Where do GCVWR numbers come from?

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Frank B

Frank B

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And don't tell me the stork brings them. :)  Though I _might_ believe that they get pulled out of a hat.


Is this just up to the manufacturers to choose a number for bragging rights, or is there some independent body that verifies these claims or tests their validity?
 
Its a mfg number based on a lot of factors,  acceleration, braking, handling under load are part of this equasion.
 
donn said:
Its a mfg number based on a lot of factors,  acceleration, braking, handling under load are part of this equasion.
Click to expand...

All of that plus how much load that the frame and axles can tolerate. Too much weight can kink the frame and/or bend the axles. 
 
Frank B said:
"Is this just up to the manufacturers to choose a number for bragging rights, or is there some independent body that verifies these claims or tests their validity?"

I have often wondered about this too. I've never seen anything from anybody on these forums who actually knows exactly how they arrive at the given specifications.

Do the numbers have built in tolerances?  Can you exceed the numbers by 10% and still be well within the safe area?

I know some of the numbers are based on laws such as the 26,000 lb weight rating. Anything over that puts the vehicle in a different class. 

So...how do they arrive at the stated numbers exactly. 
 
The vehicle GCWR is determined and set by the vehicle manufacturer. It can be changed by changing the vehicle axle ratio. For instance the GCWR for my Dodge will go from 22000# to 23500# by changing axle ratio 3.73 to 4.10. However, there is zero change in the vehicle's GVWR. That's because the GCWR is a towing weight limiter and GVWR is strictly a weight limiter.


I don't think you'll find anyone that will officially say weight and towing limits can be safely exceeded. If so, get them to certify it with their signature.
 
This is the standard that the manufacturers are supposedly going by.

https://www.sae.org/standards/content/j2807_201602/


 
Is this just up to the manufacturers to choose a number for bragging rights, or is there some independent body that verifies these claims or tests their validity?
Click to expand...

The SAE J2807 standards defines a standard method of measuring tow capacity and all the major US vehicle manufacturers have signed up to use it on their trucks.  Typically they don't use it on cars and smaller SUVs that are not expected to be routinely used as tow vehicles.

J2807 has an effect on GCWR, since obviously the GCWR won't exceed the vehicle GVWR + tow capacity.  However, the GCWR may well be less. It is determined by the vehicle manufacturers engineers based on a variety of factors which others have already listed.  Surely the vehicle marketing folks lobby for higher bragging numbers, but they have to get engineering to agree.

Be aware that towing capacity isn't restricted to RV trailers and most testing is done with low utility trailers that don't have much wind resistance and don't tend to sway as much as RVs. They aren't as long or top heavy either.  Most vehicles can tow a lot more weight in a utility trailer than as a big boxy RV.
 
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