LRG RST tires or the 17.5" tires used for such fitments before the strictly Asian 16" LRG tires hit the market.

Does your trailer's certification label list your axles as 7000# GAWR? Just curious. Usually RV trailer builders do not want to invite consumers to overload the trailer's GVWR.

IRV2 12-20-2017 10:16 AM

My tires are rated 2540# each single. My axles are 4400# and my GVWR is 10,000# seems like I need more axle:ermm:

CWtheMan 12-20-2017 10:46 AM

Quote:

Originally Posted by rhagfo (Post 267042)
Why not? They were LT's still in very good shape, always covered and the last couple of years most of our trips were about 150 miles.
I see so many post on here of those running ST tire get a blowout thinking back to maybe hitting a curb, or a bad set of tracks. Heck these tires did this many times and still preformed great.
So many on this site are so happy to get a set of ST tires for their 5er and pay less then $400 for a set of four, then bitch like crazy when one blows after a year of service. I paid over twice that for a quality set of LT same size and weight capacity as the OEM LT tires.
I see all this talk of the ST being "Special" to deal with the scuffing when turning. Well I don't buy that, had a neighbor that ran log truck for years, ran the same casings, be it be steer, drive, or trailer position. When drivers got worn a bit and still had ok tread depth, but traction was down, those were moved to trailer position, then when tread was worn out, they were capped. Could be capped for either drivers or rib tread for trailer use.

Just my personal experience.

Going back in Keystone's tire fitment history you will find that in model years 2005 & 2006 they fitted all 6000# axles with LT235/85R16E tires. Mostly Missions from China and Uniroyal's by Michelin. As the off shore ST tire manufacturer's adapted to the USA market, more sizes and increased load capacity ST tires were built and saturated the RV trailer market. Keystone, like most all other RV trailer manufacturer's took advantage of the cost difference and started using all ST tires in 2007.

This is a fact. No tire manufacturer of any design would ever recommend their tires to be used beyond 10 years. Also, most recommend professional annual inspections from the 5 year point forward.

Because Keystone fitted LT tires to a lot of their models does not mean individuals can do the same. You cannot find a LT tire with the load capacity of a like sized ST tire. Example, LT235/85R16E has a maximum load capacity of 3042# @ 80 PSI. ST235/85R16E has a maximum load capacity of 3640# @ 80 PSI. Since 2007 the prefix ST & LT are part of the tire's size.

The wording in the tire fitment standard the trailer builders must abide is such that it avoids monetary or brand consideration. It goes like this: "The size designation and the recommended cold inflation pressure for those tires such that the sum of the load ratings of the tires on each axle is appropriate for the GAWR." Remember, all the builder has to do is meet the minimum safety standard, the vehicle certified GAWR (s). If you get more than that, consider it a gift of load capacity reserves.

Once understanding how and why particular words are used in regulations/standards written in government documents it becomes clear why in the paragraph above "appropriate" also means that the recommended inflation pressures are also minimal. That is supported by the tire industry as they say to never use less tire inflation pressure than what is recommended on the tire placard. REF: US Tire Manufacturer's Association.


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