Al-Ko Trailer Tongue failure

To answer Ramy's question, I have had two trailers who's hitch would rotate 360 degrees......... Never any problem. It can't rotate off the ball and is limited to about 30 degrees by the hitch sides.
Now, I'd like to ask (and answer) why these hitch bolts are failing. AN bolts are good for 125,000 psi, but these are common hardware bolts, so let's say they are good for 100,000 psi tensil strength. A half inch bolt cross section would have .19635 area X 100,000 = 19,635 t/s. Say 50% for shear strength, no let's say 40% because the shear load in on the threads = 7854 psi shear strength...........we can expect the bolt to shear off when 7854 pounds of force is applied.
Now, let's take a look at what's happening as we exit a gas station in out motor home with a large over-hang.........tow ball 10 feet behing the rear axle. As the rear wheels approach the curb (low point at edge of road) the rear end of our motor home drops and the trailer tung skid starts dragging across the pavement, but the motor home rear wheels are not yet all the way down to the curb. At this point the a good portion of the weight of the MH is pulling down on the tow ball. Our trailer skid is now acting as a sliding fulcrum and trying to lift the trailer. Most MH weigh a good 10,000 pounds, so let's say half of that is pulling down on the tow ball.
The bolts that have sheared off are located 15" behind the skid plate and the ball is about 10" forward of the skid, so we have a one to one and a half ratio. 5000# X 1.5 ratio = 7500# shear force applied to our half inch bolt that should shear off at 7850#.
I'm sure the engineering types in this group will have fun pointing out the errors in my thought process.
;) JJ

On Sunday, July 10, 2016 at 1:20:11 AM UTC-7, wrote:
To answer Ramy's question, I have had two trailers who's hitch would rotate 360 degrees......... Never any problem. It can't rotate off the ball and is limited to about 30 degrees by the hitch sides.
Now, I'd like to ask (and answer) why these hitch bolts are failing. AN bolts are good for 125,000 psi, but these are common hardware bolts, so let's say they are good for 100,000 psi tensil strength. A half inch bolt cross section would have .19635 area X 100,000 = 19,635 t/s. Say 50% for shear strength, no let's say 40% because the shear load in on the threads = 7854 psi shear strength...........we can expect the bolt to shear off when 7854 pounds of force is applied.
Now, let's take a look at what's happening as we exit a gas station in out motor home with a large over-hang.........tow ball 10 feet behing the rear axle. As the rear wheels approach the curb (low point at edge of road) the rear end of our motor home drops and the trailer tung skid starts dragging across the pavement, but the motor home rear wheels are not yet all the way down to the curb. At this point the a good portion of the weight of the MH is pulling down on the tow ball. Our trailer skid is now acting as a sliding fulcrum and trying to lift the trailer. Most MH weigh a good 10,000 pounds, so let's say half of that is pulling down on the tow ball.
The bolts that have sheared off are located 15" behind the skid plate and the ball is about 10" forward of the skid, so we have a one to one and a half ratio. 5000# X 1.5 ratio = 7500# shear force applied to our half inch bolt that should shear off at 7850#.
I'm sure the engineering types in this group will have fun pointing out the errors in my thought process.
;) JJ

Well, one thing is that the bolts are in double shear and so should be good for twice that.

I'd go more with the fatigue argument. With a large overhang, sometimes the trailer gets into some serious pitch oscillations with large reversing stresses. Not amenable to simple analysis.

Trust me, I'm an engineer:
https://youtu.be/rp8hvyjZWHs