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Non engineer here. I'm interested in a conservative calculation of max trailer load for existing frame members for a truck I have. The truck is rated at ~8800lbs towing when stock and with a suspension and brake upgrade, it can tow 10k lbs. However, a modification was done to the rear before I owned it. The previous owner added a 1600lb capacity liftgate and flatbed to it. The photos show the mod - a 5mm flat steel plate drops down on both sides of the rear truck frame by 8 inches to which a 4x6 inch rectangular tube (2mm thick) is welded. The span of the tube between the drops is 35 in. All joins are welds. Assuming the steel is just a36.

  1. How can I calculate max load?
  2. Which, steel members, if any, are limiting capacity here?

The GVWR is 11,000 lb. The net payload is 3,437 lb. The completed vehicle weight is 7,063 lbs. It was done in the year 2000 and welds haven't had any issue since as far as I know.

From the factory the front axle rating is 4100 lb and the rear axle rating is 8250 lb.

I calculated my best way of doing this - determine the force on the rectangle channel at the max load of the 745lb liftgate when it's fully extended out and then determine what that load would Translate to with the liftgate retracted and a 10-in hitch receiver and hitch. My result is too hopeful for me at 5970 lbs applied downward at the hitch. Following a 10% rule that would translate to 59700lbs trailer weight. However my math ONLY works when stationary and totally ignores a lot - terrible! I'd like to see how you all would do things! Thx!

( (44"×1,600lbs)− ((33"÷2")×745lbs) )÷10"

side view

rear view

static view

Edit:

Reading around, I found a paper which says trailer frames should withstand 2.7g and have a safety factor of 3.0. Assuming I can just treat the hitch as an extention of trailer frame and assuming a tongue weight of 1k lbs, Is it correct to say the truck frame needs to withstand 1k x 2.7 x 3.0? Is it also correct to say the liftgate already has a safety factor built in? both of these numbers are much higher than normal according to the source. Ref: https://www.eng-tips.com/viewthread.cfm?qid=261920

Joe B
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  • A class five receiver (2.5 x 2.5 inches) is rated for 1800# of tongue weight, up to 20,000# GTW. So that should be all you really all you need to worry about. Having said that, I used to routinely haul 20,000# of concrete payload on a three axle, 28' trailer (with no brakes) using an '87 half ton Chevy with a 127hp 302. I did this in the Colorado Rockies. So safety factors should be considered carefully wrt intended use and users. – Phil Sweet Jul 05 '22 at 23:14
  • Towing capability is determined more by rear axel, transmission and engine than hitch strength , if that is the question. – blacksmith37 Jul 05 '22 at 23:15
  • @blacksmith37 yes to an extent, but in this case the frame was modified and the hitch would be attached to the modification. It's a simple mod, but I am concerned that some of the heavier loads would exceed the limits of these members. – Joe B Jul 06 '22 at 02:08
  • I would look up its KERB WEIGHT from manufacturer. This is the term used for pulling caravans and trailers. – Rhodie Aug 17 '22 at 23:44

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