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Author Topic: GVWR Rating  (Read 532 times)

ysidive

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GVWR Rating
« on: December 12, 2017, 05:36:46 AM »
 So this is my dilemna... My trailer has a GVWR rating of 7600# as per the sticker on the trailer... However I would like to know how this is possible when the axles are 3500 axles and the tires are 205/75R14... The axles total 7000 and the tires are rated at about1800# each... Is there a formula that I am not aware of to enable the GVWR to exceed the axle and tire capacity because they are in tandem?   Dose tongue weight decrease the Carrying capacity of the axles and tires?    Just does not make sense that the GVWR would exceed the total axle capacity 3500 x 2  = 7000... Can someone explain this to me...Seem that maybe this is why so many people have complained about tire problems....I am thinking about going to 215/75R14D rated to increase the tire capacity to 2040# each... Unfortunately this does not change the axle rating... Any answers to this dilemna would be appreciated..
« Last Edit: December 12, 2017, 06:19:14 AM by ysidive »

Ernie n Tara

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Re: GVWR Rating
« Reply #1 on: December 12, 2017, 08:13:51 AM »
Ten percent of the gross weight is borne by the tow vehicle so you sneak in under the limit.

Ernie
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glen54737

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Re: GVWR Rating
« Reply #2 on: December 12, 2017, 08:14:11 AM »
I think they are meaning for the extra to be supported by the hitch and tow vehicle.
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Glen,Nene
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massspike

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Re: GVWR Rating
« Reply #3 on: December 12, 2017, 08:45:58 AM »
I think you have a legitimate issue. My 7000lb TT has two (2) 3500lb axles and I assumed that they were the limiting factor on the TT's GVWR. My bil had a TT with insufficient axles (combined rating below the GVWR of the TT) and when loaded you could see the axles bend under the strain (after an argument with the dealer and manufacturer they agreed to upgrade at cost -- I think it was $1000).

The argument that the excess weight will be borne by the TV or jack does not account for the dynamics of actually towing where the full weight can/will transfer (temporarily) to the TT's axles. 

Gary RV_Wizard

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Re: GVWR Rating
« Reply #4 on: December 12, 2017, 08:54:25 AM »
You can argue the wisdom of it all you want, but the federal safety regs allow the trailer manufacturer to assume that 10% of the GVWR is carried on the ball hitch. If it's a 5W, he is allowed 20%.

Is a max-loaded axle more susceptible to bending on a pothole? Sure. The trailer brakes will also be at their max limit, so stopping distance will be greater as well. And if the tire rating is barely up to the axle GAWR, they are more likley to have early life failures as well.

Better quality (read: more expensive) rigs will generally provide at least a bit more than the bare minimum components needed for the GVWR, but it's not a guarantee. Check before buying and make your own judgement.
Gary
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Memtb

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Re: GVWR Rating
« Reply #5 on: December 12, 2017, 09:35:59 AM »
    Gary.....you beat me to the punchline. I still contend that there should be more stringent regulations in RV construction!  memtb
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Gary RV_Wizard

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Re: GVWR Rating
« Reply #6 on: December 12, 2017, 10:41:19 AM »
Quote
I still contend that there should be more stringent regulations in RV construction!

These are the "more stringent regulations"!  Up until about 2008 there were hardly any rules on trailers.

It boils down to skimping on cost and RVers share the blame for that. RV manufacturers know that price is a huge factor in most every RV purchase decision, regardless of all the talk about quality. Most prospective buyers care more about the size of the tv screen and the paint job than  axle ratings and brakes.
Gary
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Memtb

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Re: GVWR Rating
« Reply #7 on: December 12, 2017, 11:02:10 AM »
    Gary, Again....You are “spot-on”!
Todd and Marianne
Home Base: Winchester, Wy.
Miniature Schnauzers - Sundai, Nellie and Maggie Mae
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2007 Bigfoot Class C

ysidive

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Re: GVWR Rating
« Reply #8 on: December 12, 2017, 12:03:38 PM »
Thanks for all the input and answers from everyone.... It still seems to me that assuming that the ball hitch carriers 10% does not take weight off the tires?  In this case the tires are rated at around 1800 each.. Just barely marginal... Guess I need to upgrade size and carrying capacity to build in a safety factor.......

sadixon49

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Re: GVWR Rating
« Reply #9 on: December 12, 2017, 02:18:04 PM »
Thanks for all the input and answers from everyone.... It still seems to me that assuming that the ball hitch carriers 10% does not take weight off the tires?  In this case the tires are rated at around 1800 each.. Just barely marginal... Guess I need to upgrade size and carrying capacity to build in a safety factor.......

Of course it takes weight off of the tires. 7600# GVW - 10%, 760# = 6840# on the axles and tires which are rated for 7000# and 7200# respectively.
steve
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FastEagle

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Re: GVWR Rating
« Reply #10 on: December 12, 2017, 05:45:48 PM »
Gross Vehicle Weight Rating (GVWR): Is the maximum permissible weight of the vehicle or trailer when fully loaded for travel. This includes the unloaded vehicle weight, all fluids, cargo, optional equipment and accessories.

It’s the trailer manufacturer’s target weight for each individual trailer they build.

I’m just writing about RV trailers here. The National Highway Transportation Safety Administration (NHTSA) has put together a package of standards that all RV trailer manufacturer’s MUST comply with. They are the Federal Motor Vehicle Safety Standards (FMVSS). Those standards are written to insure that all RV trailers are equipped with safety items that meet the minimum safety standard for a fully loaded RV trailer. The trailer MUST be certified in accordance with the guidelines outlined in the Code of Federal Regulations (CFR 49 part 567). All RV trailers will have a certification placard/label.

After the target is set the trailer manufacturer has full authority to set each GAWR. That means they can take a certified 7000# axle and derate it to something lower, say 6840# and certify that figure to the trailer by putting it on the certification label. After doing that they can fit two 3420# tires to that axle with a minimum inflation pressure of 80 PSI and satisfy the minimum requirement for the 6840# certified axles. (Unlike automotive vehicles, RV trailer tires are not required to provide any reserve load capacity via inflation).

Within the standards there is a paragraph that directs the trailer manufacturer to set each individual tire’s recommended inflation pressure to a PSI that is appropriate for the fitment. You will see a lot of posts about that subject. The bottom line is the recommended tire inflation pressures for original equipment tires are the minimum requirement. Why? Because the vehicle manufacturer said it’s appropriate and certified it as being so.

GVWR is set. GAWRs MUST support GVWR. It is simply put into the standard sort of like this. Total GAWR when added to the trailer manufacturer’s recommended (published) hitch/pin weight MUST equal or exceed GVWR. What ever weight left in limbo becomes the trailer’s cargo capacity. It will be found on another placard inside the trailer.

From an analyst point of view; I’ve personally researched many different trailer manufacturer specs and compared their recommended hitch/pin weights against GVWR. In every case the percentages were close to minimum but not below it. As a full time fifth wheel hauler I struggled with hitch weights more than once at the scales. If you’re not over GVWR it can be worked-out by balancing moveable cargo. Sometimes we just had to send a big box of “things” to our son who would add it to our storage unit. Never thought much about individual wheel position weights until owning a big heavy fiver. You just got to know where your storage tanks are located and never forget to manage their load. A single tire 300-500# overweight will bite you when least expected.
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Gary RV_Wizard

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Re: GVWR Rating
« Reply #11 on: December 12, 2017, 06:22:11 PM »
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Guess I need to upgrade size and carrying capacity to build in a safety factor......

Well, the axle manufacturer built some margin into his GAWR numbers, since he doesn't really know what will be built on them or how the owner/driver will use/abuse them. The RV maker may derate them as Fast Eagle described, but I think in most cases he just chooses an axle set that meets the minimum requirement for the trailer he wants to build. The axle companies offer a wide variety of axle ratings to meet all needs, though some of them may be over-built while others are merely adequate.

It's not real expensive to upgrade axles if you like, but adding the matching tires can get pricey and bigger tires may need larger wheels too. Depending on what you now have, the whole package can add up to a nice sum.

If the trailer has a large CCC, you may not be loaded anywhere near GVWR, but those with skimpy CCC are also most likley to be traveling at max load.
Gary
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FastEagle

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Re: GVWR Rating
« Reply #12 on: December 12, 2017, 10:36:08 PM »
Well, the axle manufacturer built some margin into his GAWR numbers, since he doesn't really know what will be built on them or how the owner/driver will use/abuse them. The RV maker may derate them as Fast Eagle described, but I think in most cases he just chooses an axle set that meets the minimum requirement for the trailer he wants to build. The axle companies offer a wide variety of axle ratings to meet all needs, though some of them may be over-built while others are merely adequate.

It's not real expensive to upgrade axles if you like, but adding the matching tires can get pricey and bigger tires may need larger wheels too. Depending on what you now have, the whole package can add up to a nice sum.

If the trailer has a large CCC, you may not be loaded anywhere near GVWR, but those with skimpy CCC are also most likley to be traveling at max load.

Lots of people never look at their trailer's certification label. Here is a picture I took of one at a RV show some time ago. The top label is the tire label and the bottom is the certification label. Dexter does not make 6750# axles. Those were derated from the installed 7000# axles.

Certification labels for RV trailers are located on the left side, forward external section, of the trailer. Regardless of what the axle manufacturer has listed on the individual axle, the vehicle manufacturer's certified GAWR is recognized as being the official limit.  All tire & wheel fitments for RV trailers must have a load capacity that will support the vehicle manufacturer's GAWRs,(MINIMUM!).  Upgrading axles does in no way extend the certified GAWRs. Only the vehicle manufacturer or a certified vehicle modifier have the authority to change a vehicle's GVWR or any of the GAWRs.   

http://www.irv2.com/photopost/showfull.php?photo=20840
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Gary RV_Wizard

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Re: GVWR Rating
« Reply #13 on: December 13, 2017, 08:38:49 AM »
Quote
Upgrading axles does in no way extend the certified GAWRs. Only the vehicle manufacturer or a certified vehicle modifier have the authority to change a vehicle's GVWR or any of the GAWRs. 

All true but not very relevant for a private vehicle. We are discussing an owner upgrade to reduce the risk of axle & tire failure on the highway, not an attempt to re-certify for heavier loads or commercial use.

Axle manufacturers also de-rate their components to meet their customers needs. These days it is cheaper for them to manufacture a few sizes and adjust the ratings than to actually produce an infinite variety of axles with different components.  If the trailer frame manufacturer wants 6700 lb instead of 7000, they just slap a 6700# label on the 7000 lb axle when it comes off their assembly line.
« Last Edit: December 13, 2017, 08:46:59 AM by Gary RV_Wizard »
Gary
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FastEagle

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Re: GVWR Rating
« Reply #14 on: December 13, 2017, 01:34:24 PM »
If the trailer frame manufacturer wants 6700 lb instead of 7000, they just slap a 6700# label on the 7000 lb axle when it comes off their assembly line.

And that's my bottom line. Axle manufacturers build to their own specs. They do not set GAWRs. That's the sole responsibility of the trailer manufacturer. Most trailer owners don't pay attention to that. They look at the label on the axle - maybe 7000# - and assume it's okay to load them to that weight, whereas, the trailer's axle limit is always the GAWR  - say 6700# - listed on the certification label which is often different from the axle manufacturer's load specs.

As an owner you can fool around with any of the limiting factors or weak link items. However, it is never recommended to use less than the minimum. I did not, in this thread, mention tire industry standards. Most want to do it their own way, right or wrong. Because industry standards are explained in a sort of "do it this way" manner, galls a percentage of readers.

Trailers are built with limitations. When the limit is GVWR or GAWR it's the limit, period. But, isn't there a fudge factor? What is it? Why limit something and than publish a fudge factor? They don't. It's a fictitious factor that differs in value by word of mouth. Engineer #1 says 10% and number 2 says 15%. Cut the difference and have a ball. One of the safety items it's doesn't work with is tires.   

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FastEagle

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Re: GVWR Rating
« Reply #15 on: December 13, 2017, 01:51:14 PM »
All true but not very relevant for a private vehicle. We are discussing an owner upgrade to reduce the risk of axle & tire failure on the highway, not an attempt to re-certify for heavier loads or commercial use.


Is it used for off road boondocking?
Plus sizing is usually for durability, not strength.
Weak link tire plus sizing is common.
What justifies plus sizing axles?
 
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Gary RV_Wizard

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Re: GVWR Rating
« Reply #16 on: December 14, 2017, 10:43:24 AM »
Quote
Axle manufacturers build to their own specs. They do not set GAWRs.

Yes & No. The axle manufacturer sets what he considers the maximum GAWR for that model of axle, based on his design & testing.  It's the max rating he is willing to accept liability for.  Then the trailer frame builder installs that axle and may choose to set a lower GAWR, though probably not.  Then the RV trailer builder buys the frame and chooses wheels and tire for it, which may lead him to set an even lower GAWR.  He might do that to make cheaper, lower load rating, tires legal under the federal FMVSS regs.  [Technically, he could also set a higher GAWR but then he would have to assume all legal liability since he chose to exceed the axle and frame specs. Too legally risky in most all situations, so very, very rare these days.]

So, the GAWR you see on your trailer do indeed come from the trailer builder rather than the axle maker. The GAWRs may or may not be the same as what the axle manufacturer designed for. Odds are there is a sticker or inscribed code on the axle itself that identifies the original spec from the axle maker, if you wanted to find it and double-check.

With the numbers that ysidrive quoted, an upgraded axle with a higher axle rating would take him from being barely within the component specs at max load to something more comfortable. His OEM axles are at 97.7% of max capacity if he loads his trailer to the 7600 GVWR and balances it perfectly (10%/90%).  A pair of 4000 lb axles and matching tires would be at 85.5%. Is that extra margin of capacity worth the price? Only he can judge that...
« Last Edit: December 14, 2017, 10:50:36 AM by Gary RV_Wizard »
Gary
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FastEagle

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Re: GVWR Rating
« Reply #17 on: December 14, 2017, 09:54:51 PM »
Axle manufacturers build to their own specs. They do not set GAWRs.

Okay, I see my error there. We are discussing two axle ratings. The axle manufacturer's rating and the trailer builder's rating. The trailer builder's certified axle GAWRs depicted on the trailer's certification label supersedes the axle manufacturer's rating. It doesn't matter what the axle rating of the replacements might be, the trailer builder's rating will still be the only official rating.

This is an example of how axle manufacturers label their axles.

https://www.lci1.com/images/support/lipsheet/0019.pdf

This is an actual trailer builders certification label showing axle certification.

http://www.irv2.com/photopost/showfull.php?photo=27431

 
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