Al-Ko Trailer Tongue failure

Per Carlin wrote on 7/8/2016 12:43 AM:
On Friday, July 8, 2016 at 4:31:04 AM UTC+2,
wrote:
Remember that "tensile strength" measures a bolt under a
longitudinal load. What you are seeing in this instance is a
failure under "shear load." Two entirely different situations, and
one of the main reasons that aircraft bolts (AN) are not the same
as "Grade 8" or the European (DIN) equivalent. High tensile
strength bolts often exhibit less than desirable brittleness under
shear load.

I think Mark is close to the root cause of the failing bolts. It is
not the strength of the bolts itself that makes in brake (sounds
funny, I know). In the bolt-configuration in the initial post is the
function of the bolts to hold the Al-Ko tongue tight to the square
bar, the friction between the tongue and the bar makes the strength.
If the friction coefficient is low (fat, grease, dirt) or bolts is
not tight (loosen by vibrations, deformations etc) is the friction
between the tongue/bar low and a shear stress occurs on the bolts.
The bolts are not dimensioned for this and will brake by fatigue.
This is a common problem on Cobra trailers(the nose-cone / spare
wheel holder), the bolts a not tighten enough from the factory and /
or they vibrate loose on the road.

Can we be certain the missing bolt broke? I would expect the bolt hole
to show noticeable elongation or other damage from shearing the bolt,
especially on the side opposite the break (it would stay in the hole for
while, taking all the pounding, until that side also sheared, or the
bolt simply fell out), but the hole looks undamaged in the photo. Can't
see the other hole, however.

There is vertical bolt in the picture (about where Karl's seems to be),
just ahead of the missing bolt: what is it's function?

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "A Guide to Self-Launching Sailplane Operation"

https://sites.google.com/site/motorg...ad-the-guide-1
- "Transponders in Sailplanes - Dec 2014a" also ADS-B, PCAS, Flarm

http://soaringsafety.org/prevention/...anes-2014A.pdf