Thor Forums

[Chance]

Originally Posted by bigben

....cut.... The bad news - still not sure why the failure. ....cut....

The Devil is in the details, and in this case I see a few that came together -- mostly due to poor design and fabrication.

I can't be certain from a picture, but I can give you an engineering opinion of likely cause(s):

1) Brake systems may help stop the car, but a time delay of even fractions of seconds will allow very high forces to be transmitted forward momentarily through tow bar. Based on motorhome being much heavier than toad, the braking force on a sudden stop (regardless of speed -- can be at 10 MPH) could easily reach 3,000 pounds in your case.

2) That force, acting through a lever (moment arm) from hitch ball to frame attachment bolts, will obviously try to rotate the Reese hitch clockwise (as seen from driver's side of motorhome). The greater the drop the worse this becomes. Your 2-inch drop isn't much, and even if it was a lot more drop, the frame extension should have been designed to take it.

3) It's hard to confirm from picture, but it looks to me that one of the biggest issues is the way the extension was made. The 90 degree bend/curve appears too sharp, adding a stress concentration point. The correct way is to have a smooth radius bend similar to a Ford chassis or as can be seen on the Reese hitch on the first picture. If they formed this on a brake (which is possible, maybe even likely) there may be a scoring line that makes stress concentration even higher, and helps the tearing away of the flange to start.

4) The Reeese hitch itself looks short along the frame extension. The shorter it is, the higher the stresses on the frame extension. The shorter length will create additional downward forces at the rear bolts which pulled the flange away.

5) If the frame extension was a little longer beyond the hitch, it would reduce stresses at the point of failure. This alone shouldn't be necessary but contributes to the cumulative loads that increase stress at failure.


Again, it's difficult to say from a picture, but I think all these came together to exceed the frame extension's capacity. Just my 2 cents... which is all it's worth.