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Old 02-04-2018, 09:10 PM   #1
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how is GVWR computed?

I've been wondering about this for a while and I'll ask tonight. does anybody know how a manufacturer calculates the GVWR that they put on the unit? what are the factors and how do they determine the rating? for example, out trailer has a GVWR of slightly over 16,000 lbs. but it has two 7,000 lb axles. so they should only be able to carry 14,000 lbs, leaving 2,000 lbs or more to be carried by the tow vehicle via the pin and hitch. I've seen many threads that say the pin carries 15 to 20 percent of the trailer weight. to compute the GVWR do they take the actual weight as manufactured and apply some formula to it? if this were true say you added had a couple of units, one with queen bed and the other with a king bed and generator. the second one would have a heavier actual weight and if you used a formula based on that it would have a higher GVWR. this does make sense as the axles, tires, and pin box would be the same and I would expect the same GVWR for both units . I'm just wondering how this magic number that we are all supposed to adhere to is calculated. and can you change it by upgrading components. say you replaced the 7,000 lb axles with 8,000 lb axles. does that change the GVWR? just wondering!
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Old 02-04-2018, 09:14 PM   #2
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A chain is as strong as its weakest link. I don’t claim to know what all the links are, but unless the axles are the weakest link, replacing them won’t raise the GVWR.
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Old 02-04-2018, 09:15 PM   #3
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I think you forgot about the tires on the camper
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Old 02-04-2018, 09:37 PM   #4
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Originally Posted by CHICKDOE View Post
I'm just wondering how this magic number that we are all supposed to adhere to is calculated. and can you change it by upgrading components. say you replaced the 7,000 lb axles with 8,000 lb axles. does that change the GVWR? just wondering!
Typically yes- the GVWR is the sum of the axles + calculated pin weight but not always. For instance, my model year 2013 Sabre 36QBOK had a GVWR of 15,825 on 2 x 7,000 lb. axles. Either later in 2013 or early 2014, the GVWR was derated to 14,666 lbs. Another instance, a Grand Design toy hauler I looked at had 3 x 7,000 lb. axles but a GVWR of just 20,000 lbs... not even 21,000 lbs. or 21,000+ lbs. But a Prime Time Spartan toy hauler that I'm hopefully buying has 3 x 7,000 lb. axles and a GVWR of 21,000 lbs.

You can't just upgrade something and change the GVWR. As mentioned above, there's a weakest link and it's hard to know where it is. Only the manufacturer knows .... maybe. And, if they do, they certainly aren't going to spill the beans.

You can upgrade tires, wheels, axles, and suspension and still end up with frame damage (cough) been there, done that (cough).
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Old 02-04-2018, 11:08 PM   #5
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The short answer to your question of can you the owner increase the GVWR is no. Only an alterer approved by the NTHSA can.
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Old 02-04-2018, 11:49 PM   #6
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Ask 33 people get 33 answers.

And some people wonder why others don’t use these numbers to make their decision on what TV or trailer to get...

Unfortunately here isn’t one answer that’ll make everyone happy.

Good luck.
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Old 02-05-2018, 02:55 PM   #7
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The way I have seen the GVWR calculated in heavy construction machines was to design the tractor to a set of parameters (weight rating). The machine would be test or a major component such as the Rollover protection system to the maximum weight. Than the GVWR was backwards calculated to allow grow (increase weight added) of the machine over a period of years. The GVWR would be 20 to 25% under the tested maximum weight allowing for 10 to 15% grow over several years. This way the test cost of the machine could be avoided until a new design iteration was released.

Not what you wanted to hear I bet but the GVWR is a certified weight that the manufacture states for that model and year of manufacturing for that trailer. This is the safest weight with all of the attachments and loading that the unit can support as designed.
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Old 02-06-2018, 12:50 AM   #8
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Originally Posted by CHICKDOE View Post
I've been wondering about this for a while and I'll ask tonight. does anybody know how a manufacturer calculates the GVWR that they put on the unit? what are the factors and how do they determine the rating? for example, out trailer has a GVWR of slightly over 16,000 lbs. but it has two 7,000 lb axles. so they should only be able to carry 14,000 lbs, leaving 2,000 lbs or more to be carried by the tow vehicle via the pin and hitch. I've seen many threads that say the pin carries 15 to 20 percent of the trailer weight. to compute the GVWR do they take the actual weight as manufactured and apply some formula to it? if this were true say you added had a couple of units, one with queen bed and the other with a king bed and generator. the second one would have a heavier actual weight and if you used a formula based on that it would have a higher GVWR. this does make sense as the axles, tires, and pin box would be the same and I would expect the same GVWR for both units . I'm just wondering how this magic number that we are all supposed to adhere to is calculated. and can you change it by upgrading components. say you replaced the 7,000 lb axles with 8,000 lb axles. does that change the GVWR? just wondering!
This is probably done by using a computer model that includes all equipment that must be certified for that total load;; Gross Vehicle Weight Rating (GVWR): is the maximum permissible weight of the trailer when fully loaded for travel. This includes the unloaded vehicle weight, all fluids, cargo, optional equipment and accessories.

To determine the trailer's axle loads, an approved standard is used. This, in part, is how it's applied;; The trailer manufacturer MUST establish a recommended tongue weight and publish it. Then, that weight is subtracted from the trailer’s GVWR. The remainder is then divided by the number of axles to be fitted to the trailer. That sum is the minimum value for the GAWRs of those axles.

Before a trailer can be shipped from the factory to a consumer or dealer it must be certified. The certification label has the GVWR displayed on it. Once it leaves the factory the only way it can be changed is by the vehicle manufacturer or a certified vehicle modifier. So technically, there is nothing an owner can do to increase the trailer's GVWR. However, upgrades provide extra safety features referred to as load capacity reserves. They are, in most cases, desired.
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Old 02-06-2018, 10:30 AM   #9
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I don’t know how the GVWR is calculated but this a example. My Cedar Creek 32RL came equipped with 6,000lb axles and E rated tires. Payload was 2658lbs, GVWR was 14,177lbs. I option in 7,000lb axles and G614 tires. That changed everything, the payload went the 3927lbs and GVWR to 15,750lbs. Same trailer just different axles and tires. But since the rv is not going to be stopped at a dot scale it doesn’t really matter. If I wanted to carry a little more weight I would at least change the tire size
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Old 02-07-2018, 01:26 PM   #10
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I don’t know how the GVWR is calculated but this a example. My Cedar Creek 32RL came equipped with 6,000lb axles and E rated tires. Payload was 2658lbs, GVWR was 14,177lbs. I option in 7,000lb axles and G614 tires. That changed everything, the payload went the 3927lbs and GVWR to 15,750lbs. Same trailer just different axles and tires. But since the rv is not going to be stopped at a dot scale it doesn’t really matter. If I wanted to carry a little more weight I would at least change the tire size
From a safety standpoint, the vehicle manufacturer must set a maximum load capacity for any vehicle they build. They take into consideration all building materials and equipment used and their strengths. Based on their findings for all the materials weakest load bearing qualities the GVWR is established. It’s never advised or recommended to exceed a manufacturer’s GVWR limit without having a certified modifier’s approval and certification to do so. So, the options you have added for your safety do not extend the values of the vehicle’s GVWR or any of it’s GAWR values. What you did was provide your vehicle with some nice LOAD CAPACITY RESERVES.
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Old 02-07-2018, 04:12 PM   #11
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From a safety standpoint, the vehicle manufacturer must set a maximum load capacity for any vehicle they build. They take into consideration all building materials and equipment used and their strengths. Based on their findings for all the materials weakest load bearing qualities the GVWR is established. It’s never advised or recommended to exceed a manufacturer’s GVWR limit without having a certified modifier’s approval and certification to do so. So, the options you have added for your safety do not extend the values of the vehicle’s GVWR or any of it’s GAWR values. What you did was provide your vehicle with some nice LOAD CAPACITY RESERVES.


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Old 02-07-2018, 11:28 PM   #12
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I don't see anything wrong with that labeling. If the manufacturer offered optional equipment that required any label changes they have displayed it properly. They can do that prior to first sale. That certification lists the GVWR as 15,750#. The CCC label totals out less than GVWR as it should. The axles and tongue weights are above GVWR. The tires are above GAWR. There is no problem. Your limiting factor is the GVWR. Everything above that is referred to as load capacity reserves.
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Old 02-07-2018, 11:33 PM   #13
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An advanced engineering algorithm


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