Comet trailer near catastrophe

On Mar 11, 3:36 pm, raulb wrote:
I am no expert (just a dumb electrician), but this is what has always
bothered me about the German glider trailers, the single pole tongue.

Well I'm a physicist who considers electricity to be
dangerous black magic so I'm strongly disinclined
to think of any electrician who has survived his
choice of profession as being 'dumb'.


Think about it, this single pole has to absorb a lot of movement and
stress:
1. The weight of the trailer and contents (almost a ton) when parked,
often for long periods.
2. The up and down motion of the trailer as you tow it over bumps and
uneven pavement.
3. The sway of the trailer when towed behind your car.
4. The sideways stress on the tongue when turning, especially short
turns (like U-turns).

If I understand the design and use correctly the
weight being supported by the tongue when
properly parked is about the same as when
hitched and is only a fraction of the wieight of
the trailer itself. Most of the weight is carried
to the ground through the suspension, axle.
wheels, and tires--always.

Well, almost always:

http://www.altavista.com/image/resul...ch&kgs=1&kls=0

'Parked' implies a static load, except when loading
or unloading. For Structural metals, which typically
do not creep, the length of time over which a static
load is borne is not relevant to the life expectancy
of the part. It is the process of cycling a load that
causes fatigue, that is why fatigue life is measured
in terms of cycles, not time. By contrast, a
cantelevered wing spar undergoes reverse stress
at least once each flight, supporting the weight
of the wing, in one direction on the ground and
half the weight of the entire aircraft, plus half the
down force of the epenage, in the opposite direction
when airborne (canards excepted of course).

Of course in real flight the load varies constantly
with up/down movement, banking, and bumps
in the air.

Lightweight construction motivates designs with
a relatively load fatigue life, despite a high factor of
safety for static loads, which is one of many good
reasons to avoid flying in turbulence even with a
wing rated for 9 gs or more. But I digress.

The typical ball hitch allows the tongue to pivot
up, down, left, and right, so that the load on
the tongue is predominantly pure tension, or
when slowing down or driving downhill, pure
compression, with minimal bending moment.

As you note, tight turns are probably the most
stressful operation and should be minimized.


Those are tremendous arm moments.
I have never understood why these
trailers have this kind of tongue--or how
they have passed DOT and
State regulations.

They are approved because they work well,
they are most likely to fail during those tight
turns, as when backing, parking or doing a uey,
(low speed manouvers) and also, being ductile
metal, tend to fail gradually enough that the
problem is noticed before rupture.

That said, they are a single point of failure,
never a good engineering practice,and i's
personally opt for the triangular bracing and
a safety chain, whenever possible.



Also, are those poles galvanized or just painted? If just painted,
they should be inspected carefully before you tow the trailer (a good
idea even if it it galvanized). It just takes a pin hole in the paint
to allow rain and dew to seep into the steel where it can rust
unnoticed. This is not to say that galvanizing will not rust,
especially if it is welded, but the chance is considerably less and
the zinc finish will take more abuse than paint will.

Yes, unfortunately zinc reacts readily with acid.
Rainwater is naturally slightly acidic, in part due
to nitric acid created in the atmosphere by lighting,
an effect dwarfed by the ubiquitous acid rain we
have heard so much about in recent decades.

Painting galvanized steel may help it to last longer
but makes it much harder to evaluate the condition
of the underlying metal.


I have a Swan trailer (also German) for my LS-1c and it has a
triangular tongue, made of C-channel, more like what we typically see
on American trailers, and it is galvanized. A triangular tongue helps
spread out he load and stresses. You still have a single point where
the trailer hitch is located, but it is rather beefier and the arm is
considerably less than a single 3 or 4 foot pole. You also have a
single point on each leg of the tongue where it attaches to the
trailer, so you still need to be mindful of the up and down loads, but
there should be less risk than with a round pole. I have no proof for
this, but the C-channel cross-section triangular tongue should be
stronger than a round pole in all instances.

The idea of putting the chains on the trailer instead of the tongue is
a good one and even though I have more confidence in the Swan tongue,
I think I am going to investigate this idea.

Yes, the safety chain then spans what would otherwise be
single point of failure, giving you redundancy.


On Mar 9, 11:45 pm, wrote:
...

I called AAA who told me that they'll help with my car, but not with
trailer problems. So I thanked the agent (not really) and called back
to report my "car" problems, asking that they send a flat bed truck.
The driver arrived shortly thereafter, put the trailer up on the flat
bed, and delivered the trailer a few miles away to the repair shop
that recently repacked the wheel bearings.

I'm extremely impressed that you actually got through to
the AAA, not once, but twice, in the same day!

--

FF