mkirsch wrote:
How would you go about calculating how much a receiver should be de-rated with a 14" extension?
Secondarily, does anybody know what a Curt 24" receiver tube is rated for?
http://www.curtmfg.com/part/49240
That's what I used to make my extension. It is cross-drilled to provide 14" of extension, and extends through the receiver box on my truck's Reese Titan.
As This question comes up frequently, I thought I would take some time and share some knowledge for what it is worth.........
mkirsh asked 2 questions.....
1. How much would a 14" extension de-rate his stock receiver - The chart previously posted can answer this. To use the chart, you need to determine what the distance is going to be from the rear mounting bolts/weld of the receiver to the center of the ball. Without knowing exactly how he is going to build this, I am assuming that he will have 14" of extension plus 11" added by his ball mount for a total of 25". Using the chart, if he has a Reese Titan Class V receiver rated at 1200 lbs (1700 with WD), then using the 14" extension, it should be de-rated to 530 lbs. If using a properly set up weight distribution hitch, the rating will increase to 750 lbs.
2. If he used the Curt extension, what will be the revised rating if it is cut down to 14" - Using the info for the Curt 24" to 34" extension which should be the same material and manufacturing process, you can back calculate (reverse engineer) the revised rating. I estimate that it will be Max tongue weight 900 lbs. Using a weight distribution hitch, that rating will increase to 1200 lbs.
One of the problems with estimating the strength of a home made extension, is that there are too many variables to build a standard calculator. You need to know the type of metal, the manufacturing process and the fabrication process to calculate the strength. There are also several assumptions such as the safety factor to use and the calculation of dynamic loading. That is why there is no mention of the extension in the on-line calculator. It is assumed that a manufactured extension with a known rating will be used. mkirsh is smart to use a manufactured extension and cut it down. That way, you can use the manufacturers ratings as a basis as they have control of the type of material and the fabrication.
Lastly, as others have mentioned, you also need to consider the added load a trailer on an extension will add to your rear axle - This is where the previously mentioned on-line calculator can help. It allows you to enter the various measurements that are needed and performs the calculation, giving you a reasonably accurate answer.
Hope this helps. It is not offered as an engineered solution, just my understanding based on many years working in the engineering field.