Here is what our resident engineer, Ron Gratz had to say on this back in 2010: Quote:
---A first year engineering student can figure this one out. The longer draw bar and the farther back you get from the tow vehicle, the more stress you put on the hitch attachyment points and truck frame.
But, a second year engineering student probably would be aware that the OP is using a weight distribution hitch.
So, let's assume the tongue weight is 1000# and the WD bars are loaded so they exert a pitch-axis moment of about 2000# x 30" = 60,000 lb-in on the hitch head. This moment, when viewed from the driver's side, acts in a CCW direction.
Now, assume the WD system reduces the vertical load on the hitch head to 800# and let's assume this force, on the non-extended setup, acts at 10" from the receiver crosstube. This generates a CW moment of 8,000 lb-in. Combining the two moments gives 60,000 CCW plus 8,000 CW = 52,000 lb-in CCW.
Then, let's extend the tow tube so the vertical load acts at a distance of 20" from the crosstube. The vertical load now generates a CW moment of 16,000 lb-in, and combining the moments gives a CCW moment of 44,000 lb-in. The second year engineering student would conclude using the extension, with WD, results in less "stress on the hitch attachyment points and truck frame".
I don't think any one of us really knows why the use of a specific extension does or does not result in a reduced rating.
There are many examples of the "internet myths" mentioned by Larry. Gander Mountain sells an 18" extension and says, "Reduces hitch capacity by one third."
Trailerhitches.com says, "Extenders come in several different lengths from 6" to 48"." And then they state unequivocally, "Hitch extensions are mandatory for some types of towing, but they do have one large disadvantage, they towing capacity is reduced by 50 percent." Are we to assume the 50% applies to everything from 7" to 48"?
Cabela's sells 7", 12", and 18" extensions and state (regardless of length), "Use of this extender will reduce the vehicle hitch's overall tongue weight capacity to 1/3 of original capacity. Example: 900 pound tongue capacity would be reduced to 300 pounds."
So, depending on the source, the alleged capacity reduction when using probably the same hardware, might be 33% or 50% or 67%. Reese, OTOH, justs lists a capacity for the extension and doesn't say anything about reducing the capacity of the receiver.
If you're going to use an extension, there's probably not much point in speculating why there is or is not a recommended rating reduction. And, if the seller says the extension reduces the allowable tongue weight or towing capacity by 1/3 or 1/2 or 2/3, you might want to ask why.
Ron
This particular thread had a lot of discussion about generic hitch extensions, which are a totally different animal that a longer drawbar, in that WD cannot be used with them... But the math for the WD differences is enlightening
Link:WD and longer drawbars explained
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