On Thursday, July 28, 2016 at 2:59:28 PM UTC-6, Andrzej Kobus wrote:
On Wednesday, July 20, 2016 at 10:31:52 PM UTC-4, Dave Springford wrote:
Hi Tom,

Very doubtful that the nut came off from vibration. Nylock nuts are used and they are not prone to this. My trailer is towed behind a motorhome with a large axle to tow ball distance resulting a bending moment on the tongue and shear load on the (front) bolt (in particular).

After my post, another motorhome owner checked his trailer and found that the same bolt on his trailer was broken - shear plane through the threads, bad design - but fortunately the rest of the bolt was still in the tongue so he was able to take pictures of it. It was certainly a failure due to shear.

I question why AL-KO switched from 1000 MPa bolts to 800 MPa bolts at some point. I checked several other older trailers than mine and they all had 10.9 bolts. Mine had 8.8, as did the other trailer that failed.

I just checked my new trailer (delivered this year), it has bolts 10.9.

To follow up on my earlier post. My trailer, as many know, is towed behind my motor home. Its on a 6 foot tow bar which goes under my motorcycle lift which then attaches to the bottom of the lift. This whole system does allow for movement as its subject to bouncing/flexing around when traveling. But I never have seen the trailer sway thru my rear view camera.
I again checked my bolts(10.9) and their is a washer then self locking nut installed. I did take my torque breaker bar to check. The head of the 2 bolts are really tight( I never went for a max torque reading), but both nuts had less than 25 pounds of torque applied.
I did tighten both nuts down and it appears the nylon used in the nut has worn allowing them to loosen(become looser or maybe that is what they were originally torqued to?).
My trailer is a 2006 and has around 80,000 tow miles on it. Have gotten new bolts,washers, nuts to carry with me.

Thanks again to Dave for bringing this to our attention.

Best. #711.